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Thursday, April 21, 2011

Serotonin and its role in depression

Serotonin and its role in depression
Serotonin is believed to play a multifunctional role in depression which is to
be anticipated from its involvement in the physiological processes of sleep,
mood, vigilance, feeding and possibly sexual behaviour and learning, all of
which are deranged to varying extents in severe depression. However, the
involvement of precise serotonin receptor subtypes in depression, and in
the action of antidepressants, is still far from clear. One approach to
unravelling the changes in serotonin receptors in depression has been to
study the effects of chronically administered antidepressants on serotonin
receptor subtypes in rat brain. While there is evidence that most
antidepressants show only a low affinity for the 5-HT1 sites, there is
experimental evidence to show that chronic antidepressant treatment
results in a hypersensitivity of postsynaptic and a hyposensitivity of
presynaptic 5-HT1A receptors. In contrast to the 5-HT1A receptors, many
antidepressants from various chemical classes have a moderate affinity for
5-HT2 receptors although there is no apparent correlation between the
5-HT2 receptor affinity and the antidepressant potency.
Regarding the changes that occur in rat cortical 5-HT2 receptor density
following chronic antidepressant and lithium treatment, there is unequivocal
evidence that the number of receptors increases in response to chronic
drug treatment although it must be emphasized that chronic electroconvulsive
shock results in a decrease in the receptor number. Similarly, in
untreated depressed and panic patients, the density of 5-HT2 receptors on
the platelet membrane has been shown to be increased. The number of
receptors normalizes on effective, but not ineffective, treatment. Using the
serotonin-induced platelet aggregation response as a measure of the
functional activity of 5-HT2 receptors, it has been consistently shown that
the 5-HT2 receptor responsiveness is reduced in the untreated depressive
but returns to control values following effective treatment irrespective of
the nature of treatment. Thus changes in 5-HT2 receptor density andfunction appear to be disturbed in the depressed patient and return to
control values only following effective treatment. The increase in the
receptor number, and decrease in their responsiveness to serotonin, in the
untreated depressed patient may suggest an abnormality in the coupling
mechanism between the receptor site and the phosphatidylinositol second
messenger system that brings about the platelet shape change underlying
It has been hypothesized that depression could arise from a pathological
enhancement of 5-HT2 receptor function. This view would concur with the
observations that the functional activity of 5-HT2 receptors on the platelet
membrane is enhanced in depression and the increase in the density of
5-HT2 receptors in the frontal cortex of brains from suicide victims. It is
possible that enhanced 5-HT2 receptor function is associated primarily with
anxiety, a common feature of depression, and that the increased activity of
the 5-HT2 receptors results in an attenuation of the functioning of 5-HT1
receptors thereby resulting in the symptoms of depression. Whether this
change in the activity of 5-HT1 receptors is due to direct effects of the
altered 5-HT2 receptor function is uncertain. There is evidence that
hypercortisolaemia, which is a characteristic feature of depression, reduces
the activity of these receptors probably through central glucocorticoid type
2 receptors. Clearly further research is needed to determine the precise
interaction between the 5-HT2 and 5-HT1 receptor types.
More recently, it has been speculated that the 5-HT1B/1D receptors may
have a role to play in depression and in the mode of action of
antidepressants. These receptors appear to be located presynaptically
where they control the release of 5-HT; in experimental studies the nonselective
5-HT1 antagonist methiothepin has antidepressant properties.Thus it may be speculated that the 5-HT1B/1D receptors are supersensitive in
depression, thereby leading to a reduced intersynaptic concentration of
5-HT with a consequent increase in the number of postsynaptic 5-HT2
receptor sites. However, only the development of highly selective 5-HT1B/1D
antagonists will enable this hypothesis to be tested.
Although the precise mechanism whereby antidepressants produce their
therapeutic effects is incompletely understood, there is a growing body of
evidence to suggest that serotonin receptors, particularly of the 5-HT1A and
5-HT2 subtype, play a role in their actions. Only the 5-HT2A receptor has, so
far, been convincingly demonstrated to be malfunctional in depression and
to be normalized following effective treatment.

