Homeostasis of circulating androgen levels in the breeding male three-spined stickleback Gasterosteus aculeatus

Abstract

Androgens are important for stimulating male characters, but excessively high levels may suppress the immune system, and in many animals, circulating levels are homeostatically controlled by feedback mechanisms. However, it was shown that there is no compensation of plasma androgen levels in hemi-castrated threespined stickleback Gasterosteus aculeatus. In this study, we investigated to what extent androgens levels are homeostatically regulated in the breeding male three-spined stickleback, and whether aromatization of androgens plays a role in this. To that end, breeding male sticklebacks were either completely castrated, hemicastrated, or sham-operated, and then implanted with different doses of 11-ketoandrostenedione (11KA) and testosterone (T) or with the aromatase inhibitor (AI), fadrozole. Hemi-castration alone diminished androgen levels, while complete castration almost completely removed them. Low doses of 11KA and T increased plasma androgen levels in castrated but not in sham-operated fish. Both low and high doses of 11KA increased plasma 11-ketotestosterone (11KT) in hemi-castrated fish, whereas only the high dose of 11KA did so in sham-operated fish. If aromatization plays a role in homeostatic mechanisms, androgen levels would be expected to rise in sham-operated fish treated with the AI. However, this was not the case. The reduction in plasma androgen levels in fully mature hemi-castrated fish suggests that the remaining testis was unable to further increase its steroidogenesis. However, both 11KA and T treatments increased plasma levels much less in sham-operated fish than in castrated ones, indicating that homeostatic mechanisms are present and act to prevent excessively high plasma androgen levels.