THE NUMBER OF GLUTAMATE TRANSPORTER SUBTYPE MOLECULES AT GLUTAMATERGIC SYNAPSES - CHEMICAL AND STEREOLOGICAL QUANTIFICATION IN YOUNG-ADULT RAT-BRAIN

The role of transporters in shaping the glutamate concentration in the extracellular space after synaptic release is controversial because o f their slow cycling and because diffusion alone gives a rapid removal . The transporter densities have been measured electrophysiologically, but these data are from immature brains and do not give precise infor mation on the concentrations of the individual transporter subtypes. H ere we show by quantitative immunoblotting that the numbers of the ast roglial glutamate transporters G;AST (EAAT1) and GLT (EAAT2) are 3200 and 12,000 per mu m(3) tissue in the stratum radiatum of adult rat hip pocampus (CA1) and 18,000 and 2800 in the cerebellar molecular layer, respectively. The total astroglial cell surface is 1.4 and 3.8 m(2)/cm (3) in the two regions, respectively, implying average densities of GL AST and GLT molecules in the membranes around 2300 and 8500 mu m(-2) i n the former and 4700 and 740 mu m(-2) in the latter region. The total concentration of glial glutamate transporters in both regions corresp onds to three to five limes the estimated number of glutamate molecule s in one synaptic vesicle from each of all glutamatergic synapses. How ever, the role of glial glutamate transporters in limiting synaptic sp illover is likely to vary between the two regions because of differenc es in the distribution of astroglia. Synapses are completely ensheathe d and separated from each other by astroglia in the cerebellar molecul ar layer. In contrast, synapses in hippocampus (stratum radiatum) are only contacted by astroglia and are often found side by side without i ntervening glial processes.