5-HT receptors, genes and antipsychotic mechanisms

Influence of acute tryptophan depletion on gastric sensorimotor function in humans

Peripheral serotonin (5-hydrodytryptamine; 5-HT) is involved in the regulation of gastrointestinal motility and sensation, whereas centrally it plays a role in mood regulation. A dysfunctional serotonergic system may provide a plausible link between functional dyspepsia symptoms and its high psychosocial comorbidity such as anxiety and depression. The aim of this study was to evaluate the effect of decreased 5-HT synthesis by acute tryptophan depletion (ATD) on gastric sensorimotor function and nutrient tolerance, anxiety scores, and gastrointestinal mucosal 5-HT concentrations in healthy volunteers. All subjects were studied under a control condition and during ATD. Gastric sensorimotor function and nutrient tolerance were assessed using a barostat (n = 16, mean age 28.8 ± 1.4 yr) and a satiety drinking test (n = 13, mean age 27.3 ± 1.4 yr). Anxiety during the barostat was evaluated using State-Trait Anxiety Inventory (STAI) questionnaire. 5-HT concentrations were measured in fundic and duodenal mucosal biopsies by means of ELISA and immunohistochemistry. ATD significantly decreased plasma tryptophan levels compared with control in every experiment. ATD did not affect gastric sensitivity and compliance but decreased the sensation of nausea during balloon distension (AUC: 17.4 ± 4.3 vs. 11.4 ± 3.4 mm·mmHg, P = 0.030). ATD enhanced the postprandial volume increase (ANOVA, P < 0.05), but this was not accompanied by augmented nutrient tolerance (848 ± 110 vs. 837 ± 99 ml, nonsignificant). ATD had no effect on STAI state anxiety scores. No evidence was found for an effect on the number of enterochromaffin cells, but ATD reduced 5-HT levels in the duodenal mucosa. ATD alters gastric postprandial motor function and distension-induced nausea. These findings confirm involvement of 5-HT in the control of gastric accommodation and sensitivity.

Acute tryptophan depletion. Part II: clinical effects and implications

OBJECTIVE

This is the second of two articles reviewing the modern dietary technique of acute tryptophan depletion (ATD), a method of transiently reducing central serotonin levels in both healthy volunteers and clinical populations. This article details the clinical studies to date and discusses the implications of this research methodology.

METHOD

The authors present a review of clinical ATD studies collated from a MEDLINE search, unpublished communications and the authors' considerable experience with this paradigm.

RESULTS

Following from the initial use of ATD in subjects with depressive illness, studies of anxiety disorders and other psychiatric illnesses have been reported. Sleep, aggressive and cognitive effects are also active areas of research and are reviewed here.

CONCLUSIONS

Acute tryptophan depletion remains a useful psychiatric research tool. The findings from the clinical studies reviewed here are summarized and implications for future research detailed.

Monoaminergic synapses and schizophrenia: 45 years of neuroleptics

In 1952 Delay and Deniker introduced the first antipsychotic, chlorpromazine, into the treatment of mental patients. They subsequently defined the word 'neuroleptic' to describe drugs as different as reserpine and chlorpromazine which seemed to have similar effects on the mental life of patients. In the 1960s the hypothesis was developed, mainly due to Carlsson, that the principal mode of action of neuroleptics was to interfere with synaptic transmission mediated by dopamine (DA) in the brain. This concept was given substantial credence with the discovery by Seeman and Snyder in the 1970s that many of the neuroleptics acted as DA receptor blockers. Subsequently two different classes of DA receptor were defined on the basis of their coupling to adenylate cyclase by Kebabian. In the 1980s molecular biology led to the cloning of five different DA receptors, and at the end of this period vanTol and his colleagues cloned the D4 DA receptor, which has been of considerable interest in the 1990s as it is greatly elevated in the brains of schizophrenics. This historical review ends with a consideration of the possibility that in addition to DA receptors, serotonin and perhaps other transmitter receptors are involved in the aetiology of schizophrenia.

5-HT and human anxiety. Evidence from studies using acute tryptophan depletion

There is abundant evidence that serotonin (5-hydroxytryptamine, 5-HT) is involved in anxiety in both animals and humans but there is conflicting evidence for the precise role it plays. Acute tryptophan depletion provides a technique for investigating a global reduction in brain 5-HT function and we have investigated its effect on anxiety in drug-free panic disorder patients and normal volunteers. We found little effect on general levels of anxiety but it enhanced the effect of a panic challenge using 5% carbon dioxide (5%CO2) in panic disorder patients. The effect in normal volunteers was less clear with no overall effect following 5%CO2 challenge or the psychological challenge of a simulated public speaking task. These results are discussed in relation to the literature and are broadly supportive of the hypothesis that 5-HT acts to inhibit panic anxiety at the level of the periaqueductal grey but facilitates general and conditioned anxiety at the level of medial temporal lobe structures.