Prolactin and Gonadotropin Responses in Geldings to Injections of Estradiol Benzoate in Oil, Estradiol Benzoate in Biodegradable Microspheres, and Estradiol Cypionate

Abstract 

We previously reported success in inducing early ovulation in seasonally anovulatory mares with a combination of estradiol pretreatment followed by daily administration of a dopamine antagonist (sulpiride). Although every-other-day injections of estradiol benzoate (EB) were effective in that experiment, practical application of this technology would require simplification of the treatment regimen. The current experiment was designed to compare, in a gelding model, the biologic responses of two alternative, one-injection regimens for estradiol delivery to the established EB treatment used previously. Fifteen long-term geldings were sampled via jugular venipuncture from November 5 to 7, 2006, and were then administered intramuscular injections of vegetable oil (n = 4); EB, 11 mg in oil (n = 4; controls); EB in biodegradable microspheres (300 mg; n = 3); or estradiol cypionate, 100 mg in oil (n = 4). Injections of EB in oil were repeated every other day for a total of 10 injections, as was done in our previous experiment. Jugular blood samples were drawn from all geldings at 3, 6, 12, 24, 36, and 48 hours relative to injections, and then on the mornings of days 3, 4, 6, 8, 10 to 18, 22, 26, and 30. On days 10 through 13, all geldings received subcutaneous injections of 125 mg sulpiride, a dopamine receptor antagonist, to stimulate prolactin secretion. On day 12, each gelding received an intravenous injection of 30 μg gonadotropin-releasing hormone (GnRH) analog and 3 mg thyrotropin-releasing hormone (TRH); frequent blood samples were drawn to characterize the luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin responses. Relative to geldings receiving oil, all geldings receiving estradiol injections had a rise (P < .05) in estradiol concentrations lasting at least 12 days. Daily LH concentrations increased (P < 0.01) in all treated groups, but the response was delayed approximately 14 days in the geldings receiving EB in microspheres. Daily FSH concentrations decreased (P < .01) in all treated groups, with the greatest response in the geldings receiving EB in microspheres. Prolactin in daily samples increased (P < .01) similarly in all estradiol-treated groups after injection of sulpiride. The LH response to GnRH analog was greatest (P < .05) in geldings receiving EB in oil and estradiol cypionate; the FSH response was not altered by treatment. The prolactin response to TRH was greater (P < .01) in estradiol-treated geldings relative to controls, but did not differ among groups. Compared with the responses to every-other-day EB injections in oil, as we used previously, a single injection of 100 mg estradiol cypionate gave the most similar and consistent responses. Because of these similar responses in this gelding model, it is likely that a single injection of 100 mg estradiol cypionate can be used in lieu of every-other-day injections of EB in oil in the treatment regimen we reported previously for stimulating ovarian activity in seasonally anovulatory mares.

Keywords: Estradiol, Follicle-stimulating hormone, Geldings, Luteinizing hormone, Prolactin