LH and testosterone responses to five doses of a GnRH analogue (buserelin acetate) in 12-month-old Thoroughbred colts

To determine the responsiveness of the pituitary–gonadal axis of peri-pubertal colts to GnRH, buserelin (0.5, 1, 5, 10 and 40μg) was given to 13 male Thoroughbred yearlings (n=3–8 colts per dose). Jugular venous blood samples were taken at −10, 0, 10, 20, 30, 40, 60, 120 and 180min relative to buserelin administration. Increases (P<0.05) in LH concentrations occurred in colts that received 5, 10, or 40μg buserelin, but not in those that received 0.5 or 1μg. Peak LH concentrations and mean area under the curve were higher (P<0.05) in colts receiving 40μg buserelin than in those that received 0.5 or 1μg. Increases (P<0.05) in testosterone concentrations occurred in some, but not all, colts that received 1, 5, 10, or 40μg buserelin. Neither peak concentration nor area under the curve of testosterone differed significantly among doses of buserelin. The percentage of horses that responded to the buserelin increased with increasing dose, with only the highest dose eliciting LH and testosterone responses in all colts. In conclusion, peri-pubertal colts exhibited a dose–response release of LH following buserelin treatment, but individual colts responded in an “all or nothing” manner, such that each either had an LH response or did not. Some colts that exhibited a significant LH response had no subsequent increase in plasma testosterone concentrations; perhaps the pituitary LH response may not have been great enough to stimulate the Leydig cells in these individuals.