Clinical implications of Psychopharmacology

Clinical implications
Schizophrenia
Following the discovery that some antipsychotic drugs bind to sigma receptors, the suggestion arose that sigma receptors may be involved in schizophrenia and in the mode of action of antipsychotic drugs. Support for this hypothesis arose from the observation that the density of sigma receptors was dramatically reduced in several brain regions of post-mortem brains from schizophrenic patients. Such changes appeared to be restricted to the sigma receptor and did not involve the NMDA receptor or the PCP receptor. Whether such findings implicate alterations in sigma receptor function in schizophrenia is uncertain as it is possible that the changes in the density of these receptors is a function of the duration of treatment with neuroleptics. Support for the possible involvement of sigma receptors in schizophrenia, and in the action of antipsychotic drugs comes from the observation that haloperidol had a high affinity for these receptors in rat brain. Furthermore rimcazole, a putative neuroleptic, was found to have a high affinity for sigma receptors with little action on dopamine receptors. Several other sigma-selective ligands were also developed as possible neuroleptics. Unfortunately, despite convincing pre-clinical data showing that many of the sigma-selective ligands were active in animal models predictive of antipsychotic activity, none proved to have efficacy in clinical trials. It would therefore seem that the sigma ligands so far developed are unlikely to become the novel neuroleptics of the future.

Movement disorders
The most common symptomatic dystonias result from the administration of neuroleptics and occur as acute dystonic reactions or as tardive dyskinesia. The dystonias are disorders that involve sustained, involuntary muscle contractions and abnormal posture which interferes with normal motor function. Dystonias can be focal, as in the case of torticollis in which the neck involuntarily rotates, or they may be progressive and generalized as in torsion dystonia in which the body slowly becomes contorted. Torsian dystonia is familial and recent studies have identified a defective gene which may be responsible. Acute dystonic reactions occurring following the administration of potent neuroleptics are reported primarily in young men and usually develop shortly after the start of therapy. By contrast, tardive dystonia occurs following chronic neuroleptic treatments; as with tardive dyskinesia, symptoms often begin after the abrupt withdrawal of the neuroleptic. Although less severe than acute dystonic reactions, tardive dystonia is frequently permanent and difficult to treat. Until recently, the cause of dystonia has been assumed to involve a dysfunction of the basal ganglia. However, it is now known that most patients with lesions of the basal ganglia show no evidence of dystonia while those patients with dystonia exhibit little biochemical or anatomical change in basal ganglia function. More recently, there is clinical evidence that dystonia is associated with lesions of the brainstem and the cerebellum. The cerebellum is closely linked to the red nucleus which contains a high density of sigma receptors but few dopamine, serotonin or glutamate receptors. The brainstem region is also implicated in the hereditary mutant mouse model of dystonia in which the symptoms are known to be associated with both brainstem and cerebellar lesions. The presence of sigma receptors in anatomical structures that control movement and posture provides indirect evidence for the link between sigma receptors and dystonia. Further support for the involvement of these receptors is provided by the effects induced by the direct administration of sigma ligands into the red nucleus of rats; the degree of dystonia produced is directly proportional to the affinity of the drug for the sigma receptors. Additional experimental support for the involvement of sigma receptors in idiopathic dystonias comes from studies on a strain of rats which can develop a lethal dystonia but which are free of any identifiable anatomical lesions. It would appear that the density of sigma receptors is dramatically reduced compared to their non-affected litter-mates.

Regarding neuroleptic-induced dystonias, it is well known that typical neuroleptics cause catalepsy in rats and movement disorders in man. By contrast, the atypical neuroleptics clozapine and sulpiride have a low propensity to cause movement disorders in man even though they have established antipsychotic effects. These atypical neuroleptics, unlike many of the typical neuroleptics, have a low affinity for sigma receptors which lends support to the hypothesis that the dystonias produced by typical neuroleptics are related to their affinity for sigma receptors in the brainstem–cerebellar region.