Serotonin and aggression, panic attack and related disorders

Serotonin and aggression, panic attack and related disorders
The possible overlap between anxiety, depression, panic attack, aggression
and obsessive–compulsive disorders, and the involvement of serotonin in
the symptoms of these disorders, has recently led to the investigation of
various selective serotonin reuptake inhibitors (SSRIs) and selective 5-HT
receptor agonists/antagonists in the treatment of these conditions. In
experimental studies, there is evidence that drugs such as eltoprazine,
which binds with high affinity to 5-HT1A, 5-HT1B and 5-HT2C sites, are
active antiaggressive agents, whereas selective 5-HT1A agonists and 5-HT2
and 5-HT3 antagonists are inactive. There is also preliminary evidence to
suggest that SSRIs such as fluoxetine reduce impulsive behaviour which
may contribute to their therapeutic action in the treatment of obsessive–
compulsive disorders and possibly in reducing suicidal attempts.
Zohar and Insel have suggested that the symptoms of obsessive–
compulsive disorder are due to supersensitive 5-HT1-type receptors and
that the function of SSRIs such as clomipramine, fluoxetine and the
non-selective 5-HT antagonist metergoline owe their efficacy to their ability
to reduce the activity of these receptors.
It now seems generally accepted that the effects of anti-obsessional drugs
may be mediated by serotonergic mechanisms. The apparent hypersensitivity
of obsessive–compulsive patients to the trazodone metabolite m-chlorophenyl
piperazine (mCPP, a non-selective 5-HT1B, 5-HT2C and 5-HT2 agonist)
suggests that a diverse group of 5-HT1 and 5-HT2 receptors are involved.
The efficacy of buspirone, a partial agonist of 5-HT1A receptors, in
attenuating the obsessional symptoms further suggests that 5-HT1A
receptors are also involved. As the 5-HT reuptake inhibitors such as
fluoxetine and fluvoxamine are particularly effective in attenuating the
obsessive symptoms following several weeks of administration, it may be
argued that the therapeutic effect of such drugs lies in their ability to
desensitize the supersensitive 5-HT1-type receptors. Which of the 5-HT1
receptors is specifically involved is unclear, but neuroimaging studies on
patients with obsessive–compulsive disorder implicate the striatum as the
major brain region which is defective. The 5-HT receptors in the striatum
are 5-HT1D and 5-HT2 in man which may implicate these receptor subtypes
specifically in the aetiology of the condition.
With regard to generalized anxiety disorder, it has been postulated that
an overactivity of the stimulatory 5-HT pathways occurs. Drugs such as
buspirone and ipsapirone are effective in such conditions because they
stimulate the inhibitory 5-HT1A autoreceptors on the raphe´ nuclei and
thereby reduce serotonergic function. It is noteworthy that the SSRIs often
worsen anxiety initially because they temporarily enhance serotonergic
function. Adaptive changes in the pre- and postsynaptic 5-HT receptors
then occur leading to a reduction in the anxiety state.

Serotonin and anxiolytic activity

Serotonin and anxiolytic activity
Although the benzodiazepine anxiolytics primarily interact with the GABA
receptor complex, there is ample experimental evidence to show that
secondary changes occur in the turnover, release and firing of 5-HT neurons
as a consequence of the activation of the GABA-benzodiazepine receptor.
Similar changes are observed in the raphe´ nuclei where a high density of
5-HT1A receptors occurs. Such findings suggest that 5-HT may play a key
role in anxiety disorders.
Undoubtedly one of the most important advances implicating serotonin
in anxiety has been the development of the azaspirodecanone derivatives
buspirone, gepirone and ipsapirone as novel anxiolytics. All three agents
produce a common metabolite, namely 1-(2-pyrimidinyl) piperazine or 1-PP,
which may contribute to the anxiolytic activity of the parent compounds. It
soon became apparent that these anxiolytic agents do not act via the
benzodiazepine or GABA receptors but show a relatively high affinity for
the 5-HT1A sites; the 1-PP metabolite however only possesses a very low
affinity for the 5-HT1A site although it may contribute to the anxiolytic effect
of the parent compound by acting as an alpha2 adrenoceptor agonist. In
experimental studies, these atypical anxiolytics have mixed actions,
behaving as agonists in some situations and antagonists in others. For
this reason they are considered to be partial agonists at 5-HT1A receptors,
acting either as agonists on presynaptic 5-HT1A receptors or antagonists on
postsynaptic 5-HT1A receptors.
In animal models of anxiety, 5-HT2 receptor antagonists have been shown
to be active. Ritanserin appears to exhibit both anxiolytic and anxiogenic
activity in different animal models. Nevertheless, in man, preliminary
evidence suggests that ritanserin is an effective anxiolytic agent, although a
placebo-controlled trial of the 5-HT2 antagonist ritanserin has shown no
differences in the Hamilton Anxiety and the Clinical Global Impression
scales between the drug-treated and placebo-treated patients.
The anxiolytic properties of 5-HT3 receptor antagonists have been
demonstrated in several animal models of anxiety. In these models, the
5-HT3 antagonists mimic the anxiolytic effects of the benzodiazepines but
differ from the latter in their lack of sedative, muscle relaxant and
anticonvulsant action. These compounds appear to be extremely potent
(acting in the ng–mg/kg range) and, providing the initial clinical finding of
their anxiolytic activity is substantiated, this group of drugs could provide a
valuable addition to the non-benzodiazepine anxiolytics. Thus experimental
and clinical evidence suggests that 5-HT1A receptor partial agonists and
5-HT2 and 5-HT3 antagonists may be useful and novel anxiolytic agents.

