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Thursday, May 5, 2011

Treatment option of Depression

Treatment decisions for unipolar depression
. Treatment of choice – second-generation antidepressants such as a SSRI, venlafaxine, mirtazepine, reboxetine, moclobemide.
. Switching – alternative second-generation antidepressant, usually from another group.
. Augmentation of antidepressant response – add lithium, thyroid hormone, an atypical antipsychotic (e.g. risperidone, olanzepine), pindolol, buspirone.
. Other options – ECT, St John’s Wort.
Note: In a survey of 13 studies, switching from an SSRI to either another SSRI or to imipramine, venlafaxine, mirtazepine or buproprion resulted in a response rate of 30–90%.

Electroconvulsive shock treatment
One of the pioneers in the application of electroconvulsive shock treatment (ECT) was the Italian clinician Cerletti who stated that the ‘‘. . . electricity itself is of little importance . . . the important and fundamental factor is the epileptic-like seizure no matter how it is obtained’’. ECT is undoubtedly an effective treatment for a range of psychiatric diseases varying from severe depression and mania to some forms of schizophrenia. Despite the considerable use of ECT over the last 50 years, it still arouses intense emotional and scientific debate. While the opposition to the use of ECT has been more evident in some Continental European countries and the United States than in Britain or Ireland, it was a British study of the use of ECT which, following a survey of over 100 centres, found that many units were badly equipped and had poor facilities and staff training. This report resulted in a considerable improvement in the application of ECT, with the establishment of guidelines governing the managemen and use of the technique; somewhat similar guidelines were instituted by the American Psychiatric Association. It is now generally agreed that ECT is singularly effective and useful. There has been controversy over the relative merits in using unilateral or bilateral ECT. In general, it would appear that unilateral ECT is effective in the treatment of most depressed patients, whereas manic patients appear to respond best to bilateral ECT. Following a course of treatment, twice weekly for several weeks, the success rate in treating depression is about 80%. This is more successful than using antidepressants (up to 70% for a single course of treatment). Seizure monitoring is essential to ensure an adequate response. The principal side effect of ECT is a temporary cognitive deficit, specifically memory loss. There is evidence that such impairment is reduced if unilateral ECT is applied to the non-dominant hemisphere. Brief pulse-current ECT machines are now favoured in Britain and the United States to ensure opt imal efficacy and minimal side effects. As Cerletti hypothesized in 1938, chemically induced seizures are equally effective as ECT and at one time pentamethylenetetrazole or flurothyl were used to produce seizures. However, the safety and ease of application of ECT means that such methods have been largely replaced. While there are various psychological, neurophysiological and neuroendocrine theories that have been developed to explain the beneficial effects of ECT, most attention has been given to the manner in which ECT causes changes in those neurotransmitters that have been implicated in psychiatric illness. It is known that the rise in the seizure threshold during the course of treatment, and the corresponding alteration in cerebral blood flow, may reflect profound changes in cerebral metabolism that could be of crucial importance regarding the action of ECT. Changes in the hypothalamo– pituitary–adrenal axis have also been reported, but most studies suggest that such changes are secondary to the clinica l response. The major emphasis of research has therefore been in the functional changes in brain neurotransmission, but it must be emphasized that most detailed studies have been conducted in rodents and therefore their precise relevance to changes in the human brain are a matter of conjecture.

Experimental studies in rodents have largely centred on the changes in biogenic amine neurotransmitters following chronic ECT treatments. Under these conditions, noradrenaline and 5-HT have been shown to be increased; the number of presynaptic alpha2 receptors and their functional activity has been shown to be decreased, as has the functional activity of the dopamine autoreceptors. Such changes have also been found following the chronic administration of antidepressant drugs. The most consistent changes reported have been those found in postsynaptic receptor function. The functional activity of the postsynaptic beta adrenoceptors is decreased, a change which is also found with antidepressants. The postsynaptic 5-HT2 receptor sensitivity is enhanced by chronic ECT and antidepressant treatment. Thus there appears to be a consistency between the chronic effects of both ECT and antidepressants in enhancing 5-HT responsiveness and diminishing that of noradrenaline. Regarding the dopaminergic system, while there is speculation that changes in the activity of this system may be important in the action of novel antidepressants such as bupropion, the only consistent changes found following chronic application of ECT and antidepressants is a functional decrease in the dopamine autoreceptor activity. This would lead to a reduction in the release of this transmitter. In contrast to the plethora of animal studies, few clinical studies have shown consistent changes in the biogenic amines. There is evidence that the urinary and CSF concentrations of the noradrenaline metabolites normetanephrine and MHPG are decreased, suggesting that the turnover of noradrenaline is decreased, the opposite to that found in animals. Neuroendocrine challenge tests that have been used as probes to assess central noradrenergic function (e.g. with clonidine) show no consistent changes in patients following chronic ECT. Consistent changes have been reported in serotonergic function, however, with enhanced prolactin release occurring in response to a thyroid-stimulating hormone challenge. This is in agreement with the view that chronic ECT sensitizes postsynaptic 5-HT2 receptors. Furthermore, platelet imipramine binding, which according to the results of some studies is increased in the untreated depressed patient, is attenuated by both antidepressant and ECT treatments, although it must be emphasized that not all investigators can replicate these findings. The transport of [3H]5-HT into the platelets of depressed patients is also normalized following ECT and chronic antidepressant treatments. There is no evidence of any change in the dopaminergic system in depressed patients following ECT. The central cholinergic system has been implicated in the pathogenesis of affective disorder and in memory function, which is frequently found to be malfunctioning in depressed patients. The memory deficit elicited by chronic ECT in both patients and animals may be related to the decreased density and function of central muscarinic receptors, but it should be emphasized that the changes reported in cholinergic function are small and their relevance to the clinical situation remains to be established. Brain GABA is closely associated with the induction of seizures. In animals, chronic ECT decreases GABA synthesis in the limbic regions. While consistent changes in GABA-A receptor activity have not been reported, it would appear that GABA-B receptor density increases in the limbic regions following chronic ECT. This is qualitatively similar to the changes that have been reported following antidepressant treatment. The recent interest in the involvement of GABA in the aetiology of depression and in the mode of action of antidepressants is based on the hypothesis that GABA plays a key role not only in the induction of seizures but also in modulating the changes in the serotonergic system that are induced by both antidepressants and ECT. Due to the ubiquitous distribution of peptides as cotransmitters and neuromodulators in the brain, it is not surprising to find that ECT produces changes in their concentrations and in their possible functional activity. Increased metenkephalin concentrations have been reported following chronic ECT. Such changes may be due to increased opioid receptor binding sites. Opioid-mediated behavioural changes such as catalepsy and reduced pain responses are increased following ECT in animals. Whether such changes are relevant to the effects of ECT and antidepressants in depressed patients is still unknown. Other possibilities that have been suggested as a cause of the antidepressant action of ECT include an enhanced adenosine1 receptor density in the cortex; agonists at these receptor sites are known to have anticonvulsant properties, while antagonists such as caffeine can cause convulsions, at least in high doses. Thyroid-stimulating hormone activity has also been shown to be enhanced. This peptide may exert antidepressant effects in its own right, but may also act by modulating both serotonergic and dopaminergic activity.

In general overview and summary, it would appear that ECT produces a number of changes in central neurotransmission that are common to antidepressants. These include a decrease in the functional activity of beta adrenoceptors and an enhanced activity of 5-HT2 and possibly GABA-B receptors. The functional defect in central muscarinic receptors may be associated with the memory deficits caused by ECT treatment. It must be emphasized that the changes reported have largely been derived from animal experiments and their precise relevance to the mode of action of ECT in man is still a matter of conjecture. Adverse effects of drug treatment for depression TCAs Significant side effects have been estimated to occur in about 5% of patients on TCAs, most of these effects being attributed to their antimuscarinic properties, for example, blurred vision, dry mouth, tachycardia and disturbed gastrointestinal and urinary tract function. Orthostatic hypotension due to the block of alpha1 adrenoceptors and sedation resulting from antihistaminic ac tivity frequently occur at therapeutic doses, particularly in the elderly. Excessive sweating is also a fairly common phenomenon, but its precise mechanism is uncertain. In the elderly patient, the precipitation of prostatic hypertrophy and glaucoma by the TCAs is also a frequent cause of concern.

Adverse effects of the TCAs on the brain include confusion, impaired memory and cognition and occasionally delirium; some of these effects have been reported to occur in up to 30% of patients over the age of 50. These effects may occasionally be confused with a recurrence of the symptoms of depression and are probably due to the central antimuscarinic activity of these drugs. Tremor also occurs frequently, particularly in the elderly, and may be controlled by the concurrent administration of propranolol. Neuroleptics are normally not recommended to be used in combination with TCAs as they are liable to accentuate the side effects of the latter drugs. The risk of seizures, and the switch from depression to mania in bipolar patients, has also been reported following TCA administration. Weight gain is a frequent side effect and is of considerable concern, particularly in the female patient, an effect probably associated with increased appetite. Other less common side effects include jaundice (particularly with imipramine), agranulocytosis and skin rashes.

Acute poisoning
This occurs all too frequently with the TCAs and can be life threatening. Death has been reported with doses of 2000mg of imipramine, or the equivalent quantity of other TCAs, which approximates to 10 daily doses or less! Severe intoxication has been reported at doses of 1000 mg. Because of the toxicity of these drugs and the nature of the illness, in which suicidal thoughts are a common feature, it is generally recommended that no more than a 1 week’s supply should be given at any one time to an acutely depressed patient. The symptoms of overdose are to some extent predictable from the antimuscarinic and adrenolytic activity of these drugs. Excitement and restlessness, sometimes associated with seizures, and rapidly followed by coma, depressed respiration, hypoxia, hypotension and hypothermia are clear signs of TCA overdose. Tachycardia and arrhythmias lead to diminished cardiac function and thus to reduced cerebral perfusion, which exacerbates the central toxic effects. It is generally accepted that dialysis and forced diuresis are useless in counteracting the toxicity, but activated charcoal may reduce the absorption of any unabsorbed drug. The risk of cardiac arrhythmias may extend for several days after the patient has recovered from a TCA overdose. It is partly due to the toxicity of the TCAs that the newer non-tricyclic drugs have been developed. All the evidence suggests that the non-tricyclics are much safer in overdose.

Drug interactions
Another area of concern regarding the use of the TCAs is their interaction with other drugs which may be given concurrently. Such interactions may arise due to the drugs competing for the plasma protein binding sites (e.g. phenytoin, aspirin and the phenothiazines) or for the liver microsomal enzyme system responsible for the common metabolism of the drugs (e.g. steroids, including the oral contraceptives, sedatives, apart from the benzodiazepines, and the neuroleptics). All of the TCAs potentiate the sedative effects of alcohol and any other psychotropic drug with sedative properties given concurrently. Smoking potentiates the metabolism of the TCAs. There is a well-established interaction between the TCAs and the adrenergic neuron blocking antihypertensives (e.g. bethanidine and guanethidine) which results from the TCA impeding the uptake of the neuron blocker into the sympathetic nerve terminal, thereby preventing it from exerting its pharmacological effects. There is also a rare, but occasionally fatal, interaction between TCAs and MAOIs in which hyperpyrexia, convulsions and coma can occur. The precise mechanism by which this is brought about is unclear, but it may be associated with a sudden release of 5-HT. Following prolonged TCA administration, abrupt withdrawal of the drug can lead to generalized somatic or gastrointestinal distress, which may be associated with anxiety, agitation, sleep disturbance, movement disorders and even mania. Such symptoms may be associated with central and peripheral cholinergic hyperactivity that is a consequence of the prolonged muscarinic receptor blockade caused by the TCAs.

