Agonist-dependent Single Channel Current and Gating in α(4)β(2)δ and α(1)β(2)γ(2S) GABA(A) Receptors

The family of γ-aminobutyric acid type A receptors (GABA(A)Rs) mediates two types of inhibition in the mammalian brain. Phasic inhibition is mediated by synaptic GABA(A)Rs that are mainly comprised of α(1), β(2), and γ(2) subunits, whereas tonic inhibition is mediated by extrasynaptic GABA(A)Rs comprised of α(4/6), β(2), and δ subunits. We investigated the activation properties of recombinant α(4)β(2)δ and α(1)β(2)γ(2S) GABA(A)Rs in response to GABA and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3(2H)-one (THIP) using electrophysiological recordings from outside-out membrane patches. Rapid agonist application experiments indicated that THIP produced faster opening rates at α(4)β(2)δ GABA(A)Rs (β ∼1600 s(−1)) than at α(1)β(2)γ(2S) GABA(A)Rs (β ∼ 460 s(−1)), whereas GABA activated α(1)β(2)γ(2S) GABA(A)Rs more rapidly (β ∼1800 s(−1)) than α(4)β(2)δ GABA(A)Rs (β < 440 s(−1)). Single channel recordings of α(1)β(2)γ(2S) and α(4)β(2)δ GABA(A)Rs showed that both channels open to a main conductance state of ∼25 pS at −70 mV when activated by GABA and low concentrations of THIP, whereas saturating concentrations of THIP elicited ∼36 pS openings at both channels. Saturating concentrations of GABA elicited brief (<10 ms) openings with low intraburst open probability (P(O) ∼ 0.3) at α(4)β(2)δ GABA(A)Rs and at least two “modes” of single channel bursting activity, lasting ∼100 ms at α(1)β(2)γ(2S) GABA(A)Rs. The most prevalent bursting mode had a P(O) of ∼0.7 and was described by a reaction scheme with three open and three shut states, whereas the “high” P(O) mode (∼0.9) was characterized by two shut and three open states. Single channel activity elicited by THIP in α(4)β(2)δ and α(1)β(2)γ(2S) GABA(A)Rs occurred as a single population of bursts (P(O) ∼0.4–0.5) of moderate duration (∼33 ms) that could be described by schemes containing two shut and two open states for both GABA(A)Rs. Our data identify kinetic properties that are receptor-subtype specific and others that are agonist specific, including unitary conductance.