Structural basis of neurosteroid anesthetic action on GABA A receptors

Type A γ-aminobutyric acid receptors (GABA A Rs) are inhibitory pentameric ligand-gated ion channels in the brain. Many anesthetics and neurosteroids act through binding to the GABA A R transmembrane domain (TMD), but the structural basis of their actions is not well understood and no resting-state GABA A R structure has been determined. Here, we report crystal structures of apo and the neurosteroid anesthetic alphaxalone-bound desensitized chimeric α1GABA A R (ELIC-α1GABA A R). The chimera retains the functional and pharmacological properties of GABA A Rs, including potentiation, activation and desensitization by alphaxalone. The apo-state structure reveals an unconventional activation gate at the intracellular end of the pore. The desensitized structure illustrates molecular determinants for alphaxalone binding to an inter-subunit TMD site. These structures suggest a plausible signaling pathway from alphaxalone binding at the bottom of the TMD to the channel gate in the pore-lining TM2 through the TM1–TM2 linker. The study provides a framework to discover new GABA A R modulators with therapeutic potential.