Understanding your inhibitions: effects of GABA and GABAA receptor modulation on brain cortical metabolism

A targeted neuropharmacological, ¹H/¹³C NMR spectroscopy and multivariate statistical approach was used to examine the effects of exogenous GABA and ligands at the GABA_A receptor family on brain metabolism in the Guinea pig cortical tissue slice. All ligands at GABA_A receptors generated metabolic patterns which were distinct from one another with the major variance in the data arising because of metabolic work (shown by net flux into Krebs cycle byproducts and increased metabolic pool sizes). Three major clusters of metabolic signatures were identified which corresponded to: (i) activity at phasic (synaptic) GABA_A receptors, dominated by α1-containing receptors and responsive to GABA at 10 μmol/L; (ii) activity at perisynaptic receptors, dominated by response to high (40 μmol/L) GABA and the superagonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol hydrochloride, and C, activity at extrasynaptic receptors, dominated by response to low (0.1–1.0 μmol/L) GABA, zolpidem (400 nmol/L) and the non-specific allosteric modulator RO19-4603 (1 nmol/L). These results highlight the utility of a different but robust approach to study of the GABAergic system using metabolic systems analysis.     

In vivo neurochemical profiling of rat brain by 1H-[13C] NMR spectroscopy: cerebral energetics and glutamatergic/GABAergic neurotransmission

The quantification of excitatory and inhibitory neurotransmission and the associated energy metabolism is crucial for a proper understanding of brain function. Although the detection of glutamatergic neurotransmission in vivo by ¹³C NMR spectroscopy is now relatively routine, the detection of GABAergic neurotransmission in vivo has remained elusive because of the low GABA concentration and spectral overlap. Using ¹H-[¹³C] NMR spectroscopy at high magnetic field in combination with robust spectral modeling and the use of different substrates, [U-¹³C₆]-glucose and [2-¹³C]-acetate, it is shown that GABAergic, as well as glutamatergic neurotransmitter fluxes can be detected non-invasively in rat brain in vivo .     

Two pools of Triton X-100-insoluble GABA(A) receptors are present in the brain, one associated to lipid rafts and another one to the post-synaptic GABAergic complex

Rat forebrain synaptosomes were extracted with Triton X-100 at 4 degrees C and the insoluble material, which is enriched in post-synaptic densities (PSDs), was subjected to sedimentation on a continuous sucrose gradient. Two pools of Triton X-100-insoluble gamma-aminobutyric acid type-A receptors (GABA(A)Rs) were identified: (i) a higher-density pool (rho = 1.10-1.15 mg/mL) of GABA(A)Rs that contains the gamma2 subunit (plus alpha and beta subunits) and that is associated to gephyrin and the GABAergic post-synaptic complex and (ii) a lower-density pool (rho = 1.06-1.09 mg/mL) of GABA(A)Rs associated to detergent-resistant membranes (DRMs) that contain alpha and beta subunits but not the gamma2 subunit. Some of these GABA(A)Rs contain the delta subunit. Two pools of GABA(A)Rs insoluble in Triton X-100 at 4 degrees C were also identified in cultured hippocampal neurons: (i) a GABA(A)R pool that forms clusters that co-localize with gephyrin and remains Triton X-100-insoluble after cholesterol depletion and (ii) a GABA(A)R pool that is diffusely distributed at the neuronal surface that can be induced to form GABA(A)R clusters by capping with an anti-alpha1 GABA(A)R subunit antibody and that becomes solubilized in Triton X-100 at 4 degrees C after cholesterol depletion. Thus, there is a pool of GABA(A)Rs associated to lipid rafts that is non-synaptic and that has a subunit composition different from that of the synaptic GABA(A)Rs. Some of the lipid raft-associated GABA(A)Rs might be involved in tonic inhibition.