The toxic effects of these drugs may arise shortly after an overdose, the effects including agitation, hallucinations, hyperreflexia and convulsions. Somewhat surprisingly, both hypo- and hypertension may occur, the former symptoms arising due to the accumulation of the inhibitory transmitter dopamine in the sympathetic ganglia leading to a marked reduction in ganglionic transmission, while hypertension can result from a dramatic release of noradrenaline from both central and peripheral sources. Such toxic effects are liable to be prolonged, particularly when the older irreversible inhibitors such as phenelzine and tranylcypromine are used.

Treatment of such adverse effects should be aimed at controlling the temperature, respiration and blood pressure.

The toxic effects of the MAOIs are more varied and potentially more serious than those of the other classes of antidepressants in common use. Hepatotoxicity has been reported to occur with the older hydrazine type of MAOIs and led to the early demise of iproniazid; the hepatotoxicity does not appear to be related to the dose or duration of the drug administered. Excessive central stimulation, usually exhibited as tremors, insomnia and hyperhidrosis, can occur following therapeutic doses of the MAOIs, as can agitation and hypomanic episodes. Peripheral neuropathy, which is largely restricted to the hydrazine type of MAOI, is rare and has been attributed to a drug-induced pyridoxine deficiency. Such side effects as dizziness and vertigo (presumably associated with hypotension), headache, inhibition of ejaculation (which is often also a problem with the TCAs), fatigue, dry mouth and constipation have also been reported. These side effects appear to be more frequently associated with phenelzine use. They are not associated with any antimuscarinic properties of the drug but presumably arise from the enhanced peripheral sympathetic activity which the MAOIs cause.

Drug interactions
Predictable interactions occur between the MAOIs and any amine precursors, or directly or indirectly acting sympathomimetic amines (e.g. the amphetamines, phenylephrine and tyramine). Such interactions can cause pronounced hypertension and, in extreme cases, stroke. MAOIs interfere with the metabolism of many different classes of drugs that may be given concurrently. They potentiate the actions of general anaesthetics, sedatives, including alcohol, antihistamines, centrally acting analgesics (particularly pethidine due to an enhanced release of 5-HT) and anticholinergic drugs. They also potentiate the actions of TCAs, which may provide an explanation for the use of such a combination in the treatment of therapy-resistant depression. The ‘‘cheese effect’’ is a well-established phenomenon whereby an aminerich food is consumed while the patient is being treated with an irreversible MAOI. Foods which cause such an effect include cheeses, pickled fish, yeast products (red wines and beers, including non-alcoholic v arieties), chocolate and pulses such as broad beans (which contain dopa). It appears that foods containing more than 10mg of tyramine must be consumed in order to produce a significant rise in blood pressure. Furthermore, it is now apparent that there is considerable variation in the tyramine content of many of these foods even when they are produced by the same manufacturer. Therefore it is essential that all patients on MAOIs should be provided with a list of foods and drinks that should be avoided.

Changing a patient from one MAOI to another, or to a TCA, requires a ‘‘wash-out’’ period of at least 2 weeks to avoid the possibility of a drug interaction. There is evidence to suggest that a combination of an MAOI with clomipramine is more likely to produce serious adverse effects than occurs with other TCAs. Regarding the newer non-tricyclic antidepressants, it is recommended that a ‘‘wash-out’’ period of at least 5 weeks be given before a patient on fluoxetine is given an MAOI; this is due to the very long half-life of the main fluoxetine metabolite norfluoxetine. Although it is widely acknowledged that the older MAOIs have the potential to produce serious adverse effects, the actual reported incidence is surprisingly low. Tranylcypromine was one of the most widely used drugs, involving several million patients by the mid 1970s, and yet only 50 patients were reported to have severe cerebrovascular accidents and, of these, only 15 deaths occurred. Nevertheless, it is generally recommended that this drug sh ould not be given to elderly patients or to other patients with hypertension or cardiovascular disease.

Second-generation antidepressants
With the possible exception of maprotiline, which is chemically a modified TCA with all the side effects attributable to such a molecule, all of the newer non-tricyclic drugs have fewer anticholinergic effects and are less cardiotoxic than the older tricyclics. Lofepramine is an example of a modified tricyclic that, due to the absence of a free NH2 group in the side chain, is relatively devoid of anticholinergic side effects. Thus by slightly modifying the structure of the side chain it is possible to retain the efficacy while reducing the cardiotoxicity.

Of the plethora of new 5-HT uptake inhibitor antidepressants (e.g. zimelidine, indalpine, fluoxetine, fluvoxamine, citalopram, sertraline and paroxetine), the most frequently mentioned side effects following therapeutic administration are mild gastrointestinal discomfort, which can lead to nausea and vomiting, occasional diarrhoea and headache. This appears to be more frequent with fluvoxamine than the other SSRI antidepressants. Such changes are attributable to increased peripheral serotonergic function. Some severe idiosyncratic and hypersensitivity reactions such as the Gullain–Barre´ syndrome and blood dyscrasias have led to the early withdrawal of zimelidine and indalpine. For the well-established antidepressants such as fluoxetine, the side effects appear to be mild and welltolerated, although akathisia and agitation have been reported and may be more pronounced in elderly patients. Nomifensine and bupropion are examples of non-tricyclic antidepressants that facilitate catecholaminergic function. These drugs have the advantage over the TCAs of being non-sedative in therapeutic doses. The rare, although fatal, occurrence of haemolytic anaemia and pyrexia following therapeutic administration of nomifensine led to its withdrawal from the market a few years ago. Bupropion was also temporarily withdrawn from clinical use following evidence of seizure induction, but it has now returned to the market in the United States. Idiosyncratic reactions have been reported to occur with the tetracyclic antidepressant mianserin, several cases of agranulocytosis have been reported in different countries. Elderly patients would appear to be most at risk from such adverse effects. Whether such side effects are a peculiarity of the mianserin structure or will also be found with the 6-aza derivative, mirtazepine, is uncertain but preliminary evidence from post-marketing surveys suggests that this is unlikely. Other frequent side effects associated with therapeutic doses of mianserin are sedation and orthostatic hypotension; sedation and weight gain are also problems with mirtazepine. Clearly the major advantage of all the recently introduced antidepressants lies in their relative safety in overdosage and reduced side effects. These factors are particularly important when considering the need for optimal patient compliance and in the treatment of the elderly depressed patient who is more likely to experience severe side effects from antidepressants.

Treatment-resistant depression
It has been estimated that at least 30% of patients with major depression fail to respond to a 6-week course of a TCA antidepressant. A major problem arises however in the definition of ‘‘treatment resistance’’. To date, there appears to be no internationally acceptable definition of the condition. A practical definition which many clinicians find useful is that treatment resistance occurs when the patient fails to respond to:
1. An antidepressant given at maximum dose for 6–8 weeks.
2. An antidepressant from another group administered for 6–8 weeks.
3. A full course of ECT.

The following possibilities may then be considered should the patient fail to respond:
1. Add lithium to a standard antidepressant (e.g. an SSRI) maintaining the plasma lithium concentration at 0.4–0.6mmol/l. This is a wellestablished method with approximately 50% of the patients responding.
However, the plasma lithium concentration must be monitored.
2. Administer a high therapeutic dose of a ‘‘dual action’’ antidepressant such as venlafaxine or possibly mirtazepine. A discontinuation syndrome may occur if venlafaxine is abruptly withdrawn. The symptoms of the discontinuation reaction, which also occur occasionally when some of the SSRIs are abruptly withdrawn, include dizziness, ‘‘electroshock’’ sensations, anxiety and agitation, insomnia, flu-like symptoms, diarrhoea and abdominal pain, parathesis, nausea.
3. Add tri-iodothyronine to a standard antidepressant. This combination is usually well tolerated but monitoring the plasma T3 concentration is important.
4. Add tryptophan to a standard antidepressant (usually an SSRI). There is a danger that the serotonin syndrome may occur however and occasionally the eosinophilia myalgia syndrome. The symptoms that occur with increasing severity are restlessness, diaphoresis, tremor, shivering, myoclonus, confusion, convulsions, death.
5. Add pindolol (a 5-HT1D antagonist as well as a beta adrenoceptor antagonist), which is well researched but there are contradictory findings in the literature with regard to its efficacy. So far, the clinical data suggest that the response to a standard antidepressant is accelerated.
6. Add dexamethasone to a standard antidepressant. This combination is well tolerated for a short course of treatment but so far there is only limited evidence of efficacy from the literature.
7. Add lamotrigine to a standard antidepressant. Again, the support for this approach is largely anecdotal.
8. Add buspirone to a standard antidepressant (usually an SSRI). The evidence in favour of this combination is largely anecdotal.
9. Add an atypical antipsychotic (e.g. olanzapine or risperidone). There is some ‘‘open trial’’ evidence in favour of such combinations.
10. Add mirtazepine to a standard antidepressant (usually an SSRI). Again, the evidence is largely anecdotal.
There are a number of other methods mentioned in the literature, some of which (such as the combination of a TCA with an MAOI) are potentially cardiotoxic and not to be recommended. More recently, a combination of an SSRI with a TCA has become popular but is not to be recommended because of the probability of metabolic interactions involving the cytochrome P450 system that can increase the tissue concentration of even a modest dose of a TCA to a cardiotoxic level.

Physical pain and depression

Physical pain and depression
Major depression is a triad of psychological, somatic and physical symptoms. Over 75% of depressed patients report painful physical symptoms involving the neck, back, head, stomach and the skeletomuscular system. Not only can chronic pain lead to depression, but also vice versa. Fibromyalgia, accounting for 2–4% of the general population, is a common cause of chronic pain. It has been estimated that 20–40% of such patients have co-morbid depression with a lifetime prevalence of about 70%. This raises the question whether there is a common mechanism linking pain and depression. Neuroanatomically both the locus coeruleus and the raphe´ nuclei project to the spinal cord where they gate sensory pathways from the skeletomuscular areas. As there is evidence that both noradrenaline and 5-HT are dysfunctional in depression, it is perhaps not surprising to find that the pain threshold is often reduced in patients with depression. Conversely, different types of antidepressants have been shown to have an antinociceptive effect in both rodent models of neuropathic pain, and clinically in fibromyalgia, chronic fatigue syndrome, postherpetic neuralgia and diabetic neuropathy. In general, it would appear that the dual action antidepressants (such as the TCAs and SNRIs) are more effective than the SSRIs.
Physical pain and depression
Major depression is a triad of psychological, somatic and physical symptoms. Over 75% of depressed patients report painful physical symptoms involving the neck, back, head, stomach and the skeletomuscular system. Not only can chronic pain lead to depression, but also vice versa. Fibromyalgia, accounting for 2–4% of the general population, is a common cause of chronic pain. It has been estimated that 20–40% of such patients have co-morbid depression with a lifetime prevalence of about 70%. This raises the question whether there is a common mechanism linking pain and depression. Neuroanatomically both the locus coeruleus and the raphe´ nuclei project to the spinal cord where they gate sensory pathways from the skeletomuscular areas. As there is evidence that both noradrenaline and 5-HT are dysfunctional in depression, it is perhaps not surprising to find that the pain threshold is often reduced in patients with depression. Conversely, different types of antidepressants have been shown to have an antinociceptive effect in both rodent models of neuropathic pain, and clinically in fibromyalgia, chronic fatigue syndrome, postherpetic neuralgia and diabetic neuropathy. In general, it would appear that the dual action antidepressants (such as the TCAs and SNRIs) are more effective than the SSRIs.

