MELATONIN ADMINISTRATION DURING PREGNANCY RETARDS SEXUAL-MATURATION OF FEMALE OFFSPRING IN THE RAT

The influence of pineal gland function and of melatonin during pregnancy upon sexual maturation of female offspring in rats was examined. The following groups of Wistar rats were studied: (A) control, (B) melatonin treated (250-mu-g/100 g.b.w. per day) throughout pregnancy, and (c) pinealectomized (pin-x). Melatonin was injected subcutaneously 2 hr before lights off. All groups were housed in 12-hr L:12-hr D. The female offspring were observed until the onset of puberty, and blood samples were collected 5.5 hr after lights off under red light for melatonin and LH determinations by RIA. Ovary, pineal, and pituitary gland as well as body weights were recorded. The offspring of melatonin-treated rats showed later vaginal opening than did those of the other two groups (P < 0.05 vs. control and P < 0.01 vs. pin-x offspring), which was accompanied by a lower LH concentration, 0.42 + 0.05 ng LH/ml, showing statistically significant differences with the control levels [1.00 + 0.22 ng LH/ml (P < 0.05)] and with the pin-x group [1.16 + 0.22 ng LH/ml (P < 0.05)]. The percentage of rats in proestrus was higher in the offspring of pin-x rats (78.6%; P < 0.01) compared to control offspring (30%) and offspring of melatonin-treated rats (11.8%). The concentrations of melatonin, 5.5 hr after darkness, were not significantly different among groups. No differences were observed in the body, ovarian, and pineal weights of the studied offspring. The pituitary weight was significantly lower (P < 0.05) in the offspring of pin-x rats than in the other two groups. Our results seem to conform with the fact that exogenous melatonin crosses the placental barrier. In sum, the results show an inhibitory influence of melatonin during fetal life on the development of the neuroendocrine reproductive axis of the rat.