The Na+-dependent binding of [3H]l-aspartate in thaw-mounted sections of rat forebrain

Binding of [³H]l-aspartate to thaw-mounted coronal sections of frozen rat forebrain was strong in grey regions of telencephalon (neocortex, hippocampus and neostriatum), but it was weaker and unevenly distributed in diencephalon. At low nanomolar concentrations of ligand used in the present studies, [³H]l-aspartate binding was strongly inhibited by l-threo-3-hydroxyaspartate and l-trans-pyrrolidine-2,4-dicarboxylate, compounds known to be substrate/inhibitors of the high affinity uptake of l-glutamate and l-aspartate. None of the typical ligands for the glutamate and aspartate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), N-methyl-d-aspartate and kainate, produced a strong enough inhibition (only CNQX at 100 M weakly inhibited) of the Na⁺-dependent [³H]l-aspartate binding to suggest that [³H]l-aspartate was bound to the receptor binding sites. Furthermore, the binding was absolutely dependent on the presence of Na⁺ in the incubation medium. It is concluded that [³H]l-aspartate is a ligand suitable for autoradiographic studies of the distribution of Na⁺-dependent, high affinity uptake of acidic amino acids in the central nervous system (CNS). However, feasibility of using [³H]l-aspartate as a specific marker of glutamatergic and/or aspartergic synapses in the CNS requires further investigation.