Tricyclic antidepressants (TCAs), MAOIs(monoamine oxidase inhibitors), SSRI, Classification of antidepressants, and Other Drug Treatments

Tricyclic antidepressants (TCAs)
This group of drugs was introduced during the early 1960s following the chance discovery of the antidepressant effects of imipramine. The therapeutic efficacy of the TCAs has been ascribed to their ability to inhibit the reuptake of noradrenaline and serotonin into the neuron following the release of these transmitters into the synaptic cleft. In addition, these drugs inhibit the muscarinic receptors (causing dry mouth, impaired vision, tachycardia, difficulty in micturition), histamine type-1 receptors (causing sedation) and alpha-1 adrenoceptor antagonism (causing postural hypotension). Such side effects often lead to non-compliance (estimated to be at least 40% in general practice situations) and are more frequent in the elderly. The excellent clinical efficacy of the TCAs has been well documented and the pharmacokinetic profiles are favourable. The most serious disadvantage of the TCAs lies in their cardiotoxicity. Thus, with the exception of lofepramine, all the tricyclic antidepressants, including mapro tiline, block the fast sodium channels in the heart which can lead to heart block and death. Approximately 15% of all patients with major depression die by suicide and a high proportion of these (up to 25%) do so by taking an overdose of TCAs. Such a dose can be as low as 5–10 times the recommended daily dose.Lofepramine differs from the other TCAs in that its structure seems to preclude it from causing the anticholinergic, antihistaminergic and antiadrenergic effects evident with the other TCAs. In addition, it does not appear to be any more cardiotoxic than most of the second-generation antidepressants. The reason for this is an enigma, as the main metabolite of lofepramine is desipramine, a typical cardiotoxic TCA. There is a suggestion that, due to its high lipophilicity, it impedes the access of desipramine to the sodium fast channels in the heart without interfering with their normal function. Thus lofepramine would appear to fulfil many of the requirements of a safe and effective TCA; it has been widely used, particularly in elderly depressed patients, in the past in both the UK and Ireland.

Irreversible inhibitors of monoamine oxidase (MAOIs)
Iproniazid, an MAOI no longer available because of its hepatotoxicity, was the first effective antidepressant to be discovered; it was introduced shortly before the discovery of imipramine. All MAOIs are presumed to have a similar mode of action, namely to inhibit the intra- and interneuronal metabolism of the biogenic amine neurotransmitters (noradrenaline, dopamine and serotonin). These amines are primarily metabolized by MAO-A (noradrenaline and serotonin) or MAO-B (dopamine). The irreversible MAOIs are inhibitors of MAO-A while selegiline (deprenyl), used as an adjunctive treatment for Parkinson’s disease, is a selective, irreversible inhibitor of MAO-B. The main limitation to the clinical use of the MAOIs is due to their interaction with amine-containing foods such as cheeses, red wine, beers (including non-alcoholic beers), fermented and processed meat products, yeast products, soya and some vegetables. Some proprietary medicines such as cold cures contain phenylpropanolamine, ephedrine, etc. and will also interact with MAOIs. Such an interaction (termed the ‘‘cheese effect’’), is attributed to the dramatic rise in blood pressure due to the sudden release of noradrenaline from peripheral sympathetic terminals, an event due to the displacement of noradrenaline from its intraneuronal vesicles by the primary amine (usually tyramine). Under normal circumstances, any dietary amines would be metabolized by MAO in the wall of the gastrointestinal tract, in the liver, platelets, etc. The occurrence of hypertensive crises, and occasionally strokes, therefore limited the use of the MAOIs, despite their proven clinical efficacy, to the treatment of atypical depression and occasionally panic disorder. The side effects of the MAOIs include, somewhat surprisingly, orthostatic hypotension. This is thought to be due to the accumulation of dopamine in the sympathetic cervical ganglia where it acts as an inhibitory transmitter, thereby reducing peripheral vascular tone. Other side effects include psychomotor restlessness and sleep disorder. The MAOIs are cardiotoxic but probably less so than the TCAs. Potentially fatal interactions can however occur when MAOIs are combined with SSRIs or any type of drug which enhances serotonergic function. The interaction can give rise to hyperexcitability, increased muscular tone, myoclonus and loss of consciousness.Reversible inhibitors of monoamine oxidase (RIMAs) Antidepressants of this class, such as moclobemide, have a high selectivity and affinity for MAO-A. However, unlike the MAOIs, the RIMAs are reversible inhibitors of the enzyme and can easily be displaced from the enzyme surface by any primary amine which may be present in the diet. This means that the dietary amines are metabolized by MAO in the wall of the gastrointestinal tract while the enzyme in the brain and elsewhere remains inhibited. Thus the RIMAs have brought the MAOIs back into use as antidepressants in general practice. It is now evident that the RIMAs are not as potent as most currently available antidepressants.

Selective serotonin reuptake inhibitors
Zimelidine was the first SSRI antidepressant to be launched in Europe and, despite its therapeutic success, was withdrawn in the late 1980s due to severe abdominal toxicity. Zimelidine was soon replaced by fluvoxamine which only slowly received acceptance in Europe because of the high incidence of nausea and vomiting; the recommended starting dose was initially too high. Fluoxetine was the third SSRI to be launched in Europe with the advantage of a fixed daily dose (20 mg) and relatively few side effects. Sertraline, paroxetine and citalopram followed so that by the end of the 1980s, the five SSRIs were well established throughout Europe and most of the world.

As the name implies, these drugs have a high affinity for the serotonin transporter both on neuronal and also platelet membranes. There is abundant evidence that the SSRIs inhibit the reuptake of 3H-5-HT into platelets, brain slices and synaptosomal fractions, as illustrated in Table 7.10, but it is clear that there is no direct relationship between the potency of the drug to inhibit 5-HT reuptake in vitro and the dose necessary to relieve depression in the clinic. In experimental studies, it is clear that the increased release of 5-HT from the frontal cortex only occurs following the chronic (2 weeks or longer) administration of any of the SSRIs. Thus the inhibition of 5-HT reuptake may be a necessary condition for the antidepressant activity, but it is not sufficient in itself.

Despite their common ability to enhance serotonergic function in vivo, the SSRIs differ both in terms of their pharmacological profiles and their pharmacokinetics. Thus in addition to their direct inhibitory action on the serotonin transporter, they also affect other neurotransmitter systems which may have some clinical relevance. Citalopram has a modest antihistamine action which might account for its slightly sedative action. Sertraline has slight dopaminomimetic effect which may contribute to its alerting effect, while paroxetine is a muscarinic receptor antagonist. Both fluvoxamine and sertraline have affinity for sigma 1 receptors, the precise importance of which is uncertain but could contribute to the motor side effects which all the SSRIs are reputed to have, albeit very rarely. Fluoxetine, by activating 5-HT 2C receptors, may cause anxiety at least in some patients. Thus differences between the SSRIs are due not only to their different potencies as 5-HT reuptake inhibitors, but also because of their actions on other receptor systems. These differences may be of clinical importance in terms of the special populations to whom the drugs should be administered. Sertraline could also be considered for this group and while drug interactions could be more problematic it does have a slightly alerting profile which could be beneficial. Fluvoxamine has also been extensively studied in the elderly, but nausea could be a problem while fluoxetine, with its very long half-life (with its active metabolite norfluoxetine, amounting to 12 days in the elderly patient) could be beneficial for the non-compliant patient. In the elderly, fluoxetine could cause some anorexia and weight loss however. Paroxetine should be administered with more care in the elderly because of its anticholinergic action. In addition to their proven efficacy in the treatment of all types of depression, the SSRIs have been shown to be the drugs of choice in the treatment of panic disorder, obsessive–compulsive disorder, bulimia nervosa, and as an adjunct to the treatment of alcohol withdrawal and relapse prevention, premenstrual dysphoric disorder and post-traumatic stress disorder. The usefulness of these drugs in treating such a diverse group of disorders reflects the primary role of serotonin in the regulation of sleep, mood, impulsivity and food intake. All the SSRIs have qualitatively similar side effects that largely arise from the increase in serotonergic function and the resulting activation of the different 5-HT receptor types in the brain and periphery. There are differences in the frequency of these effects however which would not be anticipated if all the SSRIs were essentially the same! These effects include nausea, vomiting, diarrhoea or constipation, insomnia, tremor, initial anxiety, dizziness, sexual dysfunction and headache. Loss in body weight may occur but this is rare. The behavioural toxicity of the SSRIs as indicated by their effects on psychomotor function, memory and learning, is low, particularly when compared to the TCAs and same of the sedative secondgeneration antidepressants such as mianserin, mirtazepine and trazodone.

The SSRIs are not cardiotoxic and safety in overdose has been indicated for all these drugs. In general, the severity of the adverse effects is slight and seldom leads to non-compliance. In addition to the five SSRIs currently available, many more compounds are in development which will no doubt be marketed in the near future. Of the new arrivals, escitalopram, the S-enantiomer of citalopram, has already become available in many European countries.

Classification of antidepressants available in Europe
. Antidepressants that inhibit monoamine reuptake
Tricyclic antidepressants (TCAs) – first-generation antidepressants
Examples – tertiary amine type: imipramine, amitriptyline, dothiepin,
– secondary amine type: desipramine, nortriptyline
– other effects: potent antagonists of muscarinic, histaminic and
alpha-1 adrenergic receptors; cardiotoxic
Modified TCA-lofepramine – non-cardiotoxic; low affinity for muscarinic and
alpha-1 adrenoceptors
. Inhibitors of noradrenaline reuptake (NARIs)
Examples – maprotiline: a ‘‘bridged’’ tricyclic with affinity for histamine, H1,
and alpha-1 adrenoceptors. Causes seizures
– reboxetine: not cardiotoxic; does not have an affinity for any
neurotransmitter receptors
. Inhibitors of serotonin reuptake (SSRIs)
Examples – citalopram (1), sertraline (2), fluoxetine (3), paroxetine (4),
Slight affinity for (1) histamine, (2) dopamine, (3) serotonin, (4) muscarinic
receptors (see text)
. Specific inhibitors of noradrenaline and serotonin reuptake (SNRIs)
Examples – venlafaxine (more potent inhibitor of 5-HT than noradrenaline
– milnacipran (more potent inhibitor of noradrenaline than 5-HT
. Antidepressants that inhibit monoamine metabolism
Irreversible monoamine oxidase inhibitors (MAOIs)
Examples – phenelzine, pargyline, tranylcypromine, isocarboxazid. All
interact with dietary monoamine to cause the ‘‘cheese effect’’
(see text)
. Reversible inhibitors of monoamine oxidase A (RIMAs)
Examples – moclobemide, pirlindole. At therapeutic doses unlikely to interact
with dietary amines (see text)
. Tetracyclic antidepressants
Examples – mianserin (1), mirtazepine (6-aza-mianserin) (2)
(1) The first second-generation antidepressant; an alpha-2 adrenoceptor antagonist with some affinity for 5-HT1A, 5-HT2A and 5-HT3, alpha-1 adrenoceptors and H1 receptors
(2) Known as a noradrenaline and specific serotonin antidepressant (NaSSA).

