Opiate binding sites in the retina: properties and distribution

The presence of opiate binding sites in rat retina was confirmed and their properties and distribution were investigated. Etorphine, naloxone and methionine enkephalin exhibit high affinity, saturable binding to retinal membrane preparations. Scatchard analysis of the binding data yielded linear plots for each ligand. The P1 subcellular fraction contains the greatest density of sites per milligram of protein. The response of the binding of all ligands to sodium ions was similar in the retina to that observed in the brain. Competition experiments using naloxone and methionine enkephalin indicated that a homogeneous class of binding sites exists in the rat retina. Opiate binding sites were also demonstrated in the retina of cow, toad and skate. Methionine enkephalin-like immunoreactivity was not clearly detectable in rat retina but was present in toad retina. The physiological role of the opiate binding sites in the retina remains to be elucidated.