Agonist-dependent Single Channel Current and Gating in α4β2δ and α1β2γ2S GABAA Receptors.

The family of-γ-aminobutyric acid type A receptors (GABAARs) mediates two types of inhibit;ion in the mammalian brain. Phasic inhibition is mediated by synaptic GABAARs that are mainly comprised of α1, β2, and γ2 subunits, whereas tonic inhibition is mediated by extrasynaptic GABAARS comprised of α4/6 β2, and δ subunits. We investigated the activation properties of recombinant α4β2δ and α4β2δ GABAARs 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 experinlents indicated that THIP produced faster opening rates at α4,β2δ GABAARs (β ~ 1600 s-1) than at α1β2Sγ2S GABAARS (β ~ 460 s-1), whereas GABA activated α1β2γ2s GARAARs more rapidly (β ~ 1800 s-1) than α4β2δ GABAARS (β < 440 s-1). Single channel recordings of α1β2γ2S and α4β2δ GABAARS 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 (Po ~ 0.3) at α4β2δ GABAARs and at least two "modes" of single channel bursting activity, lasting ~100 nis at α1β2γ2S GABAARs. The most prevalent bursting mode had a Po of ~0.7 and was described by a reaction scheme with three open and three shut states, whereas the "high" Po mode (~0.9) was characterized by two shut and three THIP states. Single channel activity elicited by THIP in α4β2δ and α1β2γ2S GABAARS occurred as a single population of bursts (Po~0.4-0.5) of moderate duration (~33 ms) that could be described by schemes containing two shut and two open states for both GABAARS. Our data identify kinetic properties that are receptor-subtype specific and others that are agonist specific, including unitary conductance. [ABSTRACT FROM AUTHOR]