Serotonin and the antipsychotic activity of neuroleptics

Serotonin and the antipsychotic activity of neuroleptics
Given the complexity of the serotonergic system and its interaction with
multiple neurotransmitter systems in the mammalian brain, it is not
surprising to find that 5-HT plays a role in the aetiology of
schizophrenia. Meltzer has suggested that in schizophrenia a malfunction
of the mechanism whereby 5-HT modulates the release of dopamine (for
example, due to the decreased inhibition by 5-HT of the release of
dopamine in the mesencephalon and frontal cortex) might contribute to
the enhanced neocortical dopaminergic function which probably forms
the biochemical basis of the disease. The antipsychotic activity of atypical
neuroleptics such as clozapine and risperidone may therefore lie in the
normalization of the relationship between the malfunctioning 5-HT and
dopaminergic systems.
The novel antipsychotic drug clozapine has a very complicated
neurochemical profile in that it has a high affinity for 5-HT2A, 5-HT2C,
5-HT3, 5-HT6 and 5-HT7 receptors in addition to its action on D4 and D3
receptors. Risperidone likewise has a high affinity for 5-HT2A receptors as
well as acting as an antagonist of D2 receptors. Such drugs have received
attention recently because of their reduced propensity to cause extrapyramidal
side effects and for their efficacy in treating the negative symptoms of
schizophrenia. These properties may partly reside in the antagonistic actions
of the atypical neuroleptics on the various sub-populations of 5-HT receptors
of which the 5-HT2A receptor may be of primary importance.
In experimental studies, many clinically effective neuroleptics have been
shown to act as 5-HT2A receptor antagonists. Studies on post-mortem brain
from schizophrenic patients have shown that the decrease in the number of
5-HT2A receptors in the prefrontal cortex might be related to the disease
process. It therefore seems unlikely that the antipsychotic activity of
neuroleptics can be explained solely in terms of their action on 5-HT2A
receptors. Furthermore, no correlation exists between the average
therapeutic doses of a neuroleptic and its affinity for 5-HT2A receptors. It
does seem possible, however, that several atypical neuroleptics such as
amperozide, risperidone and possibly ritanserin do owe at least part of the
pharmacological profile to their ability to inhibit 5-HT2A receptors.
Following the discovery that selective 5-HT3 antagonists reduce the
behavioural effects of the infusion of dopamine into the nucleus accumbens,
there has been considerable interest in the possible role of 5-HT3 receptor
antagonists as potential neuroleptic agents. While there is a growing body
of evidence to suggest that 5-HT3 antagonists may be therapeutically
valuable for the treatment of disorders of the gastrointestinal tract, as
antiemetics and possibly anxiolytic agents, there is currently little evidence
to suggest that such drugs are effective in the treatment of schizophrenia.
However, experimental studies of the 5-HT3 antagonists on dopamine
autoreceptors may eventually offer new leads to the development of novel
antipsychotic drugs.

Serotonin and drugs of abuse

Serotonin and drugs of abuse
The role of 5-HT in the control of alcohol intake has received considerable
attention following the discovery that 5-HT reuptake inhibitors reduce
alcohol intake in alcohol dependent rats. Similar effects have been found for
intracerebroventricularly administered 5-HT or its precursor 5-HTP.
Regarding the type of 5-HT receptor involved, there is experimental
evidence that the 5-HT1A partial agonists buspirone and gepirone are
effective. Differences were found between the effects of the 5-HT3
antagonist ondansetron and the 5-HT2A/5-HT2C antagonist ritanserin.
Thus the 5-HT3 antagonist ondansetron reduces alcohol intake without
affecting the alcohol preference of rats, while ritanserin reduces both the
alcohol preference and intake. This suggests that, at least in rats, different
populations of 5-HT receptors may be involved in alcohol intake and
Regarding other types of drugs of abuse, the 5-HT3 antagonist MDL72222
has been shown to block place preference conditioning induced in rodents
by morphine or nicotine without affecting the preference for amphetamine.
It is possible that these effects of 5-HT3 antagonists are associated with the
reduction in dopamine release as it is well established that the rewarding
effects of many drugs of abuse are due to increased dopaminergic activity
in limbic regions. On the strength of the experimental findings, it has been
proposed that 5-HT3 antagonists might be useful in treating drug abuse in
man. Only appropriate placebo-controlled studies of 5-HT3 antagonists will
clarify the therapeutic value of such agents in different types of drug abuse.

Serotonin and hallucinogenic activity

Serotonin and hallucinogenic activity
There is abundant experimental evidence to show that serotonin plays a
major role in the mechanism of action of hallucinogens, but it is presently
unclear whether the actions of hallucinogens can be explained by their
agonistic or antagonistic actions. LSD, for example, may behave either as an
agonist or antagonist depending on the particular tissue, concentration and
experimental condition, whereas the tryptamine type of hallucinogens usually act as agonists. Experimental evidence nevertheless suggests that
the behavioural effects of a number of indole alkylamine (e.g. LSD-like) and
phenylalkylamine (e.g. mescaline-like) hallucinogens can be attenuated by
5-HT2A antagonists and that the potency of these classes of hallucinogens at
5-HT2A (and possibly 5-HT2C) sites correlate with their hallucinogenic
potency in man. It seems unlikely however that all hallucinogens owe their
activity to their potency in stimulating 5-HT2A receptors; LSD and
5-methoxydimethyltryptamine for example interact with 5-HT2C sites, while
phenyclidine may owe its hallucinogenic potency to an action on N-methyl-
D-aspartate (NMDA) and a subclass of sigma receptors. Nevertheless, the
balance of evidence suggests that most ‘‘classical’’ hallucinogens such as
LSD, mescaline and psilocybin act as partial agonists on 5-HT2A receptors.

