Selective Excitatory Amino Acid Uptake in Glutamatergic Nerve Terminals and in Glia in the Rat Striatum: Quantitative Electron Microscopic Immunocytochemistry of Exogenous D‐Aspartate and Endogenous Glutamate and GABA

To characterize glutamate/aspartate uptake activity in various cellular and subcellular elements in the striatum, rat striatal slices were exposed to 10 and 50 μM exogenous D‐aspartate. After fixation with glutaraldehyde/ formaldehyde the distribution of D‐aspartate was analysed by postembedding immunocytochemistry and the ultrastructural distribution was compared with the distributions of endogenous glutamate and GABA. Light microscopically, D‐aspartate‐like immunoreactivity was localized in conspicuous dots along very weakly labelled dendritic profiles and neuron cell bodies. At the electron microscope level gold particles signalling D‐aspartate occurred at highest density in nerve terminals making asymmetrical contacts with postsynaptic spines (i.e. resembling synapses of cortical afferents). Astrocytic processes also contained gold particles, but at a lower density than nerve endings. In contrast, dendritic spines were only weakly D‐aspartate–positive. The difference in labelling at 10 and 50 μM D‐aspartate was consistent with‘high‐affinity’uptake. Neighbouring sections processed with other antibodies showed that the D‐aspartate labelling occurred in nerve terminals strongly immunoreactive for glutamate, rather than in terminals very weakly glutamate‐immunopositive or in nerve endings immunoreactive for GABA. Glutamate labelling of perfusion‐fixed striatum confirmed that terminals forming asymmetrical synaptic contacts with spines were enriched with gold particles, suggesting that these terminals use glutamate as a transmitter. This study demonstrates that high‐affinity uptake sites for excitatory amino acids in the striatum are most strongly expressed on presumed glutamatergic nerve terminals and on astrocytes.