Oocyte maturation in the toad Rhinella arenarum (Amphibia, Anura): Evidence of cAMP involvement in steroid production and action

Abstract

In this work, we describe the participation of the adenylate cyclase/3′-5′-cyclic adenonsine monophosphate (cAMP) pathway in the seasonal follicular secretion of progesterone (P 4 ) and testosterone (T), and its relationship with the maturation of Rhinella arenarum oocytes. Under gonadotropin stimulation, P 4 secretion was the dominant steroid produced during the reproductive period, resulting in 100% germinal vesicle breakdown (GVBD) in oocytes in vitro; in contrast, T and estradiol (E 2 ) secretion increased (∼16 nM/20 follicles and ∼80 pM/20 follicles, respectively) during the non-reproductive period, but only yielded 50% GVBD. Treatment of the follicles with dibutyryl-cAMP or forskolin induced a significant increase in T secretion during both periods, but P 4 secretion did not significantly change and GVBD did not occur. These results suggest that high cAMP levels in the oocyte maintain meiotic arrest and prevent the induction effect of follicular steroids. An increase in cAMP levels in denuded oocytes, however, negatively regulated T-induced maturation since treatment with increasing db-cAMP or forskolin inhibited their maturation. Therefore, we hypothesize that an elevation in T during the non-reproductive period favors its aromatization to E 2 , leading to follicle growth. During the reproductive period, P 4 production might promote oocyte maturation when environmental conditions are favorable for reproduction. Together, the results indicate that steroidogenesis is seasonal and depends on gonadotropic activity in R. arenarum.