Neurodegenerative disorders
So far all the evidence implicating the neuroprotective action of sigma ligands has been based on animal models of stroke or neurodegeneration. Several sigma ligands such as igmesine (JO 1784), NPC26377, ifenprodil and eliprodil have been shown to protect gerbils against ischaemic insult resulting from the bilateral occlusion of the carotid arteries; this is a popular experimental model of stroke. Similarly, ifenprodil and eliprodil, which have high affinity for sigma receptors in rat brain, are effective in protecting the mouse against focal cerebral ischaemia when administered after the induction of ischaemia. It would appear that the neuroprotective action is due to modulation of the polyamine site on the NMDA-glutamate receptor. However, as sigma ligands such as DTG, 3-PPP and BM4 14802 (which lack affinity for the NMDA glutamate receptor) have no neuroprotective action in the mouse model of focal cerebral ischaemia, it is uncertain whether highly selective sigma ligands would be effective in focal ischae mia in man. In other experimental studies, the potent sigma ligand igmesine has been shown to potentiate the potassium-evoked release of acetylcholine from rat hippocampal slices in vitro, an effect which is blocked by haloperidol. This suggests that igmesine may act as a sigma-1 agonist and may facilitate memory formation. Further evidence for this possibility is provided by the anti-amnestic action of igmesine in scopolamine-treated rats. These experimental studies suggest that sigma ligands, particularly sigma-1 agonists, may have therapeutic potential in the treatment of stroke and possibly in facilitating memory formation in the aged brain. Only doubleblind clinical trials of drugs such as igmesine, which appear to be relatively devoid of peripheral organ toxicity, will determine whether the various animal models of memory deficit and neurodegeneration are really predictive of potential therapeutic activity.

Anxiety and depression
There is experimental evidence to show that representative drugs for most classes of antidepressants have a modest affinity for sigma-1 receptors in vitro. Some antidepressants, such as sertraline and the monoamine oxidase- A inhibitor clorgyline, are moderately potent ligands for their receptor site. However, more recent studies have indicated that the most important final common pathway for the action of antidepressants involves the modulation of the NMDA-glutamate receptor possibly via the sigma receptor. It therefore seems uncertain that potent and selective sigma ligands will form the basis of a new group of antidepressants. However, there is more convincing experimental evidence to suggest that sigma ligands could have anxiolytic or anti-stress activity. Thus igmesine and DTG have been shown to block environmentally induced stress or corticotrophin-releasing factor induced colonic activity in the rat. Recently there has been renewed interest in the clinical development of igmesine as an antidepressant. Other experimental studies have shown that selective sigma ligands such as Lu 28-178 are potent anxiolytics in rodent models of anxiety.

The future of sigma receptor ligands
Besides the obvious need to develop highly potent and selective drugs for the sigma-1 and sigma-2 receptor sites, knowledge of the precise structures of the sigma receptors is required in order to establish firmly their identity. The presence of sigma receptors in the brain, in the gastrointestinal tract and endocrine and immune systems suggests that there must be endogenous factors that act as agonists and antagonists for these receptors. To date the nature of these endogenous factors is unknown but there is experimental evidence to implicate some neuropeptides (such as neuropeptides- Y and PYY) and steroids such as progesterone and deoxycorticosterone as putative ligands. In addition to the need for more detailed experimental studies to characterize the cellular mechanism of action of the different types of sigma receptors it is also essential to broaden the clinical profile of these drugs. So far, attention has been almost exclusively directed at the action of relatively non-selective sigma ligands in the treatment of psychotic disorders. The experimental findings that sigma compounds may have putative neuroprotective and anxiolytic/anti-stress effects will hopefully encourage the further development of the highly selective sigma compounds for their therapeutic application.

Endocoids and their Role in Psychopharmacology

The empirical evidence implicating naturally occurring substances which occur within the mammalian brain and which appear to produce their psychotropic effects by activating specific receptors within the brain. Such substances are termed endocoids and they include the enkephalins and endorphins, which activate specific opioid receptors, the anandamide related compounds, which activate cannabinoid receptors, the endopsychosins and related compounds that activate sigma receptors and natural agonists and antagonists that show an affinity for the benzodiazepine receptors. These different types of endocoids will be discussed in terms of their possible physiological effects.

