Steroid production in toads

In Bufo arenarum, androgen biosynthesis occurs through a complete 5-ene pathway, including 5-androstane-3β,17β-diol as the immediate precursor of testosterone. Besides, steroidogenesis changes during the breeding period, turning from androgens to C 21-steroids such as 5α-pregnan-3α,20α-diol, 3α-hydroxy-5α-pregnan-20-one and 5α-pregnan-3,20-dione. In B. arenarum, steroid hormones are not involved in hCG-induced spermiation, suggesting that the steroidogenic shift to C 21-steroids during the breeding be not related to spermiation. The activity of 17-hydroxylase-C 17–20 lyase (CypP450 c17) decreases during the reproductive season, suggesting that this enzyme would represent a key enzyme in the regulation of seasonal changes. However, the increase in the affinity for pregnenolone of 3β-hydroxysteroid dehydrogenase (3αHSD)/isomerase could also be involved. Moreover, the reduction in CypP450 c17 leading to a reduction in C 19-steroids, among them dehydroepiandrosterone (DHE), would contribute to the conversion of pregnenolone into progesterone, avoiding the non-competitive inhibition exerted by DHE on this transformation. Additionally, CypP450 c17 possesses a higher affinity for pregnenolone than for progesterone, explaining the predominance of the 5-ene pathway for testosterone biosynthesis. Animals in reproductive condition showed a significant reduction in circulating androgens, enhancing the physiological relevance of all the in vitro results. The in vitro effects of mGnRH and hrFSH on testicular steroidogenesis revealed that both hormones inhibited CypP450 c17 activity. In summary, these results demonstrate that, in B. arenarum, the change in testicular steroidogenesis during the reproductive period could be partially due to an FSH and GnRH-induced decrease in CypP450 c17 activity.