Abstract
Several isoenzymes of the Na+,K+-ATPase are expressed in brain but their specific roles are poorly understood. Recently, it was suggested that an isoenzyme of the Na+,K+-ATPase containing the [alpha]2 subunit, together with the glutamate transporters GLAST and GLT-1, participate in a coupling mechanism between neuronal activity and energy metabolism taking place in astrocytes. To substantiate this hypothesis, we compared the distribution of [alpha]2, GLAST and/or GLT-1 in the rat cerebral cortex using double immunofluorescence and confocal microscopy, and immunocytochemistry at the electron microscopic level. We also investigated the relationship between [alpha]2, GLAST or GLT-1 and asymmetrical synaptic junctions (largely glutamatergic) and GABAergic nerve terminals. Results show that the [alpha]2 subunit has an exclusive astroglial localization, and that it is almost completely co-distributed with GLAST and GLT-1 when evaluated by confocal microscopy. This similar distribution was confirmed at the ultrastructural level, which further showed that the vast majority of the [alpha]2 staining (73% of all labelled elements), like that of GLAST and GLT-1, was located in glial leaflets surrounding dendritic spines and the dendritic and/or axonal elements of asymmetrical (glutamatergic) axo-dendritic synapses. Synapses ensheathed by [alpha]2, GLAST or GLT-1 virtually never included (< or =2%) GABAergic nerve terminals or synaptic junctions. However, a subset of GABAergic nerve terminals (10-14%) were directly apposed to asymmetrical axo-dendritic junctions surrounded by [alpha]2, GLAST or GLT-1. Altogether these results demonstrate that [alpha]2, GLAST and GLT-1 have comparable perisynaptic distribution within cortical astrocytes most likely associated with glutamatergic synapses.