Endogenous cannabinoids and cannabinoid receptors
The Chinese emperor Shen Nung is believed to have produced the first written account of the medicinal properties of cannabis over 2000 years ago and various formulations of herbal cannabis have been used over the centuries to treat seizures, neuralgia, dysmenorrhoea, insomnia and even gonorrhoea. The hemp plant, Cannabis sativa, from which cannabis and many of the related compounds are obtained, has a long history in medicine. Thus over the centuries the cannabinoids have been used for the treatment of pain, asthma, dysentery, as sedatives, for the suppression of nausea and vomiting and as anticonvulsants. Although the clinical uses of the cannabinoids declined in the 20th century there has been a renewed interest in these natural compounds in recent years for the control of spasticity associated with multiple sclerosis and in the treatment of chronic pain. Such renewed interest coincided with greater attention being paid by the medical profession and society at large to herbal remedies. Understanding the mec hanism of action of the cannabinoids has been advanced by the identification and cloning of specific cannabinoid receptors in the mammalian brain and spleen and the identification of endogenous substances which bind to these receptors. Thus the cannabinoid receptors in the brain are primarily of the CB1 type. These receptors are widely distributed in areas concerned with motor activity (basal ganglia and cerebellum), memory and cognition (cerebral cortex and hippocampus), emotion (amygdala and hippocampus), sensory perception (thalamus) and with endocrine function (hypothalamus and pons). The distribution of radio-labelled tetrahydrocannabinol, the main active ingredient of Cannabis sativa, is similar to the distribution of the CB1 receptors and there is good evidence that the cannabinoids exact their action through these receptors. In addition to the CB1 receptors, CB2 receptors have been identified on macrophages in the spleen where they probably mediate the immunological effects of the cannabinoids. CB1 receptors have also been detected in peripheral tissues.

The discovery of cannabinoid receptors has raised the possibility that therapeutic agents could be developed that may combine the therapeutic uses of the cannabinoids with lack of abuse and drug dependency. The first endogenous substances to be shown to have a high affinity for the cannabinoid receptors were the anandamides, named after the Sanskrit word for ‘‘bliss’’=ananda. Structurally the endogenous ligands for the cannabinoid receptors are unlike those of plant origin. The The system comprising the cannabinoid receptors and endogenous anandamide-related compounds is referred to as the anandamide system. However, it must be borne in mind that endogenous ligands for cannabinoid receptors may exist with properties that differ from those of the anandamide series of compounds. Endogenous parent compound is a derivative of the endogenous fatty acid arachidonic acid, arachidonyl ethanolamide. More recently, two other endogenous unsaturated fatty acid ethanolamides with a high affinity for cannabinoid receptors have been identified in brain tissue. These are homogamma-linolenylethanolamide and docotetraenylethanolamide. While there is convincing evidence that endogenous compounds exist in the mammalian brain that have properties which resemble those of tetrahydrocannabinol, the most potent cannabinoid from a plant source, the question arises regarding the need to postulate the existence of specific receptors for these natural ligands. After all, although opioid peptides have been isolated from brain extracts, the search for other receptor ligands, including those which bind to the benzodiazepine and sigma receptors, has not been nearly as successful. Nevertheless, due to the special nature of receptors which are coupled to G proteins, it is highly probable that there are natural ligands for all such receptors. This is because G proteins are single molecules that do not contain allosteric binding sites, unlike the benzodiazepine–GABA receptor where the benzodiazepine binding site is an allosteric regulatory site for GABA.

For all G protein-coupled receptors, every receptor has an endogenous ligand associated with its binding site. Thus it is reasonable to conclude that the binding sites for the anandamide system in the mammalian brain are true receptor sites through which the physiological changes initiated by the cannabinoids are expressed. Despite the recent advances in molecular biology, the mechanisms of action and the physiological functions of the anandamide system remain obscure. It would appear that the cannabinoid receptors and the anandamides reside within the neurons. Thus unlike the classical neurotransmitters noradrenaline and serotonin, the anandamides are not released into the synaptic cleft and are not involved in interneuronal communication. Instead the anandamides modulate the excitability and inhibitory responsiveness of neurons by acting on cannabinoid heteroceptors located on inhibitory and excitatory terminals. In this way, the cannabinoid receptors reduce the activity of these neurons by decreasing the i nflux of calcium through the calcium channels and increasing the efflux of potassium ions through the potassium channels located on the neuronal membrane. In some regions such as the cerebellum, there is a convergence of the G protein-linked receptors such as the GABA-B, adenosine A1, cannabinoid and kappa opioid receptors that inhibit the activity of adenylate cyclase thereby leading to a reduction in the release of glutamate. Thus it seems possible that the anandamide system modulates the activity of the major neurotransmitter systems including the opioid, prostenoid and glucocorticoid systems.

