Molecular arrangement and modulation of extrasynaptic αβδ GABA_A receptors

γ-aminobutyric acid Type-A (GABA_A) αβδ receptors regulate neuronal excitability and contribute to sleep, mood and motor coordination. However, the molecular arrangements of these receptors and the basis of GABA activation and δ-selective drug modulation remain unclear. We solve cryo-EM structures of α4β3δ receptors in an α-β-α-β-δ arrangement that contains two GABA binding pockets. GABA binding supports a classical β-subunit tilt and an outward configuration of the β-subunit M2-M3 loops. However, the δ subunit M2-M3 loop is orientated inward and the 9' activation gate in the pore is closed, consistent with the low efficacy for these receptors. Addition of a δ-selective positive allosteric modulator (PAM), DS2-Me, reveals binding to an α-δ pocket that causes the 9' activation gate to open. In this work we provide key insights into the stoichiometry, arrangement and molecular modes of GABA activation and δ-selective modulation of these critical regulators of neuronal excitability.