Effect of GnRH and hCG administration on plasma LH and testosterone concentrations in normal stallions, aged stallions and stallions with lack of libido

Gonadotrophin-releasing hormone (GnRH) (a single intravenous injection with 0.042 mg busereline acetate) was administered to control stallions (n=5), aged stallions (n=5) and stallions with lack of libido (n=5), Jugular blood samples were taken at -10, 0, 10, 20, 40 and 80 minutes after treatment an d measured for luteinizing hormone (LH) and testosterone concentrations. A single intravenous injection of hCG (3000 TE) was given 1 day later. Venous blood samples were taken at -60, 0, 15, 30, 60, 120, and 240 minutes after treatment and measured for the testosterone concentration. The experiment was performed in the breeding season. There was a wide variation between st allions in basal concentrations of LH and testosterone, The treatment group s all showed a significant increase in LH and testosterone concentrations a fter treatment with GnRH, There was a significant difference (P<0.05) betwe en the control, the lack of libido stallions and the aged stallions in the production of LH before and after stimulation with GnRH. The aged stallions had higher basal LH concentrations, GnRH induced a rise in plasma LH in al l groups, but the greatest response was observed in aged stallions. No resp onse to GnRH was seen with respect to plasma testosterone. There was an inc rease in plasma testosterone following hCG; however, this increase was very small in aged stallions. After stimulation,vith hCG the control and lack o f libido stallions had a significant increase (P<0.05) in testosterone prod uction. Tn conclusion, stimulation with either GnRH or hCG can be a valuabl e method to test whether the function of the stallion's reproductive endocr ine system is optimal.