In dogs, a slow‑release GnRH agonist (deslorelin) temporarily shrinks the epididymis and stops sperm production, but once the implant is removed the tissue returns to normal within a few weeks as testosterone levels rise again.

Slow-release GnRH agonist implants containing deslorelin (SRI) are registered for temporary suppression of male fertility. The effect of SRI treatment on canine testicular function is well characterised, although the effect of downregulation and subsequent recovery on epididymal function has not been studied yet. Therefore, twenty-nine healthy male dogs were treated with a 4.7 mg SRI for five months. Subsequent to implant removal, groups of 4-5 dogs were surgically castrated either at implant removal (week 0) or 2, 4, 6, or 10 weeks later. Three subgroups were categorised according to pre-surgical testosterone levels. Five healthy untreated dogs served as control. Epididymides were separated into head, body and tail. Epididymal duct diameter and epithelial height were measured using haematoxylin-eosin-stained sections of each dog and part of epididymides. Besides, the presence of spermatozoa, the cilial height, the thickness of the muscle layers and the relative amount of connective tissue were semiquantitatively assessed. The downregulated epididymis was characterised by a reduced epithelial height and epididymal duct diameter, lower cilia and absence of sperm, but more connective tissue, supporting that epididymal function is significantly altered by SRI treatment. At recovery subsequent to implant removal, the histomorphology was comparable with untreated controls. The study indicates that recovery of the epididymal function, like spermatogenesis, depends on testicular testosterone production.