Benzodiazepine Modulation of GABAA Receptors: A Mechanistic Perspective

Abstract

Benzodiazepines (BZDs) are a class of widely prescribed psychotropic drugs that target GABA_A receptors (GABA_ARs) to tune inhibitory synaptic signaling throughout the central nervous system. Despite knowing their molecular target for over 40 years, we still do not fully understand the mechanism of modulation at the level of the channel protein. Nonetheless, functional studies, together with recent cryo-EM structures of GABA_A(α1)₂(βX)₂(γ2)₁ receptors in complex with BZDs, provide a wealth of information to aid in addressing this gap in knowledge. Here, mechanistic interpretations of functional and structural evidence for the action of BZDs at GABA_A(α1)₂(βX)₂(γ2)₁ receptors are reviewed. The goal is not to describe each of the many studies that are relevant to this discussion nor to dissect in detail all the effects of individual mutations or perturbations but rather to highlight general mechanistic principles in the context of recent structural information.

Keywords: GABAA receptor; benzodiazepines; mechanism; structure function.

Figure 1

Structures of synaptic GABA_ARs in complex with the BZD PAM diazepam (DZ) at both high-affinity ECD and lower-affinity TMD sites. (A) Top-down view with GABA bound at the β+/α− interfaces and DZ bound at the α+/γ− interface in the ECD. Cryo-EM map of GABA_A(α1)₂(β3)₂(γ2)₁ from PDB 6HUP. (B) Side view of the structure in A, additionally showing one of several binding sites for DZ in the TMD. Only three subunits are shown for clarity. (C) Same perspective as in (A) for a slice through the TMD. Cryo-EM map of GABA_A(α1)₂(β2)₂(γ2)₁ from PDB 6X3X. Three binding sites for DZ are highlighted at the β+/α− and γ+/β− intersubunit pockets between the TM helices and below the M2-M3 linker of one of the subunits. The central pore-gate-forming 9′ leucine residues are shown as sticks near the middle of the pore-lining M2 helices. (D) Biphasic modulation by DZ at the canonical high-affinity site in the ECD and lower-affinity sites in the TMD.

Figure 2

Canonical ECD binding α+/γ− interface, with diazepam (DZ) shown as white sticks bound behind loop C. Other loops implicated in binding or intersubunit interactions are indicated. Cryo-EM map for GABA_A(α1)₂(β3)₂(γ2)₁ from PDB 6HUP. Features discussed in the main text are indicated with spheres for residue Cα atoms and dashed lines for cross-linked residue pairs (numbered labels are referenced in the main text). The critical histidine residue in the β4−β5 loop within the BZD binding pocket (i) and the central valine residue in the α1 M2-M3 linker (viii) are shown as sticks.