The GABA-ARs belong to the large pentameric ligand gated ion channel (previously referred to as "Cys-loop" receptors), which is the major inhibitory neurotransmitter in the central nervous system ^[1]. GABA-AR is a chloride channel family with 19 distinct subunits: six α (alpha1-6), three β (beta1-3), three γ (gamma1-3), three ρ (rho1-3), and one each of the δ (delta), ε (epsilon), π (pi), and θ (theta) which result in the production of a considerable number of receptor isoforms ^[2]. In humans, the units are as follows: six types of α subunits (GABRA1, GABRA2, GABRA3, GABRA4, GABRA5, GABRA6); three βs (GABRB1, GABRB2, GABRB3); three γs (GABRG1, GABRG2, GABRG3); as well as αδ (GABRD), an ε (GABRE), απ (GABRP), and αθ (GABRQ); three ρ units (GABRR1, GABRR2, GABRR3) ^[3].

The majority of GABA-ARs are composed of two α subunits, two β subunits, and one γ subunit arranged as γ2β2α1β2α1 counterclockwise around the center. Each subunit comprises four transmembrane domains with both the N- and C-terminus located extracellularly ^[4].

The Benzodiazepine Site (yellow sphere in the 3D structure viewer): The antagonists and inverse agonists bind the α4 and α6 (bc2) subtypes, and the binding site on all 6 α subunits was believed to contain the same residues as flunitrazepam in α1, confirmed by photoaffinity labeling with [3H]Ro15-4513, identified as attached covalently to Y209 (Loop C, ECD). The BZ binding site was shown to be located at the α+/γ- interface, as opposed to the GABA site at the β+/α- interface. The surprising observation was that the BZ binding sites were in the same homologous sequences (Loops A-H) as the GABA binding sites, which were the same utilized for agonist binding in all members of the cys-loop LGIC super-Family ^[5]. H101 residue (in a1, rat) that was R in the nonBZ binding α4 and α6 ^[6]. The GABA Site (white sphere in the 3D structure viewer): The GABA binding site is located at the β + α- extracellular interface ^[7].

Picrotoxinin, the active ingredient in picrotoxin, is a non-nitrogenous natural product from plants that has Picrotoxinin blocks GABA function non-competitively, by binding at a site within the TMD channel and nowhere near the GABA binding site, which has been identified on the extracellular domain of the protein ^[10]. The GABA-AR channel blocking activity is shared by some close structural analogues, tutin and coriamyrtin, and similar compounds found in related plants ^[11].

GABA itself can adopt a variety of almost equi-energetic conformations, but which analogues showed high potency on GABA-AR was noted to include trans-aminocrotonic acid, muscimol, isonipecotic acid, 3-aminopropane sulfonate, isoguvacine, piperidine-4-sulfonate, and the synthetic isoxazole, THIP (aka gaboxadol). Some of these mentioned as well as analogues with different conformations were synthesized by this group to add to the armamentarium for ligand structure–activity comparisons. ^[8] The results on compounds more rigid than GABA itself suggested a rather extended conformation of GABA for the receptor-specific conformation at the GABA-AR; furthermore, this differed from the active conformation required for the GABA uptake sites. Krogsgaard-Larsen distributed these compounds to numerous labs, who soon concurred in the results defining the GABA-AR ^[9].

A large number of BZ and non-BZ ligands were found active on the 'BZ receptor' sites identified as allosteric modulatory sites on GABA-ARs, and many more were synthesized due to the widespread clinically prescribed use of the BZs as anxiolytics ^[12]. U-89843A interacts with an allosteric site on GABA-ARs distinct from the sites for benzodiazepines, barbiturates, neurosteroids, substituted pyrazinones or loreclezole. ^[13] Not only are GABA-ARs the targets of agonist depressants and antagonist convulsants, but most GABA-AR drugs act at other (allosteric) binding sites on the GABA-AR proteins. Some anxiolytic and sedative drugs, like benzodiazepine and related drugs, act on GABA-AR subtype-dependent extracellular domain sites. General anesthetics including alcohols and neurosteroids act at GABA-AR subunit-interface trans-membrane sites. Ethanol at high anesthetic doses acts on GABA-AR subtype-dependent trans-membrane domain sites. Ethanol at low intoxicating doses acts at GABA-AR subtype-dependent extracellular domain sites ^[14].