Behavioural and pharmacological investigations of 5-HT hypophagia and hyperdipsia.

Treatment with 5-hydroxytryptamine (5-HT) reliably induced hypophagia in non-deprived rats and in rats tested following a period of food-deprivation, regardless of the presence or absence of water during testing. The hyperdipsic effect of 5-HT, however, was sensitive to changes in the length of food-deprivation, suggesting a possible interaction between 5-HT hyperdipsia and prandial drinking. Both 5-HT hypophagia and hyperdipsia were attenuated by methysergide pretreatment, thus confirming the involvement of peripheral post-synaptic 5-HT receptors in both effects. Pretreatment with propranolol blocked 5-HT hyperdipsia, but did not alter 5-HT hypophagia, thus suggesting that 5-HT hypophagia and hyperdipsia are mediated by different mechanisms at some point subsequent to the stimulation of peripheral 5-HT receptors. These results are consistent with other evidence that 5-HT hyperdipsia is mediated by stimulation of the renin-angiotensin system. It is tentatively suggested that 5-HT hypophagia could result from 5-HT-induced inhibition of cephalic phase insulin secretion.