Dipsogenic effect of L-5-hydroxytryptophan in rats

The precursor of serotonin, l-hydroxytryptophan (5HTP), is a potent dipsogen in the rat. Peripheral administration of increasing doses of this compound increased water intake in a dose-dependent fashion. Peripheral administration of other analogs of tryptophan, including d-tryptophan, l-tryptophan and acetyltryptophan, failed to affect water intake at a dose at which 5HTP induced maximal drinking (25 mg/kg, SC). The dipsogenic effect of melatonin, one of the metabolites of serotonin, was also tested. At any of 6 different doses (0.5 to 50 mg/kg, SC), melatonin failed to affect water intake in the rat. The mechanism by which 5HTP induces drinking is not known with certainty but could involve its conversion to serotonin, a known dipsogenic agent.