3β-OH-5β-Pregnan-20-One Enhances [³H]Gaba Binding in Developing Chick Optic Lobe

Abstract

The neurosteroids are synthesized in the CNS and act mainly through allosteric modulation of the GABA_A receptor. Structure-activity relationship studies in mammalian CNS have shown that a 3α-hydroxyl group and a 5α-reduced A-ring are striking features for their biological activity, while the 3β,5β structures as in 3β,5β-P are completely inactive. In this work we report the enhancing activity of epipregnanolone on [³H]GABA binding to its receptor sites in the chick optic lobe. Concentration-effect curves for this neurosteroid showed a concentration-dependent activity with different potencies at the three developmental stages studied, the hatching stage being the most sensitive to the steroid stimulatory effect. The displacement of a potent 3α,5α steroid by epipregnanolone indicated that this steroid behaved as a partial agonist of the steroid recognition site. Considering the developmental profile for steroidogenesis in avian tissues and the biological relevance of 5β-reduced steroids in early development, we propose that 3 or its 3α-epimer, pregnanolone, instead of the potent 3α,5α neurosteroids, modulates GABA_A receptors in the chick optic lobe during development.