Functional consequences of GABA_A receptor α4 subunit deletion on synaptic and extrasynaptic currents in mouse dentate granule cells

Background: The α4 subunit-containing γ-aminobutyric acid (A) receptors (GABA_ARs) are highly expressed primarily at extrasynaptic sites in the dentate gyrus (DG) and thalamus and are suspected to contribute to tonic inhibition that is sensitive to potentiation by gaboxadol and ethanol (EtOH). Global α4 subunit knockout (KO) mice exhibit greatly reduced tonic currents and insensitivity to ataxic, sedative and analgesic effects of gaboxadol compared to wild type (WT) controls. The α4 KO mice were also significantly more sensitive to pentylenetetrazol-induced seizures. However, no differences were observed between α4 KO and WT mice in other baseline behaviors or in the effects of EtOH on these behaviors. To examine possible functional and pharmacological GABA_AR alterations, and search for causes for the lack of differences in EtOH behaviors we studied the effects of acute EtOH application on GABA_AR-currents of DG cells from α4 KO and WT control mice complemented by Western blot measurements. Methods: We studied the consequences of α4 subunit deletion using Western immunoblotting and whole cell patch recordings from DG cells in brain slices from α4 KO and WT mice. Results: The magnitude of tonic current and its potentiation by EtOH (10 to 100 mM), alphaxalone (3 μM), and Ro15-4513 (0.3 μM) was greatly attenuated in α4 KO mice. The kinetics of miniature inhibitory postsynaptic currents (mIPSCs) in α4 KO mice were significantly slower compared to WT mice. Potentiation of mIPSCs by alphaxalone was greatly reduced in α4 KO mice. Ro15-4513 had no effect on mIPSCs from WT or KO mice. However, mIPSCs of α4 KO mice were significantly more sensitive to EtOH than those from WT mice. The γ2 subunit protein levels were selectively increased in hippocampus and thalamus, but not cortex of α4 KO mice. Conclusions: These data suggest that the global loss of α4 subunits leads to region- and cell location-specific compensatory increases in γ2 subunits, which in turn alter the pharmacological sensitivity of synaptic and extrasynaptic GABA_AR-currents. Our data also suggests that while enhancement of tonic inhibitory currents by gaboxadol, alphaxalone, and EtOH are reduced, and behavioral sensitivity to gaboxadol and alphaxalone may be reduced, compensatory changes in synaptic GABA_AR subunits may prevent similar reductions in behavioral sensitivity to EtOH.