An endogenous serotonergic rhythm acting on 5-HT2A receptors may be involved in the diurnal changes in tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in female rats

The central serotonergic system has long been known to have a stimulatory r ole on the secretion of prolactin (PRL). The integrity of serotonergic neur otransmission is essential for the expression of the estrogen-induced after noon PRL surge. Whether its effect on PRL involves change in the activity o f tuberoinfundibular dopaminergic (TIDA) neurons has not been ascertained. in adult ovariectomized rats treated with estrogen, depletion of central se rotonin (5-HT) by 5,7-dihydroxytryptamine (5,7-DHT, 200 mu g/rat, i.c.v.) e ffectively prevented the afternoon fall in TIDA neuronal activity (using th e levels of 3,4-dihydroxyphenylalanine and 3,4-dihydroxyphenyl-acetic acid (DOPAC), and the ratio of DOPAC/dopamine in the median eminence as indices) , and blunted the afternoon PRL surge. A single injection of a 5-HT2A recep tor antagonist, ketanserin (5 mg/kg, i.p. at 12.00 h), also had the same ef fects on the diurnal changes in TIDA neuronal activity and PRL secretion as the treatment with 5,7-DHT did. Intracerebroventricular injection of a 5-H T2 receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) in the morning in hibited the TIDA neuronal activity and stimulated PRL secretion in a dose-d ependent manner; while injection of a 5-HT1 agonist, 8-hydroxy-dipropylamin otetralin, was without effect. Injection of DOI in 5,7-DHT-pretreated rats at 14.30 h also lowered the TIDA neuronal activity and reinstated the PRL s urge. In all, endogenous 5-HT, acting through the 5-HT2A receptor, appears to exhibit an inhibitory effect on TIDA neuronal activity during the aftern oon, which is essential for the PRL surge. copyright (C) 2000 S. Karger AG, Basel.