Effects of melatonin injections on the ability of golden hamster pituitaries to secrete prolactin and luteinizing hormone.

Chronic afternoon (PM) but not morning injections of melatonin (MEL) induced significant reductions in testicular and seminal vesicle weights as well as attenuating serum prolactin (PRL) and luteinizing hormone (LH) levels. Although there were no treatment-induced effects on hemipituitary weights, PM-Mel injections led to significant reductions in in vitro PRL secretion and tended to increase the ability of dopamine to inhibit PRL release. It was also shown that LH-releasing hormone (LHRH) could inhibit in vitro PRL release from hamster pituitaries. Basal or LHRH-stimulated LH secretion from incubated pituitaries was not affected by Mel in vivo. From these results we conclude that properly timed Mel injections do not reduce pituitary's ability to secrete LH but severely attenuate PRL secretion. These findings are similar to those observed in pituitaries from hamsters housed in short-photoperiod conditions.