Agonist-dependent single channel current and Gating in α4β2δ and α1β2γ2S GABAA receptors

The family of gamma-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 alpha(1), beta(2), and gamma(2) subunits, whereas tonic inhibition is mediated by extrasynaptic GABA(A)Rs comprised of alpha(4/6), beta(2), and delta subunits. We investigated the activation properties of recombinant alpha(4)beta(2)delta and alpha(1)beta(2)gamma(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 alpha(4)beta(2)delta GABA(A)Rs (beta similar to 1600 s(-1)) than at alpha(1)beta(2)gamma(2S) GABA(A)Rs (beta similar to 460 s(-1)), whereas GABA activated alpha(1)beta(2)gamma(2S) GABA(A)Rs more rapidly (beta similar to 1800 s(-1)) than alpha(4)beta(2)delta GABA(A)Rs (beta < 440 s(-1)). Single channel recordings of alpha(1)beta(2)gamma(2S) and alpha(4)beta(2)delta GABA(A)Rs showed that both channels open to a main conductance state of similar to 25 pS at -70 mV when activated by GABA and low concentrations of THIP, whereas saturating concentrations of THIP elicited similar to 36 pS openings at both channels. Saturating concentrations of GABA elicited brief (< 10 ms) openings with low intraburst open probability (P-O similar to 0.3) at alpha(4)beta(2)delta GABA(A)Rs and at least two modes of single channel bursting activity, lasting similar to 100 ms at alpha(1)beta(2)gamma(2S) GABA(A)Rs. The most prevalent bursting mode had a P-O of similar to 0.7 and was described by a reaction scheme with three open and three shut states, whereas the high P-O mode (similar to 0.9) was characterized by two shut and three open states. Single channel activity elicited by THIP in alpha(4)beta(2)delta and alpha(1)beta(2)gamma(2S) GABA(A)Rs occurred as a single population of bursts (PO similar to 0.4-0.5) of moderate duration (similar to 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.