Sodium-dependent D-aspartate 'binding' is not a measure of presynaptic neuronal uptake sites in an autoradiographic assay

Abstract

The binding of D-[3H]aspartate to sections of rat brain was examined in an autoradiographic assay. Binding was entirely dependent on the presence of sodium ions, but not chloride ions, and was optimal at 2 degrees C. D-Aspartate bound rapidly, reached equilibrium within 20 min and remained stable for 45 min. The rate of dissociation was relatively rapid with a t1/2 of 56 s, but was not as fast as anticipated, perhaps because of some sequestration of ligand. Binding had a Kd of 6.8 +/- 1.2 microM and a Bmax of 49.4 +/- 8.6 pmol/mg protein. The high Bmax value may further indicate some sequestration of D-aspartate. L-Glutamate, unlabeled D-aspartate, and D,L-threo-hydroxyaspartate, a potent inhibitor of synaptosomal uptake, each competed for D-[3H]aspartate binding with IC50s of 7.0 +/- 4.3 microM, 5.4 +/- 1.5 microM, and 2.5 +/- 1.0 microM, respectively. N-methyl-D-aspartate (NMDA), quisqualate, and kainate had no affinity for this site. The regional distribution of D-aspartate binding sites was unique and did not conform to the distribution of neuronal uptake sites described by others. Striatal D-aspartate binding was unaffected by unilateral decortication or striatal quinolinic acid lesions. In contrast, binding to NMDA, quisqualate, and kainate receptors was reduced by 80-90% by quinolinate lesions of the striatum. The results of D-aspartate binding after lesions strongly suggest that this site is not associated with either lesioned glutamatergic afferents or intrinsic neurons of the striatum; it may be associated with glia.