The effects of continuous treatment of stallions with high levels of a potent GnRH analogue.

The effect of long-term treatment of stallions with a powerful gonadotrophin-releasing hormone (GnRH) analogue was investigated. In the first part of the study, 9 sexually mature pony stallions were allocated to 1 of 3 groups, each of which was treated with 240 micrograms or 60 micrograms Buserelin per day administered by subcutaneously sited osmotic pumps or 30-50 micrograms Buserelin per day via solid, slow-release implants injected subcutaneously. Peripheral blood plasma and serum samples were collected frequently and assayed for follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and testosterone. No significant differences were seen between either high of low dose rates: in all treatment groups, serum and plasma concentrations of FSH, LH and testosterone rose sharply following insertion of pumps ofr implant. The levels remained elevated for 3-10 days in most animals, after which they returned to pre-treatment of lower values. In a second experiment, single implants, formulated to deliver 120 micrograms Buserelin per day for 28 days, were injected subcutaneously in to 2 pony stallions on 4 occasions at 5-day intervals. Gonadotrophin and testosterone secretions were temporarily stimulated in response to the first implant, but thereafter returned to pre-treatment or lower levels and showed no further stimulation following injection of subsequent implants. In a third experiment designed to establish the effects on libido and spermatogenesis of chronic treatment with levels of Buserelin (shown previously to depress gonadotrophin concentrations in stallions), 3 fertile pony stallions received subcutaneous implants at 2- to 5-week intervals for 12 months. Three untreated pony stallions acted as controls. Weekly blood samples were collected from both groups to monitor levels of FSH, LH, testosterone and oestrone sulphate. Semen was collected daily for 6-10 days both before treatment and at 3-month intervals thereafter in order to establish daily sperm output (DSO) and assess libido. Gonadotrophin, testosterone and oestrone sulphate levels were all lower in the treated group compared with either pre-treatment or control values. DSO was reduced by at least 50% throughout the treatment period in treated stallions but 7 months after insertion of the last implant it had returned to pre-treatment values. No significant effects on other semen characteristics were noted and there were no noticeable changes in libido of treated animals throughout the period of the experiment.