More potent affinity for alpha-2 adrenoceptors and 5-HT receptors than mianserin; H1 antagonist
. Other antidepressants (sometimes called ‘‘atypical’’)
Examples – trazodone, nefazodone: 5-HT1A and 5-HT2 antagonists, weak SSRI activity; alpha-1 and H1 antagonism

Noradrenaline reuptake inhibitors (NRIs)
Reboxetine is the only selective and reasonably potent noradrenaline reuptake inhibitor available clinically at the present time. Reboxetine has a chemical structure not dissimilar from viloxazine, an antidepressant which was of only limited clinical interest in the 1970s because of its weak efficacy and unacceptable side effects (nausea, vomiting and occasionally seizures). Unlike the secondary amine TCA antidepressants, such as maprotiline, desipramine, nortriptyline and protriptyline, reboxetine does not affect any other transporter or receptor system and therefore is largely devoid of TCA and SSRI-like side effects. In clinical trials, reboxetine has been shown to be as effective as the SSRIs in the treatment of depression but, unlike the SSRIs, reboxetine does not inhibit any of the cytochrome P450 enzymes in the liver.

In contrast to the widespread interest in 5-HT in depression research and in the development of antidepressants, there would appear to be little interest in developing antidepressants that selectively modulate the noradrenergic system. At the present time, there do not appear to be any drugs of this type in development. For completeness, buproprion should be mentioned even though it is not widely registered as an antidepressant in Europe, partly because of its propensity to cause seizures in some patients. Buproprion, quite widely used in the USA as an antidepressant, appears to inhibit the reuptake of both dopamine and noradrenaline and therefore tends to have a slightly alerting action. In many European countries it has recently been introduced, at a lower than antidepressant dose, in the treatment of nicotine withdrawal in smoking cessation programmes. Lastly, nomifensine was an interesting antidepressant that also had noradrenaline, dopamine and, due to its 4-hydroxy metabolite, serotonin reuptake properties. It was withdrawn some years ago because of the occurrence of haemolytic anaemia in a small number of patients. It was a particularly effective drug in the treatment of depression in patients with epilepsy as, unlike many antidepressants available at that time, it did not affect the seizure threshold.

Selective serotonin and noradrenaline reuptake inhibitors (SNRIs)
In an attempt to combine the clinical efficacy of the TCAs with the tolerability and safety of the SSRIs and NRIs, drugs showing selectivity in inhibiting the reuptake of both noradrenaline and serotonin were developed. Being structurally unrelated to the TCAs however, they lacked their side effects including their cardiotoxicity. To date, venlafaxine is the most widely available of the SNRIs. Although it is known to enhance both serotonergic and noradrenergic function, at the lower clinical dose range it primarily enhances serotonergic function and therefore has the characteristic side effects of an SSRI. At higher therapeutic doses, venlafaxine also inhibits noradrenaline reuptake and therefore resembles a TCA antidepressant. While there is no evidence that venlafaxine is as cardiotoxic as the TCAs, recent studies have indicated that it is at least threefold more likely than the SSRIs to result in death if taken in overdose. Hypertension may occur in some patients when given a high therapeutic dose of venlafaxine. A more potent, but qualitatively similar antidepressant to venlafaxine, duloxetine, is currently in advanced clinical development. Milnacipran is also a dual-action antidepressant which, like venlafaxine, has been shown to be more effective than the SSRIs in the treatment of severe, hospitalized and suicidally depressed patients. At lower therapeutic doses, milnacipran blocks the noradrenaline transporters and therefore resembles the NRI antidepressants. Higher doses result in the serotonergic component becoming apparent (i.e. an SSRI-like action). The main problem with milnacipram appears to be its lack of linear kinetics with some evidence that it has a U-shaped dose–response curve.

Tetracyclic antidepressants
Mianserin was the first of the second-generation antidepressants to be developed. It lacked the amine reuptake inhibitory and MAOI actions of the first-generation drugs and also lacked the cardiotoxicity and anticholinergic activity of the TCAs. However, it was sedative (antihistaminic), caused postural hypotension (alpha-1 blockade) and also caused blood dyscrasias and agranulocytosis in a small number of patients. This has limited the use of mianserin in recent years. Mirtazepine (called a Noradrenaline and Selective Serotonin Antidepressant; NaSSA) is the 6-aza derivative of mianserin and shares several important pharmacological properties with its predecessor, namely its antihistaminic and alpha-1 adrenoceptor antagonistic actions. Like mianserin, mirtazepine also causes weight gain. Nevertheless, mirtazepine is better tolerated and there is no evidence of blood dyscrasias associated with its clinical use. Regarding the mode of action of these tetracyclic compounds, both are potent alpha-2 adrenoceptor antagonists which cause an enhanced release of noradrenaline. The action of mirtazepine on serotonin receptors (5- HT1A, 5-HT2A, 5-HT3) is both direct (5-HT2A and 5-HT3) and indirect (5- HT1A). The complexity of the interaction of the drug with both adrenoceptors and serotonin receptors helps to emphasize the importance of the ‘‘cross talk’’ between these two neurotransmitter systems. Thus the antidepressant effects of both mirtazepine and mianserin are related to the enhancement of noradrenaline release (alpha-2 blockade) and 5-HT2A receptor antagonism. In addition, mirtazepine (and to a lesser extent mianserin) blocks 5-HT3 receptors therefore reducing the anxiety and nausea normally associated with drugs that enhance serotonergic function. The anti-anxiety effect of mirtazepine is ascribed to its indirect activation of the 5-HT1A receptors, an effect also seen following the administration of an SSRI.

Other, or atypical, antidepressants
These include trazodone and a derivative of its metabolite nefazodone, both of which are strongly sedative, an effect which has been attributed to their potent alpha-1 receptor antagonism rather than to any antihistaminic effects. A main advantage of these drugs in the treatment of depression is that they appear to improve the sleep profile of the depressed patient. Their antidepressant activity is associated with their weak 5-HT reuptake inhibition and also a weak alpha-2 antagonism. However, unlike most of the second-generation antidepressants, neither drug is effective in the treatment of severely depressed patients. Furthermore, there is some evidence that trazodone can cause arrythmias, and priapism, in elderly patients.

Herbal antidepressants – St John’s Wort (Hypericum officinalis)
St John’s Wort in recent years has become widely used in Europe and North America for the treatment of mild depression. Unlike all other antidepressants mentioned above, St John’s Wort is obtained through herbalists and health food shops in such countries, the exception being Ireland where it can only be obtained on prescription like any other antidepressant. There are at least 12 placebo-controlled studies proving the efficacy of St John’s Wort against standard antidepressants; all these studies show that the mixture of compounds present in St John’s Wort is effective in mild to moderate, but not severe, depression. Of the main active ingredients of the plant, it would appear that hyperfolin is largely responsible for the antidepressant activity. This compound is an inhibitor of the reuptake of noradrenaline, dopamine and serotonin. In addition, it appears to have some NMDA-glutamate receptor antagonist activity, a property which it shares with many other antidepressants.

Antidepressants and changes in neuronal structure

Antidepressants and changes in neuronal structure
Another possible mechanism whereby antidepressants may change the physical relationship between neurons in the brain is by inhibiting neurite outgrowth from nerve cells. In support of this view, it has been shown that the tricyclic antidepressant amitriptyline, at therapeutically relevant concentrations, inhibited neurite outgrowth from chick embryonic cerebral explants in vivo. While the relevance of such findings to the therapeutic effects of amitriptyline in man is unclear, they do suggest that a common mode of action of all antidepressants could be to modify the actual structure of nerve cells and possibly eliminate inappropriate synaptic contacts that are responsible for behavioural and psychological changes associated with depression. There are several mechanisms whereby antidepressants can modify intracellular events that occur proximal to the postsynaptic receptor sites. Most attention has been paid to the actions of antidepressants on those pathways that are controlled by receptor-coupled second messengers (such as cyclic AMP, inositol triphosphate, nitric oxide and calcium binding). However, it is also possible that chronic antidepressant treatment may affect those pathways that involve receptor interactions with protein tyrosine kinases, by increasing specific growth factor synthesis or by regulating the activity of proinflammatory cytokines. These pathways are particularly important because they control many aspects of neuronal function that ultimately underlie the ability of the brain to adapt and respond to pharmacological and environmental stimuli. One mechanism whereby antidepressants could increase the synthesis of trophic factors is by the activation of cyclic AMP-dependent protein kinase which indirectly increases the formation of the transcription factors. There is experimental evidence to show that the infusion of one of these transcription factors (brain derived neutrophic factor) into the midbrain of rats results in antidepressant-like activity, an action associated with an increase in the synthesis of tryptophan hydroxylase, the rate-limiting enzyme in the synthesis of serotonin.

Changes in neuronal structure in depression
1. There is evidence that inadequately treated, or untreated, major depression is associated with a decrease in the hippocampal volume. This could be a consequence of the increase in proinflammatory cytokines and hypercortisolaemia.
2. Experimental evidence suggests that chronic antidepressant treatments increase the formation of transcription factors within the brain which increases neuronal plasticity and leads to recovery.

The effects of antidepressants on endocrine-immune functions

The effects of antidepressants on endocrine-immune functions
Stress is frequently a trigger factor for depression in vulnerable patients. There is clinical evidence to show that CRF is elevated in the cerebrospinal fluid of untreated depressed patients, which presumably leads to the hypercortisolaemia that usually accompanies the condition. One of the consequences of elevated plasma glucocorticoids is a suppression of some aspects of cellular immunity. It is now established that many cellular (for example, natural killer cell activity, T-cell replication) and non-cellular (for example, raised acute phase proteins) aspects are abnormal in the untreated depressed patient. Such observations could help to explain the susceptibility of depressed patients to physical ill health. A link between CRF, the cytokines which orchestrate many aspects of cellular immunity, and the prostaglandins of the E series has been the subject of considerable research in recent years. There is clinical evidence to show that prostaglandin E2 (PGE2) concentrations are raised in the plasma of untreated depressed patients and are normalized following effective treatment with tricyclic antidepressants. Raised PGE2 concentrations in the brain and periphery reflect increased proinflammatory cytokines (particularly tumour necrosis factor, interleukins 1 and 6) which occur as a consequence of increased macrophage activity in the blood and brain. In the brain the microglia functions as macrophages and produces such cytokines locally. Thus the increased synthesis of PGE2 may contribute to the reduction in amine release in the brain that appears to underlie the pathology of depression. It has recently been postulated that several types of antidepressants (e.g. tricyclics, monoamine oxidase inhibitors) normalize central neurotransmission by reducing brain concentrations of both the cytokines and PGE2 by inhibiting central and peripheral macrophage activity together with cyclooxygenase type 2 activity in the brain. Cyclooxygenase is the key enzyme in the synthesis of the prostaglandins. It is not without interest that the usefulness of tricyclic antidepressants in severe rheumatoid arthritis can now be explained by the inhibitory action of such drugs on cyclooxygenase activity in both the periphery and brain. Such changes, together with those in glucocorticoid receptor function, may therefore incrementally bring about the normalization of defective central neurotransmission as a consequence of antidepressant treatment. Whether the inhibition of cyclooxygenase is a common feature of all classes of antidepressants is presently unknown.

