NEUROSTEROID MODULATION OF GABA BINDING-SITES IN DEVELOPING AVIAN CENTRAL-NERVOUS-SYSTEM

Our aim was to examine the effect of the potent neurosteroid 3 alpha-h ydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) on [H-3]-GABA bindin g to its receptor sites in the chick optic lobe. Binding was performed on synaptic membranes isolated at different stages of development and two different membrane preparation procedures were applied to expose high and low affinity GABA binding sites. The addition of 3 alpha,5 al pha-P was shown to increase [H-3]-GABA binding in an age-and concentra tion-dependent manner. Maximal stimulation for low affinity GABA bindi ng sites was observed at hatching (130% enhancement), in fresh-washed as well as in frozen membranes. Saturation analysis performed on both membrane types disclosed that 3 alpha,5 alpha-P increases the affinity of low affinity GABA binding sites without altering their maximal bin ding capacity. On the other hand, the augmenting effect at high affini ty sites, displayed only in frozen membranes, was roughly 50% for all developmental stages. However, their saturation binding parameters rem ained unaltered in the presence of the steroid, suggesting that stimul ation of such sites seems due to interference exerted by the low affin ity site population. Findings indicate that 3 alpha,5 alpha-P acts as an allosteric modulator only for low affinity GABA binding sites, disp laying an age-dependent profile probably related to plastic events dur ing visual pathway development. (C) 1998 Elsevier Science Ltd. All rig hts reserved.