Effect of chronic treatment with a GnRH agonist to suppress pulsatile LH secretion on the ability of exogenous melatonin to advance oestrous cyclicity in ewes

Abstract

Thirty-two Scottish Blackface ewes that lambed outdoors in March and were weaned at the end of April and individually penned indoors under the natural photoperiod at 57°N were used to determine whether the ovine ovary that was deprived of gonadotrophic support was capable of early activation by melatonin. From 5 May (day 0), 16 of the ewes received an oral dose of 3 mg melatonin in a 4:1 (v:v) mixture of water and ethanol daily at 15:00 h. The remaining 16 ewes received the vehicle alone. Within each of these groups, eight were implanted s.c. on day 0 with an osmotic minipump which infused 50 μg of the gonadotrophin releasing hormone agonist (GnRH(a)), buserelin day-1. On day 25, a second minipump was inserted to ensure continued infusion of the agonist and on day 50 (24 June) both minipumps were removed. Ovarian activity was assessed by laparoscopy at intervals of 3 weeks from day 29 until the experiment was terminated on day 200 (21 November). Blood samples taken by jugular venepuncture three times a week for the first 50 days, daily from days 51-78 and thereafter three times a week were analysed for progesterone, prolactin and LH. Samples taken at intervals of 15 min for 10 h on days 1, 14, 28, 49, 56, 70 and 91 were assayed for LH. Treatment with GnRH(a) reduced LH concentrations and abolished pulsatile LH secretion. The onset of ovarian activity (progesterone > 3.8 nmol l-1) was not affected by the 50-day GnRH(a) treatment and occurred for the melatonin-treated ewes at mean (±SEM) intervals from 5 May of 66 ± 2.9 (range 51-75) and 71 ± 0.9 (range 68-75) days for non-GnRH(a) and GnRH(a) ewes, respectively. For the ewes not receiving melatonin the corresponding intervals were 113 ± 11.6 and 125 ± 9.2 days, respectively. The mean numbers of corpora lutea at first oestrus were not affected by GnRH(a) treatment and were 1.4 ± 0.13 and 1.5 ± 0.13 for control and melatonin-treated ewes, respectively. First ovulation following GnRH(a) treatment resulted in luteal concentrations of progesterone of normal duration and magnitude but, within the melatonin-treated ewes, those that received GnRH(a) returned to anoestrus 21 days earlier (P < 0.03) than did their non-GnRH(a) counterparts. After their initial suppression by melatonin, prolactin concentrations began to increase after 80 days of melatonin treatment in both non-GnRH and GnRH(a) ewes and approximately three months before the ewes returned to anoestrus. The results demonstrate that a period of isolation of the ovaries of the ewe from the pituitary gonadotrophins does not alter their ability to respond to the melatonin-induced activation of the GnRH pulse generator.