Stimulatory and entraining effect of melatonin on tuberoinfundibular dopaminergic neuron activity and inhibition on prolactin secretion

The aims of the present study were to determine if melatonin exerts an effe ct on prolactin (PRL) secretion via the tuberoinfundibular dopaminergic (TI DA) neurons and if endogenous or exogenous melatonin has an entraining effe ct on the rhythmic changes of TIDA neuronal activity and PRL secretion. Mel atonin given in the morning (10:00 h), dose- (0.01-1 mg/kg, ip) and time- ( at 15 and 60 min, but not at 30 min) dependently stimulated TIDA neuronal a ctivity in ovariectomized (OVX), estrogen-treated rats as determined by 3,4 -dihydroxyphenylacetic acid (DOPAC) levels in the median eminence (ME). Ser um PRL was concurrently inhibited by the injection. Melatonin administered in the afternoon (15:00 h) was even more effective in stimulating the lower ed TIDA neuronal activity and inhibiting the increased PRL level than that given in the morning (10:00 h). S-20098, a melatonin agonist was also effec tive in stimulating the TIDA neurons. In contrast, S-20928, a putative mela tonin antagonist, while it had no effect by itself, blocked the effect of S -20098. Although S-20928 failed to prevent melatonin's effect on ME DOPAC l evels, six interspaced injections of S-20928, from 18:00 to 01:30 h, signif icantly blocked the increase of ME DOPAC levels at 03:00 h, indicating that the endogenous melatonin may play a role. We further used rats that receiv ed daily injection of melatonin (1 mg/kg, ip) at 18:00 h for 10 days and fo und that the injection augmented basal TIDA neuronal activity at 11:00 h an d blunted the afternoon PRL surge. In all, melatonin can have an inhibitory effect on PRL secretion by stimulating the TIDA neurons, and it may help t o entrain the circadian rhythms of both TIDA neuronal activity and PRL secr etion.