Androgens directly stimulate spermatogonial differentiation in juvenile Atlantic salmon (Salmo salar).

We studied the effects of androgens on early stages of spermatogenesis along with androgen receptor binding characteristics and the expression of selected testicular and pituitary genes. To this end, immature Atlantic salmon postsmolts received testosterone (T), adrenosterone (OA, which is converted in vivo into 11-ketotestosterone, 11-KT) or a combination of the two androgens (T+OA). Treatment with OA and T elevated the plasma levels of 11-KT and T, respectively, and co-injection of OA with T lead to high 11-KT levels but prevented plasma T levels to reach the levels observed after injecting T alone. Clear stimulatory effects were recorded as regards pituitary lhb and gnrhr4 transcript levels in fish receiving T, and to a lesser extent in fish receiving OA (but for the lhb transcript only). The two androgen receptors (Ara1 and Ara2) we cloned bound T and 11-KT and responded to these androgens in a similar way. Both androgens down-regulated testicular amh and increased igf3 transcript levels after 1 week of treatment, but effects on growth factor gene expression required sustained androgen stimulation and faded out in the groups with the decreasing T plasma levels. In fish exhibiting a sustained elevation of 11-KT plasma levels (OA and T+OA groups) for 2 weeks, the number of differentiating spermatogonia had increased while the number of undifferentiated spermatogonia decreased. Previous work showed that circulating gonadotropin levels did not increase following androgen treatments of gonad-intact immature male salmonids. Taken together, androgen treatment of immature males modulated testicular growth factor expression that, when sustained for 2 weeks, stimulated differentiation, but not self-renewal, of undifferentiated type A spermatogonia.