Sites of D-[3H]aspartate accumulation in mouse cerebellar slices.

Slices of mouse cerebellar vermis, cut in the parasagittal plane, were incubated for various times (up to 3 h) in the presence of 1 microM D-[3H]aspartate, a non-metabolized substrate for the glutamate/aspartate carrier in brain tissue. Light microscopic autoradiography indicated that in regions away from the cut edges of the slices the amino acid accumulated in glia and granule cells. Relatively few grains were seen over Purkinje, Golgi, stellate and basket cells or over white matter. Grain counts over the granule cell layers in the middle parts of the slices indicated that after short (15 min) exposures to the labelled substrate, non-granule cell areas (which included glia) contained, on average, slightly more grains than granule cells but with longer exposures (1.5 and 3 h) the relative grain density over granule cells became much higher, possibly because glial uptake prevents D-[3H]aspartate gaining access to neuronal sites in adequate amounts during short incubations and/or because the longer incubations allow time for retrograde migration of the label from parallel fibre terminals to occur. The demonstration of selective uptake of D-[3H]aspartate into granule cells contrasts with previous autoradiographic results (possible reasons for which are discussed) and supports the notion that L-glutamate is the transmitter of granule cells. The results also have a bearing on the importance of the metabolic compartmentation of glutamate in relation to its proposed transmitter role.