Analysis of GABA_A Receptor Activation by Combinations of Agonists Acting at the Same or Distinct Binding Sites

Abstract

Under both physiologic and clinical conditions GABA A receptors are exposed to multiple agonists, including the transmitter GABA, endogenous or exogenous neuroactive steroids, and various GABAergic anesthetic and sedative drugs. The functional output of the receptor reflects the interplay among all active agents. We have investigated the activation of the concatemeric a1b2g2L GABA A receptor by combinations of agonists. Simulations of receptor activity using the coagonist concerted transition model demonstrate that the response amplitude in the presence of agonist combinations is highly dependent on whether the paired agonists interact with the same or distinct sites. The experimental data for receptor activation by agonist combinations were in agreement with the established views of the overlap of binding sites for several pairs of orthosteric (GABA, b-alanine, and piperidine-4-sulfonic acid) and/or allosteric agents (propofol, pentobarbital, and several neuroactive steroids). Conversely, the degree of potentiation when two GABAergic agents are coapplied can be used to determine whether the compounds act by binding to the same or distinct sites. We show that common interaction sites mediate the actions of 5a-and 5b-reduced neuroactive steroids, and natural and enantiomeric steroids. Furthermore, the results indicate that the anesthetics propofol and pentobarbital interact with partially shared binding sites. We propose that the findings may be used to predict the efficacy of drug mixtures in combination therapy and thus have potential clinical relevance.

Steady‐state activation and modulation of the synaptic‐type α 1 β 2 γ 2L GABA A receptor by combinations of physiological and clinical ligands

“…The latter can be considered to reflect the net concentration of all species of potentiating steroids, for example, pregnanolone (3 α 5 β P), tetrahydrodeoxycorticosterone (THDOC) and others, in addition to 3 α 5 α P. The sum concentration of all potentiating steroids species is almost certainly higher than the concentration of 3 α 5 α P employed in the calculations above. Because the steroids interact with the same set of sites, the effects are additive in terms of concentrations (Shin et al ). For example, five potentiating steroid species with similar affinities and efficacies present at 10 nmol/L concentration each, effectively act as a single potentiating steroid at 50 nmol/L.…”

“…The sum concentration of all potentiating steroids species is almost certainly higher than the concentration of 3a5aP employed in the calculations above. Because the steroids interact with the same set of sites, the effects are additive in terms of concentrations (Shin et al 2019). For example, five potentiating steroid species with similar affinities and efficacies present at 10 nmol/L concentration each, effectively act as a single potentiating steroid at 50 nmol/L.…”

“…The predictions for peak current responses were made as described in detail previously (Shin et al 2019). In brief, the P Open,Peak was calculated using the state function that pertains to the two-state MWC model (Forman 2012;Steinbach and Akk 2019).…”

Monod-Wyman-Changeux Allosteric Shift Analysis in Mutant α1β3γ2L GABAA Receptors Indicates Selectivity and Crosstalk among Intersubunit Transmembrane Anesthetic Sites

“…Assuming that each mutation obliterated one site, their results were consistent with independent, additive, and approximately equal gating energies per site. Additionally, MWC analyses of wild-type receptor activation with pairs of modulators that act via distinct sites provide more evidence of independent and additive energy contributions (Li et al, 2013(Li et al, , 2014Shin et al, 2017Shin et al, , 2019. Thus, we hypothesized that independence and additivity are general features of anesthetic modulator sites.…”

Binding site location on GABAA receptors determines whether mixtures of intravenous general anaesthetics interact synergistically or additively in vivo

“…They postulated that allosteric interactions between binding sites were responsible. These studies on the interaction of steroid anaesthetics with intravenous agents were later extended to alphaxalone + propofol and to 3α5βP + etomidate and parallels with actions on heterologously expressed GABA A receptors were established (Cao et al, ; Li et al, ; Shin, Germann, Covey, Steinbach, & Akk, ; Shin, Germann, Steinbach, & Akk, ). In this study, we have extended the pharmacology to include synergy between the three distinct TMD sites shown in Figure .…”