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.

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 degrees 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 micrograms of the gonadotrophin releasing hormone agonist (GnRHa), 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 GnRHa reduced LH concentrations and abolished pulsatile LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)