The role of serotonin in eating disorders.

Recent pharmacological studies have more precisely characterised the nature of the inhibitory effect of brain serotonin (5-hydroxytryptamine) on feeding behaviour. Thus, the brain sites and receptors involved have been identified, and a possible physiological role of endogenous serotonin in controlling natural patterns of eating and nutrient selection has been defined. The medial hypothalamus is believed to be a critical location in the mediation of serotonin’s action. Specifically, the paraventricular and ventromedial nuclei are known to be involved in controlling energy balance, while the suprachiasmatic nucleus determines circadian patterns of eating. Serotonergic stimulation of these 3 nuclei with exogenous serotonin or drugs that release endogenous serotonin, preferentially reduces carbohydrate intake in naturally feeding animals through satiety mechanisms involved in the termination of feeding. This phenomenon is mediated by serotonin and possibly serotonin receptors, in contrast to serotonin autoreceptors which potentiate feeding possibly by inhibiting serotonin release. The activity of serotonergic function in the medial hypothalamus exhibits a circadian rhythm which is characterised by a peak at the beginning of the active cycle when the motivation to eat is strongest and is triggered by deficits in energy stores. At this time, carbohydrate is found to be the naturally preferred macronutrient, and it appears that serotonin becomes most activated under these conditions to terminate the carbohydrate-rich meal, possibly by activating satiety neurons localised in the medial hypothalamus. In this process, serotonin may interact antagonistically with noradrenaline (norepinephrine) and its α2-noradrenergic receptors that normally function to enhance carbohydrate intake at the onset of the natural feeding cycle. Moreover, while inducing satiety for carbohydrate, serotonin may also play a role in switching the animal’s preference towards protein. The regulation of this macronutrient is closely linked to that of carbohydrate, and it is normally preferred in the second meal of the natural feeding cycle. Most of the pharmacological evidence to date generally supports the hypothesis that disturbances in serotonin function occur in eating disorders. Decreases in plasma tryptophan, urinary 5-hydroxyindoleacetic acid (5-HIAA), platelet serotonin binding and basal cerebrospinal fluid 5-HIAA in anorexia nervosa normalise upon weight restoration and appear to be starvation effects. These alterations in serotonergic function may however perpetuate the symptomatology of anorexia nervosa once the illness is set in motion. Some drugs which in part affect serotonergic function facilitate weight gain in conjunction with an integrated psychotherapeutic and behavioural programme. Patients with bulimia nervosa, regardless of the presence of anorexia nervosa or major depression, who have been relatively weight stable and free of binge/vomit episodes for at least 3 weeks, have significantly blunted prolactin responses to the serotonin agonists. These findings indicate that post-synaptic responsiveness in hypothalamic-pituitary serotonergic pathways is reduced in bulimia. Similar alterations in other serotonin pathways at or above the level of the hypothalamus may contribute to binge eating and other behavioural symptoms in bulimic patients. The clinical response to several psychotropic agents known to potentiate serotonergic transmission further substantiates a serotonin dysregulation hypothesis of bulimia nervosa. Serotonergic function is subject to seasonal alterations and may also be implicated in changes of mood and eating behaviour which accompany the seasonal affective disorder. Serotonin agonists such as dexfenfluramine are effective in suppressing excessive caloric intake in carbohydrate cravers.