γ‐Hydroxybutyrate and the GABAergic footprint: a metabolomic approach to unpicking the actions of GHB

Abstract

Gamma-hydroxybutyrate is found both naturally in the brain and self-administered as a drug of abuse. It has been reported to act at endogenous γ-hydroxybutyrate (GHB) receptors and GABA(B) receptors [GABA(B)R], and may also be metabolized to GABA. Here, the metabolic fingerprints of a range of concentrations of GHB were measured in brain cortical tissue slices and compared with those of ligands active at GHB and GABA-R using principal components analysis (PCA) to identify sites of GHB activity. Low concentrations of GHB (1.0 μM) produced fingerprints similar to those of ligands active at GHB receptors and α4-containing GABA(A)R. A total of 10 μM GHB clustered proximate to mainstream GABAergic synapse ligands, such as 1.0 μM baclofen, a GABA(B)R agonist. Higher concentrations of GHB (30 μM) clustered with GABA(C)R agonists and the metabolic responses induced by blockade of the GABA transporter-1 (GAT1). The metabolic responses induced by 60 and 100 μM GHB were mimicked by simultaneous blockade of GAT1 and GAT3, addition of low concentrations of GABA(C)R antagonists, or increasing cytoplasmic GABA concentrations by incubation with the GABA transaminase inhibitor vigabatrin. These data suggest that at concentrations > 30 μM, GHB may be active via metabolism to GABA, which is then acting upon an unidentified GABAergic master switch receptor (possibly a high-affinity extrasynaptic receptor), or GHB may itself be acting directly on an extrasynaptic GABA-R, capable of turning off large numbers of cells. These results offer an explanation for the steep dose-response curve of GHB seen in vivo, and suggest potential target receptors for further investigation.

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

“…Our finding is supported by several studies, indicating a possible role for GHB at extrasynaptic α4-containing GABA A receptors. A study using metabolic fingerprinting recently reported that the actions of GHB are similar to several ligands acting at extrasynaptic GABA A receptors (25). A link between GHB and α4 has also been indicated by the ability of GHB to antagonize increases in α4 mRNA levels induced by ethanol withdrawal (24) and the association of chronically elevated GHB levels seen in succinic semialdehyde dehydrogenase KO mice with increased tonic inhibition at extrasynaptic GABA A receptors (23).…”

“…In fact, several studies infer a role for GHB at extrasynaptic GABA A receptors, such as a correlation between elevated GHB levels and increased tonic extrasynaptic inhibition through GABA A receptors (23). More specifically, effects involved receptor subtypes containing α4-and δ-subunits (24)(25)(26).…”

Brain Uptake of the Drug of Abuse γ-Hydroxybutyric Acid in Rats

“…In this study, the microdialysis probe placement was in the frontal cortex for sampling after intravenous GHB administration. The frontal cortex was used to assess GHB toxicokinetics because GABA b receptors [a likely site of action for GHB (Mathivet et al, 1997;Nasrallah et al, 2010)] are highly expressed in the cerebral cortex (Bowery et al, 1987). In addition, GABA concentrations can be measured in the frontal cortex, and a previous report indicated a reduction in GABA release in the frontal cortex after the subcutaneous administration of ␥-butyrolactone, a precursor of GHB (Hu et al, 2000).…”

The Neurobiological Mechanisms of Gamma-Hydroxybutyrate Dependence and Withdrawal and Their Clinical Relevance: A Review

“…The properties of dependence/tolerance and behavioural effects of GHB seem to be consistent with the involvement of the GABAergic receptors, mostly the GABA B R [61,86,87]. GHB activates the GABA B R directly or indirectly through the metabolic conversion of GHB to GABA and the GHB-mediated feedback control of GABA release [21,67]. Subtypes of the GABA B R (subunits GABA B1 and GABA B2 ) are important in mediating the effects of GHB [21,88,89,90,91,92].…”

“…Nevertheless, the exact involvement of GHBR is still unclear. The effects of GHB on sleep, as has been suggested, have to be mediated by the β 1 -subunit containing the α 4 -GABA A receptor (GABA A R) [65,66] which is highly sensitive to low concentrations of GHB (levels of 1.0 µ M GHB or below in neuronal level); when the concentration increases an additional GABA A R δ-subunit would putatively become involved [66,67]. In summary, GHB ingested in low doses could initiate some physiological effects mediated mainly by GHBR, the β 1 -subunits of α 4 -GABA A R. These effects which have positive therapeutic functions may unfortunately provide a potential risk for abuse.…”