Glutamate transporters in glial plasma membranes: Highly differentiated localizations revealed by quantitative ultrastructural immunocytochemistry

The glutamate transporters GLT-1 and GLAST were studied by immunogold labeling on ultrathin sections of rat brain tissue embedded in acrylic resins at low temperature after freeze substitution. Both proteins were selective markers of astrocytic plasma membranes. GLT-1 was much higher in hippocampal astrocytes than in cerebellar astrocytes. Astroglial membrane GLAST densities ranked as follows: Bergmann > cerebellar granular layer ≈ hippocampus > cerebellar white matter. No astrocyte appeared unlabeled. Astrocytic membranes facing capillaries, pia, or stem dendrites were lower in glutamate transporters than those facing nerveterminals, axons, and spines. Parallel fiber boutons (glutamatergic) synapsing on interneuron dendritic shafts were surrounded by lower transporter densities than those synapsing on Purkinje cell spines. Our findings suggest the localizations of glutamate transporters are carefully regulated.