Sites of action of the cannabinoids
CB1 receptors are present in a high density in the hippocampus and cerebral cortex and the effects of cannabinoids on cognition and memory are undoubtedly related to their activation of the receptors in this brain region. These regions also mediate the effects of the cannabinoids on perception of time, sound, colour and taste. With regard to the motor effects, and effects on posture, of the cannabinoids it would appear that this is related to their agonist action on CB1 receptors located in the basal ganglia and cerebellum. Other central actions of the anandamide system include the hypothalamus (effect on body temperature), the spinal cord (antinociception) and the brain stem (suppression of nausea and vomiting). The discovery that cells of the immune system contain both cannabinoid binding sites and cannabinoid receptor mRNA suggests that the immunosuppressive actions of the naturally occurring cannabinoids are receptor mediated. There is now evidence that cannabinoid receptors occur on spleen cells in rodents and man and in human thymus cells and monocytes, but the receptor density is lower than that occurring in the brain. The B-lymphocytes have been shown to contain the highest quantity of cannabinoid receptor mRNA. The specific binding of cannabinoids to the small intestine and testis has also been reported to occur in different mammalian species. As the peripheral cannabinoid receptor appears to be of the CB2 type which appears to be absent from the brain, there have been attempts to develop selective agonists which would lack psychotropic properties but which would be of therapeutic value as immunosuppressants and in the control of such autoimmune diseases as rheumatoid arthritis. Conversely, CB2 receptor antagonists may act as drugs to enhance immune function. To date, no compounds have reached clinical application despite showing promising pharmacological profiles in the preclinical stages of their development. There is hope that a new approach in which analogues of the anandamides are developed will be more fruitful.

Physiological processes in that endogenous cannabinoids may be act as mediators
The possible physiological importance of the endogenous cannabinoids has largely been based on an extrapolation from the pharmacological properties of the THC-like compounds that are known for their psychotropic effects. Such drugs may differ in action from the endogenous cannabinoids because of their broad range of activity that follows the activation of both the CB1 and CB2 receptors, but also their ability to inhibit membrane bound enzymes and to cause a disruption of the normal function of the phospholipid compounds of neuronal and other membranes. Thus it would be anticipated that endogenous cannabinoids would show more selective actions both in the brain and periphery.

Tolerance is known to develop rapidly to many of the effects of the psychotropic cannabinoids but little is known regarding the mechanisms responsible for the development of tolerance to these drugs. One possibility to account for the development of tolerance is that compensatory decreases in the sensitivity or density of cannabinoid receptors occurs following the prolonged stimulation of these receptors, perhaps by inducing changes in the genetic expression of the receptor protein. This could occur as a result of a decrease in the signal transduction mechanism or in the affinity of the receptor sites for the cannabinoids. There are several in vitro and in vivo experimental studies in support of such mechanisms, but it is presently unproven whether such mechanisms apply to the components of the anandamide system.

Endozepines as endogenous anxiolytic and anxiogenic agents
It has been postulated that, at the cellular level, the symptoms of anxiety can arise because:
1. There is inadequate activity of an endogenous anxiolytic ligand.
2. There is excessive activity of an endogenous inverse agonist at the benzodiazepine receptor site.
3. There is a dysfunctional GABA-A receptor causing a shift in the GABAA complex towards inverse agonist activity.
It is uncertain which of these three possibilities apply to patients with anxiety disorders. There is evidence that the binding of the benzodiazepine receptor antagonist, flumazenil, is lower than normal in patients with panic disorder and that it increases the panic attack frequency in these patients but not in normal subjects. This has been interpreted as a slight shift in the benzodiazepine receptor towards the inverse agonist state.

Three types of endozapines have been isolated. It is known that the betacarbolines can be synthesized in the mammalian brain and that, in vitro, they act as inverse agonists at benzodiazepine receptor sites. Theoretically such compounds could induce anxiety. However, none of these compounds has been isolated in vivo and the original detection of a beta-carboline in the urine of anxious patients was later found to be an artifact, possibly caused by bacterial contamination. A diazepam binding inhibitor has been isolated from mammalian brain and found to be a mixture of two peptides (an octodecaneuropeptide and a trikontatetra neuropeptide) which stimulates neurosteroid synthesis by acting on peripheral benzodiazepine receptors. There are two main neurosteroids present in the mammalian brain which are antagonists of GABA-A receptors, namely dehydroepiandrosterone and its sulphate form (DHEA and DHEAS). These neurosteroids are also synthesized in the adrenal glands. These neurosteroids are known to have multiple effects of brain function by affecting mood, cognition and sleep; they also enhance neuronal plasticity and are neuroprotective. The third group of compounds are the naturally occurring benzodiazepines. Desmethyldiazepam has been isolated from human brains which were stored frozen in the 1930s, at least two decades before the benzodiazepines were developed. While there is no evidence that the benzodiazepine structure can be synthesized enzymatically in the mammalian brain, several other compounds of this type have since been isolated from cattle brain and from human breast milk. One possibility is that gastrointestinal flora can partially synthesize the benzodiazepine molecule and it is also known that plants such as wheat and potatoes are a potential source of diazepam, desmethyldiazepam and lormetazepam. If it is eventually shown that the local brain concentration of these benzodiazepines is sufficiently high to activate the benzodiazepine receptors then the possibility arises that anxiety disorders could result from a lack of these endozepines.

Several species of plant also contain compounds that have been shown to act as agonists on benzodiazepine receptors. These include: Valeriana officinalis which contains hydroxypinoresinol, Matricaria recutita which contains 5,7,4’-trihydroxyflavone, Passiflora coeruleus which contains chrysin and Karmelitter Geist which contains amentoflavin. Hypericum perforatum (St John’s Wort) also contains unknown compounds which have affinity for these receptors. Extracts of these drugs are commonly recommended by herbalists for the treatment of insomnia and anxiety.

Endogenous sleep factors
Early in the 20th century, Pierin in Paris infused the CSF of sleep-deprived dogs into normal dogs and showed that the CSF contained a sleep-inducing (somnogenic) factor. This was thought to be a muramyl peptide but later suggested to be the result of bacterial contamination as these peptides cannot be synthesized by the mammalian brain. Pro-inflammatory cytokines can also induce sleep, the effect depending on the concentration of the cytokine and the time of day. The effect on the sleep profile (increased non-REM and decreased REM sleep) appears to depend on the increased synthesis of prostaglandin D2 and nitric oxide which then alter the circadian rhythm. It is also known that some pro-inflammatory cytokines can affect the reuptake of 5-HT which plays an important role in regulating the sleep–wake profile. The endogenous fatty acid, oleamide, can cause sedation and induce sleep by activating cannabinoid receptors but also by potentiating the action of benzodiazepines on their receptor sites. Whether such action is of physiological relevance is presently unknown.

Function and therapeutic effects of sigma receptors
The sigma opiate receptor were originally proposed by the American neuropharmacologist William R. Martin as the site that mediates the psychotomimetic and stimulatory effects of cyclazocine, pentazocine, Nallyl normetazocine (SKF 10047) and related opiates in humans and dogs. However, there is now considerable evidence to suggest that these effects are not mediated by opioid receptors. Many of the opiates that have psychotomimetic properties also bind with a high affinity to phencyclidine (PCP) receptor sites situated in the channel of the N-methyl-D-aspartate (NMDA) receptor. It now appears from electrophysiological, biochemical, anatomical and molecular studies that there are two distinct sites that bind opioid analgesics that have an affinity for sigma receptors. One site is on the PCP receptor situated in the NMDA receptor. The other sigma site is defined as non-opioid, non-dopaminergic and shows a high affinity for haloperidol and N-allyl normetazocine. Using a highly selective ligand for sigma receptors such as ditolyguanidine (DTG), it has now been possible to separate sigma receptors into two major types. Sigma-1 receptors are the main neuronal type and exhibit a high affinity for centrally acting antitussive and anticonvulsant drugs. The other site has a low affinity for most sigma ligands except DTG and haloperidol. This site is found in the red nucleus and cerebellum (as well as many other brain regions) where it may mediate the motor (dystonic) effects of different types of sigma ligand. Biochemically the sigma-1 and sigma-2 receptors may also be distinguished by the nature of the second messenger to which they are attached. Thus the sigma-1 receptors appear to be linked to guanylyl nucleotide binding proteins (G proteins) whereas the sigma-2 sites are not and may bring about their physiological effects by modulating K+ channels.

Sigma receptors and psychosis
Some 20 years ago, Martin and coworkers proposed that the psychotomimetic effects of pentazocine and related opiate analgesics was due to their effect on sigma receptors. It is now known that the sigma receptors are quite distinct from PCP, opioid, serotonin and dopamine receptors. However, many psychotropic drugs that bind to dopamine, serotonin and PCP receptors also have a high affinity for sigma receptors. For example, haloperidol and the novel benzamide neuroleptic remoxipride bind with high affinity for both D2 and sigma receptors. Nevertheless, there are many potent neuroleptics that have a negligible affinity for sigma receptors and conversely, many sigma ligands that do not apparently have any neuroleptic activity, but it remains a possibility that there could be an involvement of sigma receptors in the pathology of schizophrenia. Thus receptor autoradiographic studies of post-mortem schizophrenic brain have demonstrated a significant reduction of sigma binding sites in the frontal cortex, amygdala and hippocampus without any significant change in the density of PCP binding sites. Therefore, the evidence linking a malfunctional sigma receptor system to schizophrenia, or the use of selective sigma receptor ligands as putative neuroleptics, is inconclusive.

Sigma receptors and the immune and endocrine systems
Experimental evidence suggests that sigma receptors play an important role in regulating and integrating both immune and endocrine functions. In experimental studies, it has been shown that the selective sigma ligand N-allyl-normetazocine stimulates the hypothalamic–pituitary–adrenal axis but suppresses luteinizing hormone and prolactin secretion. A high density of sigma receptors has been identified on human leucocytes and in the rat spleen, testis, ovary and adrenal gland. In human leucocytes it has also been shown that sigma receptors are involved in the second signalling mechanisms that are essential for cellular activation. In addition, sigma receptors have been identified on human and rat T and B cells. There is experimental evidence to show that the suppression of T cell replication, and enhanced activity of monocyte phagocytosis, that occurs in some rodent models of depression, can be effectively reversed by the chronic administration of selective sigma ligands such as igmesine. This suggests that such compounds may be of benefit in correcting the diverse immune and possibly endocrine defects that characterize depression.

Inter-relationship Between Psychopharmacology and Psychoneuroimmunology

An adverse effects of stress and depression, the effects of bereavement, unemployment and social isolation on mental and physical health have been known since antiquity. Aristotle advised physicians, ‘‘Just as you ought not to attempt to cure eyes without head or head without body, so you should not treat body without soul.’’ One of the fathers of modern medicine put it more scientifically in the 19th century when he recommended that when attempting to predict health outcomes from tuberculosis in patients, it is just as important to know what is going on in a man’s head as it is in his chest. These are two of the numerous examples, largely anecdotal, that document the complex and intimate connection between the mind and the body. In the past 20 years this has given rise to a new science of psychoneuroimmunology that is devoted to the study of the inter-relationship between the brain, behaviour and the immune system. Interest in this area of neuroscience has undoubtedly been due to the impact of acquired immune deficiency syndrome (AIDS) in which it has been estimated that at least 10% of these patients will develop mood, behavioural, cognitive and memory changes before they develop somatic signs of the illness. Similarly, studies have shown that 6 months before patients with pancreatic cancer develop clinical signs of the disease, a significant proportion develop depression. Such observations suggest that not only does the brain influence the immune system by way of the endocrine and efferent neuronal pathways but also that products of immune cell activity, such as the cytokines, play a role in modifying human behaviour by directly modulating central neurotransmitter pathways.

Basic structure of the immune system
It is not the purpose of this short introduction to psychoneuroimmunology to give a comprehensive view of the immune system. Most of the cells comprising the immune system can be divided into one of two categories depending on the targets of their action. Thus the immune cells are either primed to eliminate specific pathogens or to respond to any type of cell that is not recognized as being a normal body component. The first category of cells comprises the different types of lymphocytes which are divided into the B-lymphocytes (B cells) that are responsible for antibody production, and the T-lymphocytes (T cells) that directly phagocytose pathogens or release specific biologically active proteins, the cytokines, that regulate the activity of other cells in the immune system. Both T and B cells respond in a highly specific manner when attacking pathogens. In addition to these specific immune cells, there are phagocytic cells, such as the monocytes and neutrophils, that respond to any cell type or foreign molecule that is not recognized as being a normal constituent of the body. The phagocytic cells such as the monocytes and neutrophils are basically scavenger white blood cells that ingest invading bacteria or viruses. Some of the monocytes also enter the tissues where they become macrophages. They can also provide signals enabling T cells to respond more efficiently to the pathogen. In this situation the antigen becomes attached to the monocyte membrane which is then presented to a T-lymphocyte together with the cytokine interleukin-1 (IL-1). This initiates a further activation of T-lymphocytes. Monocytes also produce mediators of inflammation, the complement proteins, which help to create a hostile environment for foreign organisms. In addition to complement proteins other mediators of the immune response include histamine (which acts as a local hormone to cause capillary dilatation), the prostaglandins and leukotrienes which act to initiate and terminate the activities of the macrophages and T cells.

Lymphocytes are derived from bone marrow but, whereas some of the cells remain in the bone marrow until they reach maturity (the B cells), others migrate early in their development to the thymus gland to become T cells. Thus B (from bursa) and T (from thymus) cells learn to distinguish between the normal constituent cells of the body and foreign objects, due to the presence of specific memory cells which are under genetic control. B and T cells circulate throughout the vascular system before concentrating in lymphoid tissue (spleen and lymph nodes) where they remain inactive until stimulated by specific antigens. Because of the specificity of function imparted on the T and B cells by the memory cells, the lymphocytes are highly selective in responding to relatively few antigens.

Main properties of the immune cells that are altered in psychiatric illnesses :
Natural killer cells (NKCs):
Recognize changes on cell-membrane virus-infected and cancer cells and destroy the cells. NKCs bind to surfaces of target cells and inject cytotoxic molecules into the cell membrane, destroying the cells. There are several types of cells that have NKC activity.

Phagocytes:
Two major classes of WBCs are involved in removing invading microorganisms by a process of phagocytosis. These are polymorphonuclear leukocytes and mononuclear phagocytes, or monocytes. In tissues, monocytes differentiate into macrophages and, in the brain, into microglia.

T and B lymphocytes:
Produced by lymphoid tissue. Lymphocytes represent about 20% of the WBCs in adults; they have a long life span (sometimes several years). They probably serve as memory cells for the immune system. These mononuclear cells may be small, agranular structures (T and B cells) or large, granular cells (NKCs). Different types of T cells may only be differentiated by their cell-surface markers (CD markers – clusters of differentiation). CD markers are identified using labelling antibodies.

T cells exist in several different forms. Thus the T-helper cells (Th cells) play a regulatory role by facilitating the antibody production by B cells and also activate the macrophages. Other types of T cells can directly attack pathogens or normal cells that have been infected with a virus or bacterium for example. These are the cytotoxic T cells, or natural killer cells (NKCs). Not only can such cells destroy pathogens but they also secrete such cytokines as IL-1 which have a key role to play in orchestrating the immune system both peripherally and in the brain. The immunoglobulins (the most important in man being IgM, IgG, IgE, IgD and IgA) are produced following the activation of B cells by specific antigens. Fever and sleep are important events which assist recovery following an infection by helping to destroy heat-sensitive foreign microorganisms. One of the key promoters of sleep and fever following an infection is IL-1. This cytokine can penetrate some areas of the blood–brain barrier and raise the temperature ‘‘set point’’ in the hypothalamus thereby producing a fever. Similarly IL-1 promotes slow-wave sleep and thereby facilitates tissue repair due to the secretion of growth hormone during that sleep phase. In addition to facilitating tissue repair, growth hormone can also boost the immune system. Whereas the precise mechanism whereby the cytokines can enter the brain and initiate subtle changes in brain function is uncertain, CNS changes initiated by peripherally produced IL-1 (and also by the microglial cells within the brain) provides convincing evidence that the immune system directly impacts upon the brain.