The possible role of prostaglandins and cytokines in depression
1. There is evidence that both cellular and non-cellular immunity are abnormal in the depressed patient.
2. The proinflammatory cytokines (interleukins 1 and 6 and tumour necrosis factor alpha) from macrophages are raised in depression. This leads to increased PGE2 synthesis and release which may lead to a reduction in central monoamine release.
3. Chronic antidepressant treatments reduce both the proinflammatory cytokines and PGE2.

The role of the glutamatergic system in the action of antidepressants

The role of the glutamatergic system in the action of antidepressants
Whereas much emphasis has been placed on the monoamine neurotransmitters with respect to the mechanism of action of antidepressants, little attention has been paid to the changes in the glutamate system, the primary excitatory neurotransmitter pathway in the brain. Experimental evidence shows that tricyclic antidepressants inhibit the binding of dizolcipine to the ion channel of the main glutamate receptor, the N-methyl-D-aspartate receptor in the brain. The initial studies have more recently been extended to show that both typical and atypical antidepressants have a qualitatively similar effect by reducing the binding of dizolcipine to the NMDA receptors. Whether this is due to direct action of the antidepressants on the ion channel receptor sites, or an indirect effect possibly involving the modulation of the glycine receptor site, is uncertain, but there is evidence that glycine and drugs modulating the glycine site have antidepressant-like activity in animal models of depression. These results suggest that antidepressants act as functional NMDA receptor antagonists.

Intracellular changes that occur following chronic antidepressant treatment
The recent advances in molecular neurobiology have demonstrated how information is passed from the neurotransmitter receptors on the outer side of the neuronal membrane to the secondary messenger system on the inside. The coupling of this receptor to the secondary messenger is brought about by a member of the G protein family. Beta-adrenoceptors are linked to adenylate cyclase, and, depending on the subtype of receptors, 5-HT is linked to either adenylate cyclase (5-HT1A, 5-HT1B) or phospholipase (5-HT2A, 5-HT2C). Activation of phospholipase results in an intracellular increase in the secondary messengers diacylglycerol and inositol triphosphate (IP3), the IP3 then mobilizing intraneuronal calcium. The net result of the activation of the secondary messenger systems is to increase the activity of the various protein kinases that phosphorylate membrane-bound proteins to produce a physiological response. Some researchers have investigated the effect of chronic antidepressant treatment on the phosphorylation of proteins associated with the cytoskeletal structure of the nerve cell. Their studies suggest that antidepressants could affect the function of the cytoskeleton by changing the component of the associated protein phosphorylation system. In support of their hypothesis, these researchers showed that both typical (e.g. desipramine) and atypical (e.g. (+) oxaprotiline, a specific noradrenaline reuptake inhibitor, and fluoxetine, a selective 5-HT uptake inhibitor) antidepressants increased the synthesis of a microtubule fraction possibly by affecting the regulatory subunit of protein kinase type II. These changes in cytoskeletal protein synthesis occurred only after chronic antidepressant treatments and suggest that antidepressants, besides their well-established effects on pre- and postsynaptic receptors and amine uptake systems, might change neuronal signal transduction processes distal to the receptor. Glucocorticoid receptors: adaptive changes following antidepressant treatment Interest in the possible association of glucocorticoid receptors with central neurotransmitter function arose from the observation that such receptors have been identified in the nuclei of catecholamine and 5-HT-containing cell bodies in the brain. Experimental studies have shown that glucocorticoid receptors activate as DNA binding proteins which can modify the transcription of genes. The link to antidepressant treatments is indicated by the chronic administration of imipramine which increases glucocorticoid receptor immunoreactivity in rat brain, the changes being particularly pronounced in the noradrenergic and serotonergic cell body regions. Preliminary clinical studies have shown that lymphocyte glucocorticoid receptors are subsensitive in depressed patients. The failure of the negative feedback mechanism that regulates the secretion of adrenal glucocorticoids further suggests that the central glucocorticoid receptors are subsensitive.

This leads to the hypersecretion of cortisol, a characteristic feature of many patients with major depression. Such findings lend support to the hypothesis that the changes in central neurotransmission occurring in depression are a reflection of the effects of chronic glucocorticoids on the transcription of proteins that play a crucial role in neuronal structure and function. If the pituitary–adrenal axis plays such an important role in central neurotransmission, it may be speculated that glucocorticoid synthesis inhibitors (e.g. metyrapone) could reduce the abnormality in neurotransmitter function by decreasing the cortisol concentration. Recent in vitro hybridization studies in the rat have demonstrated that typical antidepressants increase the density of glucocorticoid receptors. Such an effect could increase the negative feedback mechanism and thereby reduce the synthesis and release of cortisol. In support of this hypothesis, there is preliminary clinical evidence that metyrapone (and the steroid synthes is inhibitor ketoconazole) may have antidepressant effects. Recently several lipophilic antagonists of corticotrophin releasing factor (CRF) type 1 receptor, which appears to be hyperactive in the brain of depressed patients, have been shown to be active in animal models of depression. Clearly this is a potentially important area for antidepressant development. Glucocorticoid receptors are present in a high density in the amygdala and neuroimaging studies have shown that the amygdala is the only structure in which the regional blood flow and glucose metabolism consistently correlate positively with the severity of depression. This hypermetabolism appears to reflect an underlying pathological process as it also occurs in asymptomatic patients and in the close relatives of the patients.

Possible role of excitatory amino acids and intracellular second messengers in the action of antidepressants
1. In experimental studies, chronic antidepressant treatments have been shown to reduce the behavioural effects of the NMDA-glutamate receptor antagonist dizolcipine. This suggests that antidepressants may act as functional NMDA receptor antagonists and thereby reduce excitatory glutamate transmission which is mediated by NMDA receptors.
2. Intracellular protein phosphorylation is enhanced by chronic antidepressant treatment. This leads to the increased synthesis of microtubules that form an important feature of the cellular cytoskeleton. Thus antidepressants might change signal transduction with the neurone.
3. Enhanced synthesis and transport of neurotransmitter synthesizing enzymes (e.g. tyrosine and tryptophan hydroxylase).

Role of glucocorticoids in modulating brain amines in depression
1. Glucocorticoid receptors occur on catecholamine and 5-HT cell bodies in the brain.
2. There is evidence that the glucocorticoid receptors are hyposensitive in the depressed patients.
3. Chronic antidepressant treatment sensitizes these receptors, thereby normalizing the noradrenergic and serotonergic function that is reduced by the hypercortisolaemia which occurs in major depression.

Link between the serotonergic and noradrenergic systems

Link between the serotonergic and noradrenergic systems
Considerable attention has recently been focused on the interaction between serotonergic and beta-adrenergic receptors, which may be of particular relevance to our understanding of the therapeutic effect of antidepressants. Thus the chronic administration of antidepressants enhances the inhibitory response of forebrain neurons to micro-iontophoretically applied 5-HT. This enhanced response is blocked by lesions of the noradrenergic projections to the cortex. This dual effect could help to explain enhanced serotonergic function that arises after chronic administration of antidepressants or ECT. Conversely, impairment of serotonergic function by means of a selective neurotoxin (e.g. 5,7-dihydroxytryptamine) or 5-HT synthesis inhibitor (e.g. parachlorophenylalanine) largely prevents the decrease in functional activity of cortical beta-adrenoceptors that usually arises following chronic antidepressant treatment. 5-HT1B receptors are located on serotonergic nerve terminals that act as autoreceptors, and, on stimul ation by serotonin, decrease the further release of this amine. It has been hypothesized that the chronic administration of selective serotonin reuptake inhibitor antidepressants (such as fluoxetine, paroxetine, sertraline, citalopram and fluvoxamine) slowly desensitize the inhibitory 5-HT1B receptors and thereby enhance serotonin release. In addition to the importance of the 5-HT1B autoreceptors in the regulation of serotonergic function, there is experimental and clinical evidence that the 5-HT1A receptors play a fundamental role in both anxiety and depression. In brief, the 5-HT1A somatodendritic receptors inhibit the release of serotonin and it is postulated that the enhanced release of the transmitter following the chronic administration of the selective serotonin reuptake inhibitors is a consequence of the adaptive down-regulation of the inhibitory 5-HT1A receptors. The validity of this hypothesis is supported by the pharmacological effect of 5-HT1A antagonists. Thus the beta-adrenoceptors antagonist an 5-HT1A antagonist pindolol, in combination with fluoxetine or paroxetine, enhance the therapeutic efficacy of the SSRI and, in some studies, reduce the time of onset of the peak therapeutic effect. However, several investigators have not been able to replicate such findings.

Both clinical and experimental studies have provided evidence that 5-HT can also regulate dopamine turnover. Thus several investigators have shown that a positive correlation exists in depressed patients between the homovanillic acid (HVA), a major metabolite of dopamine, and 5-HIAA concentrations in the CSF. In experimental studies, stimulation of the 5-HT cell bodies in the median raphe´ causes reduced firing of the substantia nigra where dopamine is the main neurotransmitter. There is thus convincing evidence that 5-HT plays an important role in modulating dopaminergic function in many regions of the brain, including the mesolimbic system. Such findings imply that the effects of some antidepressants that show an apparent selectivity for the serotonergic system could be equally ascribed to a change in dopaminergic function in mesolimbic and mesocortical regions of the brain. It has been postulated that the hedonic effect of antidepressants may be ascribed to the enhanced dopaminergic function in the mesocor ex.

Mechanism of action of antidepressants: changes in serotonergic function
1. There is experimental evidence that the chronic administration of antidepressants or ECT enhances the inhibitory effect of micro-iontophoretically applied 5-HT.
This effect is blocked by lesions of the noradrenergic projections to the frontal cortex.
2. SSRIs after chronic administration down-regulate the inhibitory 5-HT1A receptors on the serotonergic cell body, thereby leading to an enhanced release of the transmitter from the nerve terminal.
3. 5-HT can also decrease dopamine release from the substantia nigra (an important dopaminergic nucleus). This may account for the observation that some SSRIs may cause dystonias and precipitate the symptoms of parkinsonism if given to such patients who are responding to L-dopa. Sertraline appears to differ from other SSRIs in this respect and may slightly enhance dopaminergic function by reducing the reuptake of this transmitter.

