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

摘要

Most of the actions of neurosteroids on the central nervous system are mediated through allosteric modulation of the γ-aminobutyric acid type A (GABA_A) receptor, but a direct effect of GABA on the regulation of neurosteroid biosynthesis has never been investi- gated. In the present report. we have attempted to determine whether 3β-hydroxysteroid dehydrogenase (3β-HSD)-containing neurons, which secrete neurosteroids in the frog hypothalamus. also express the GABA_A receptor, and we have investigated the effect of GABA on neurosteroid biosynthesis by frog hypothalamic explants. Double immunohistochemical labeling revealed that most 3β-HSD-positive neurons also contain GABA_A receptor α3 and β2/β3 subunit-like immunoreactivities. Pulse-chase experiments showed that GABA inhibited in a dose-dependent manner the conversion of tritiated pregnenolone into radioactive steroids. ihcluding 17-hydroxy-pregnenolone, progesterone, 17-hydroxy- progesterone, dehydroepiandrosterone, and dihydrotestosterone. The effect of GABA on neurosteroid biosynthesis was mimicked by the GABA_A receptor agonist muscimol but was not affected by the GABA_B receptor agonist baclofen. The selective GABA_A receptor antagonists bicuculline and SR9553l reversed the inhibitory effect of GABA on neurosteroid formation. The present results indicate that steroid-producing neurons of the frog hypothalamus express the GABA_A receptor α3 and β2/β3 subunits. Our data also demon- strate that GABA, acting on GABA_A receptors at the hypothalamic level, inhibits the activity of several key steroidogenic enzymes, including 3β-HSD and cytochrome P450_c17 (17α-hydroxylase).