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 we re selective markers of astrocytic plasma membranes. GLT-1 was much hi gher in hippocampal astrocytes than in cerebellar astrocytes. Astrogli al membrane GLAST densities ranked as follows: Bergmann > cerebellar g ranular layer approximate to hippocampus > cerebellar white matter. No astrocyte appeared unlabeled. Astrocytic membranes facing capillaries , pia, or stem dendrites were lower in glutamate transporters than tho se facing nerve terminals, axons, and spines. Parallel fiber boutons ( glutamatergic) synapsing on interneuron dendritic shafts were surround ed by lower transporter densities than those synapsing on Purkinje cel l spines. Our findings suggest the localizations of glutamate transpor ters are carefully regulated.