Binding characteristics of the gamma-hydroxybutyric acid receptor antagonist [(3)H](2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid in the rat brain.

Radioligand binding studies with (3)H-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid ([(3)H]NCS-382), an antagonist of gamma-hydroxybutyric acid (GHB) receptor, revealed specific binding sites in the rat cerebral cortex and hippocampus. However, there was very little binding in the rat cerebellum, heart, kidney, liver, and lung membranes. Binding was rapid and reached equilibrium in about 5 min. Scatchard analysis of saturation isotherms revealed two different populations of binding sites in the rat cerebral cortex (K(d1), 795 nM, B(max1), 25.4 pmol/mg of protein; K(d2), 21 microM; B(max2), 178 pmol/mg of protein) as well as in the rat hippocampus (K(d1), 441 nM; B(max1), 16.2 pmol/mg of protein; K(d2), 9.8 microM; B(max2), 255 pmol/mg of protein). (+/-)Baclofen (500 microM) and gamma-aminobutyric acid (100 microM) inhibited the binding only partially, whereas (+)bicuculline, muscimol, picrotoxinin, and phaclofen did not modify the binding. Interestingly, potassium chloride (100-300 mM) inhibited [(3)H]NCS-382 binding (34-56%), and this inhibitory effect was not affected by picrotoxinin. GHB and NCS-382 completely inhibited the [(3)H]NCS-382 (16 nM) binding in the rat cerebrocortical and hippocampal membranes, and NCS-382 was found to be about 10 times more potent than GHB in this regard. A variety of ligands for other receptors did not modify the [(3)H]NCS-382 binding, thereby suggesting selectivity of this radioligand for the GHB receptor sites in the brain. Based on these observations, [(3)H]NCS-382 seems to be a better radioligand than [(3)H]GHB for investigating the role of the GHB receptors in various pharmacological actions.