Strychnine-insensitive [3H] glycine binding to synaptosomal membranes from the chick retina

The pharmacology and kinetics of strychnine-insensitive [3H] glycine binding to synaptic membranes from the outer (P1) and the inner (P2) plexiform layers of chick retina was studied. Inhibition curves for glycine, D-serine, 1-amincyclopropanecarboxylic acid (ACPC) and strychnine were analyzed by non-linear regression. Hill slopes for glycine and D-serine were not different from unity, whereas those for ACPC were < 1 in both fractions, revealing heterogeneity of binding sites in these membranes. Non-linear regression analysis of time course and saturation experiments strengthen the idea that [3H] glycine binds to more than one class of sites, with similar affinities at equilibrium. Antagonists of strychnine-insensitive glycine receptors in the CNS did not inhibit [3H] glycine binding to these membranes, which demonstrates that NMDA receptors in the retina have different structural requirements for ligand interaction at these sites. pH affected the specific binding, in agreement with the participation of specific amino acid residues at glycine binding sites on NMDA receptors, and also with functional studies in which the modulation of affinity at this site by protons has been observed. These results support previous studies regarding CPP and MK-801 binding, and provide evidence which indicates that the pharmacophore for glycine and other NMDA-related ligands is distinct for the retina, compared to the CNS, mainly regarding the effects of glycine-site antagonists.