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

The allosteric regulation of specific [H-3]-muscimol binding by neuroa ctive steroids to the GABA-binding sites of membrane fractions prepare d from five different brain areas was characterized in order to elucid ate if the regionally variable subunit composition of GABA(A) receptor s is reflected in the responsiveness of the GABA binding site to neuro -steroid modulatory effects. At a final concentration of 1muM progeste rone 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 differ ences in potency of these two steroids between frontal cortex (FC) and hypothalamus (HYP), since the affinity was enhanced in FC only by pro gesterone and in HYP only by HPO. While the magnitude of progesterone- induced alterations in affinity were similar in FC, MED and HIP, HPO a ffected 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 n umber 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 s mallest enhancement was found in CER, followed by HYP. Neurosteroidal activity exhibited also THDOC and alphaxalone, the synthetic HPO analo gue. A significant different potency of THDOC was found in FC versus C ER, 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 d iffers between brain areas. These findings give evidence that certain endogenous steroid metabolites are potent and highly selective modulat ors of the GABA(A) receptor thus supporting a physiologic role of thes e steroids in the regulation of brain excitability.