In vitro pituitary responsiveness to LHRH in young and old female rats. Influence of melatonin

The effect of aging and melatonin on in vitro pituitary responsiveness to l uteinizing hormone-releasing hormone (LHRH) was studied. Young cyclic (3-mo nths-old) control (cyclic-control, N=15), and melatonin (MEL) treated for 2 months (150 mu g/100 g BW) (cyclic-MEL, N=15), old acyclic (23-months-old) control (acyclic-control, N=13), and MEL-treated (acyclic-MEL, N = 18) rat s were used. The hormones analyzed were luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL). The results showed a differe nt influence of the reproductive status as well as of melatonin on the basa l secretion rate of both gonadotropins, i.e. LH and FSH. Only the basal FSH release was significantly reduced in cyclic-MEL and acyclic-controls compa red to cyclic-controls. The hemipitutary FSH content raised to values simil ar to those observed for FSH secretion and only the cyclic-MEL group showed significantly higher FSH pituitary content than for release. LHRH addition to the incubation medium resulted in increased LH release for both cyclic and acyclic rats, but FSH release was only stimulated in acyclic rats. Mela tonin treatment blunted this response in both cases. In addition, melatonin treatment inhibited prolactin release in acyclic-MEL group after LHRH stim ulation but not the basal levels. Pituitary LH and prolactin contents, were significantly higher than the pituitary LH and prolactin levels released f rom all groups studied, and were not affected by reproductive senescence no r by exogenous melatonin. These data indicate that aging influences more th e secretory than the biosynthetic processes. Melatonin influences is endocr ine status-dependent, being inhibitory when pituitary hormones reach their higher values. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.