Perisynaptic localization of delta subunit-containing GABA(A) receptors and their activation by GABA spillover in the mouse dentate gyrus.

In cerebellar granule cells, delta subunit-containing GABA(A) receptors are found exclusively at extrasynaptic sites, but their subcellular distribution in other brain areas is poorly understood. We examined the anatomical localization and physiological activation of these receptors in adult mouse dentate gyrus granule cells. Immunocytochemistry revealed a high density of delta subunits in the molecular layer and a much lower density in the cell body layer. At the ultrastructural level, immunogold-labeled delta subunits were found at the edge of symmetric synapses on granule cell dendrites. Functional correlates of this perisynaptic localization were obtained by comparing inhibitory responses in delta subunit-deficient (delta-/-) and wild-type (wt) mice. In whole-cell recordings at 22 degrees C, the weighted decay time constants (tau(w)) of spontaneous IPSCs (sIPSCs) were significantly longer in wt mice but were similar at 34 degrees C, reflecting the role of temperature-dependent GABA uptake in shaping sIPSC decay. IPSCs evoked by minimal stimulation (eIPSCs) near the somata had similar tau(w) in delta-/- and wt mice, but eIPSCs elicited from dendritic sites decayed significantly more slowly in wt mice, consistent with a higher density of delta subunit-containing receptors in the molecular layer. The tau(w) of dendritic eIPSCs of wt mice were shortened by ZnCl2 (10 microm), reflecting the high Zn2+ sensitivity of delta subunit-containing GABA(A) receptors, and were prolonged by the GAT-1 GABA transporter inhibitor NO711 (10 microm). Our results demonstrate a perisynaptic localization of delta subunit-containing GABA(A) receptors and indicate that these receptors can be activated by GABA overspill in the molecular layer.