Effects of ageing and exogenous melatonin on pituitary responsiveness to GnRH in rats

The effect of age and melatonin on the activity of the neuroendocrine repro ductive system was studied in young cyclic (3-5 months-old), and old acycli c (23-25 month-old) female rats. Pituitary responsiveness to a bolus of GnR H (50 ng per 100 g body weight) was assessed at both reproductive stages in control and melatonin-treated (150 mu g melatonin per 100 g body weight ea ch day for 1 month) groups. After this experiment, female rats were treated for another month to study the influence of ageing and melatonin on the re productive axis. Plasma LH, FSH, prolactin, oestradiol and progesterone wer e measured. A positive LH response to GnRH was observed in both control gro ups (cyclic and acyclic). However, a response of greater magnitude was obse rved in old acyclic rats. Melatonin treatment reduced this increased respon se in acyclic rats and produced a pituitary responsiveness similar to that of young cyclic rats. FSH secretion was independent of GnRH administration in all groups, indicating desynchronization between LH and FSH secretion in response to GnRH in young animals and during senescence. No effect on prol actin was observed. Significantly higher LH (3009.11 +/- 1275.08 pg ml(-1); P < 0.05) and FSH concentrations (5879.28 +/- 1631.68 pg ml(-1); P < 0.01) were seen in acyclic control rats. After melatonin treatment, LH (811.11 /- 89.71 pg ml(-1)) and FSH concentrations (2070 +/- 301.62 pg ml(-1)) decr eased to amounts similar to those observed in young cyclic rats. However, p lasma concentrations of oestradiol and progesterone were not reduced. In co nclusion, the results of the present study indicate that, during ageing, th e effect of melatonin is exerted primarily at the hypothalamo-pituitary axi s rather than on the ovary. Melatonin restored the basal concentrations of pituitary hormones and pituitary responsiveness to similar values to those observed in young rats.