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

Abstract

gamma-Hydroxybutyrate (GHB), a metabolite of gamma-aminobutyric acid (GABA), is proposed to function as a neurotransmitter or neuromodulator. gamma-Hydroxybutyrate and its prodrug, gamma-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 GABAB and/or specific GHB receptors, the latter corresponding to high-affinity [3H]GHB-binding sites coupled to G-proteins. To investigate the contribution of GABAB receptors to GHB actions we studied the effects of GHB in GABAB(1)-/- mice, which lack functional GABAB receptors. Autoradiography reveals a similar spatial distribution of [3H]GHB-binding sites in brains of GABAB(1)-/- and wild-type mice. The maximal number of binding sites and the KD 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 GABAB(1)-/- compared with wild-type mice, demonstrating that GHB- are distinct from GABAB-binding sites. In the presence of the GABAB receptor positive modulator 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol GHB induced functional GTPgamma[35S] responses in brain membrane preparations from wild-type but not GABAB(1)-/- mice. The GTPgamma[35S] responses in wild-type mice were blocked by the GABAB 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 GTPgamma[35S] responses are mediated by GABAB receptors. Following GHB or GBL application, GABAB(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 GABAB receptor dependent. The molecular nature and the signalling properties of the specific [3H]GHB-binding sites remain elusive.

Baclofen and Gamma-Hydroxybutyrate Withdrawal

“…After GHB or gamma-butyrolactone (a GHB precursor) application, GABA B receptor deficient mice did not show the behavioral or electroencephalogram changes seen in wild-type mice. This suggests that all the effects of GHB studied were GABA B receptor dependent [22]. Furthermore, chronic administration of GHB caused cross-tolerance to baclofen, but not to flunitrazepam (a GABA A agonist) [23].…”

Cataleptic effects of γ-hydroxybutyrate (GHB) and baclofen in mice: mediation by GABAB receptors, but differential enhancement by N-methyl-d-aspartate (NMDA) receptor antagonists

“…At present, there appears to be little evidence for a role for GHB receptors in the in vivo effects of GHB (Wong et al 2004). Instead, many studies suggest that GABA B receptors are particularly important for various behavioral effects of GHB, including discriminative stimulus effects (e.g., Winter 1981, Colombo et al 1998, Carter et al 2003, decreased operant responding (Goodwin et al 2005), and directly observable effects such as hypolocomotion (Kaupmann et al 2003), catalepsy (Carter et al 2005), ataxia (Goodwin et al 2005), and loss of righting (Carai et al 2001). Consistent with the involvement of GABA B receptors, all of these effects of GHB are produced also by the prototypical GABA B receptor agonist baclofen (Carter et al 2003(Carter et al , 2005.…”

α4βδ GABA A receptors are high-affinity targets for γ-hydroxybutyric acid (GHB)

“…In addition to the validated GABA B receptor effects and other suggested receptors (7), GHB binds with nanomolar to micromolar affinity to a remarkably abundant protein of distinct spatial distribution and ontogenesis (4), representing an additional functional target. Interestingly, this high-affinity binding protein is preserved in GABA B(1) KO mice (6) and can be specifically probed with BnOPh-GHB) (8). Furthermore, several reports point to GHBinduced effects that cannot be consequences of GABA B receptor activation alone: Fos expression studies with GHB indicate a unique pattern of neuronal activation, which in several ways, is different from the pattern produced by the GABA B receptor agonist baclofen (9).…”

“…When GHB is ingested in high doses and reaches millimolar concentrations in the brain, it induces behavioral effects such as sedation, motor incoordination and hypothermia (3). These actions are largely mediated by metabotropic GABA B receptors, because effects are prevented by GABA B receptor antagonist pretreatment (5) and completely abolished in GABA B(1) KO mice (6). In addition to the validated GABA B receptor effects and other suggested receptors (7), GHB binds with nanomolar to micromolar affinity to a remarkably abundant protein of distinct spatial distribution and ontogenesis (4), representing an additional functional target.…”