Structural basis of neurosteroid anesthetic action on GABAA receptors

Type A γ-aminobutyric acid receptors (GABAARs) control neuronal excitability1. They are targets for the treatment of neurological diseases and disorders and also for general anesthetics. The underlying mechanisms of these drugs’ action on GABAARs remain to be determined.
One of the mechanisms is to potentiate function of GABAARs via binding to the transmembrane domain (TMD)2. Ample experimental evidence suggests that the TMD of GABAARs harbors sites for the primary actions of general anesthetics and neurosteroids. The TMD plays an essential role in functional transitions among the resting, activated, and desensitized states of these Cl-conducting channels.
Alphaxalone (5α-pregnan-3α-ol-11,20 dione) is a potent neurosteroid anesthetic. The anxiolytic, anticonvulsant, analgesic, and sedative-hypnotic effects of alphaxalone have been linked to its potentiation of GABA-evoked currents and direct activation of GABAARs3. However, the data about the alphaxalone binding site in GABAARs and the underlying structural basis of alphaxalone’s action are sparse.

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Figure: Alphaxalone-induced structural changes at the bottom of the TMD (a) Bottom view of overlaid TM1-TM2 structures of the apo (orange) and alphaxalone-bound (cyan) α1GABAAR chimera. (b) Side view of overlaid structures of apo (principal subunit – gold; complementary subunit – orange) and alphaxalone-bound (principal subunit – blue; complementary subunit – cyan) α1GABAAR chimera. For clarity, only TM2 and TM3 are shown in the principal subunit and only TM1 and TM2 are shown in the complementary subunit. The arrow highlights structural perturbations originating from the alphaxalone binding site near W246 through the TM1-TM2 linker to the pore-lining residues P253 (-2′) and V257 (2′). (c) The 2FO-FC electron density maps (blue mesh, contoured at 1 σ) covering TM1-TM2 in the apo (left) and alphaxalone-bound (right) α1GABAAR chimera. The sidechains are shown only for residues W246 to V257 (2′).