What is a GABA Receptor?

Introduction

The GABA receptors are a class of receptors that respond to the neurotransmitter gamma-aminobutyric acid (GABA), the chief inhibitory compound in the mature vertebrate central nervous system.

There are two classes of GABA receptors: GABAA and GABAB. GABAA receptors are ligand-gated ion channels (also known as ionotropic receptors); whereas GABAB receptors are G protein-coupled receptors, also called metabotropic receptors.

Ligand-Gated Ion Channels

Ionotropic GABA receptors (iGABARs) are ligand-gated ion channel of the GABA receptors class which are activated by gamma-aminobutyric acid (GABA), and include:

  • GABAA receptors.
  • GABAA-ρ receptors.

The GABAB receptor, a G protein-coupled receptor, is the only metabotropic GABA receptor and its mechanism of action differs significantly from the ionotropic receptors. Functionally, in mature organisms, activation of these receptors typically results in neural inhibition, primarily via the influx of chloride ions, although exceptions to this general principle exist, such as during early development. Structurally, iGABARs are pentameric transmembrane ion channels, meaning they are made up of five subunits. Since there are several classes of subunits and a variety of genes encoding many members of these classes, a wide variety of structurally, and therefore functionally, distinct channels of iGABARs is observed.

GABAA Receptor

It has long been recognised that the fast response of neurons to GABA that is stimulated by bicuculline and picrotoxin is due to direct activation of an anion channel. This channel was subsequently termed the GABAA receptor. Fast-responding GABA receptors are members of a family of Cys-loop ligand-gated ion channels. Members of this superfamily, which includes nicotinic acetylcholine receptors, GABAA receptors, glycine and 5-HT3 receptors, possess a characteristic loop formed by a disulfide bond between two cysteine residues.

In ionotropic GABAA receptors, binding of GABA molecules to their binding sites in the extracellular part of the receptor triggers opening of a chloride ion-selective pore. 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 GABAA 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 GABAA 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 or in certain cell populations. 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 remodelling 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, or that the excitatory GABA was an artefact of neuronal damage. Subsequent studies from originators and proponents of the excitatory GABA theory have questioned these results, 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, and instead leading to a decrease of activity.

GABA receptors influence neural function by coordinating with glutamatergic processes.

GABAA-ρ Receptor

A subclass of ionotropic GABA receptors, insensitive to typical allosteric modulators of GABAA receptor channels such as benzodiazepines and barbiturates, was designated GABAС receptor. Native responses of the GABAC receptor type occur in retinal bipolar or horizontal cells across vertebrate species.

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

G Protein-Coupled Receptors

GABAB Receptor

A subclass of ionotropic GABA receptors, insensitive to typical allosteric modulators of GABAA receptor channels such as benzodiazepines and barbiturates, was designated GABAС receptor. Native responses of the GABAC receptor type occur in retinal bipolar or horizontal cells across vertebrate species.

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

GABA Receptor Gene Polymorphisms

Two separate genes on two chromosomes control GABA synthesis – glutamate decarboxylase and alpha-ketoglutarate decarboxylase genes – though not much research has been done to explain this polygenic phenomenon. GABA receptor genes have been studied more in depth, and many have hypothesized about the deleterious effects of polymorphisms in these receptor genes. The most common single nucleotide polymorphisms (SNPs) occurring in GABA receptor genes rho 1, 2, and 3 (GABBR1, GABBR2, and GABBR3) have been more recently explored in literature, in addition to the potential effects of these polymorphisms. However, some research has demonstrated that there is evidence that these polymorphisms caused by single base pair variations may be harmful.

It was discovered that the minor allele of a single nucleotide polymorphism at GABBR1 known as rs1186902 is significantly associated with a later age of onset for migraines, but for the other SNPs, no differences were discovered between genetic and allelic variations in the control vs. migraine participants. Similarly, in a study examining SNPs in rho 1, 2, and 3, and their implication in essential tremor, a nervous system disorder, it was discovered that there were no differences in the frequencies of the allelic variants of polymorphisms for control vs. essential tremor participants. On the other hand, research examining the effect of SNPs in participants with restless leg syndrome found an “association between GABRR3rs832032 polymorphism and the risk for RLS, and a modifier effect of GABRA4 rs2229940 on the age of onset of RLS” – the latter of which is a modifier gene polymorphism. The most common GABA receptor SNPs do not correlate with deleterious health effects in many cases, but do in a few.

One significant example of a deleterious mutation is the major association between several GABA receptor gene polymorphisms and schizophrenia. Because GABA is integral to the release of inhibitory neurotransmitters which produce a calming effect and play a role in reducing anxiety, stress, and fear, it is not surprising that polymorphisms in these genes result in more consequences relating to mental health than to physical health. Of an analysis on 19 SNPs on various GABA receptor genes, five SNPs in the GABBR2 group were found to be significantly associated with schizophrenia, which produce the unexpected haplotype frequencies not found in the studies mentioned previously.

Several studies have verified association between alcohol use disorder and the rs279858 polymorphism on the GABRA2 gene e, and higher negative alcohol effects scores for individuals who were homozygous at six SNPs. Furthermore, a study examining polymorphisms in the GABA receptor beta 2 subunit gene found an association with schizophrenia and bipolar disorder, and examined three SNPs and their effects on disease frequency and treatment dosage. A major finding of this study was that functional psychosis should be conceptualised as a scale of phenotypes rather than distinct categories.

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