Serotonin and sleep

Serotonin and sleep

It has been known for some years that the functional activity of 5-HT
neurons in the brain changes dramatically during the sleep–wake arousalcycle. Thus from a stable, slow and regular discharge pattern during quiet
wakening, neuronal activity gradually declines as the animal becomes
drowsy and enters slow-wave sleep. During rapid eye movement (REM)
sleep, 5-HT activity is totally suppressed but in anticipation of awakening
the neuronal activity returns to its basal level several seconds before the end
of the REM episode. During arousal or wakening, the 5-HT neuronal
discharge pattern increases considerably above the quiet waking state.
Koella has reviewed the evidence implicating the involvement of
serotonin in the sleep–wake cycle but the involvement of specific serotonin
receptor subtypes in sleep mechanisms is unclear. Experimental evidence
suggests that 5-HT1A agonists delay the onset of REM sleep while 5-HT2
antagonists suppress REM and have variable effects on non-REM sleep.
It must be emphasized that most studies of the relationship between the
serotonergic system and sleep have been conducted in rats and therefore
the relevance of such findings to man remains unproven. From such
experimental studies, it has been shown that blockade of 5-HT2 receptors
increases the proportion of slow-wave sleep and decreases the quantity of
REM sleep. Whether this effect of 5-HT2 antagonists can be ascribed to a
specific effect on slow-wave sleep is, however, a matter of conjecture as any
increase in time spent in one stage of sleep will be reflected in a decrease in
the time spent in other stages of sleep. However, experimental evidence
suggests that most drugs that alter serotonergic transmission reduce REM
sleep. There is evidence that the 5-HT2 antagonist ritanserin improves sleep
quality in those suffering from ‘‘jet lag’’ which suggests that the 5-HT2
receptors may be involved in adjusting the sleep–wake cycle to the
photoperiod. Furthermore, experimental data suggest that activation of 5-
HT2 receptors may vary according to the sleep–wake cycle. Such findings
suggest that 5-HT2 receptors are involved in the regulation of circadian
rhythms and the sleep–wake cycle. With regard to the overall role of 5-HT
in sleep, Koella has postulated that serotonin may produce its various
effects on sleep architecture by influencing cognition and vigilance.

5-HT receptor subtypes

5-HT receptor subtypes
Current knowledge of 5-HT receptors has been derived from advances in
medicinal chemistry, from the synthesis of ligands that show considerable
specificity for subpopulations of 5-HT receptors. The application of such
ligands to our understanding of the distribution of the 5-HT receptor
subtypes has been largely due to quantitative in vitro autoradiographic
techniques and the application of such imaging techniques as positron
emission tomography. Functional studies undoubtedly lag behind but the
development of sophisticated electrophysiological techniques and studies
of changes in secondary messenger systems which respond to the binding
of selective ligands to the 5-HT receptor subtypes have opened up the
probability that the physiological importance of the numerous receptor
subtypes will soon be clarified.
As a consequence of the application of these various techniques, the
International Union of Pharmacological Societies (IUPHAR) Commission
on serotonin nomenclature has published two major reports which attempt
to classify the various receptor subtypes according to their ligand binding
properties and secondary messenger systems. The first report classified
5-HT receptors into 5-HT1-like (comprising 5-HT1A, 1B, 1C and 1D), 5-HT2
(formerly the 5-HT-D receptor) and 5-HT2 (formerly the 5-HT-M receptor).
The detection of a novel 5-HT receptor, that could not be classified as 5-HT1,
5-HT2 or 5-HT3, in both the peripheral and central nervous systems,
extended the receptor types to 5-HT4. The application of molecular biology
techniques has led to the cloning and sequencing of at least six different
5-HT receptors, namely 5-HT1A, 5-HT1B, 5-HT1C, 5-HT1D, 5-HT2 and 5-HT3.
Further studies of the second messenger systems to which these receptor
subtypes are attached have shown that the 5-HT1-like, 5-HT2 and 5-HT4 receptors belong to the G protein coupled receptor superfamily, whereas
the 5-HT3 receptor belongs to the same family as the nicotinic, gammaaminobutyric
acid-A (GABA-A) and glycine receptors which are ion gated
channel receptors.
The most recent publication of the IUPHAR Commission has redefined
the 5-HT receptor subtypes according to their second messenger
associations and thereby helped to stress the functional role of the receptor
subtypes rather than relying primarily on the specificities of ligands that
bind to them. This approach has led to the classification of 5-HT receptors
into those linked to adenylate cyclase (5-HT1A, 5-HT1B, 5-HT1D, 5-HT4),
those linked to the phosphatidyl inositol system (5-HT2A, 5-HT2B and
5-HT2C), and those linked directly to ion channels (5-HT3). Table 6.1
summarizes the accepted classification of the 5-HT receptor subtypes, all of
which occur in the brain, together with the most specific agonists and
antagonists which have been developed. The structures of the seven
subtypes of the serotonin receptor have now been determined. Apart from
the ionotropic 5-HT3 receptors (Figure 6.2), the other receptors are of the
metabotropic type (Figure 6.3, 5-HT2 receptor). Figure 6.4 illustrates the
molecular structure of the 5-HT4 receptor.
More recently, the family of 5-HT receptors has been dramatically
increased to include 5-HT4, 5-HT5A and 5-HT5B, 5-HT6 and 5-HT7. The
5-HT6 and 5-HT7 receptors are positively linked to adenylate cyclase. Of
these, only the 5-HT4 receptor has so far not been cloned. Of these newly
discovered receptors, only the 5-HT4 receptor has been investigated in some
detail. This receptor is quite widely distributed in the brain and peripheral
tissues where they are positively coupled to adenylate cyclase. In the brain,
the 5-HT4 receptors facilitate acetylcholine release and may play a role in
peristalsis. It has been hypothesized that in the brain 5-HT4 receptors may
also play a role in facilitating cholinergic transmission and thereby have a
potential role to play in preventing cognitive deficits which are associated
with cortical cholinergic malfunction. The possible clinical significance of
5-HT4 receptors must await the development of specific agonists and
antagonists. So far, such compounds have not been developed. Figures 6.5,
6.6 and 6.7 illustrate the distribution of 5-HT3, 5-HT4, 5-HT6 and 5-HT7
receptors in the human brain.
Despite the dramatic advances which have taken place in the
identification and characterization of 5-HT receptor subtypes, it is evident
that many of the ligands used to characterize these receptor subtypes are
not completely selective. It must also be emphasized that receptors are the
products of genes and are therefore subject to genetic changes and, as a
consequence, variability in physiological and pharmacological responsiveness.
Thus affinity, potency and intrinsic activity of a drug at one receptor
may vary depending on the time, species and receptor–effector coupling. It is already known, for example, that ipsapirone, buspirone, spiroxatrine and
lysergic acid diethylamide (LSD) may behave either as agonists or
antagonists depending on the functional model being used to assess their
activity. A similar problem arises with intrinsic activity which is usually
assumed to be a direct reflection of the pharmacological properties of the
drug. It seems possible that the affinity can also be influenced by the nature
of the genetically determined receptor–effector coupling and is therefore
tissue (and species) dependent. Such factors may help to explain why the
identification and subclassification of 5-HT receptor subtypes is complex
and often confusing.
This dilemma can be illustrated by the attempts being made to identify
the functional role of 5-HT receptor subtypes using ligands which are
believed to be specific in their binding properties. Such ligands may prove
to be non-selective, more selective for an as yet unidentified 5-HT receptor
subtype or more selective for a non-5-HT receptor site. Conversely several
non-5-HT ligands are known to bind to 5-HT receptors with a high affinity.
For example, the alpha1 adrenoceptor antagonist WB4101, and the beta
adrenoceptor antagonist pindolol, have a high affinity to 5-HT1A receptors.