Historical development of antidepressants

Historical development of antidepressants
The use of cocaine, extracted in a crude form from the leaves of the Andean coca plant, has been used for centuries in South America to alleviate fatigue and elevate the mood. It was only relatively recently, however, that the same pharmacological effect was discovered when the amphetamines were introduced into Western medicine as anorexiants with stimulant properties. Opiates, generally as a galenical mixture, were also widely used for centuries for their mood-elevating effects throughout the world. It is not without interest that while such drugs would never now be used as antidepressants, there is evidence that most antidepressants do modulate the pain threshold, possibly via the enkephalins and endorphins. This may help to explain the use of antidepressants in the treatment of atypical pain syndromes and as an adjunct to the treatment of terminal cancer pain. Finally, alcohol in its various forms has been used to alleviate anguish and sorrow since antiquity. Whilst the opiates, alcohol and the stimulants offer some temporary relief to the patient, their long-term use inevitably leads to dependence and even to an exacerbation of the symptoms they were designed to cure. The development of specific drugs for the treatment of depression only occurred in the early 1950s with the accidental discovery of the monoamine oxidase inhibitors (MAOIs) and the tricyclic antidepressants (TCAs). This period marked the beginning of the era of pharmacopsychiatry. Although the iminodibenzyl structure, which forms the chemical basis of the TCA series, was first synthesized in 1889, its biological activity was only evaluated in the early 1950s following the accidental discovery that the tricyclic compound chlorpromazine had antipsychotic properties. Imipramine is also chemically similar in structure to chlorpromazine, but was found to lack its antipsychotic effects. It was largely due to the persistence of the Swiss psychiatrist Kuhn that imipramine was not discarded and was shown to have specific antidepressant effects. It is not without interest that the first report of the antidepressant effects of imipramine was presented to an audience of 12 as part of the proceedings of the Second World Congress of Psychiatry in Zurich in 1957! The introduction of the first MAOI in the early 1950s was equally inauspicious. Iproniazid had been developed as an effective hydrazide antitubercular drug, but was subsequently found to exhibit mood-elevating effects. This was shown to be due to its ability to inhibit MAO activity and was unconnected with its antitubercular action. Thus by the late 1950s, psychiatrists had at their disposal two effective treatments for depression, a TCA and an MAOI. But it was only in attempting to discover how these drugs may work, together with the evidence that the recently introduced antipsychotic drug resperine caused depression in a small number of patients, that the hypothesis was developed that depression was due to a relative deficit of biogenic amine neurotransmitters in the synaptic cleft and that antidepressants reversed this deficit by preventing their inactivation.
While this hypothesis has been drastically revised in the light of research into the biochemical nature of depression, at that time it had the advantage of unifying a number of disparate clinical and experimental observations and in laying the basis for subsequent drug development. Aspects of the biochemical basis of depression Research into the chemical pathology of depression has mainly concentrated on four major areas:
1. Changes in biogenic amine neurotransmitters in post-mortem brains from suicide victims.
2. Changes in cerebrospinal fluid (CSF) concentrations of amine metabolites from patients with depression.
3. Endocrine disturbances which appear to be coincidentally related to the onset of the illness.
4. Changes in neurotransmitter receptor function and density on platelets and lymphocytes from patients before and following effective treatment.
Approximately 30 years ago, Schildkraut postulated that noradrenaline may play a pivotal role in the aetiology of depression. Evidence in favour of this hypothesis was provided by the observation that the antihypertensive drug reserpine, which depletes both the central and peripheral vesicular stores of catecholamines such as noradrenaline, is likely to precipitate depression in patients in remission. The experimental drug alpha-methylparatyrosine that blocks the synthesis of noradrenaline by inhibiting the rate-limiting enzyme tyrosine hydroxylase was also shown to precipitate depression in patients during remission. While such findings are only indirect indicators that noradrenaline plays an important role in human behaviour, and may be defective in depression, more direct evidence is needed to substantiate the hypothesis. The most obvious approach would be to determine the concentration of noradrenaline and/or its major central metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) in the brains of suicide victims. The problem with such post-mortem studies is that (a) the precise diagnosis may be uncertain, (b) there is usually a considerable postmortem delay before the brain is removed at autopsy and (c) suicide is often committed by taking an overdose of alcohol together with drugs which grossly affect central monoamine neurotransmitter function. Unless these variables are carefully controlled, the value of the results obtained from such analyses is uncertain. Nevertheless, there is evidence that the neurotransmitter receptors in post-mortem brain are less labile than the neurotransmitters that act upon them. The finding that the density of beta adrenoceptors is increased in cortical regions of the brains from suicide victims who had suffered from depression is evidence of disturbed noradrenergic function which is associated with some of the symptoms of the illness. Such observations are further supported by the increase in the density of beta adrenoceptors on the lymphocytes of untreated depressed patients. As t he density of these receptors is normalized by effective antidepressant treatment, it has been postulated that changes in the beta receptor density may be a state marker of the condition. Other studies have shown that the elevation of growth hormone in the plasma following the administration of the alpha-2 adrenoceptor agonist clonidine is diminished in depressed patients, which suggests that central postsynaptic alpha-2 adrenoceptors are also subfunctional in such patients. This is perhaps the most consistent finding to have emerged in studies of the hypothalamic–pituitary axis in depression. As the clonidine response does not return to normal after effective antidepressant treatment, this is possibly a trait marker of depression. It should be emphasized that the reduced growth hormone response to clonidine cannot be accounted for by drug treatment, age or gender of the patient, which supports the view that the noradrenergic system is dysregulated in depression. Lastly, determination of the urine or plasma concentrations of MHPG (an indicator of central noradrenergic activity) suggests that central noradrenergic function is suboptimal in depression. Taken together, these results suggest that central noradrenergic function is decreased in depression, an event leading to the increase in the density of the postsynaptic beta adrenoceptors that show adaptive changes in response to the diminished synaptic concentration of the transmitter. It should be emphasized however that none of the studies of noradrenergic function in post-mortem material or tissues from depressed patients are entirely satisfactory. Many of the findings cannot be replicated, the number of patients studied is relatively small and the tritiated ligands used to determine the receptor density for example vary in their selectivity.

There is also evidence that the density of muscarinic receptors is increased in limbic regions of depressed patients who have committed suicide. If it is assumed that such a change reflects an increased activity of the cholinergic system, it could help to explain the reduced noradrenergic function as there is both clinical and experimental evidence to suggest that increased central cholinergic activity can precipitate depression and reduce noradrenergic activity. The role of serotonin (5-hydroxytryptamine, 5-HT) has also been extensively studied in depressed patients. Whereas the overall psychophysiological effects of noradrenaline in the CNS appear to be linked to drive and motivation, 5-HT is primarily involved in the expression of mood. It is not surprising therefore to find that the serotonergic system is abnormal in depression. This is indicated by a reduction in the main 5-HT metabolite, 5-hydroxyindole acetic acid (5-HIAA), in the cerebrospinal fluid of severely depressed patients and a reduction in 5-HT and 5-HIAA in the limbic regions of the brain of suicide victims. The 5-HT receptor function also appears to be abnormal in depression. This is indicated by an increase in the density of cortical 5-HT2A receptors in the brains of suicide victims and also on the platelet membrane of depressed patients. Platelets may be considered as accessible models of the nerve terminal. Thus platelets are, like neurons, of ectodermal origin and contain enzymes such as enolase that are otherwise restricted to neurons. In addition, platelets contain storage vesicles for 5-HT from which the amine is released by a calcium-dependent mechanism. An energy-dependent transport site for 5-HT also occurs on the platelet membrane, the structure of which is identical to that found on neurons in the brain. Furthermore, the platelet membrane contains 5-HT2A and alpha-2 adrenergic receptors that are functionally involved in platelet aggregation; there is evidence that the densities of these receptors are increased in depressed patients and largely normalized following effective treatment. Thus a number of important biochemical parameters may be determined from a platelet-rich plasma sample. It has been found, for example, that the transport of 3H-5-HT into the platelet is significantly reduced in the untreated depressed patient but largely returns to normal following effective treatment. This change occurs irrespective of the nature of the antidepressant used to treat the patient and may therefore be considered as a state marker of the illness. The function of the 5-HT2A receptor also appears to be subnormal in the untreated patient as shown by diminished aggregatory response to the addition of 5-HT in vitro, but normalizes when the patient recovers. As the number of 5-HT2A receptors on the platelet membrane of depressed patients is increased (as shown by the increased binding of a specific ligand such as 3H-ketanserin) this finding suggests that the G protein transducer mechanism, which links the receptor to the second messenger phosphatidyl inositol system within the platelet, is possibly defective in depression. There is also evidence that the modulatory site on the 5-HT transporter, the imipramine binding site, is decreased in depression, but unlike the changes in 5-HT uptake, remains unchanged by effective treatment. This suggests that the number of imipramine binding sites on the platelet membrane is a trait marker of the condition. However, the precise relevance of this finding is uncertain as the binding of the more specific ligand, 3H-paroxetine, is unchanged in depression. These studies of platelet function before, during and following treatment can give important information of the biochemical processes which may be causally related to depression. However, it is still uncertain how the changes in platelet function precisely reflect those occurring in the brain. Platelets are unconnected with the nervous system and therefore the changes observed could be a reflection of the hormonal changes (for example, glucocorticoids) that occur in depression. There is also evidence that some of the alterations in platelet function are a consequence of low molecular weight plasma factors that occur in the depressed patient and which are absent following effective antidepressant treatment.

Changes in brain and tissue amine neurotransmitters in depressed patients which may be indicative of the mechanism of action of antidepressants
. Evidence from the brains of depressed patients who committed suicide
. Increased density of beta adrenoceptors in cortical regions
. Increased density of 5-HT2A receptors in limbic regions
. Decreased concentration of 5-HIAA in several brain regions
. Increased muscarinic receptor density in limbic regions

Other clinical studies implicating an abnormal biogenic amine function in depression
. Decreased 5-HIAA concentration in CSF
. Decreased HVA (main dopamine metabolite) in CSF
. Decreased urinary excretion of the main central noradrenaline metabolite MHPG (?)
. Rapid relapse following administration of a tryptophan-free amino acid drink to depressed patients being treated with an SSRI
. Rapid relapse following administration of the tyrosine hydroxylase inhibitor alpha-methyl-tyrosine to depressed patients who respond to a noradrenaline reuptake inhibitor such as desipramine

While there has been considerable attention devoted to changes in noradrenergic and serotonergic function in depression, less attention has been paid to the possible involvement of dopamine in this disorder. This is surprising as anhedonia is a characteristic feature of major depression and a defect in dopaminergic function is thought to be causally involved in this symptom. Several studies have shown that the concentration of the main dopamine metabolite, homovanillic acid (HVA), is decreased in the CSF of depressed patients, particularly those with psychomotor retardation. These depressed patients who attempted suicide were also found to have a decreased urinary excretion of HVA and the second major dopamine metabolite, dihydroxyphenylacetic acid (DOPAC). It is of course possible that the dopamine deficit is more a reflection of the degree of retardation rather than the psychological state as similar changes in CSF HVA concentrations have been reported to occur in patients with Parkinson’s disease. This imp lies that the changes in the basal ganglia probably overshadow any changes in the mesolimbic dopaminergic system as the contribution of this area is relatively minor. With regard to the specific action of antidepressants on the dopaminergic system, there is evidence that buproprion (not marketed in most countries in Western Europe as an antidepressant but available in North America), amineptine and nomifensin (withdrawn because of the rare occurrence of haemolysis) owed their antidepressant efficacy to their ability to increase central dopaminergic function. There are also open label studies to suggest that the novel dopamine receptor agonist roxindole and the selective dopamine uptake inhibitor pramipexole may have antidepressant action. Thus when the results of the studies on platelets, lymphocytes, changes in cerebrospinal fluid metabolites of brain monoamines and the post-mortem studies are taken into account it may be concluded that a major abnormality in both noradrenergic and serotonergic function occu rs in depression, and that such changes could be causally related to the disease process.

