Evidence for a G Protein-Coupled g-Hydroxybutyric Acid Receptor

Abstract

g-Hydroxybutyric acid (GHB) is a naturally occurring metabolite of GABA that has been postulated to exert ubiquitous neuropharmacological effects through GABAB receptor (GABABR)-mediated mechanisms. The alternative hypothesis that GHB acts via a GHB-specific, G protein-coupled presynaptic receptor that is different from the GABABR was tested. The effect of GHB on regional and subcellular brain adenylyl cyclase in adult and developing rats was determined and compared with that of the GABABR agonist (2)-baclofen. Also, using guanosine 59-O-(3-[35S]thiotriphosphate) ([35S]GTPgS) binding and low-Km GTPase activity as markers the effects of GHB and (2)-baclofen on G protein activity in the brain were determined. Neither GHB nor baclofen had an effect on basal cyclic AMP (cAMP) levels. GHB significantly decreased forskolin-stimulated cAMP levels by 40–50% in cortex and hippocampus but not thalamus or cerebellum, whereas (2)-baclofen had an effect throughout the brain. The effect of GHB on adenylyl cyclase was observed in presynaptic and not postsynaptic subcellular tissue preparations, but the effect of baclofen was observed in both subcellular preparations. The GHB-induced alteration in forskolin-induced cAMP formation was blocked by a specific GHB antagonist but not a specific GABABR antagonist. The (2)-baclofen-induced alteration in forskolin-induced cAMP formation was blocked by a specific GABABR antagonist but not a specific GHB antagonist. The negative coupling of GHB to adenylyl cyclase appeared at postnatal day 21, a developmental time point that is concordant with the developmental appearance of [3H]GHB binding in cerebral cortex, but the effects of (2)-baclofen were present by postnatal day 14. GHB and baclofen both stimulated [35S]GTPgS binding and low-Km GTPase activity by 40– 50%. The GHB-induced effect was blocked by GHB antagonists but not by GABABR antagonists and was seen only in cortex and hippocampus. The (2)-baclofen-induced effect was blocked by GABABR antagonists but not by GHB antagonists and was observed throughout the brain. These data support the hypothesis that GHB induces a G protein-mediated decrease in adenylyl cyclase via a GHB-specific G protein-coupled presynaptic receptor that is different from the GABABR. Key Words: g-Hydroxybutyric acid—GABAB receptor—Presynaptic receptor—G protein—Cyclic AMP