Involvement of melatonin in the seasonal changes of the gonadal function and prolactin secretion in female goats.

Abstract

Ovarian cyclicity in the goat continues under short days (8L: 16D), while it ceases for 150-200 days under long days (16L: 8D). During the anovulatory period under long days, prolactin secretion is enhanced. Experiments were conducted to investigate the role of the pineal gland in the photoperiodic control of the gonadal function and prolactin secretion in the goat. Firstly, the effects of the abolition of the diurnal change in melatonin secretion on the photoperiodic responses of the gonadal axis and prolactin secretion were examined. Female Saanen goats, reared under short days (8L:16D, 22 +/- 2 degrees C) for 5 months, were bilaterally superior cervical ganglionectomized (SCGX) to denervate their pineal gland. One month after surgery, both SCGX and intact control goats were divided into two groups. Animals in group 1 were maintained under short days and those in group 2 were transferred to long days (16L:8D). In group 1, both SCGX and intact goats ovulated periodically and basal plasma levels of prolactin were maintained throughout the day. In intact controls, exposure to long days blocked ovulation and increased prolactin secretion for the first 150-200 days of exposure. The animals became photorefractory after this time; ovulation recurred and prolactin secretion was suppressed. The continuous melatonin secretion appeared simultaneously with the photorefractoriness. Superior cervical ganglionectomy abolished or weakened the suppression of gonadal axis and eliminated the increase in prolactin secretion induced by the exposure to long days in intact animals. Secondly, the effect of melatonin replacement by timed melatonin infusion on prolactin secretion was examined in SCGX goats to establish that the effect of SCGX could be ascribed to the abolition of the diurnal change in melatonin. Ovariectomized SCGX Shiba goats (n = 5) were infused with melatonin (20 micrograms/h, s.c.) daily for 8 h (the long-day-type infusion) and for 16 h (the short-day-type infusion) to mimic the nocturnal profiles of plasma melatonin under long days and short days, respectively. The long-day-type melatonin infusion for 9 days accelerated prolactin secretion, inducing a nocturnal rise in plasma prolactin; this was comparable to that observed in intact controls under long days. On the other hand, by the short-day-type infusion, the plasma prolactin concentrations were maintained at a low level throughout the day as were observed in intact goats under short days. The prevailing photoperiod appeared to have no distinct effect on these prolactin responses to exogenous melatonin, which were indistinguishable under long and short days.(ABSTRACT TRUNCATED AT 400 WORDS)