Evidence from depressed patients
. Increased density of 5-HT2A receptors on the platelet membrane
. Decreased uptake of 3H-5-HT into platelet
. Increased beta adrenoceptor density on lymphocyte membrane
. Increased density of alpha-2 adrenoceptors on platelet membranes (*)
. Blunted growth hormone response to a clonidine challenge
. Blunted prolactin response to a fenfluramine challenge

Both clinical and experimental studies have shown that a number of transmitter receptors and amine transport processes show circadian changes. It is well established that depression is associated with a disruption of the circadian rhythm as shown by changes in a number of behavioural, autonomic and neuroendocrine aspects. One of the main consequences of effective treatment is a return of the circadian rhythm to normality. For example, it has been shown that the 5-HT uptake into the platelets of depressed patients is largely unchanged between 0600 and 1200 hours, whereas the 5-HT transport in control subjects shows a significant decrease over this period. The normal rhythm in 5-HT transport is only reestablished when the depressed patient responds to treatment. Thus it may be hypothesized that the mode of action of antidepressants is to normalize disrupted circadian rhythms. Only when the circadian rhythm has returned to normal can full clinical recovery be established. Chronobiological studies have shown tha circadian rhythms occur in the responsivity of animals to light, dark and psychotropic drugs. This implies that the timing of drug administration so that the drug reaches the target organ at its optimum sensitivity could help to improve its therapeutic efficiency. The results of experimental studies suggest that most antidepressants delay the circadian phase and lengthen the circadian period. These changes could be due to the drugs acting on the circadian pacemaker in the superchiasmatic nucleus. However, other brain regions could also be responsible together with antidepressantinduced changes in the retina which would lead to a modification of the processing of light stimuli; the lateral geniculate nucleus may also have a role to play. Seasonal affective disorder (SAD) generally consists of recurrent depressive episodes in autumn and winter that alternate with euthmia or hypomania in spring and summer. The seasonal rhythms of mood, sleep and weight change seen in SAD patients resemble hibernation seen in animals. This led to the hypothesis that extension of the photoperiod in winter could counteract the depressive symptoms. Exposure to bright light had indeed been shown to be efficacious. Clinical studies show that carbohydrate craving, a common feature of SAD, is possibly linked to a decreased serotonin turnover. Such a hypothesis is supported by the fact that the serotonin releasing agent D-fenfluramine is effective in treating SAD. Chronobiology is clearly an important area of research for the psychopharmacologist which needs more attention.

Why is there a delay in the onset of the antidepressant response?
In an attempt to explain the reason for the delay in the onset of the therapeutic effect of antidepressants, which is clearly unrelated to the acute actions of these drugs on monoamine reuptake transporters or intracellular metabolizing enzymes, emphasis has moved away from the presynaptic mechanism governing the release of the monoamine transmitters to the adaptive changes that occur in pre- and postsynaptic receptors that govern the physiological expression of neurotransmitter function. Antidepressant therapy is usually associated with a gradual onset of action over 2 to 3 weeks before the optimal beneficial effect is obtained. Much of the improvement seen early in the treatment with antidepressants is probably associated with a reduction in anxiety that often occurs in the depressed patient and improvement in sleep caused by the sedative action of many of these drugs. The delay in the onset of the therapeutic response cannot be easily explained by the pharmacokinetic profile of the drugs as peak plasma (and presumably brain) concentrations are usually reached in 7 to 10 days. Furthermore, the 2–3 weeks delay is also seen in many, though not all, patients given electroconvulsive therapy (ECT). It is apparent that adaptational changes occur in adrenoceptors, serotonin, dopamine and GABA-B receptors. There is evidence that GABA-B receptors play a role in enhancing noradrenaline release in the cortex and in this respect differ fundamentally from the inhibitory GABA-A receptors which facilitate central GABAergic transmission. A decrease in the activity of GABA-B receptors may therefore contribute to the reduced central noradrenergic tone reported to occur in depression.

Changes in cholinergic and aminergic receptors in depression and following antidepressant treatment
1. Evidence that central muscarinic receptors are supersensitive in depressed patients and that chronic antidepressant treatments normalize the supersensitivity of these receptors. This effect does not depend on any intrinsic anticholinergic activity of the antidepressant (i.e. it is an indirect, adaptive effect).
2. Following chronic administration to rats, there is evidence that most antidepressants cause adaptive changes in 5-HT1A, 5-HT2A alpha-1, alpha-2 and beta adrenoceptors, GABA-B receptors and possibly the NMDA-glutamate receptors.

Changes in cholinergic function
In addition to these changes, recent evidence has shown that a decrease in cortical muscarinic receptors occurs in the bulbectomized rat model of depression that, like most of the changes in biogenic amine receptors, returns to control values following treatment with either typical (e.g. tricyclic antidepressants) or atypical (e.g. mianserin) antidepressants. Such findings are of particular interest as the anticholinergic activity of the tricyclic antidepressants is usually associated with their unacceptable peripheral side effects and most second generation antidepressants have gained in therapeutic popularity because they lack such side effects. Nevertheless, support for the cholinergic hypothesis of depression is provided by the finding that the short-acting reversible cholinesterase inhibitor pyridostigmine, when administered to drug-free depressed patients, causes an enhanced activation of the anterior pituitary gland as shown by the release of growth hormone secretion. This suggests that the muscarinic receptors are supersensitive in the depressed patient. However, the mechanism whereby the receptors are normalized by chronic (but not acute) antidepressant treatment vary and in most cases are unlikely to be due to a direct anticholinergic action. It has been postulated that depression arises as the result of an imbalance between the central noradrenergic and cholinergic systems; in depression the activity of the former system is decreased and, conversely, in mania it is increased. As most antidepressants, irrespective of the presumed specificity of their action on the noradrenergic and serotonergic systems, have been shown to enhance noradrenergic function, it is hypothesized that the functional reduction in cholinergic activity arises as a consequence of the increase in central noradrenergic activity.

Drug Treatment of Depression

Drug Treatment of Depression
The Oxford Dictionary defines depression as a state of ‘‘low spirits or vitality’’. Clearly, this state has been experienced by most people at some stage during their lives. However, the psychiatrist is seldom concerned with such a mood change unless it persists for such a long time that it incapacitates the individual. Should the depressed mood be associated with feelings of guilt, suicidal tendencies and disturbed bodily functions (such as weight loss, anorexia, loss of libido or a disturbed sleep pattern characterized by early morning wakening) and persist for weeks or even months, often with no initiatory cause, then psychiatric assistance is usually required. It is not proposed to discuss the various types of depression that have been identified because the drug treatment is essentially similar irrespective of whether or not there appears to be an initiatory cause. For example, bereavement is often associated with a severe depressive episode, particularly in the elderly, and while counselling may be of c onsiderable assistance in enabling the patient to adjust to the changed circumstances the use of an antidepressant is often advisable.

Many psychiatrists still divide depression into the endogenous (i.e. no apparent external cause) and reactive (i.e. an identifiable external cause) types and, while such a division may be of some value regarding ancillary treatment, there is presently no evidence to suggest that the biochemical changes that may be causally linked to the illness differ nor is there any evidence that the way in which the patient should be assisted by drugs differs substantially. Other international classifications of depression are based on the mono- and bipolar dichotomy, a system of classification that separates those patients with depressive symptoms only from those that fluctuate between depression and mania (i.e. manic-depression) or have only manic symptoms. In such cases treatment strategies differ as specific and antimanic drugs such as lithium or the neuroleptics would be used to abort an acute attack of mania, while antidepressants are the drugs of choice to treat the depressive episodes and anxiety associated with de pression. Readers are referred to the various classification manuals, such as the Diagnostic and Statistical Manual of Mental Disorders of the American Psychiatric Association (DSM-IV) or the International Classification of Diseases, 10th Revision (ICD10), for further details.
Serotonin receptor subtypes and disease states
Physiological or pathological -- condition Serotonin receptor subtype implicated
Feeding behaviour: 5-HT1A agonists enhance food consumption in experimental animals
5-HT1B/5-HT2C agonists decrease food consumption in experimental animals
Thermoregulation: 5-HT1A agonists cause hypothermia in experimental animals
5-HT1B and 5-HT2 agonists cause hyperthermia in experimental animals
Sexual behaviour: 5-HT1A agonists both facilitate and inhibit sexual behaviour in male rats 5-HT1B agonists inhibit sexual behaviour in the male but facilitate this behaviour in the female rat Cardiovascular system: 5-HT1, 5-HT2 and 5-HT3 receptors may be involved in the complex action of serotonin on blood pressure 5-HT2 agonists appear to be hypertensive agents whereas the antagonists are hypotensives
Sleep: 5-HT1A agonists delay the onset of REM sleep
5-HT2 antagonists suppress REM sleep
Hallucinogenic activity: Most ‘‘classical’’ hallucinogens such as LSD and mescaline are antagonists at 5-HT2 receptors
Antipsychotic activity: Many atypical neuroleptics (e.g. amperozide and risperidone) are 5-HT2 receptor antagonists. In animals,
5-HT3 antagonists have profiles similar to chronically active neuroleptics
Anxiolytic activity: Several novel anxiolytics (e.g. buspirone, ipsapirone) are
5-HT1A partial agonists
5-HT2 and 5-HT3 antagonists have anxiolytic properties
Depression: 5-HT1A receptors are functionally sensitized by chronic antidepressant treatments in rats
5-HT2 receptor numbers are increased and activity decreased, in depression; return to control values in response to treatment

