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"GABA" redirects here. For other uses, see GABA (disambiguation).
γ-Aminobutyric acid (GABA) (IPA: [ˈgæmə əˈmiːnoʊbjuːˈtɪrɨk ˈæsɨd]) is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays an important role in regulating neuronal excitability throughout the nervous system. In humans, GABA is also directly responsible for the regulation of muscle tone.[1] In insect species GABA acts only on excitatory nerve receptors. Although chemically it is an amino acid, GABA is rarely referred to as such in the scientific or medical communities, because the term "amino acid," used without a qualifier, refers to the alpha amino acids, which GABA is not, nor is it incorporated into proteins. In spastic diplegia in humans, GABA absorption by some nerves becomes impaired, which leads to hypertonia of the muscles signaled by those nerves.
[edit] Function[edit] NeurotransmitterIn vertebrates, GABA acts at inhibitory synapses in the brain by binding to specific transmembrane receptors in the plasma membrane of both pre- and postsynaptic neuronal processes. This binding causes the opening of ion channels to allow the flow of either negatively charged chloride ions into the cell or positively charged potassium ions out of the cell. This action results in a negative change in the transmembrane potential, usually causing hyperpolarization. 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 CNS GABAergic cells. In contrast, GABA exhibits excitatory 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.[2] GABAA receptors are chloride channels, that is, when activated by GABA, they allow the flow of chloride ions across the membrane of the cell. Whether this chloride flow is excitatory/depolarizing (makes the voltage across the cell's membrane less negative), shunting (has no effect on the cell's membrane) or inhibitory/hyperpolarizing (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 excitatory or depolarizing; when the net chloride flows into the cell, GABA is inhibitory or hyperpolarizing. 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 minimises the effect of any coincident synaptic input essentially by reducing the electrical resistance of the cell's membrane (essentially equivalent to Ohm's law). A developmental switch in the molecular machinery controlling concentration of chloride inside the cell and hence the direction of this ion flow, is responsible for the changes in the functional role of GABA between the neonatal and adult stages. That is to say, GABA's role changes from excitatory to inhibitory as the brain develops into adulthood.[3] [edit] Development In hippocampus and neocortex of the mammalian brain, GABA has primarily excitatory effects early in development, and is in fact the major excitatory neurotransmitter in many regions of the brain before the maturation of glutamate synapses - See developing cortex.[3] 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.[5][6] GABA regulates the proliferation of neural progenitor cells[7][8] the migration[9] and differentiation[10][11] the elongation of neurites[12] and the formation of synapses.[13] 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.[14] GABA activates the GABAA receptor, causing cell cycle arrest in the S-phase, limiting growth.[15] [edit] Beyond the nervous systemGABAergic mechanisms have been demonstrated in various peripheral tissues and organs including, but not restricted to the intestine, stomach, pancreas, Fallopian tube, uterus, ovary, testis, kidney, urinary bladder, lung and liver.[16] In 2007, an excitatory GABAergic system was described in the airway epithelium. The system activates following exposure to allergens and may participate in the mechanisms of asthma.[17] GABAergic systems have also been found in the testis[18] and in the eye lens.[19] [edit] Structure and conformationGABA is found mostly as a zwitterion, that is, 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 favored because of the electrostatic attraction between the two functional groups. The stabilization is about 50 kcal/mol, according to quantum chemistry calculations. In the solid state, a more 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 neighboring 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.[20][21] [edit] HistoryGamma-aminobutyric acid was first synthesized in 1883,[citation needed] and was first known only as a plant and microbe metabolic product. In 1950, however, GABA was discovered to be an integral part of the mammalian central nervous system.[22] [edit] SynthesisOrganisms synthesize GABA from glutamate using the enzyme L-glutamic acid decarboxylase and pyridoxal phosphate (which is the active form of vitamin B6) as a cofactor. This process converts glutamate, the principal excitatory neurotransmitter, into the principal inhibitory neurotransmitter (GABA).[23][24] [edit] PharmacologyDrugs that act as agonists 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.[25] Many of the substances below are known to cause anterograde amnesia and retrograde amnesia. It has been suggested that orally administered GABA increases the amount of Human Growth Hormone, but this is questionable since it is unknown whether GABA can pass the blood-brain barrier. However, orally administered GABA does have effects outside of the central nervous system (e.g. increased muscle tone).[citation needed] [edit] GABAergic Drugs
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