GABAA Receptor α and γ Subunits Shape Synaptic Currents via Different Mechanisms

Joseph W. Lynch,Angelo Keramidas,Christine Dixon,Pankaj Sah

doi:10.1074/jbc.m113.514695

Abstract

Synaptic GABAA receptors (GABAARs) mediate most of the inhibitory neurotransmission in the brain. The majority of these receptors are comprised of α1, β2, and γ2 subunits. The amygdala, a structure involved in processing emotional stimuli, expresses α2 and γ1 subunits at high levels. The effect of these subunits on GABAAR-mediated synaptic transmission is not known. Understanding the influence of these subunits on GABAAR-mediated synaptic currents may help in identifying the roles and locations of amygdala synapses that contain these subunits. Here, we describe the biophysical and synaptic properties of pure populations of α1β2γ2, α2β2γ2, α1β2γ1 and α2β2γ1 GABAARs. Their synaptic properties were examined in engineered synapses, whereas their kinetic properties were studied using rapid agonist application, and single channel recordings. All macropatch currents activated rapidly (<1 ms) and deactivated as a function of the α-subunit, with α2-containing GABAARs consistently deactivating ∼10-fold more slowly. Single channel analysis revealed that the slower current decay of α2-containing GABAARs was due to longer burst durations at low GABA concentrations, corresponding to a ∼4-fold higher affinity for GABA. Synaptic currents revealed a different pattern of activation and deactivation to that of macropatch data. The inclusion of α2 and γ1 subunits slowed both the activation and deactivation rates, suggesting that receptors containing these subunits cluster more diffusely at synapses. Switching the intracellular domains of the γ2 and γ1 subunits substantiated this inference. Because this region determines post-synaptic localization, we hypothesize that GABAARs containing γ1 and γ2 use different mechanisms for synaptic clustering.

Highlights

GABAA receptors (GABAARs) ␣2 and ␥1 subunits are highly expressed in amygdala but their influence on synaptic currents is unknown
3 pA α1β2 200 ms Incorporation of the ␥-Subunit into GABAARs—On the basis of conductance and kinetic properties, GABAARs comprising of ␣, ␤, and ␥ subunits are clearly distinguishable on the single channel level from those composed of ␣ and ␤ subunits. ␣␤␥
We wished to investigate the presence of GABAARs comprised only of ␣ and ␤ subunits in our standard ␣␤␥ receptor transfections to determine whether our transfections produced pure populations of ␣␤␥ receptors

Summary

Background

GABAAR ␣2 and ␥1 subunits are highly expressed in amygdala but their influence on synaptic currents is unknown. The amygdala, a structure involved in processing emotional stimuli, expresses ␣2 and ␥1 subunits at high levels The effect of these subunits on GABAAR-mediated synaptic transmission is not known. The amygdala is a temporal lobe structure that plays a key role in processing fear, and amygdala dysfunction is associated with anxiety-related disorders such as generalized anxiety, depression, and post-traumatic stress These disorders are commonly managed using benzodiazepines, which produce their therapeutic actions by enhancing the action of GABA at GABAARs containing ␥2 subunits [16, 17]. By providing new insights into the functional properties of ␣2- and ␥1-containing GABAARs, our study facilitates investigations into whether these GABAARs contribute to synaptic currents in brain regions that mediate anxiety-related disorders such as fear, depression, and post-traumatic stress

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION