Plasma LH concentrations in the ewe in response to repeated injections of low doses of gonadotrophin releasing hormone during the follicular phase of the oestrous cycle

Abstract

Oestrus was synchronized in 15 naturally cyclic ewes by the administration of a prostaglandin F2 alpha analogue. Groups of five ewes were then treated i.v. with either small doses of gonadotrophin releasing hormone (GnRH; 125 or 250 ng/injection) or saline, at 2-h intervals from day 14 of the subsequent cycle until 24 h after the onset of oestrus. Treatment with GnRH induced episodic LH release which continued until the onset of a preovulatory LH surge. Mean plasma LH concentrations over this period were significantly (P less than 0.001) higher in animals receiving 250 ng GnRH (2.44 +/- 0.11 microgram/1) than in those receiving either 125 ng GnRH (1.17 +/- 0.06 microgram/1) or saline (1.14 +/- 0.05 microgram/1). However, GnRH treatment did not influence the timing of oestrus or mean ovulation rates.