Putative acidic amino acid transmitters in the cerebellum. II. Electron microscopic localization of transport sites.

In structurally preserved cerebellar slices, the sites of high affinity uptake of acidic amino acids were analyzed using the nonmetabolizable analogue, D-[3H]aspartate. Electron microscopic autoradiography showed the greatest accumulation of grains to be over glial structures. The labelling of the perikarya, dendrites and axons of the putatively glutamatergic granule cells was very low. However, "hypothetical grain" analysis indicated that the terminals of these cells are probable sources of radioactivity even though they contained less than 9% of the total grains in the molecular layer. The resolution of the autoradiographic technique did not permit definitive conclusion, as the parallel fibre terminals are too small and are ensheathed by thin glial processes. Nevertheless, further supporting evidence for some D-[3H]aspartate uptake into parallel fibre terminals was obtained using mechanically chopped cerebellar slices in which compared with glia presynaptic structures are selectively preserved. It is concluded that in line with hypotheses relating to the compartmentation of glutamate metabolism, the principal sites of uptake of acidic amino acids in the cerebellum are the glial cells. The results have clear implications regarding the use of high affinity uptake as a marker for glutamatergic nerve terminals.