Dentate Gyrus Basket Cell GABAAReceptors Are Blocked by Zn2+via Changes of Their Desensitization Kinetics: AnIn SituPatch-Clamp and Single-Cell PCR Study

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Abstract
Although GABA type A receptors (GABAARs) in principal cells have been studied in detail, there is only limited information about GABAARs in interneurons. We have used the patch-clamp technique in acute rat hippocampal slices in combination with single-cell PCR to determine kinetic, pharmacological, and structural properties of dentate gyrus basket cell GABAARs. Application of 1 mmGABA (100 msec) to nucleated patches via a piezo-driven fast application device resulted in a current with a fast rise and a marked biexponential decay (time constants 2.4 and 61.8 msec). This decay could be attributed to strong receptor desensitization. Dose–response curves for the peak and the slow component yielded EC50values of 139 and 24 μm, respectively. Zn2+caused a marked blocking effect on both the peak and the slow component via a noncompetitive mechanism (IC50values of 8 and 16 μm). This led to an acceleration of the slow component as well as a prolongation of recovery from desensitization. Zn2+sensitivity was suggested to depend on the absence of γ-subunits in GABAARs. To test this hypothesis we performed single-cell reverse transcription PCR that revealed primarily the presence of α2-, β2-, β3-, γ1-, and γ2-subunit mRNAs. In addition, flunitrazepam increased the receptor affinity for its agonist, indicating the presence of functional benzodiazepine binding sites, i.e., γ-subunits. Thus, additional factors seem to co-determine the Zn2+sensitivity of native GABAARs. The modulatory effects of Zn2+on GABAAR desensitization suggest direct influences on synaptic integration via changes in inhibition and shunting at GABAergic synapses.