Alprazolam

Neurotransmission relies on excitatory and inhibitory signalling. γ-aminobutyric acid (GABA) type-A receptors (GABAARs) are members of the pentameric ligand-gated ion channel (PLGIC) superfamily located synaptically and perisynaptically to mediate phasic inhibition and extrasynaptically to mediate tonic inhibition. GABAARs comprise a variety of subunits from a homologous family whose members are named based on sequence identity as one of α1-6, β1-3, γ1-3, δ, ε, θ, π, and ρ1-3. Each subunit possesses an extracellular (ECD), transmembrane (TMD), and intracellular (ICD) domain; inter-subunit interfaces are the primary points of neurotransmitter and modulator binding, described by coordination of the principal (+) and complementary (-) sites in each subunit. Binding of GABA to GABAARs induces pore opening, rapid flow of chloride ions, and synaptic hyperpolarization, which in turn manifests as an inhibitory signal.

The most prevalent GABAARs in vivo are the α1β2γ2 receptors, which contain both GABA (β+/α-) and benzodiazepine (BZD, α+/γ-) binding sites in the intersubunit interfaces of the relevant subunits. In general, any receptors containing an αx/γz interface, where x = 1-3,5 and z = 1-3, have potential high-affinity BZD binding sites, although small sequence differences between subunits may alter binding affinity to individual molecules. The α4 and α6 subunits, in which an otherwise conserved histidine is replaced by arginine, do not bind traditional BZD ligands such as diazepam and hence are considered "diazepam-insensitive". GABA binding results in a series of conformational changes in the ECDs of GABAAR β subunits, "locking" each to its neighbouring α- interface. The binding of alprazolam in the high-affinity BZD site stabilizes the α+/γ- interface and facilitates the conformational changes that lead to pore opening, hence functioning as a positive allosteric modulator.

The exact manner in which GABAAR allosteric modulation mediates the therapeutic and unwanted effects of benzodiazepines remains unclear. Earlier studies suggested that the primary factor was the α subunit composition, with α1-containing receptors mediating the sedative effects, α2/3-containing receptors the anxiolytic effects, and α5-containing receptors the memory effects of benzodiazepines. More recent studies suggest a more complex set of factors including subunit composition, physiological location, neuronal circuit, and nerve cell type. To further complicate matters, there may be up to five distinct BZD binding sites on GABAARs, with site 1 corresponding to the classical high-affinity α+/γ- interface. The effects of binding at sites 2-4 are not fully understood and likely impart greater complexity to benzodiazepine pharmacological action.