Psychotropic Drugs that Modifythe Serotonergic System

Psychotropic Drugs that Modifythe Serotonergic System
Over a century ago, a substance was recognized in clotted blood which was
found to cause vasoconstriction. This substance was still present following
adrenalectomy therapy suggesting that it differed from adrenaline and
noradrenaline. Eventually, Rapaport, Green and Page in 1947, purified the
vasoconstrictor factor from serum and identified it as serotonin (‘‘serum
tonic’’). Independently of the American investigators, Erspamer and
colleagues in Italy had identified a substance they termed ‘‘enteramine’’
from the intestine. ‘‘Enteramine’’ was subsequently found to be identical to
serotonin and was subsequently synthesized by Hamlin and Fisher in 1951.
Chemically, serotonin or enteramine is the indoleamine 5-hydroxytryptamine
Following the isolation and synthesis of serotonin in the early 1950s, there
has been increasing interest in the physiological function of this amine.
Initially, it was assumed that its main function was that of a peripheral
hormone because of the relatively high concentrations that were found in
the gastrointestinal tract and blood. Twarog and Page soon showed,
however, that it was also present in the mammalian brain thereby
suggesting that it may have a neurotransmitter role there. Interest in the
physiological role of serotonin in the central nervous system has
preoccupied neurobiologists since that time.
The detection of serotonin in nervous and non-nervous tissue was aided
by the development of the Falck–Hillarp histochemical technique, a method
whereby freeze-dried sections of tissue, when exposed to formaldehyde
vapour cause indoleamines to emit a yellow fluorescence. Dahlstrom and
Fuxe used this technique to show that the highest concentration of serotonin
in the brain is located in the raphe´ nuclei, projections from these cell bodies
ascending to the forebrain via the medial forebrain bundle. Descending
fibres were also shown to project to the dorsal and lateral horns and the
intermediolateral column of the spinal cord. Detailed observation of the
distribution of the serotonergic system in the brain became possible with the development of specific antibodies to the amine and the introduction of
autoradiographic methods for both the human and rodent brain.
For serotonin to be considered as a neurotransmitter, it was essential to
establish that it produced its physiological effects by activating specific
receptors located on the intestinal wall, platelet membrane or on nerve cells.
A major development occurred in 1957 when Gaddum and Picarelli
showed that the action of serotonin on the guinea-pig ileum could be
blocked by either phenoxybenzamine (dibenzyline) or morphine. These
investigators termed the two types of serotonin receptors on the intestinal
wall ‘‘D’’ (for dibenzyline) or ‘‘M’’ (for morphine) receptors, the ‘‘M’’ type
receptors being associated with the nerves supplying the intestine that
produced contraction of the smooth muscle by facilitating acetylcholine
release, while the ‘‘D’’ receptors were located on the smooth muscle wall.
More recently, it has been realized that the ‘‘D’’ receptors are widely
distributed in the body and coincide with 5-HT2 receptors which, when
activated by selective agonists, contract smooth muscle and aggregate
platelets. They also occur in synaptosomal membranes where they are
possibly associated with postsynaptic membrane structures. By contrast,
the ‘‘M’’ receptor has not been unequivocally identified in neuronal
membranes. However, increasing evidence now suggests that the
peripheral ‘‘M’’ receptor is identical to the 5-HT3 receptor in the brain.
Thus in a period of some 20 years, the distribution of serotonin in both
nervous and non-nervous tissue has been determined, many of its
physiological properties explained and the types of receptors upon which
it acts to produce its diverse physiological effects evaluated.

