The Actions of Drug Combinations on the GABA_AReceptor Manifest as Curvilinear Isoboles of Additivity

Abstract

Drug interactions are often analyzed in terms of isobolograms. In the isobologram, the line connecting the axial points corresponding to the concentrations of two different drugs that produce an effect of the same magnitude is termed an isobole of additivity. Although the isobole of additivity can be a straight line in some special cases, previous work has proposed that it is curvilinear when the two drugs differ in their maximal effects or Hill slopes. Modulators of transmitter-gated ion channels have a wide range of maximal effects as well as Hill slopes, suggesting that the isoboles for drug actions on ion channel function are not linear. In this study, we have conducted an analysis of direct activation and potentiation of the human 122L GABA receptor to demonstrate that: 1) curvilinear isoboles of additivity are predicted by a concerted transition model where the binding of each GABAergic drug additively and independently reduces the free energy of the open receptor compared with the closed receptor; and 2) experimental data for receptor activation using the agonist pair of GABA and propofol or potentiation of responses to a low concentration of GABA by the drug pair of alfaxalone and propofol agree very well with predictions. The approach assuming independent energetic contributions from GABAergic drugs enables, at least for the drug combinations tested, a straightforward method to accurately predict functional responses to any combination of concentrations.

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 peak P Open in the presence of 1 mmol/L GABA is 0.91 (Fig. ) whereas coapplication of propofol with saturating GABA increases the P Open to near unity (Ruesch et al ; Shin et al ). Hence, coapplication of propofol is also expected to enhance P Open,S.S.…”

“…The activation parameters (K GABA , c GABA ) were determined from the analysis of peak currents from 5 to 8 cells per agonist concentration using equation 4. With the number of GABA binding sites constrained to 2 (Amin and Weiss 1993) and L held at 8000 (Shin et al 2017), we estimate a K GABA of 16 AE 3 lmol/L (best-fit parameter AE SE of the fit) and a c GABA of 0.0042 AE 0.0003. These estimates are similar to those reported previously (Chang and Weiss 1999;Rusch et al 2004;Shin et al 2017).…”

“…The best-fit value for Q was 0.29 AE 0.02. The value of L was constrained to 8000 (Shin et al 2017), the number of binding sites for GABA was constrained to 2 (Amin and Weiss 1993), and the value of d GABA was assumed to be 1 (see text for discussion). response was 0.21 AE 0.05.…”

“…First, the α 1 β 2 γ 2L receptor is minimally active with a P Open,S.S. of ~ 0.007 when exposed to ambient GABA and taurine, although this is greater than the P Open in the absence of any agonist (constitutive P Open = 0.00012; (Shin et al )). Second, exposure to endogenous potentiating steroids has a relatively small effect on receptor activity.…”

GABAA Receptor β2E155 Residue Located at the Agonist-Binding Site Is Involved in the Receptor Gating

“…These findings provide additional evidence for P openMax decrease caused by β 2 E155C mutation when activating these receptors with GABA. β 2 E155 Mutation Enhances the Receptor Spontaneous Activity WT α 1 β 2 γ 2 receptors are characterized with a very low spontaneous activity (Shin et al, 2017;Jatczak-Śliwa et al, 2018;Kisiel et al, 2018). In the present study, we assessed the extent of spontaneous activity using two different compounds: PB and picrotoxin (PTX).…”