Effect of exogenous melatonin on neuroendocrine-reproductive function of middle-aged female rats

The possible role of melatonin in the regulation of the reproductive system of female rats during ageing was investigated in middle-aged female rats s howing irregular duration of the oestrous cycle (n = 30). Blood samples wer e obtained by jugular venepuncture during the oestrous cycle in control rat s. After this experiment was completed, the female rats were treated with m elatonin for 2 months and blood samples were obtained at different stages o f the oestrous cycle. Plasma LH, FSH and prolactin concentrations were sign ificantly increased in the afternoon of the day of pro-oestrus after melato nin treatment compared with control rats. Moreover, FSH concentrations too were significantly increased on the morning of pro-oestrus and oestrus in m elatonin treated rats compared with control rats. Similarly, oestradiol con centrations were significantly higher on the morning of pro-oestrus in mela tonin treated rats compared with controls. Another group of rats showing ir regular duration of the oestrous cycle was used to study the possible effec t of melatonin treatment on the timing of progestrous surges of LH and FSH. The results showed that LH and FSH peak values occurred at 5 h after melat onin treatment. Pituitary responsiveness to LHRH in a 90 min test was also studied in middle-aged rats showing irregular duration of the oestrous cycl e that had been injected for 1 month with either melatonin or saline. Prola ctin response was unaffected by exogenous melatonin, but a stimulatory effe ct of melatonin on LH and FSH pituitary responsiveness to LHRH was observed . The results indicate an improved function of the neuroendocrine-reproduct ive axis in middle-aged rats after melatonin treatment.