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.