It is now over 30 years since Gaddum & Picarelli (1957) described and characterised tryptamine receptors in the periphery. The setting in which they were working was what to us now is an antique department of pharmacology whose most sophisticated equipment was a smoked drum to record the response of a variety of smooth muscle preparations to the extracts of tissue containing the sought after transmitters. Gaddum the Professor of Pharmacology was the ideas man, Crawford a brilliant though highly obsessional methodologist, and Amin an indefatigable Indian PhD student. That group (Amin et al, 1954) described the presence of 5-HT in brain. They had been looking for substance P but the contractile response of the preparations to the extracts of brain did not resemble that peptide but serotonin. In many ways that was an initiator of psychopharmacology which has progressed exponentially to the present day.

Although suicide was traditionally considered an extreme response to stress, with the most frequent stress being depressive illness, severity of depression does not distinguish those who commit suicide from non-suicide attempters. A biological role involving the serotonergic system, possibly associated with a genetic risk factor, has been postulated. Low levels of 5-HT and 5-HIAA have been found in post-mortem examinations of brain-stem tissues of suicide victims (levels in cortical tissue were generally normal). An increased number of 5-HT2 receptors was found in the pre-frontal cortex of suicide victims, such upregulation having been demonstrated in induced 5-HT deficiency states; 5-HT1A receptors were also increased. Receptor populations may be altered by chronic psychotropic medication; e.g. 5-HT2 downregulation occurs following administration of antidepressants. There is some indication that the adrenergic system may be involved as well, but this will require further study. Antidepressant drugs may be effective in preventing suicide in patients with non-depressive syndromes who exhibit suicidal behaviour.

Involvement of the brain serotonin (5-HT) neurotransmitter system in obsessive-compulsive disorder (OCD) was originally suggested on the basis of therapeutic effects found with the semi-selective serotonin uptake inhibitor, clomipramine. More recent studies directly comparing clomipramine with non-selective or norepinephrine-selective uptake inhibitors, such as desipramine or nortriptyline, as well as studies with new, more selective serotonin uptake inhibitors, including fluvoxamine and fluoxetine, have supported that hypothesis. Clomipramine's antiobsessional effect has been augmented with the serotonin precursor, L-tryptophan, or with lithium, which has prominent serotonergic effects. Patients whose OCD symptoms improved on clomipramine worsened when the drug was discontinued (regardless of duration of therapy) and improved when clomipramine was reinstituted. OCD symptoms also worsened when metergoline, a 5-HT antagonist, was given to patients who had improved with clomipramine. Metergoline given alone had no effect. Administration of m-chlorophenylpiperazine (m-CPP), a 5-HT receptor agonist, to untreated OCD patients increased their anxiety, depression, and dysphoria, and exacerbated their OC symptoms. After 4 months of clomipramine therapy, m-CPP failed to produce the same behavioural effects, suggesting an alteration of a 5-HT subsystem (possibly downregulation of some 5-HT receptors). The data reviewed suggest an important role for an abnormal brain 5-HT subsystem in patients with OCD.

Various irregularities in serotonin (5-HT) function have been postulated as causes of affective disorders. Serotonin has been related to many of the major symptoms of depression, e.g. mood, appetite, sleep, activity, and cognitive dysfunction. Interference with 5-HT synthesis or storage has been shown to induce depression in vulnerable individuals. Decreased levels of 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid, decreased plasma tryptophan, low tryptophan neutral amino acid ratio, abnormalities in serotonergic function indicated by neuroendocrine challenge tests and various platelet measures, have been reported in depressed patients. Concentrations of 5-HIAA, the major metabolite of 5-HT in plasma, were found to be significantly negatively correlated with severity of depression as measured by the Hamilton Rating Scale for Depression score and specific depressive symptoms, despite the fact that plasma 5-HIAA is largely peripheral in origin. Blood platelets, which have been suggested as models for serotonergic nerve terminals, have a significantly decreased number of 5-HT uptake sites and 3H-imipramine binding sites in depressed patients. Antidepressant drugs may act, in part, by enhancing serotonergic activity. The serotonergic deficit may occur at any of several levels: diminished availability of precursor, impaired activity of tryptophan hydroxylase, abnormalities in 5-HT release or uptake, 5-HT receptor abnormalities or interactions with other neurotransmitters.

The functional roles of monaminergic transmitters in depression have been widely studied during the past decade. Data from that research suggest that lower levels of the 5-HT metabolite, 5-HIAA, in the cerebrospinal fluid; 5-HT uptake in human platelets; and platelets [3H]-imipramine binding sites occur in depressed patients. In recent years several potent and selective 5-HT uptake inhibitors have become available for clinical studies. The first shown to have antidepressant effects, zimelidine, was followed by similar compounds such as femoxetine, fluvoxamine, Citalopram, indalpine, fluoxetine, paroxetine, and sertraline. The effectiveness of serotonin inhibitors in treating other disorders, such as obsessive–compulsive disorder, anxiety, and panic disorder, has also been demonstrated. This review reports the data from clinical studies with these agents. The 5-HT uptake inhibitors are devoid of anticholinergic properties and have not produced weight gain or sedative side-effects, but may have another profile of side-effects. Headache, nausea, and vomiting have been reported, however.

Serotonergic anorectics are correctly defined only if they enhance 5-HT transmission and have their anorectic effects inhibited by drugs that block 5-HT receptors. Fenfluramine, the prototype indirect 5-HT agonist, and its metabolite, norfenfluramine, act as 5-HT releasers and uptake inhibitors and are both more effective in the dextro form. They lack the stimulant activity and do not cause hyperthermia or stereotypical behaviour as is characteristic of amphetamines. The anorectic effect of those drugs is attenuated by metergoline, a 5-HT receptor antagonist, in animals and man; 5-HT uptake inhibitors such as fluoxetine, zimelidine, SL 810385, and sertraline also cause anorexia, but only sertraline antagonism by metergoline has been reported. The effect of serotonergic anorectics on 5-HT release has been poorly investigated. Quipazine and RU-24969 cause anorexia by acting directly on 5-HT post-synaptic receptors. Serotonergic nerve terminals take up [3H]-D-fenfluramine and bind with high affinity in rat brain; uptake, an active process, appears to occur at a different site than binding, which is not affected by ouabain or low temperature. Anorexia is probably induced by interaction with 5-HT1B receptors in the rat; the human equivalent of this receptor is not known, but the 5-HT1D type is a likely candidate.

The role of central serotonergic (5-HT) system dysfunction in the regulation of aggression in both animals and man has been investigated for more than the past two decades. Evidence for reduced central 5-HT in the mediation of aggression comes from both behavioural and correlative studies. Functional reduction and augmentation of 5-HT activity is respectively associated with increased and decreased aggression in various animal models of aggression. While similar studies in man have not been performed, strong and consistent associations between indices reflecting reduced pre-synaptic 5-HT activity and aggression have been reported. Evidence of post-synaptic receptor upregulation in the brains of suicide victims has also been reported leaving the functional status 5-HT activity in such patients an open question. However, reduced neuroendocrine (i.e. prolactin) responses to fenfluramine, a 5-HT uptake inhibitor/releaser, which activates both pre- and post-synaptic sides of the 5-HT synapse, strongly suggest that overall central 5-HT activity is reduced in mood and/or personality disorder patients with history of suicidal and/or impulsive aggressive behaviour. Preliminary data with the 5-HT receptor agonist m-chlorophenylpiperazine further suggest that reduced activity of post-synaptic 5-HT receptors may be an important correlate of impulsive aggressive behaviour. Pharmacological agents with potent 5-HT pre- and/or post-synaptic augmenting effects should be tested clinically to determine their efficacy in the treatment of impulsive aggressive behaviour in psychiatric patients.

Identification of 5-HT receptor subtypes — 5-HT1A, 5-HT1B, 5-HT1c, 5-HT1D, 5-HT2 (possibly A and B), 5-HT3 subtypes, and possibly 5-HT4 — has encouraged the manufacture of 5-HT receptor inhibitors with greater subtype specificity. However, it appears that the receptors interact, and drugs initially thought to be specific may have multiple actions. For some conditions such as anxiety/depression, almost all receptors are implicated. Clinical studies provide clear evidence that manipulation of the 5-HT system has a role in treating depression, anxiety, obsessional illness, migraine, and eating disorders. Interactions between the various receptor subtypes make it difficult to identify specific clinical functions. The 5-HT1A receptors may be involved in aggression, anorexia, and hypotension. The 5-HT1B receptors may be involved in aggression, while the 5-HT1C receptors may play a role in central aversion systems and anxiety/depression. The role of the 5-HT1D receptors remains speculative; 5-HT2 receptors appear to be involved in depression, anxiety, appetite, sleep, vasoconstriction, and hypertension. Many drugs that are effective in treating migraine are potent 5-HT2 antagonists. 5-HT3 antagonists at high doses are effective in treating nausea and at low doses in treating anxiety. Treatment of aggression, suicidal behaviour, addiction behaviour, memory impairment, dementia, and schizophrenia with 5-HT inhibitors requires further testing.