The function of the pituitary‐testicular axis in dogs prior to and following surgical or chemical castration with the GnRH‐agonist deslorelin

Chemical castration, that is the reduction of circulating testosterone concentrations to castrate levels by administration of a GnRH‐agonist implant, is a popular alternative to surgical castration in male dogs. Detailed information concerning the pituitary‐testicular axis following administration of a GnRH‐agonist implant is still scarce. Therefore, GnRH‐stimulation tests were performed in male dogs, prior to and after surgical and chemical castration. This approach also allowed us to determine plasma concentrations of testosterone and oestradiol in intact male dogs for future reference and to directly compare the effects of surgical and chemical castration on the pituitary‐testicular axis. In intact male dogs (n = 42) of different breeds GnRH administration induced increased plasma LH, FSH, oestradiol and testosterone concentrations. After surgical castration basal and GnRH‐induced plasma FSH and LH concentrations increased pronouncedly. Additionally, basal and GnRH‐induced plasma oestradiol and testosterone concentrations decreased after surgical castration. After chemical castration, with a slow‐release implant containing the GnRH‐agonist deslorelin, plasma LH and FSH concentrations were lower than prior to castration and lower compared with the same interval after surgical castration. Consequently, plasma oestradiol and testosterone concentrations were lowered to values similar to those after surgical castration. GnRH administration to the chemically castrated male dogs induced a significant increase in the plasma concentrations of LH, but not of FSH. In conclusion, after administration of the deslorelin implant, the plasma concentrations of oestradiol and testosterone did not differ significantly from the surgically castrated animals. After GnRH‐stimulation, none of the dogs went to pre‐treatment testosterone levels. However, at the moment of assessment at 4,4 months (mean 133 days ± SEM 4 days), the pituitary gonadotrophs were responsive to GnRH in implanted dogs. The increase of LH, but not of FSH, following GnRH administration indicates a differential regulation of the release of these gonadotrophins, which needs to be considered when GnRH‐stimulation tests are performed in implanted dogs.