Steady-state activation of the high-affinity isoform of the α4β2δ GABAA receptor

Abstract

Activation of GABAA receptors consisting of α4, β2 (or β3), and δ subunits is a major contributor to tonic inhibition in several brain regions. The goal of this study was to analyze the function of the α4β2δ receptor in the presence of GABA and other endogenous and clinical activators and modulators under steady-state conditions. We show that the receptor has a high constitutive open probability (~0.1), but is only weakly activated by GABA that has a maximal peak open probability (POpen,peak) of 0.4, taurine (maximal POpen,peak = 0.4), or the endogenous steroid allopregnanolone (maximal POpen,peak = 0.2). The intravenous anesthetic propofol is a full agonist (maximal POpen,peak = 0.99). Analysis of currents using a cyclic three-state Resting-Active-Desensitized model indicates that the maximal steady-state open probability of the α4β2δ receptor is ~0.45. Steady-state open probability in the presence of combinations of GABA, taurine, propofol, allopregnanolone and/or the inhibitory steroid pregnenolone sulfate closely matched predicted open probability calculated assuming energetic additivity. The results suggest that the receptor is active in the presence of physiological concentrations of GABA and taurine, but, surprisingly, that receptor activity is only weakly potentiated by propofol.

Probing solution structure of the pentameric ligand-gated ion channel GLIC by small-angle neutron scattering

“…Due in part to its low conductance in single-channel recordings [31], the open probability of GLIC has not been well established. However, there is precedence in this family for flickering between states even at high agonist concentrations [32,33] and for open probabilities well below 100% at activating conditions [34,35,36,37]. In recent computational work, we estimated the low-pH open probability of GLIC to 17% using Markov state modeling [38] -notably similar to the tolerated contribution (18%) of the open structure in our linear-combination fits, Supplementary Figure S1.…”

Reduced Activation of the Synaptic-Type GABAA Receptor Following Prolonged Exposure to Low Concentrations of Agonists: Relationship between Tonic Activity and Desensitization

“…Some portion of the synaptic response likely results from activation of δ-subunit containing receptors (Herd et al, 2013;Sun et al, 2018;Wei et al, 2003). The GABA A receptor comprising α4, β2 and δ subunits shows less desensitzation to a given concentration of GABA, with an estimated value of Q = 0.8 (Pierce et al, 2019). As indicated in Fig.…”

“…The critical aspects of the RAD model are: i) a ligand can have different affinities to different states; ii) the individual sites for a ligand in a given state have identical binding properties; and iii) the receptor undergoes a concerted state transition in which all sites change properties simultaneously. We have previously employed the RAD model to analyze steady-state activity in α1β2γ2L and α4β2δ GABA A receptors (Germann et al, 2019a;Germann et al, 2019b;Pierce et al, 2019).…”

“…3B for At Q (=A/D) of 1000, the equilibrium between active and desensitized states is strongly biased towards active and there is minimal desensitization. Accordingly, the sum of fractions of (Germann et al, 2019b;Pierce et al, 2019).…”

Enhancement of muscimol binding and gating by allosteric modulators of the GABAA receptor: relating occupancy to state functions

“…The presence of this additional state requires an extension of the two-state model treated so far. We have already described and used the "resting-active-desensitized" model in analyzing the steady-state responses of GABA A receptors (Germann et al, 2019a;Germann et al, 2019b;Pierce et al, 2019). We now set out to determine whether it is able to predict steady-state binding data using parameters derived from functional studies.…”