Steady-State Activation and Modulation of the Concatemeric α1β2γ2L GABA_A Receptor

Abstract

The two-state coagonist model has been successfully used to analyze and predict peak current responses of the g-aminobutyric acid type A (GABA A) receptor. The goal of the present study was to provide a model-based description of GABA A receptor activity under steady-state conditions after desensitization has occurred. We describe the derivation and properties of the cyclic three-state resting-active-desensitized (RAD) model. The relationship of the model to receptor behavior was tested using concatemeric a1b2g2 GABA A receptors expressed in Xenopus oocytes. The receptors were activated by the orthosteric agonists GABA or b-alanine, the allosteric agonist propofol, or combinations of GABA, propofol, pentobarbital, and the steroid allopregnanolone, and the observed steady-state responses were compared with those predicted by the model. A modified RAD model was employed to analyze and describe the actions on steady-state current of the inhibitory steroid pregnenolone sulfate. The findings indicate that the steady-state activity in the presence of multiple active agents that interact with distinct binding sites follows standard energetic additivity. The derived equations enable prediction of peak and steady-state activity in the presence of orthosteric and allosteric agonists, and the inhibitory steroid pregnenolone sulfate. SIGNIFICANCE STATEMENT The study describes derivation and properties of a three-state resting-active-desensitized model. The model and associated equations can be used to analyze and predict peak and steady-state activity in the presence of one or more active agents.

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

“…Curve‐fitting yielded a K PS of 1.9 ± 1.5 μ mol/L, and a d PS of 0.11 ± 0.06 with the number of PS binding sites constrained to 1. These are similar to the values (3.5 μ mol/L and 0.054, respectively) reported recently for the concatemeric α 1 β 2 γ 2L GABA A receptor (Germann et al ). When N PS was held at 2, the fitted K PS was 1.0 ± 0.6 μ mol/L, and d PS 0.35 ± 0.07.…”

“…We next investigated receptor modulation by the endogenous steroid PS. Previous work has indicated that the steroid inhibits steady‐state activity by promoting entry to the desensitized state (Akk et al ; Eisenman et al ; Germann et al ). Receptors were activated by 1 mmol/L GABA.…”

“…Analysis and predictions of receptor activity were done in the framework of a three‐state Resting‐Active‐Desensitized (RAD) model (Fig. ; (Germann et al )). We infer that tonic activity due to exposure to ambient GABA, taurine, and neuroactive steroids contributes significantly to overall function of the synaptic GABA A receptor.…”

“…Quantitative analyses and predictions of receptor activity were made in the framework of the three‐state cyclic Resting‐Active‐Desensitized model (termed: the RAD model; Fig. ) (Germann et al ). The data indicate that the steady‐state open probability (P Open,S.S. )…”

A physiological role for GABAAreceptor desensitization – induction of long-term potentiation at inhibitory synapses

“…Although entry of GABAARs into the desensitized state is commonly studied in the context of orthosteric agonists, allosteric modulators will also affect the occupancy of this state during synaptic inhibition. In the case of one negative allosteric modulator, pregnenolone sulfate, inhibition of GABAARs is caused by directly acting on the desensitized state, enabling either stabilisation or increased entry of GABAARs into this state [49][50][51][52] . However, given that entry into the desensitized state is thought to follow receptor activation 10,53 , it is also likely that drugs which do not act directly on the desensitized state will also alter its occupancy indirectly, as a consequence of effects on receptor kinetics.…”