Distribution and selectivity of drugs for 5-HT receptor subtypes

Distribution and selectivity of drugs for 5-HT receptor subtypes
5-HT1A receptors
Hippocampus, septum, amygdala, cortical limbic area
Agonists: buspirone, gepirone, ipsapirone, flesinoxan
Antagonists: WAY100135, BMY7378, NAN-190
Possible clinical use of agonists: anxiolytics, antidepressants
5-HT1B receptors
Substantia nigra, globus pallidus, dorsal subiculum, superior colliculi
Non-selective: pinodol, propanolol
? selective partial agonists: CP-93, 129
Possible clinical use of antagonists: antidepressants
5-HT1D receptors
Caudate nucleus but widely distributed in human dry and GP brain, similar to
5-HT1B of rat brain
Non-selective: rauwolscine, yohimbine
? selective: L-694, 247
Possible clinical use of antagonists: antidepressants
5-HT1E receptor
In mammalian brain
Possible clinical use: ?
Neocortex but widely distributed in the mammalian brain
Agonists: DOI, DOB
Non-selective antagonists: ketanserin, ritanserin
Possible clinical use of antagonists: antidepressants, anxiolytics, ? neuroleptics
In mammalian brain
Possible clinical use: ?
Choroid plexus
Non-selective antagonists: spiperone, amperozide, pimozide
Possible clinical use of antagonists: neuroleptics
Area postrema, entorhinal and frontal cortex, hippocampus
Agonists: ondansetron, granisetron, zacopride
Possible clinical use of antagonists: antiemetics, anxiolytics, ? antidementia
Collicular and hippocampal neurons
Antagonists SC-205-557, SC-53606
Agonist: SC-49518
Possible clinical use: ?
8-OHDPAT= Dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthylene
RU-24969 = 5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl) 1H indol
mCPP = 1-(3 chlorophenyl) piperazine
TFMPP = 1-(m-trifluoromethylphenyl) piperazine
DOI = 1-(2,5-dimethoxy-1-iodophenyl) 2-aminopropane
MDL73005 = 8,2 (2,3-dihydro-1,4-benzodioxin-2yl) methylamino-ethyl-8-azaspirol (4,5) decan-7,9-dione
NAN190 = 1-(2-methoxyphenyl) 4-(4(2-phthalimido)entyl-piperazine)
5-CT = 5-carboxamidotryptamine
ICI 169369 = (2-(2-dimethylamino-ethylthio-3-phenylquinoline))
ICS 205-930 = (3-tropanyl)-IH-indole-2-carboxylic acid ester
DOM = 2,5-dimethoxy-4-dimethylbenzene ethamine

Principal methods used in molecular genetics

Principal methods used in molecular genetics

Brain mRNAs
Reverse transcriptase transcribes these different mRNAs into single complementary strands of DNA

Single-stranded brain cDNAs formed
Single-stranded cDNAs form template for double-stranded DNA

Double-stranded brain cDNAs formed
Double-stranded cDNAs added to bacterial plasmids that insert the cDNA into the plasmid

Recombinant DNA plasmid containing the brain DNA
Recombinant DNA then inserted into bacteria which reproduce the brain DNA

cDNA library formed with each bacterium multiplying the specific cDNA it contains

Individual bacteria isolated and cultured to produce clones that yield
specific cDNAs
Specific cDNAs (for example a receptor) inserted in plasmid that transfects a mammalian cell (e.g. fibroblasts) in culture

Mammalian cell containing the specific cDNA then exposed in a culture medium to a toxin which destroys all non-transformed mammalian cells
Add radioligand that identifies the receptor of the mammalian cell

Identification and isolation of transformed cells which can then be cultured to provide unlimited quantities of the receptor protein

Molecular Genetics and Psychopharmacology

Molecular Genetics and Psychopharmacology
Psychopharmacologist has paid increasing attention to the examination of brain proteins with which psychotropic drugs react, and also the molecular mechanisms that control the synthesis and cellular function of these proteins. For this reason, any understanding of psychopharmacology requires some knowledge of the basic techniques of molecular genetics.

Genes are composed of deoxyribonucleic acid (DNA) which is a long polymer composed of deoxyribonucleotides. Each deoxyribose nucleotide has one of the following purine or pyrimidine bases, namely adenosine, guanine, thymine or cystosine. A single gene may contain from a few thousand to several hundred thousand bases that are arranged in a specific sequence according to the information contained in the gene. It is this sequence of bases which determines the structure of the gene product which is a protein. In addition, the gene also contains information regarding the way in which the gene is expressed during development and in response to environmental stimuli. The role of DNA in storing and transferring genetic material is dependent on the properties of the four bases. These bases are complementary in that guanine is always associated with cytosine, and adenosine with thymine.
Watson and Crick, some 40 years ago, showed that the stability of DNA is due to the double helix structure of the molecule that protects it from major perturbations. Information is ultimately transferred by separating these strands which then act as templates for the synthesis of new nucleic acid molecules. There are two ways in which DNA molecules may act as templates.
Firstly, DNA is used as a template for replicating additional copies during cell division. This occurs by free deoxyribonucleotides binding to the complementary bases of the exposed DNA strand and then being linked by the enzyme DNA polymerase to form a new DNA double helix.
Secondly, small sections of the DNA molecule are used as a template for the synthesis of messenger ribonucleotides (mRNAs) which are responsible for carrying the message for the synthesis of specific proteins. mRNAs differ from DNA in that they are much shorter (generally 7000 base pairs in length) and are single stranded. mRNAs contain the information necessary for the synthesis of a specific protein and also contain the pentose sugar moiety ribose instead of deoxyribose found in DNA. In addition, thymine is replaced by the pyrimidine base uracil which, like thymine, is complementary to adenine.

The human genome contains approximately 100 000 genes which are distributed with a total DNA sequence of 3 billion nucleotides. The DNA of the human genome is divided into 24 exceptionally large molecules each of which is a constituent of a particular chromosome, of which 22 are autosomes and two are sex chromosomes (X and Y chromosomes). Translation of the information encoded in DNA, expressed as a particular nucleotide sequence, into a protein, expressed as an amino acid sequence, depends on the genetic code. In this code, sequences of three nucleotides (termed a codon) represent one of the 20 amino acids that compose the protein molecule. Because there are 64 codons which can be constructed for the four different bases, and only 20 different amino acids that are coded for, several amino acids may be coded for by more than one codon. There are also three codons, called stop codons, that terminate the transfer of information. Furthermore, although all cells contain the same complement of genes, certain cells (for example, the neurons) have specialized genes that encode specific proteins for the synthesis of specific transmitters. The expression of such genes is under the control of regulatory proteins called transcription factors which control the transcription of mRNAs from the genes they control.

The expression of enzymes that control neurotransmitter systems is controlled not only by factors operating during embryonic development, but also by the degree of neuronal activity. Thus the more active the nervous system, the greater the genetically controlled synthesis of the neurotransmitters which clearly play an important role in the behaviour of the organism. Regulation of the genes also determines the response of the brain to drugs, hence the importance of molecular genetics to psychopharmacology. One of the most important areas of molecular genetics concerns the role of specific base sequences, called regulatory sequences, that surrounded the sections of the gene that encode the amino acid sequence of a protein. These regulatory sequences are activated or inactivated by specific transcription factors and it is the complex interaction of regulatory sequences and transcription factors that underlies the adaptation of brain function to the effects of some psychotropic drugs. For example, it is well known that the optimal response to an antidepressant or neuroleptic drug requires several weeks of treatment. Such adaptive changes are probably a reflection of the molecular genetics of neurotransmitter function and may help to explain the lack of success in developing antidepressants or neuroleptics that have a rapid therapeutic action.

Manipulation of Genes
Molecular genetics is to determine the base sequence of the human genome. This is the purpose of the Human Genome Project, an international collaborative research programme aimed at providing a complete analysis of the human genome within the next decade. The first step in such an analysis is to isolate the small sequences of bases in DNA that are transcribed into mRNAs. The information contained in the mRNAs can be isolated and amplified by a technique called cDNA cloning. In this technique, mRNAs from brain tissue, for example, are purified and then treated with reverse transcriptase which converts mRNAs into single complementary strands of DNA. This is called complementary DNA (cDNA). The cDNA provides a template for producing a second strand that is complementary to the first. This double-stranded cDNA is then incubated with bacterial plasmids to produce recombination DNA plasmids. Plasmids may be considered as bacterial viruses that can reproduce themselves when inserted into the appropriate bacteria so that during the process of bacterial cell division multiple copies of the cDNA that had been inserted in the plasmid are formed. As each bacterium is likely to be infected with a plasmid, containing a different type of cDNA, the resulting medium will contain a mixed population of cDNAs from the original brain tissue. This is called a cDNA library.
The individual components of the cDNA library may be obtained by grouping individual bacteria on a culture medium so that they reproduce to form identical clones. This enables a large quantity of specific cDNAs to be produced in a pure form. The cDNA within these plasmid-containing bacteria can then be removed, and the precise nucleotide sequence determined by standard automated analytical procedures. Since the brain expresses many mRNAs that are also found in nonnervous tissue and are therefore of little interest to the psychopharmacologist, it is necessary to isolate only those cDNAs that, for example, encode for a specific enzyme or receptor protein. Several techniques have been developed to achieve this. For example, a specific cDNA plasmid may be inserted into cultured mammalian cells such as fibroblasts that can express the specific receptor or enzyme. Once this has been expressed in the culture medium, the receptor or enzyme can be identified by adding a specific ligand or substrate. This enables those cells that expressed the specific macromolecule of interest to be identified and subsequently isolated. Once a particular cDNA has been isolated in this way it can be used to make unlimited quantities of the macromolecule whose sequence it encodes. As mammalian cells are generally used for this method of amplification, the amino acid sequence is the same as that used in the limbic brain. Furthermore, if, for example, the cDNA encodes a neurotransmitter receptor, it is likely that it will be integrated into the plasma membrane of the cell surface and therefore largely reflect the portion of the receptor in the neuron. This enables such receptor-containing cells to be used for screening the affinity of putative psychotropic drugs on receptors that were derived from human brain. This method is now commonly used in the pharmaceutical industry to screen numerous compounds for their potential therapeutic application: for example, screening compounds for their affinity for the human D4 receptor as potential atypical neuroleptics.

Another important application of cDNAs is to identify specific proteins in a tissue homogenate or tissue section. Since cDNAs undergo complementary base pairing, adding a radioactively labelled cDNA to a homogenate or tissue slice will bind it to the complementary sequence by a process of hybridization. Thus the amount of radioactive cDNA that hybridizes to the tissue or tissue extract is a measure of the amount of mRNA that is complementary to it. When this procedure is undertaken on slices of brain, it is known as in situ hybridization. In this way it is possible to determine the distribution of specific receptors in a tissue by accurately determining the distribution of mRNA that encodes for the receptor protein. This is a particularly valuable technique for the administration of psychotropic drugs. A variety of techniques have now been developed to manipulate gene expression using cDNAs. For example, it is possible to introduce copies of a new gene (in the form of cDNAs) into a cultured cell line by a process of transfection. This is achieved by means of plasmids that transfect the human or mammalian cells in culture. Those cells that have had the DNA sequence integrated into their chromosomes can then be separated from those cells in which integration has not occurred by incubating the mixed cell population with a toxin to which the engineered cells are resistant whereas the normal cells are not. In this way clones of cells that contain the new genetic material can eventually be isolated.

A major advance in this technique has arisen through the development of transgenic mice. This technique involves injecting foreign DNA into the genome of the mouse embryo. As a consequence, the foreign DNA can give rise to a line of mice that contain the foreign DNA. Using this technique, mice have now been produced whose brains express the A4 protein – a marker for Alzheimer’s disease. A variant of this technique is to replace a normal gene with a foreign gene in the chromosome, thereby giving rise to a progeny that lack both the normal gene and its function. Sibling mating then gives rise to offspring which have two defective genes. This method has so far largely been confined to mice which are termed ‘‘knock-out’’ mice. This method could prove to be particularly useful for determining the physiological role of specific neurotransmitter receptors.