Effects of melatonin on estrogen receptor expression in the forebrain of outbred (Lak.LVG) golden hamsters

Studies have shown that the pineal gland via its hormone, melatonin, induces the involution of male and female reproductive systems in seasonally reproducing animals. Melatonin has direct inhibitory effects on both hypothalamic and pituitary functions, which are also exquisitely sensitive to the feedback effects of estradiol. Since melatonin can modulate estrogen receptor (ER) expression in other tissues, immunocytochemical and ribonuclease protection analyses were used to examine the effects of 12 weeks of daily late afternoon injections of melatonin on ER protein and mRNA levels in the hypothalamus of Lak.LVG golden hamsters. Significant decreases in ER-immunoreactivity were noted in the medial preoptic area (MPOA) and bed nucleus of the stria terminalis (BNST) in response to melatonin, while other hypothalamic areas which express ER, e.g. the anterior hypothalamus, showed less dramatic changes. Hypothalamic ER mRNA was decreased in response to melatonin in both intact and ovariectomized animals by 25%. In intact, cycling female hamsters, there was a significant reduction in uterine weight after melatonin treatment. These results suggest that melatonin exerts its anti-reproductive effects in hamsters by modulating ER levels in neurons of the MPOA and BNST, thereby influencing steroid feedback mechanisms.