The importance of genomics to psychopharmacology

The importance of genomics to psychopharmacology

Virtually every physical and psychiatric disorder has a genetic component.
However, the vast majority of these diseases have a complex pattern of
inheritance and there is no evidence that a single genetic locus is
responsible for any of the major psychiatric disorders. Rather it appears
that multiple alleles (gene products) occurring at multiple sites within the
genome interact to produce a vulnerability to the disorder. The enthusiastic
reception for the unravelling of the human genome rests largely on the
promise that it will soon lead to an understanding of the pathological basis
of most diseases which, in turn, will aid the development of more effective
therapeutic treatments.
Following the sequencing of the human genome it was found that there
were between 30 000 and 40 000 genes that code for proteins, only twice as
many as occur in the fruit fly or the nematode worm! However, it does
appear that human genes are more complex than those of flies and worms
in that they generate a large number of proteins due to the alternative waysof splicing the molecules. Hopefully knowledge of the human genome will
enable genes to be identified that convey a risk for psychiatric diseases in
addition to those genes which are linked to a therapeutic response to drug
treatment. Knowledge of the latter forms the basis of pharmacogenomics
which, hopefully, will eventually lead to the development of specific
treatments for the individual patient.
The potential value of pharmacogenomics can be illustrated by two
examples involving the response of individual patients to antidepressants.
In this approach, the potential importance of the cDNA microarray
technique for identifying changes in thousands of individual genes that
are expressed in the mouse brain is now widely accepted. Experimental
studies have indicated that different antidepressants exert distinct effects on
gene expression in the mouse brain, these differences becoming more
marked as the duration of the treatment increased. Such findings may
eventually lead to an individualized treatment strategy for depressed
patients based upon their cDNA analysis.
At the practical clinical level, individual differences in the pharmacokinetic
characteristics of antidepressant drugs have been more successful. It
is well established that the enzymatic activity of different allelic forms of the
cytochrome P450 oxidase system in the liver is particularly important in
the metabolism of many psychotropic and non-psychotropic drugs (see
pp. 91–94). Of the major forms of cytochrome P450 in man, the 2D6 isozyme
is particularly important in the metabolism of antidepressants and a
potential cause of drug interactions. Three of the five commonly available
SSRI antidepressants (fluoxetine, paroxetine and sertraline) undergo
autoinhibition of this isozyme and can therefore increase the tissue
concentration of a more toxic drug (for example, an antiarrhythmic or
beta-blocker) should it be given concurrently.
Over 50 allelic variants of the cytochrome P450 2D6 gene have been
identified, including individuals who lack the gene and others who have
multiple copies of the gene. This means that an individual (the functional
genotype) can either be normal, a slow or an ultra-fast metabolizer of a drug
that passes through the 2D6 pathway in the liver. Slow metabolizers will
therefore be at an increased risk for adverse effects while the rapid
metabolizers will have little benefit from the normal doses. Thus
genotyping the enzymes that metabolize the commonly used psychotropic
drugs could help to optimize the response, and to indicate the potential for
adverse drug effects, of the individual patient.
A new term has recently been introduced to cover the application of
pharmacogenomics to the design of drugs for the individual patient,
namely theranostics (from therapeutics+diagnostics). This approach
involves creating tests that can identify which patients are most suited to
a particular therapy and also to provide information on how effective thisdrug is in optimizing the treatment. Theranostics is said to adopt a broad
dynamic and integrated approach to therapeutics which may be of
practical relevance in differentiating diseases which are closely associated
diagnostically (for example, Alzheimer’s disease and Lewy body dementia)
by applying a combination of immunoassays that enhance the differential
diagnosis. Several biotechnological companies now specialize in designing
immunoassays for application to infectious diseases such as hepatitis by
genotyping the hepatitis C virus for example. There are six genotypes of
the virus known: genotype 1 is more resistant to standard therapy
(requiring at least one year of continuous therapy) whereas the other
genotypes usually respond to treatment within 6 months. Clearly a
knowledge of which viral genotype is present is important in determining
the duration of treatment in the individual patient and hopefully it will
soon be possible to extend such approaches to the drug treatment of
central nervous system disorders.
Applying pharmacogenomics to the pharmacodynamic aspects of
psychopharmacology is still at a very early stage of development, largely
because so little is known of the psychopathological basis of the major
psychiatric disorders or of the mechanisms whereby psychotropic drugs
work. In depression, for example, it is widely assumed that the inhibition of
the serotonin transporter on the neuronal membrane is ultimately
responsible for the enhanced serotonin function caused by the SSRI
antidepressants. The serotonin transporter is structurally complex. The
promoter region of the transporter, to which serotonin is linked before it is
transported back into the neuron following its release into the synaptic cleft,
exists in several polymorphic forms which are broadly categorized into the
long and short forms. It is known that when the polymorphic form occurs in
which an additional 44 pairs of nucleotide bases are inserted, there is a
higher transcription rate and a greater degree of binding of serotonin to the
promoter region. The practical importance of this finding is that depressed
patients with the long form of the transporter show a better response to
SSRIs than those with the short form. In bipolar patients, there is an
indication that the short form of the promoter is more likely to result in the
precipitation of a manic episode if given an SSRI during the depressive
phase of the disorder. There is also some evidence that the short and long
forms of the transporter may be correlated with the frequency of
extrapyramidal side effects and akathisia, which is sometimes caused by
There are two caveats that should be taken into account with regard to
the application of pharmacogenomics. Drug response is as complex as the
underlying genetic basis of the disease due not only to the genotypic
variation taking place at mostly unknown chromosomal loci, but also from
variations in gene expression, post-translational modification of proteins,pharmacokinetic features of the drugs, the effect of diet, drug interactions,
etc. One would therefore anticipate that the effects of individual genes on
the drug response are relatively slight. Thus it has been shown in studies of
pharmacogenetic markers that they only confer a twofold increased
likelihood of predicting drug response. However, the widespread
application of microarray technology, whereby information on thousands
of genes can be determined simultaneously, may help to overcome the
limitations of the candidate gene approach, the method which until now
has been used to obtain information on a few genes presumed to be
involved in the underlying pathology of a disease or its response to drug
Another aspect requiring attention concerns the statistical evaluation of
the results. For example, recently it has been shown that in a study of
asthma one genotype had a 100% positive predictive value for non-response
to a drug. However, because the susceptibility genotype only occurs in less
than 9% of patients, in practice less than 10% of the non-response to
treatment can be attributed to this abnormal genotype. In this case, it has
been calculated that avoidance of the drug as a result of pharmacogenomic
profiling would only improve its efficacy from 46% to 51%. Thus the
reliance on candidate gene variation, which ranges from 2% to 7%, is
currently not in the range for practical application.

