GABA receptor

GABA_A receptor

It has long been recognized that, for neurons that are stimulated by bicuculline and picrotoxin, the fast inhibitory response to GABA is due to direct activation of an anion channel.^{[1][2][3][4][5]} This channel was subsequently termed the GABA_A receptor.^[6] Fast-responding GABA receptors are members of a family of Cys-loop ligand-gated ion channels.^[7][8][9] Members of this superfamily, which includes nicotinic acetylcholine receptors, GABA_A receptors, glycine and 5-HT₃ receptors, possess a characteristic loop formed by a disulfide bond between two cysteine residues.^[10]

In ionotropic GABA_A receptors, binding of GABA molecules to their binding sites in the extracellular part of the receptor triggers opening of a chloride ion-selective pore.^[11] The increased chloride conductance drives the membrane potential towards the reversal potential of the Cl¯ ion which is about –75 mV in neurons, inhibiting the firing of new action potentials. This mechanism is responsible for the sedative effects of GABA_A allosteric agonists. In addition, activation of GABA receptors lead to the so-called shunting inhibition, which reduces the excitability of the cell independent of the changes in membrane potential.

There have been numerous reports of excitatory GABA_A receptors. According to the excitatory GABA theory, this phenomenon is due to increased intracellular concentration of Cl¯ ions either during development of the nervous system^[12][13] or in certain cell populations.^[14][15][16] After this period of development, a chloride pump is upregulated and inserted into the cell membrane, pumping Cl⁻ ions into the extracellular space of the tissue. Further openings via GABA binding to the receptor then produce inhibitory responses. Over-excitation of this receptor induces receptor remodeling and the eventual invagination of the GABA receptor. As a result, further GABA binding becomes inhibited and inhibitory postsynaptic potentials are no longer relevant.

However, the excitatory GABA theory has been questioned as potentially being an artefact of experimental conditions, with most data acquired in in-vitro brain slice experiments susceptible to un-physiological milieu such as deficient energy metabolism and neuronal damage. The controversy arose when a number of studies have shown that GABA in neonatal brain slices becomes inhibitory if glucose in perfusate is supplemented with ketone bodies, pyruvate, or lactate,^[17][18] or that the excitatory GABA was an artefact of neuronal damage.^[19] Subsequent studies from originators and proponents of the excitatory GABA theory have questioned these results,^[20][21][22] but the truth remained elusive until the real effects of GABA could be reliably elucidated in intact living brain. Since then, using technology such as in-vivo electrophysiology/imaging and optogenetics, two in-vivo studies have reported the effect of GABA on neonatal brain, and both have shown that GABA is indeed overall inhibitory, with its activation in the developing rodent brain not resulting in network activation,^[23] and instead leading to a decrease of activity.^[24][25]

GABA receptors influence neural function by coordinating with glutamatergic processes.^[26]

GABA_A-ρ receptor

A subclass of ionotropic GABA receptors, insensitive to typical allosteric modulators of GABA_A receptor channels such as benzodiazepines and barbiturates,^[27][28][29] was designated GABA_С receptor.^[30][31] Native responses of the GABA_C receptor type occur in retinal bipolar or horizontal cells across vertebrate species.^{[32][33][34][35]}

GABA_С receptors are exclusively composed of ρ (rho) subunits that are related to GABA_A receptor subunits.^[36][37][38] Although the term "GABA_С receptor" is frequently used, GABA_С may be viewed as a variant within the GABA_A receptor family.^[7] Others have argued that the differences between GABA_С and GABA_A receptors are large enough to justify maintaining the distinction between these two subclasses of GABA receptors.^[39][40] However, since GABA_С receptors are closely related in sequence, structure, and function to GABA_A receptors and since other GABA_A receptors besides those containing ρ subunits appear to exhibit GABA_С pharmacology, the Nomenclature Committee of the IUPHAR has recommended that the GABA_С term no longer be used and these ρ receptors should be designated as the ρ subfamily of the GABA_A receptors (GABA_A-ρ).^[41]

GABA_B receptor

A slow response to GABA is mediated by GABA_B receptors,^[42] originally defined on the basis of pharmacological properties.^[43]

In studies focused on the control of neurotransmitter release, it was noted that a GABA receptor was responsible for modulating evoked release in a variety of isolated tissue preparations. This ability of GABA to inhibit neurotransmitter release from these preparations was not blocked by bicuculline, was not mimicked by isoguvacine, and was not dependent on Cl¯, all of which are characteristic of the GABA_A receptor. The most striking discovery was the finding that baclofen (β-parachlorophenyl GABA), a clinically employed muscle relaxant^[44][45] mimicked, in a stereoselective manner, the effect of GABA.

Later ligand-binding studies provided direct evidence of binding sites for baclofen on central neuronal membranes.^[46] cDNA cloning confirmed that the GABA_B receptor belongs to the family of G-protein coupled receptors.^[47] Additional information on GABA_B receptors has been reviewed elsewhere.^{[48][49][50][51][52][53][54][55]}