Hyperpolarization-Activated Current (Ih) Is Reduced in Hippocampal Neurons from Gabra5−/− Mice

Robert P Bonin,Jieying Yu,Beverley A Orser,Douglas A Bayliss,Agnieszka A Zurek,Fabien Tell

doi:10.1371/journal.pone.0058679

Abstract

Changes in the expression of γ-aminobutyric acid type A (GABAA) receptors can either drive or mediate homeostatic alterations in neuronal excitability. A homeostatic relationship between α5 subunit-containing GABAA (α5GABAA) receptors that generate a tonic inhibitory conductance, and HCN channels that generate a hyperpolarization-activated cation current (Ih) was recently described for cortical neurons, where a reduction in Ih was accompanied by a reciprocal increase in the expression of α5GABAA receptors resulting in the preservation of dendritosomatic synaptic function. Here, we report that in mice that lack the α5 subunit gene (Gabra5−/−), cultured embryonic hippocampal pyramidal neurons and ex vivo CA1 hippocampal neurons unexpectedly exhibited a decrease in Ih current density (by 40% and 28%, respectively), compared with neurons from wild-type (WT) mice. The resting membrane potential and membrane hyperpolarization induced by blockade of Ih with ZD-7288 were similar in cultured WT and Gabra5−/− neurons. In contrast, membrane hyperpolarization measured after a train of action potentials was lower in Gabra5−/− neurons than in WT neurons. Also, membrane impedance measured in response to low frequency stimulation was greater in cultured Gabra5−/− neurons. Finally, the expression of HCN1 protein that generates Ih was reduced by 41% in the hippocampus of Gabra5−/− mice. These data indicate that loss of a tonic GABAergic inhibitory conductance was followed by a compensatory reduction in Ih. The results further suggest that the maintenance of resting membrane potential is preferentially maintained in mature and immature hippocampal neurons through the homeostatic co-regulation of structurally and biophysically distinct cation and anion channels.

Highlights

Proper functioning of the central nervous system depends on the delicate control of neuronal excitability through a balance of excitation and inhibition
We found the opposite, where a reduction in the expression of a5 subunit-containing GABAA (a5GABAA) receptors was associated with a reduction of Ih that contributes to homeostatic maintenance of resting membrane potential in these cells
We tested the hypothesis that reduced expression of a5GABAA receptors would be accompanied by a reciprocal increase in Ih [15]

Summary

Introduction

Proper functioning of the central nervous system depends on the delicate control of neuronal excitability through a balance of excitation and inhibition. Pathological brain states can result when this balance is disrupted, such as the development of seizures following the loss of neuronal inhibition [3,4]. Ample evidence suggests that homeostatic mechanisms exist to compensate for the loss of neuronal inhibition to maintain normal brain function [5,6]. Activation of synaptically-localized type A GABA (GABAA) receptors results in rapid transient inhibition of postsynaptic neurons whereas activation of extrasynaptic GABAA receptors by low concentrations of ambient GABA generates a tonic inhibitory conductance [8]. A tonic GABAergic conductance in the hippocampus is predominantly generated by GABAA receptors that contain either the a5 subunit (a5GABAA) or d subunit (dGABAA) [9,10]. Tonic GABAergic inhibition can exert powerful regulatory constraints on neuronal firing, excitability, and plasticity of excitatory synapses of hippocampal pyramidal neurons [11,12,13]

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Conclusion