Genetically modified mice and their importance in psychopharmacology

Genetically modified mice and their importance in psychopharmacology

Just as adding genes from a complex to a simpler organism (for example,
from man to a fruit fly) may be helpful in understanding the function of a
gene, so it may help to understand how a gene functions by eliminating it.
To date, most gene ‘‘knock-out’’ studies have been undertaken in mice
because of:
(a) the relative ease with which genes can be manipulated and eliminated;
(b) the relatively rapid rate at which mice breed;
(c) their well established and relatively complex behaviour.
The success, and also the limitations of the gene elimination strategy can be
illustrated by studies on the molecular basis of memory and learning. In the
early 1980s it had been shown that the glutamate NMDA receptor was an
essential component of memory formation, the term ‘‘long-term potentiation’’
(LTP) being applied to the molecular mechanism involved. The drugs
which were then available were limited in their specificity for the NMDA
receptor but by selectively deleting genes thought to be involved in memory
it was possible to identify the precise components of the NMDA-linked
messenger complex located in the hippocampus. Further studies enabled
genes ranging from those encoding neurotransmitter receptors, protein kinases and transcription factors to be identified. However, there are
limitations to these techniques which should be considered.
A major problem with ‘‘knock-out’’ technology relates to the need to
delete the gene at the very early stage of embryonic development. Often this
results in the death of the neonate. Even if the gene is not essential for
survival, it could have a key role to play in development that is unrelated to
neuronal plasticity. Thus the deficits in learning and memory seen in the
mature mouse could be the result of a developmental defect rather than a
specific abnormality in the NMDA receptor complex. Alternatively, the
deletion of a gene that from experimental studies might be expected to have
a major effect on learning and memory in practice may have no apparent
effect. This is due to the mechanism of compensation whereby other genes
take over the function of the deleted gene.
Thus developing ‘‘knock-out’’ mice to understand the function of a
particular gene gives little information on the timing when the gene
becomes active. Nor does it necessarily reflect the location of the gene in the
intact (wild-type) mouse or indeed, the long-term effect of the nervous
system on its function. Nevertheless, these are largely technical drawbacks
that will undoubtedly shortly be solved. In principle, studying the actions
of psychotropic drugs on genetically modified animals will allow the
detrimental effects of a deleted gene on the general health of the animal to
be avoided. Such an approach will also allow investigations of the
interactions between neuronal signalling pathways by assessing the
synergistic interactions between the behavioural and other biological effects
of the deleted gene and drugs.

