The anorectic action of peripherally administered 5-HT is enhanced by vagotomy

Peripherally administered 5-HT produced a greater suppression of food intake in rats with subdiaphragmatic vagotomy than in sham-operated controls. The enhanced anorexia to 5-HT in vagotomised rats and the anorexia in sham-operated controls were reversed by methysergide, indicating the involvement of 5-HT receptors in the observed anorexia in both groups of animals. Thus the increased suppression of food intake in vagotomised rats cannot be explained in terms of non-specific effects of 5-HT. Both vagotomised and sham-operated rats showed an equivalent degree of anorexia when treated with fenfluramine suggesting that the receptor mechanism responsible for the anorectic action of 5-HT plays little or no part in the action of fenfluramine. Systemic administration of 5-HT was found to slow the rate of gastric clearance. Unlike the anorexia induced by 5-HT this effect was not reversed by methysergide. Thus it appears that peripherally administered 5-HT interacts with the vagus nerve but the mechanism responsible for 5-HT anorexia is independent of any action on gastric clearance.