Gamma-aminobutyric acid (GABA) is an amino acid that functions as the primary inhibitory neurotransmitter for the central nervous system (CNS). It functions to reduce neuronal excitability by inhibiting nerve transmission. GABAergic neurons are located when the hippocampus, thalamus, basal ganglia, hypothalamus, and brainstem. The balance between inhibitory neuronal transmission via GABA and excitatory neuronal transmission via glutamate is essential for proper cell membrane stability and neurologic function.
Various diseases have been associated with low levels of GABA. Many psychiatric illnesses have been linked to low concentrations of GABA. Generalized anxiety is one example. As GABA is an inhibitory neurotransmitter, decreased concentration of it would produce a feeling of anxiousness. It has also been associated with schizophrenia, autism spectrum disorder, and major depressive disorder. It is important to note that although GABA concentrations may be altered in these psychiatric diseases, treatment using GABAa receptor agonists are not first-line therapy, due to high addition potential and potentially fatal adverse effect. Valproic acid, a GABA analog, can be used for mood instability due to the enhancement of GABA concentrations. ,
Seizures and epilepsy are associated with low levels of GABA. With decreased levels of inhibition in the cerebral cortex, cells become depolarized leading to seizure activity. GABA agonists, such as Valproic acid, are used for the treatment of seizures. Abrupt withdrawal from medications such as benzodiazepines, a GABAa receptor agonist, can provoke seizures. Also, GABA antagonists are pro-convulsant. 
Inherited disorders of GABA metabolism are rare and therefore require an increase in clinical suspension. The most common diseases are GABA-transaminase deficiency, succinic semialdehyde dehydrogenase deficiency (SSADH), and homocarnosinosis. SSADH is the most common of the neurotransmitter deficiency. It presents with vague phenotype, varying neurological manifestations, and psychiatric illness. GABA is unable to be converted to succinic acid, and gamma-hydroxybutyrate (GHB) accumulates. Elevated concentrations of GABA and GHB are found within serum and urine. Diagnosis can be made with urinary excretion of GABA and increased signaling in the globus pallidus on MRI. Characteristics include expressive language impairment, hypotonia, and seizures. The most common neuropsychiatric problem is sleep disturbance; other issues include inattention, hyperactivity, and obsessive-compulsive disorder (OCD). There is currently no standard treatment for SSADH deficiency. 
GABA-transaminase deficiency and homocarnosinosis are much rarer. GABA-transaminase deficiency is an autosomal recessive disorder. Patients may have seizures presenting in the neonatal period; other manifestations include hypotonia, hyperreflexia, severely delayed psychomotor development, and a high-pitched cry. High concentrations of GABA are found in serum and cerebrospinal fluid (CSF). Cerebrospinal fluid is needed for diagnosis. Homocarnosinosis has only been reported in one family. Characteristics include progressive spastic diplegia, intellectual disability, and retinitis pigmentosa. 
Drugs that increase the amount of GABA are commonly used as anticonvulsants, sedatives, and anxiolytics. Due to the increase in GABA, CNS depression is a common adverse effect. Some GABA agonist has addiction potential, and use should be monitored closely. 
Drugs that bind to but do not increase the amount of GABA are considered antagonist. Examples include picrotoxin or bicuculline methiodide. Both are mainly used for research. GABA antagonists are pro-convulsant and stimulants. ,