Tag: GABAB

What is a GABA Receptor?

Published on by Andrew Marshall22 Comments

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.

What is Gamma-Aminobutyric Acid?

Published on by Andrew Marshall38 Comments

Introduction

gamma-Aminobutyric acid (or γ-aminobutyric acid or GABA), is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system.

GABA is sold as a dietary supplement in many countries. It has been traditionally thought that exogenous GABA (i.e. taken as a supplement) does not cross the blood-brain barrier, however data obtained from more current research indicates that it may be possible.

Brief History

In 1883, GABA was first synthesized, and it was first known only as a plant and microbe metabolic product.

In 1950, GABA was discovered as an integral part of the mammalian central nervous system.

In 1959, it was shown that at an inhibitory synapse on crayfish muscle fibres GABA acts like stimulation of the inhibitory nerve. Both inhibition by nerve stimulation and by applied GABA are blocked by picrotoxin.

Function

Neurotransmitter

Refer to GABA Receptor.

Two general classes of GABA receptor are known:

  • GABAA in which the receptor is part of a ligand-gated ion channel complex; and
  • GABAB metabotropic receptors, which are G protein-coupled receptors that open or close ion channels via intermediaries (G proteins).

Neurons that produce GABA as their output are called GABAergic neurons, and have chiefly inhibitory action at receptors in the adult vertebrate. Medium spiny cells are a typical example of inhibitory central nervous system GABAergic cells. In contrast, GABA exhibits both excitatory and inhibitory actions in insects, mediating muscle activation at synapses between nerves and muscle cells, and also the stimulation of certain glands. In mammals, some GABAergic neurons, such as chandelier cells, are also able to excite their glutamatergic counterparts.

Release, Reuptake, and Metabolism Cycle of GABA.

GABAA receptors are ligand-activated chloride channels: when activated by GABA, they allow the flow of chloride ions across the membrane of the cell. Whether this chloride flow is depolarising (makes the voltage across the cell’s membrane less negative), shunting (has no effect on the cell’s membrane potential), or inhibitory/hyperpolarising (makes the cell’s membrane more negative) depends on the direction of the flow of chloride. When net chloride flows out of the cell, GABA is depolarising; when chloride flows into the cell, GABA is inhibitory or hyperpolarising. When the net flow of chloride is close to zero, the action of GABA is shunting. Shunting inhibition has no direct effect on the membrane potential of the cell; however, it reduces the effect of any coincident synaptic input by reducing the electrical resistance of the cell’s membrane. Shunting inhibition can “override” the excitatory effect of depolarising GABA, resulting in overall inhibition even if the membrane potential becomes less negative. It was thought that a developmental switch in the molecular machinery controlling the concentration of chloride inside the cell changes the functional role of GABA between neonatal and adult stages. As the brain develops into adulthood, GABA’s role changes from excitatory to inhibitory.

Brain Development

While GABA is an inhibitory transmitter in the mature brain, its actions were thought to be primarily excitatory in the developing brain. The gradient of chloride was reported to be reversed in immature neurons, with its reversal potential higher than the resting membrane potential of the cell; activation of a GABA-A receptor thus leads to efflux of Cl− ions from the cell (that is, a depolarising current). The differential gradient of chloride in immature neurons was shown to be primarily due to the higher concentration of NKCC1 co-transporters relative to KCC2 co-transporters in immature cells. GABAergic interneurons mature faster in the hippocampus and the GABA signalling machinery appears earlier than glutamatergic transmission. Thus, GABA is considered the major excitatory neurotransmitter in many regions of the brain before the maturation of glutamatergic synapses.

In the developmental stages preceding the formation of synaptic contacts, GABA is synthesized by neurons and acts both as an autocrine (acting on the same cell) and paracrine (acting on nearby cells) signalling mediator. The ganglionic eminences also contribute greatly to building up the GABAergic cortical cell population.

GABA regulates the proliferation of neural progenitor cells, the migration and differentiation the elongation of neurites and the formation of synapses.

GABA also regulates the growth of embryonic and neural stem cells. GABA can influence the development of neural progenitor cells via brain-derived neurotrophic factor (BDNF) expression. GABA activates the GABAA receptor, causing cell cycle arrest in the S-phase, limiting growth.

Beyond the Nervous System

Besides the nervous system, GABA is also produced at relatively high levels in the insulin-producing β-cells of the pancreas. The β-cells secrete GABA along with insulin and the GABA binds to GABA receptors on the neighbouring islet α-cells and inhibits them from secreting glucagon (which would counteract insulin’s effects).

GABA can promote the replication and survival of β-cells and also promote the conversion of α-cells to β-cells, which may lead to new treatments for diabetes.

Alongside GABAergic mechanisms, GABA has also been detected in other peripheral tissues including intestines, stomach, Fallopian tubes, uterus, ovaries, testes, kidneys, urinary bladder, the lungs and liver, albeit at much lower levels than in neurons or β-cells.

Experiments on mice have shown that hypothyroidism induced by fluoride poisoning can be halted by administering GABA. The test also found that the thyroid recovered naturally without further assistance after the Fluoride had been expelled by the GABA.

Immune cells express receptors for GABA and administration of GABA can suppress inflammatory immune responses and promote “regulatory” immune responses, such that GABA administration has been shown to inhibit autoimmune diseases in several animal models.

In 2018, GABA has shown to regulate secretion of a greater number of cytokines. In plasma of T1D patients, levels of 26 cytokines are increased and of those, 16 are inhibited by GABA in the cell assays.

In 2007, an excitatory GABAergic system was described in the airway epithelium. The system is activated by exposure to allergens and may participate in the mechanisms of asthma. GABAergic systems have also been found in the testis and in the eye lens.

GABA occurs in plants.

