Specific γ-hydroxybutyrate-binding sites but loss of pharmacological effects of γ-hydroxybutyrate in GABA_B(1)-deficient mice

γ-Hydroxybutyrate (GHB), a metabolite of γ-aminobutyric acid (GABA), is proposed to function as a neu neuronsmitter or neuromodulator. γ-Hydroxybutyrate and its prodrug, γ-butyrolactone (GBL), recently received increased public attention as they emerged as popular drugs of abuse. The actions of GHB/GBL are believed to be mediated by GABA_B and/or specific GHB receptors, the latter corresponding to high-affinity [ ³H]GHB-binding sites coupled to G-proteins. To investigate the contribution of GABA_B receptors to GHB actions we studied the effects of GHB in GABA_B(1)-/- mice, which lack functional GABA _B receptors. Autoradiography reveals a similar spatial distribution of [₃H]GHB-binding sites in brains of GABA_B(1)-/- and wild-type mice. The maximal number of binding sites and the K_D values for the putative GHB antagonist [³H]6,7,8,9-tetrahydro-5- hydroxy-5H-benzocyclohept-6-ylidene acetic acid (NCS-382) appear unchanged in GABA_B(1)-/- compared with wild-type mice, demonstrating that GHB- are distinct from GABA_B-binding sites. In the presence of the GABA_B receptor positive modulator 2,6-di-tert-butyl-4-(3-hydroxy-2,2- dimethyl-propyl)-phenol GHB induced functional GTPγ[³⁵S] responses in brain membrane preparations from wild-type but not GABA _B(1)-/- mice. The GTPγ[³⁵S] responses in wild-type mice were blocked by the GABA_B antagonist [3-[[1-(S)-(3, 4dichlorophenyl)ethyl]amino]-2-(S)-hydroxy-propyl]-cyclohexylmethyl phosphinic acid hydrochloride (CGP54626) but not by NCS-382. Altogether, these findings suggest that the GHB-induced GTPγ[³⁵S] responses are mediated by GABA_B receptors. Following GHB or GBL application, GABA _B(1)-/- mice showed neither the hypolocomotion, hypothermia, increase in striatal dopamine synthesis nor electroencephalogram delta-wave induction seen in wild-type mice. It, therefore, appears that all studied GHB effects are GABA_B receptor dependent. The molecular nature and the signalling properties of the specific [³H]GHB-binding sites remain elusive.