In the ewe, GnIH acts to inhibit the synthesis and secretion of the gonadotrophins from the anterior pituitary gland via regulation of gonadotrophin releasing hormone secretion in the hypothalamus and directly on gonadotroph cells in the pituitary gland. The situation in rams has not been investigated. We tested the hypothesis that intravenous administration of GnIH will decrease LH secretion in the merino ram.

In an initial study, intact rams (n=8) were infused with either GnIH or saline in a cross over design. Jugular blood samples were collected every 10 minutes during a 4 hour pre-infusion period, during the infusion (1.5mg GnIH in 6ml saline over 2 hours, following a bolus 1mg priming dose of GnIH) and during a 3 hour post infusion period. In a second study, 9 rams were castrated 3 weeks prior to GnIH infusion and the experiment  run as previously, except that two different sources of GnIH were used and the dose of GnIH increased by infusing 1mg over 1 hour. Each animal received each treatment (GnIH1, GnIH2 and saline) in random order at 3 day intervals. In both studies, no significant differences in plasma LH levels (mean LH, pulse frequency, amplitude, time to first pulse or amplitude of first pulse in infusion period) were seen between treatments, these results were demonstrated in both intact and castrated rams. The high dose of GnIH used in this study suggests that GnIH may not have an important role in the regulation of LH secretion in the male sheep.