Structure and Conformation

GABA is found mostly as a zwitterion (i.e. with the carboxyl group deprotonated and the amino group protonated). Its conformation depends on its environment. In the gas phase, a highly folded conformation is strongly favoured due to the electrostatic attraction between the two functional groups. The stabilisation is about 50 kcal/mol, according to quantum chemistry calculations. In the solid state, an extended conformation is found, with a trans conformation at the amino end and a gauche conformation at the carboxyl end. This is due to the packing interactions with the neighbouring molecules. In solution, five different conformations, some folded and some extended, are found as a result of solvation effects. The conformational flexibility of GABA is important for its biological function, as it has been found to bind to different receptors with different conformations. Many GABA analogues with pharmaceutical applications have more rigid structures in order to control the binding better.

Biosynthesis

GABA is primarily synthesized from glutamate via the enzyme glutamate decarboxylase (GAD) with pyridoxal phosphate (the active form of vitamin B6) as a cofactor. This process converts glutamate (the principal excitatory neurotransmitter) into GABA (the principal inhibitory neurotransmitter).

GABA can also be synthesized from putrescine by diamine oxidase and aldehyde dehydrogenase.

Traditionally it was thought that exogenous GABA did not penetrate the blood–brain barrier, however more current research indicates that it may be possible, or that exogenous GABA (i.e. in the form of nutritional supplements) could exert GABAergic effects on the enteric nervous system which in turn stimulate endogenous GABA production. The direct involvement of GABA in the glutamate-glutamine cycle makes the question of whether GABA can penetrate the blood-brain barrier somewhat misleading, because both glutamate and glutamine can freely cross the barrier and convert to GABA within the brain.

Metabolism

GABA transaminase enzymes catalyse the conversion of 4-aminobutanoic acid (GABA) and 2-oxoglutarate (α-ketoglutarate) into succinic semialdehyde and glutamate. Succinic semialdehyde is then oxidized into succinic acid by succinic semialdehyde dehydrogenase and as such enters the citric acid cycle as a usable source of energy.

Pharmacology

Drugs that act as allosteric modulators of GABA receptors (known as GABA analogues or GABAergic drugs), or increase the available amount of GABA, typically have relaxing, anti-anxiety, and anti-convulsive effects. Many of the substances below are known to cause anterograde amnesia and retrograde amnesia.

In general, GABA does not cross the blood-brain barrier, although certain areas of the brain that have no effective blood–brain barrier, such as the periventricular nucleus, can be reached by drugs such as systemically injected GABA. At least one study suggests that orally administered GABA increases the amount of human growth hormone (HGH). GABA directly injected to the brain has been reported to have both stimulatory and inhibitory effects on the production of growth hormone, depending on the physiology of the individual. Certain pro-drugs of GABA (e.g. picamilon) have been developed to permeate the blood-brain barrier, then separate into GABA and the carrier molecule once inside the brain. Prodrugs allow for a direct increase of GABA levels throughout all areas of the brain, in a manner following the distribution pattern of the pro-drug prior to metabolism.

GABA enhanced the catabolism of serotonin into N-acetylserotonin (the precursor of melatonin) in rats. It is thus suspected that GABA is involved in the synthesis of melatonin and thus might exert regulatory effects on sleep and reproductive functions.

Chemistry

Although in chemical terms, GABA is an amino acid (as it has both a primary amine and a carboxylic acid functional group), it is rarely referred to as such in the professional, scientific, or medical community. By convention the term “amino acid”, when used without a qualifier, refers specifically to an alpha amino acid. GABA is not an alpha amino acid, meaning the amino group is not attached to the alpha carbon. Nor is it incorporated into proteins as are many alpha-amino acids.

GABAergic Drugs

GABAA receptor ligands are shown in the following table.

Activity at GABAALigand
Orthosteric AgonistMuscimol, GABA, gaboxadol (THIP), isoguvacine, progabide, piperidine-4-sulfonic acid (partial agonist).
Positive allosteric modulatorsBarbiturates, benzodiazepines, neuroactive steroids, niacin/niacinamide, nonbenzodiazepines (i.e. z-drugs, e.g. zolpidem, eszopiclone), etomidate, etaqualone, alcohol (ethanol), theanine, methaqualone, propofol, stiripentol, and anaesthetics (including volatile anaesthetics), glutethimide.
Orthosteric (competitive) AntagonistBicuculline, gabazine, thujone, and flumazenil.
Uncompetitive antagonist (e.g. channel blocker)Picrotoxin, and cicutoxin.
Negative allosteric modulatorsNeuroactive steroids (Pregnenolone sulfate), furosemide, oenanthotoxin, and amentoflavone.

Additionally, carisoprodol is an enhancer of GABAA activity. Ro15-4513 is a reducer of GABAA activity

GABAergic pro-drugs include chloral hydrate, which is metabolised to trichloroethanol, which then acts via the GABAA receptor.

Skullcap and valerian are plants containing GABAergic substances[citation needed]. Furthermore, the plant kava contains GABAergic compounds, including kavain, dihydrokavain, methysticin, dihydromethysticin and yangonin.

Other GABAergic modulators include:

  • GABAB receptor ligands.
    • Agonists: baclofen, propofol, GHB, and phenibut.
    • Antagonists: phaclofen and saclofen.
  • GABA reuptake inhibitors: deramciclane, hyperforin, and tiagabine.
  • GABA transaminase inhibitors: gabaculine, phenelzine, valproate, vigabatrin, and lemon balm (Melissa officinalis).
  • GABA analogues: pregabalin, gabapentin, picamilon, and progabide.

In Plants

GABA is also found in plants. It is the most abundant amino acid in the apoplast of tomatoes. Evidence also suggests a role in cell signalling in plants.