Pregnenolone, pregnenolone sulfate, and cytochrome P450 side-chain cleavage enzyme in the amphibian brain and their seasonal changes.
To clarify whether the amphibian brain synthesizes de novo neurosteroids, we examined pregnenolone, pregnenolone sulfate ester, and cytochrome P450 side-chain cleavage enzyme (cytochrome P450scc), an enzyme converting cholesterol to pregnenolone, using amphibians. Pregnenolone and its sulfate ester in the brain, gonad, and plasma of Xenopus laevis were measured by a specific pregnenolone RIA. The concentrations of these two steroids in the female brain were significantly larger than those in the ovary and plasma. A similar tendency was evident in the male. In both sexes, pregnenolone and its sulfate ester were concentrated more highly in the cerebellum than in the telencephalon, diencephalon, or midbrain. An immunoreactive protein band of electrophoretic mobility in the proximity of bovine adrenal P450scc was detected in the Xenopus brain as well as the testis by Western blot analysis. Immunohistochemical analysis indicated that Purkinje cells in the Xenopus cerebellum were specifically immunostained with the P450scc antibody. P450scc-like immunoreactive cells were further found in several telencephalic and diencephalic regions, such as the pallium mediale and nucleus preopticus, in the Xenopus brain. A similar localization of P450scc-like immunoreactive cells was evident in Rana nigromaculata, a seasonally breeding amphibian. In the present study, seasonal changes in pregnenolone and its sulfate ester were further examined as a possible physiological change using R. nigromaculata. In both sexes, pregnenolone concentrations in the brain were almost constant during the seasonally breeding cycle. In contrast, the pregnenolone sulfate concentration in the brain was significantly lower in the hibernating quiescent phase and higher in the breeding and postbreeding active phases, independent of the plasma steroid level. These results taken together suggest that the amphibian brain possesses steroidogenic enzyme P450scc and produces pregnenolone and its sulfate ester. Pregnenolone sulfate may function well during the breeding and postbreeding active phases of the year in the seasonal breeder.