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Gamma-aminobutyric acid (GABA)

Explanations > Brain stuff > Brain chemistry > Neurotransmitters > Gamma-aminobutyric acid (GABA)

Composition | Function | Dysfunction | See also

Gamma-aminobutyric acid (GABA) is one of the most common neurotransmitters found in mammals, where it acts as an inhibitor.

Composition

Chemically, Gamma-aminobutyric acid (GABA) is also known as 4-aminobutanoic acid. It is created from glutamate using the enzyme L-glutamic acid decarboxylase (GAD) and pyridoxal phosphate.

Function

GABA, as a general inhibitor and plays an important role in regulation, preventing over-excitation of neuronal channels. It does this by opening ion channel that allow inflow of Cl^- or outflow of K⁺ions, leading to hyperpolarization.

It is important in brain development, particularly before glutamate synapses arise.

GABA_A and GABA_C are ionotropic receptors, whilst GABA_B are metabotropic receptors. GABA_A receptors accepts various drugs including benzodiazepines, barbiturates, ethanol, inhaled anaesthetics and neuroactive steroids.

GABA_B receptors are found in the CNS and ANS and are inhibitory. GABA_C receptors are similar to GABA_A but is particularly prevalent in the retina of the eye and conducts Cl ions.

Dysfunction

In spastic diplegia, GABA absorption by some nerves becomes damaged, leading excessive muscular tensing.

Unrestricted synaptic excitation across the brain causes a seizure. It is thought that GABA is implicated in epilepsy.

Drugs

Allylglycine is an antagonist that inactivates GAD and hence removes its inhibitory effects.

Muscimol (derived muscarine) is a direct agonist.

Dicuculline blocks a GABA binding site and is a direct antagonist.

Benzodiasepines, including diazapam (Vallium) and chlorodiazepoxide (Librium) act as tranquillizers, binding to a GABA_A site, reducing anxiety, relaxing muscles and promoting sleep. Alcohol is believed to bind with this site too.

Another GABA_A site binds with barbiturates whilst another binds with steroids which are used as general anesthetics. In lower doses, barbituates are calming. In increasingly higher doses they cause difficulty in walking and talking, then unconsciousness, coma and death.

Picotoxin (a shrub-derived poison) binds to yet another site and inhibits GABA_A. In high doses it causes convulsions.

Baclofen acts on GABA_B as an antagonist, thus acting as a muscle relaxant.