Homeostatic Competition between Phasic and Tonic Inhibition

Xia Wu,Lanting Huang,Zheng Wu,Ce Zhang,Dongyun Jiang,Yuting Bai,Yun Wang,Gong Chen

doi:10.1074/jbc.m113.491464

Xia Wu, Lanting Huang + Show 6 more

Open Access

https://doi.org/10.1074/jbc.m113.491464

Copy DOI

Abstract

The GABAA receptors are the major inhibitory receptors in the brain and are localized at both synaptic and extrasynaptic membranes. Synaptic GABAA receptors mediate phasic inhibition, whereas extrasynaptic GABAA receptors mediate tonic inhibition. Both phasic and tonic inhibitions regulate neuronal activity, but whether they regulate each other is not very clear. Here, we investigated the functional interaction between synaptic and extrasynaptic GABAA receptors through various molecular manipulations. Overexpression of extrasynaptic α6β3δ-GABAA receptors in mouse hippocampal pyramidal neurons significantly increased tonic currents. Surprisingly, the increase of tonic inhibition was accompanied by a dramatic reduction of the phasic inhibition, suggesting a possible homeostatic regulation of the total inhibition. Overexpressing the α6 subunit alone induced an up-regulation of δ subunit expression and suppressed phasic inhibition similar to overexpressing the α6β3δ subunits. Interestingly, blocking all GABAA receptors after overexpressing α6β3δ receptors could not restore the synaptic GABAergic transmission, suggesting that receptor activation is not required for the homeostatic interplay. Furthermore, insertion of a gephyrin-binding-site (GBS) into the α6 and δ subunits recruited α6(GBS)β3δ(GBS) receptors to postsynaptic sites but failed to rescue synaptic GABAergic transmission. Thus, it is not the positional effect of extrasynaptic α6β3δ receptors that causes the down-regulation of phasic inhibition. Overexpressing α5β3γ2 subunits similarly reduced synaptic GABAergic transmission. We propose a working model that both synaptic and extrasynaptic GABAA receptors may compete for limited receptor slots on the plasma membrane to maintain a homeostatic range of the total inhibition.

Highlights

Synaptic and extrasynaptic GABAA receptors mediate phasic and tonic inhibition, respectively
We examined whether the increased expression of extrasynaptic GABAA receptors might affect synaptic GABAergic transmission. miniature inhibitory postsynaptic currents (mIPSCs) were recorded in the presence of TTX (0.5 ␮M) and DNQX (10 ␮M) to block action potentials and glutamatergic responses, respectively
How synaptic and extrasynaptic GABAA receptors interact with each other to maintain the homeostasis of GABAergic inhibition has not been well studied

Summary

Background

Synaptic and extrasynaptic GABAA receptors mediate phasic and tonic inhibition, respectively. Insertion of a gephyrin-binding-site (GBS) into the ␣6 and ␦ subunits recruited ␣6GBS␤3␦GBS receptors to postsynaptic sites but failed to rescue synaptic GABAergic transmission It is not the positional effect of extrasynaptic ␣6␤3␦ receptors that causes the down-regulation of phasic inhibition. It has been reported that in transgenic mice with ectopic expression of the ␣6 subunit, tonic conductance increases, but synaptic GABA responses decrease simultaneously [27] These studies suggest a possible interplay between synaptic and extrasynaptic GABAA receptors, but the molecular mechanism underlying such interaction is currently unknown. Overexpression of ␣6␤3␦ receptors in cultured hippocampal pyrimidal neurons resulted in a dramatic decrease of synaptic GABAergic events as well as a significant reduction of presynaptic GABAergic terminals Such a functional outcome did not require the activation of extrasynaptic GABAA receptors. On the basis of these results, we propose that synaptic and extrasynaptic GABAA receptors will compete for limited receptor slots on the cell surface so that the total GABA inhibition can be regulated in a homeostatic manner

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION