gamma-aminobutyric acid, acting through gamma-aminobutyric acid type A receptors, inhibits the biosynthesis of neurosteroids in the frog hypothalamus

Most of the actions of neurosteroids on the central nervous system are medi ated through allosteric modulation of the gamma -aminobutyric acid type A ( GABA(A)) receptor, but a direct effect of GABA on the regulation of neurost eroid biosynthesis has never been investigated. In the present report, we h ave attempted to determine whether 3 beta -hydroxysteroid dehydrogenase (3 beta -HSD)-containing neurons, which secrete neurosteroids in the frog hypo thalamus, also express the GABA(A) receptor, and we have investigated the e ffect of GABA on neurosteroid biosynthesis by frog hypothalamic: explants. Double immunohistochemical labeling revealed that most 3 beta -HSD-positive neurons also contain GABA(A) receptor alpha (3) and beta (2)/beta (3) subu nit-like immunoreactivities. Pulse-chase experiments showed that GABA inhib ited in a dose-dependent manner the conversion of tritiated pregnenolone in to radioactive steroids, including 17-hydroxy-pregnenolone, progesterone, 1 7-hydroxy-progesterone, dehydroepiandrosterone, and dihydrotestosterone. Th e effect of GABA on neurosteroid biosynthesis was mimicked by the GABA(A) r eceptor agonist muscimol but was not affected by the GABA(B) receptor agoni st baclofen. The selective GABA(A) receptor antagonists bicuculline and SR9 5531 reversed the inhibitory effect of GABA on neurosteroid formation. The present results indicate that steroid-producing neurons of the frog hypotha lamus express the GABA(A) receptor alpha (3) and beta (2)/beta (3) subunits , Our data also demonstrate that GABA, acting on GABA(A) receptors at the h ypothalamic level, inhibits the activity of several key steroidogenic enzym es, including 3 beta -HSD and cytochrome P450(C17) (17 alpha -hydroxylase).