Absence of Photoperiodic Modulation of Gonadotrophin Secretion in HPD Rams Following Chronic Pulsatile Infusion of GnRH

Abstract

Hypothalamo-pituitary disconnected Soay rams (HPD rams) were treated chronically with a pulsatile infusion of GnRH (250 ng/2 h) for 10 weeks to reactivate the pituitary-gonadal axis under long days (16L:8D; LD group n=8) and short days (8L:16D, SD group n=8). The aim was to establish whether photoperiod modulates the secretion of gonadotrophins (presumably through the daily melatonin signal) by an action at the level of the pituitary gland. The treatment of the HPD rams with GnRH induced a marked, long-term increase in the peripheral blood concentrations of FSH, LH and testosterone, growth of the testes and reappearance of the androgen-dependent sexual skin colouration. There were parallel changes in the short-term, pulsatile fluctuations in the blood concentrations of LH and FSH, the pituitary responsiveness to GnRH and the testicular responsiveness to a standardized injection of oLH, as observed previously in normal intact rams during reactivation of the reproductive axis induced by a switch from long to short days. There were no significant differences in any of the reproductive parameters between the LD and SD HPD groups, although the blood concentrations of prolactin were significantly higher in the LD group, and GnRH provoked a chronic increase in the blood concentrations of prolactin, particularly in the SD group. Overall, the results support the conclusion that (i) melatonin does not act within the pituitary gland to mediate effects of photoperiod on GnRH-induced gonadotrophin secretion; and (ii) changes in prolactin secretion which result from the local action of melatonin in the pituitary gland do not affect gonadotrophin secretion in the ram.