Androgen and Progesterone Effects on Follicle-stimulating Hormone and Luteinizing Hormone Secretion in Anestrous Mares

Anestrous lighthorse mares were treated in December with dihydrotestosterone (DHT; 150 μg/kg of body weight), progesterone (P; 164 μg/kg), both DHT and P (DHT+P), testosterone (T; 150 μg/kg), or vehicle (n = 4/group). Daily blood sampling was started on Day 1, and on Day 4 all mares were administered a pretreatment injection of gonadotropin-releasing hormone (GnRH) and were bled frequently to characterize the responses of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) concentrations. Treatment injections were given on Day 4 and then daily through Day 17. On Day 18, all mares were again administered GnRH and were bled frequently. Treatment of mares with DHT, P, or T increased (p<0.01) plasma concentrations of these steroids to approximately 1.5 ng/ml during the last 10 days of treatment. There was no effect (p>0.10) of treatment on LH or FSH concentrations in daily blood samples. Relative to the pretreatment GnRH injection, mares treated with T or DHT+P secreted approximately 65% more (p<0.01) FSH in response to the post-treatment GnRH injection; FSH response to the second GnRH injection was not altered (p>0.10) in control mares or in DHT- or P-treated mares. There was no effect of any steroid treatment on LH secretion after administration of GnRH (p>0.10). Averaged over all mares, approximately 94 times more FSH than LH was secreted in response to injection of GnRH. We conclude that: 1) anestrous mares respond to T treatment in a manner similar to intact cyclic mares and ovariectomized mares with regard to FSH secretion; 2) there is no effect on FSH of DHT or P treatment alone in anestrous mares; but 3) there is an interaction between DHT and P treatment on FSH secretion after exogenous GnRH.