The impact of molecular neurobiology on psychopharmacology: from genes to drugs

The impact of molecular neurobiology on psychopharmacology: from genes to drugs

About 150 years ago, Charles Darwin observed that ‘‘those who make many
species are the ‘splitters’, and those who make few are the ‘lumpers’’’.
Today, the ‘‘splitters’’ dominate research in the life sciences. Such
researchers can generate massive quantities of data on genes and their
networks, proteins and their pathways and the numerous cascades of
messenger molecules that ultimately result in a physiological response.
Technological progress in recent years has enabled the genome of species as
diverse as the nematode worm Caenorhabditis elegans and the fruit fly
Drosophila melanogaster to the mouse and man to be unravelled, thereby
opening up the possibility not only of identifying genes that are responsible
for physiological processes but also those that are aberrant and cause
genetically based diseases.
Few would deny the importance of such research, but the very success
of the ‘‘splitters’’ has had a seriously detrimental effect on the equally
important role of the ‘‘lumpers’’, who attempt to integrate the
molecular/cellular approach with the behavioural/psychological consequences.
As a consequence, the ‘‘lumpers’’ are becoming a threatened
species of researchers. There are several reasons for this, not the least of
which is the widespread opposition to vivisection and the lack of
training in behavioural pharmacology in university courses. As a
consequence, research (and funding for behavioural research) has
declined in prestige. This has had an adverse impact not only in areas
of basic life science research but also in the pharmaceutical industry
where the ultimate validation of the therapeutic potential of a new
molecule depends on behavioural pharmacology. As a senior neuropharmacologist
has recently remarked ‘‘Many can genotype but few can
Despite this unfortunate disparity between molecular neurobiology and
behavioural pharmacology, it is essential that the neuropharmacologist and
biological psychiatrist are fully conversant with the basic concepts of the
subject in order to appreciate both its success and limitations.
To understand the basis of cloning, it isnecessary to consider how bacteria have evolved to resist infection by
external sources of genetic material. It has long been recognized that if a
virus could infect one strain of bacteria, it could then also infect other
bacteria of the same strain but not those of a different strain. Thus virus
infection was shown to be restricted to a particular strain, a restriction now
known to be due to two classes of enzyme, namely the methylases, which
modify bacterial DNA marking them as ‘‘self’’, and the destruction
enzymes, which act as molecular ‘‘scissors’’ and can destroy foreign DNA.
Restriction enzymes are sequence-specific in that they cut DNA at specific
locations along the nucleotide chain. While some of these enzymes yield
‘‘blunt’’ ends to the resulting DNA fragment, others make staggered cuts in
the DNA chain to produce ‘‘sticky’’ ends. Over 250 restriction enzymes are
now commercially available.
Cloning would not be possible without restriction enzymes. DNA chains
with a ‘‘sticky’’ end act like molecular ‘‘Velcro’’, thereby enabling two
pieces of DNA with complementary nucleotide sequences to be joined
together. The linking of the DNA strands is brought about by the enzyme
DNAligase which permanently joins the assembled DNA sequences with
covalent bonds, thereby producing a recombinant DNA molecule.
The next stage is to ensure that the recombinant DNA molecule is
copied by the enzymes which synthesize nucleic acids. These DNA and
RNA polymerases synthesize an exact copy of either DNA or RNA from
a pre-existing molecule. In this way the DNA polymerase duplicates the
chromosome before each cell division such that each daughter cell will
have a complete set of genetic instructions which are then passed to the
newly formed RNA by RNA polymerase. While both DNA and RNA
polymerase require a preformed DNA template, some viruses (such as
HIV) have an RNA genome. To duplicate that genome, and incorporate
it into a bacterial or mammalian cell, the viruses encode a reverse
transcriptase enzyme which produces a DNA copy from an RNA
Thermostable DNA polymerases have now been produced for
polymerase chain reaction (PCR) studies in which specific segments of
the DNA molecule can be mass produced from minute quantities of
material. RNA polymerases are then used to create RNA transcripts from
cloned genes in vitro. Reverse transcriptases have their specific uses in
molecular biology. These enzymes are used to form ‘‘cDNA libraries’’
which are batteries of molecules each one representing a single gene
expression. Such DNA libraries can then be analysed to determine which
genes are active under different conditions and in different tissues. cDNA
libraries are now used experimentally in microarray assemblies to detect
gene changes following drug treatment.
In a typical experimental situation, the gene of interest is incorporated
into a plasmid, which is a natural vector used by either a bacterium or other
cell type. To transfer the DNA fragment of a gene, the plasmids are digested
with one or two restriction enzymes and the desired fragment joined into a
single DNA recombinant molecule using DNA ligase. To express the new
gene in vitro, the plasmid containing the recombinant DNA is then
incubated with an RNA polymerase to form new RNA which is then
used to programme an in vitro system which translates the information
necessary for the synthesis of a new protein.
The foregoing is only intended to give a brief overview of the
mechanisms behind cloning. So far, the impact on diseases in man has
been limited to experimental approaches to the treatment of cystic
fibrosis and rare conditions in which a recessive gene is responsible.
However, cloning techniques have provided important information in
producing animals, usually mice, which have been manipulated to
express or remove genes that are implicated in psychiatric disorders.
Such ‘‘knock-out’’ and ‘‘knock-in’’ mice now provide important
information in which specific genes can be studied for their effects
on behaviour, which may ultimately be an important contribution to
understanding the genetic basis of psychiatric and neurological