BRAIN REGION-SPECIFIC EFFECTS OF NEUROACTIVE STEROIDS ON THE AFFINITY AND DENSITY OF THE GABA-BINDING SITE

The allosteric regulation of specific [H-3]-muscimol binding by neuroactive steroids to the GABA-binding sites of membrane fractions prepared from five different brain areas was characterized in order to elucidate if the regionally variable subunit composition of GABA(A) receptors is reflected in the responsiveness of the GABA binding site to neuro-steroid modulatory effects. At a final concentration of 1muM progesterone and its metabolite 3-alpha-hydroxy-5-alpha-pregnane-20-one (HPO) reduced the affinity in hippocampus (HIP), enhanced it in medulla (MED) and did not affect it in cerebellum (CER). However, there are differences in potency of these two steroids between frontal cortex (FC) and hypothalamus (HYP), since the affinity was enhanced in FC only by progesterone and in HYP only by HPO. While the magnitude of progesterone-induced alterations in affinity were similar in FC, MED and HIP, HPO affected the affinity significantly stronger in HIP than in HYP and MED. Concerning the density of the binding sites progesterone exerted no significant modulatory effect in contrast to HPO which increased the number of binding sites (B(max)) in all five brain areas investigated. However, the enhancements in B(max) were regionally different. The HIP reached the maximal increase of B(max), followed by FC and MED. The smallest enhancement was found in CER, followed by HYP. Neurosteroidal activity exhibited also THDOC and alphaxalone, the synthetic HPO analogue. A significant different potency of THDOC was found in FC versus CER, whereas alphaxalone did not display regionally different efficacy. The present investigation shows that steroidal modulation of the GABA binding site highly depends on the kind of steroid investigated and differs between brain areas. These findings give evidence that certain endogenous steroid metabolites are potent and highly selective modulators of the GABA(A) receptor thus supporting a physiologic role of these steroids in the regulation of brain excitability.