Structural basis of neurosteroid anesthetic action on GABAA receptors

Type A gamma-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 alpha 1GABA(A)R (ELIC-alpha 1GABA(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.