5HT2 RECEPTORS MEDIATE THE EFFECTS OF STRESS ON THE ACTIVITY OF PERIVENTRICULAR HYPOPHYSEAL DOPAMINERGIC-NEURONS AND THE SECRETION OF ALPHA-MELANOCYTE-STIMULATING HORMONE

The roles of 5-hydroxytryptaminergic (5HT) neurons and receptor subtyp es in mediating the effects of stress on the activity of periventricul ar hypophysial dopaminergic (PHDA) neurons and the secretion of alpha- melanocyte-stimulating hormone (alphaMSH) were examined in female rats . Periventricular hypophysial dopaminergic neuronal activity was estim ated by measuring concentrations of 3,4-dihydroxyphenylacetic acid in the intermediate lobe of the pituitary. Brief exposure to diethylether followed by 30 min of supine restraint decreased intermediate lobe 3, 4-dihydroxyphenylacetic acid concentrations and increased plasma conce ntrations of alphaMSH. These stress-induced effects were not observed in animals in which 5HT neurons had been previously destroyed by 5,7-d ihydroxytryptamine or inhibited by the administration of the 5HT1A rec eptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin. Pretreatment of rats with the 5HT2 receptor antagonist MDL-11,939 blocked the inhibit ory effects of stress on intermediate lobe 3,4-dihydroxyphenylacetic a cid concentrations and the corresponding increase in plasma alphaMSH c oncentrations, whereas the 5HT3 receptor antagonist ondansetron was wi thout effect. These results reveal that 5HT neurons, acting via 5HT2 r eceptors, mediate the inhibitory effects of stress on periventricular hypophysial dopaminergic neurons and the consequent increase in secret ion of alphaMSH.