Other studies indicated that the individual subunits exhibit a distinct subcellular distribution. For instance, in cerebellar granule cells α1, α6, β2/3 and γ2 subunits have been found by immunogold localizations to be concentrated in GABAergic Golgi synapses and also are present in the extrasynaptic membrane at lower concentration. In constrast, δ subunits could not be detected in synaptic junctions, although they were abundantly present in the extrasynaptic dendritic and somatic membranes (Nusser et al., 1998). Receptors containing the δ subunit also contain α6 and β subunits (Jechlinger et al., 1998; Pöltl et al., 2003). Receptors containing δ subunits exhibit a smaller single channel conductance and a much longer open time, and do not desensitize on the prolonged presence of GABA (Saxena and Macdonald, 1994). Together with the exclusive extrasynaptic localization of these receptors, these properties indicate that tonic inhibition observed in these cells is mediated mainly by the persistent activation of α6βδ receptors by GABA, that is present in the extracellular space of glomeruli (Nusser et al., 1998; Brickley et al., 1999).
