Testis and brain steroidogenesis during the reproductive cycle of pelophylax esculentus

The anuran amphibian Pelophylax esculentus shows an annual cycle of both sexual steroid production and spermatogenesis. Plasma testosterone levels are low in the non-breeding period and high during the breeding period, while 17ß-estradiol levels show an opposite trend. This chapter reports the existing work on the steroidogenic pathways activated in reproductive and post-reproductive phases in both testis and brain. Seasonal fluctuations in expression levels of steroidogenic enzyme genes as star, hsd3b1 (more commonly known as 3ß-hydroxysteroid dehydrogenase), hsd17b1 (more commonly known as 17ß-hydroxysteroid dehydrogenase) and srd5a1 (more commonly known as 5a-reductase) occur in both testis and brain (Diencephalon-Mesencephalon region), with the highest levels in reproductive period. High testosterone and 5a- dihydrotestosterone concentrations in the reproductive testis and brain strongly confirm that the androgenic pathway is necessary for spermatogenesis activation. While cyp19a1 (more commonly known as P450 aromatase) expression is still high in brain Diencephalon- Mesencephalon region of the reproductive frogs, during the postreproductive period the highest P450 aromatase protein level in the brain together with higher cyp19a1 levels in the testis, and paralleling with 17ß- estradiol titres, indicate that the estrogenic pathway is essential for the interruption of the reproductive processes. Seasonal changes were also observed in gene expression for androgen (ar) and estrogen (esr1, esr2) receptors in both testis and brain. The highest ar levels are expressed in the reproductive phase whereas the highest esr1 and esr2 levels are observed in the post-reproductive phase. In conclusion, the steroidogenic process in the testis and the brain of P. esculentus exhibits seasonal changes with the promotion of androgenic pathway in the reproductive period and estrogenic pathway in the postreproductive period. This study strongly indicates a close correlation between steroidogenic process in the testis and in the brain, suggesting a synergic mechanism of regulation.