The endocrine immune relationship
One of the major pathways whereby the central nervous system regulates the immune system is via the hypothalamic–pituitary–adrenal (HPA) axis. Various neurotransmitters (e.g. serotonin, noradrenaline, acetylcholine) regulate the secretion of corticotrophin releasing factor (CRF) which controls the release of adrenocorticotrophic hormone (ACTH) from the anterior pituitary. ACTH directly activates the adrenal cortex to produce glucocorticoids (e.g. cortisol). Following the rise in the plasma concentration of the glucocorticoids, a negative feedback mechanism normally operates to block the further release of ACTH from the pituitary. In depression, however, there would appear to be an insensitivity of the central glucocorticoid receptors to this feedback regulation. As a consequence, the plasma concentration remains elevated and cannot be easily suppressed by a potent synthetic glucocorticoid such as dexamethasone. This forms the basis of the dexamethasone suppression test (DST) which is often used as a biological marker of depression. T cells are particularly sensitive to the inhibitory effects of the glucocorticoids. In particular, the nascent T cells, which represent about 90% of all T cells in the thymus gland, are very sensitive to the inhibitory effects of these steroids; high steroid concentrations can also prematurely induce the migration of T cells from the thymus to other immune tissues. This leads to a decrease in the size of the thymus gland. It should be emphasized that the effects of the glucocorticoids on the immune system are biphasic; in high concentrations they suppress major components of the immune system whereas in low concentration they activate it. In addition to glucocorticoid receptors, T cells also contain receptors for prolactin and growth hormones which suggests ways in which the endocrine system can directly affect the immune system. The adrenal gland secretes glucocorticoids in a pulsatile rhythmical way with the highest plasma concentrations being reached during the day. It has been shown that the lowest plasma concentration of the glucocorticoids coincides with the time at which the lymphocytes respond most actively to antigens. As the hypersecretion of cortisol is a characteristic feature of depression and other psychiatric conditions, it is perhaps not surprising to find that components of the immune system are also abnormal in this condition.

Anatomical links between the brain and the immune system
What is the mechanism whereby the nervous system can influence the immune system? Two major routes serve to link the brain with the immune system. The first is via the HPA axis, already referred to. The second is via the autonomic nervous system. It has been known for over 20 years that there were adrenoceptors on T cells, B cells and macrophages. In addition, noradrenergic fibres directly innervate the bone marrow, thymus, spleen, lymph nodes and virtually all other immune organs. These sympathetic nerve terminals not only release noradrenaline but also possibly neuropeptides as well. There is evidence that many sympathetic nerve terminals innervating the immune organs make direct contact with the parenchyma, ending adjacent to the cells of the immune system. In the spleen for example, the sympathetic terminals penetrate the areas that contain a high density of helper T cells and also cytotoxic and suppressor T cells. Electron microscopic evidence suggests that the sympathetic nerve terminals can form direct physical contact with T-lymphocytes and macrophages.

The functional connection between the peripheral sympathetic system and the immune system can be illustrated by the changes which take place in ageing. It is known that in the aged animal the sympathetic innervation of the spleen is dramatically reduced. This appears to be associated with deficiencies in T cell function and in cellular immunity. At the cellular level, immunosenescence is associated with a change in responsiveness of the immune cells and in their ability to regulate the beta adrenoceptors on their cell surfaces. Such changes appear to shift the metabolism of the sympathetic nervous system to a state that encourages apoptosis (or programmed cell death) possibly by inducing an increase in the production of cytotoxic metabolites. Experimental evidence suggests that the monoamine oxidase-B (MAO-B) inhibitor deprenyl (selegiline) can reduce these neurodegenerative changes in the peripheral sympathetic system and lead to the restoration of sympathetic innervation of the spleen.

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.