USE OF GONADOTROPIN-RELEASING-HORMONE, ESTROGEN, OR A COMBINATION TO INCREASE RELEASABLE PITUITARY LUTEINIZING-HORMONE IN EARLY TRANSITIONAL MARES

A lack of pituitary LH stores has been implicated as the cause of seas onal anestrus and failure to ovulate during the spring transition peri od in mares. In this experiment, 40 mares were used to study the effec ts of GnRH, estrogen, and an estrogen-GnRH combination on increasing r eleasable pituitary LH. Mares were stratified based on their ability t o secrete LH:in response to a 950-mu g challenge of GnRH (n = 10 per g roup) and then assigned to one of four treatment groups: 1) controls, given no treatment; 2) 1 mg of estradiol-17 beta in oil i.m. daily for 8 d; 3) 200 mu g of GnRH analogue des-Gly(10), [D-ala(6)]-LHRH ethyla mide in saline i.m. twice daily for 8 d; or 4) estradiol for 4 d then estradiol plus GnRH for four subsequent days. Blood was collected on d 1, 3, 5, and 7 of treatment, and serum was assayed for LH. On d 10 af ter initiation of treatment, mares were again challenged with GnRH (95 0 mu g), and blood was collected for 4 h. Concentrations of serum LH d id not very significantly in control, estradiol-treated, or estradiol plus GnRH-treated mares among treatment days. In contrast, administrat ion of GnRH alo;ne increased (P < .05) concentrations of LH on d 5 and 7. Response to GnRH challenge, as measured by area under the LH curve (AUC) and peak LH, was greater (P < .05) for mares administered GnRH (7,307.1,67.6 ng/mL, respectively) and GnRH plus estradiol (5,691.4,60 .3 ng/mL) than for mares given estradiol alone (1,519.4,22.1 ng/mL) or no treatment (1,213.8,19.4 ng/mL). Thus, either GnRH alone or GnRH af ter estradiol treatment increased releasable pituitary LH. Gonadotropi n-releasing hormone alone increased circulating concentrations of LH d uring treatment above concentrations achieved by estradiol or estradio l plus GnRH; the latter two treatments did not improve circulating con centrations of LH above those in untreated controls.