Major inhibitory neurotransmitter system. GABA binding to pentameric GABA-A chloride channels (α/β/γ subunit composition) increases Cl⁻ conductance, hyperpolarizes the neuron, and dampens excitability. Distinct allosteric sites accept benzodiazepines (α1/2/3/5 + γ2 interface — positive allosteric modulators that enhance GABA's effect but require GABA to act), barbiturates (β subunit — at higher doses become direct channel openers without GABA, hence the larger toxicity tail), Z-drugs (α1-selective benzodiazepine-site binders, biased toward sedation over anxiolysis), neurosteroids (allopregnanolone — δ subunit-containing extra-synaptic receptors), and propofol / etomidate (β3 subunit — direct openers like barbiturates). Alcohol's CNS-depressant action is partly GABA-A potentiation at α4/δ extra-synaptic receptors. The GABA-A vs GABA-B split matters — baclofen is a metabotropic GABA-B agonist (Gi-coupled), not a GABA-A modulator, despite both being "GABA-ergic".
Organ Systems
nervous
Pathway Steps
glutamate → gaba — via GAD (glutamic acid decarboxylase) — vitamin B6 (P5P) is the cofactor. Decarboxylation of glutamate by GAD is the rate-limiting step of GABA synthesis and requires pyridoxal-5′-phosphate (vitamin B6); B6 deficiency lowers GABA tone. Two isoforms exist — GAD67 (cytosolic, constitutive, supplies metabolic GABA) and GAD65 (vesicle-associated, activity-dependent, supplies the phasic neurotransmitter pool).
gaba → gaba-a-receptor-activation — via orthosteric binding at α/β interface → Cl⁻ channel opening → hyperpolarization. The GABA-A receptor is a pentameric ligand-gated chloride channel, most commonly 2α/2β/1γ. Two GABA molecules bind at the β–α subunit interfaces (orthosteric site); the benzodiazepine site is a distinct α–γ interface. Gating is fast (sub-millisecond), mediating phasic synaptic inhibition, while extrasynaptic α4/α6–δ receptors mediate tonic inhibition.
gaba-a-receptor-activation → neuronal-inhibition — via Cl⁻ influx → IPSP → reduced firing probability. Whether Cl⁻ influx inhibits depends on the transmembrane Cl⁻ gradient. Mature neurons extrude Cl⁻ via KCC2, so the reversal potential sits below threshold and GABA-A activation hyperpolarizes (inhibitory). Immature neurons (and some pathology) accumulate Cl⁻ via NKCC1, making GABA-A transiently depolarizing/excitatory.
gaba → succinate — via GABA transaminase + succinic semialdehyde dehydrogenase → GABA shunt → TCA cycle. The GABA shunt: GABA transaminase (GABA-T) converts GABA to succinic semialdehyde, which SSADH oxidizes to succinate feeding the TCA cycle. GABA-T is the irreversible target of vigabatrin and is inhibited by valproate; SSADH deficiency causes γ-hydroxybutyric (GHB) aciduria.
Known Modulators
diazepam (activator) — BZD site (α1/2/3/5). classic long-acting BZD; active metabolites (nordiazepam, oxazepam, temazepam) extend effect to ~100h
alprazolam (activator) — BZD site (α1/2/3/5). short-to-intermediate BZD; widely prescribed for panic; CYP3A4 substrate (DDI hot spot)
zopiclone (activator) — α1-selective BZD-site (Z-drug). cyclopyrrolone; eszopiclone is the active S-enantiomer
eszopiclone (activator) — α1-selective BZD-site (Z-drug). S-enantiomer of zopiclone; metallic-taste signature side effect
phenobarbital (activator) — β-subunit (barbiturate site). long-acting barbiturate; at high doses direct channel opener without GABA — wider toxicity tail than BZDs
butalbital (activator) — β-subunit (barbiturate site). intermediate barbiturate; component of Fioricet for tension headache; chronic-use rebound risk
propofol (activator) — β3-subunit. IV anesthetic; direct GABA-A channel opener like barbiturates; rapid onset/offset due to lipid redistribution
etomidate (activator) — β3-subunit. IV anesthetic; cardiovascular stability favored for shock + RSI; adrenocortical suppression limits infusion use
alcohol (activator) — α4/δ extra-synaptic GABA-A. partial enhancement at low doses; high doses also inhibit NMDA — combined produces the disinhibition + sedation phenotype
ethanol (activator) — α4/δ extra-synaptic GABA-A. chemistry-canonical slug for the same molecule; kept distinct from alcohol stub for solver-PK separation
valproate (activator) — GABA-T inhibition + Na+/Ca++ channel block. NOT a direct GABA-A modulator; raises CNS GABA by inhibiting GABA-aminotransferase + multiple non-GABA mechanisms
vigabatrin (inhibitor) — GABA-T (irreversible). suicide inhibitor of GABA-aminotransferase → elevated CNS GABA; irreversible visual field defects limit use
baclofen (activator) — GABA-B (metabotropic, Gi-coupled). NOT GABA-A; included here for completeness since users often expect both GABA receptor families together — distinct binding site + signal transduction
gaba (substrate) — GABA-A orthosteric agonist (endogenous). parent neurotransmitter; PO supplemental GABA has minimal BBB penetration so peripheral PO GABA does not reliably modulate central GABA-A
temazepam (activator) — BZD site (α1/2/3/5). intermediate BZD; insomnia; glucuronidated (no CYP DDIs)
oxazepam (activator) — BZD site. short BZD; phase II metabolism only (UGT) — preferred in liver disease