Characterization of [L-3H]aspartate binding to chick retinal subcellular fractions

Binding of [L-3H]aspartate to synaptic receptors was examined in membranes from whole chick retina and subcellular fractions enriched with photoreceptor terminals (P1) or terminals from the inner plexiform layer (P2), Na+-independent, stereospecific, high affinity binding was concentrated in the P1 fraction (Kb = 40 nM). P2 fraction also showed a high affinity binding system (KB = 11.8 nM) with lower capacity than in the P1 fraction. Comparative studies with [L-3H]-aspartate, [L-3H]-glutamate and [H3]-kainate showed that L-aspartate and L-glutamate are the most potent inhibitors of the binding of the three ligands. Aspartate and glutamate binding were effectively displaced by N-methyl-DL-aspartate and alpha-amino adipate, whereas only [3H]-glutamate binding was significantly inhibited by glutamate-diethyl-ester. Kainic acid exhibited negligible affinity for aspartate and glutamate binding sites. Results indicate the presence of different receptors for glutamate and aspartate in both plexiform layers of the retina.