γ‐Hydroxybutyric Acid, Unlike γ‐Aminobutyric Acid, Does Not Stimulate G i /G o Proteins in Rat Brain Membranes

γ‐Hydroxybutyric acid is a naturally occurring substance that may act as a neurotransmitter or neuromodulator to elicit several biological effects. Although the existence of a specific γ‐hydroxybutyric acid receptor has been postulated, the receptor protein itself has not been cloned yet. The current study was designed to elucidate whether γ‐hydroxybutyric acid receptors are functionally coupled with heterotrimeric G‐proteins, especially G i /G o family, by means of high‐affinity GTPase activity and guanosine 5′‐ O ‐(3‐[ 35 S]thiotriphosphate) ([ 35 S]GTPγS) binding assays in rat brain membranes. The stimulatory effects of GABA B receptor activation were always determined in parallel as a positive control. The selective GABA B receptor agonist (±)‐baclofen stimulated the high‐affinity GTPase activity in cerebral cortical, hippocampal, and striatal membranes, whereas γ‐hydroxybutyric acid was inactive up to 1 mM in these brain regions. The optimum assay conditions for [ 35 S]GTPγS binding to detect a receptor‐mediated activation of G‐proteins at the greatest signal to noise ratio were then probed as to the concentrations of constituents in the assay mixture (GDP, MgCl 2 , and NaCl) and incubation period. Even under such an optimized experimental condition, [ 35 S]GTPγS binding was not altered by γ‐hydroxybutyric acid in the membranes prepared from cerebral cortex or hippocampus. On the other hand, the specific [ 35 S]GTPγS binding was increased by GABA B receptor agonists in a concentration‐dependent manner, which was competitively inhibited by CGP54626, a selective GABA B receptor antagonist. These results indicate that γ‐hydroxybutyric acid receptors, if any, are not associated with G‐proteins, at least G i /G o family.