PRESENCE OF NOCICEPTIN (ORPHANIN FQ) RECEPTORS IN RAT RETINA - COMPARISON WITH RECEPTORS IN STRIATUM

Nociceptin (orphanin FQ), a heptadecapeptide with some sequence homolo gy to dynorphin A, has been proposed as an endogenous ligand for a pre viously cloned orphan receptor with significant homology to opioid rec eptors. Utilizing [I-125][Tyr(14)]nociceptin as ligand, saturable and high affinity nociceptin binding sites were detected and characterized in rat retina and striatum. For retina, B-max = 34.0 +/- 4.5 fmol/mg and K-d = 32.4 +/- 2.7 pM; for striaturn, B-max = 51.6 +/- 7.7 fmol/mg and K-d = 98.6 +/- 11.3 pM. In competition studies, nociceptin bound with picomolar affinity, dynorphin A with nanomolar affinity, naloxone and dynorphan A-(1-8) with micromolar affinity, while [des-Tyr(1)]dyn orphin (dynorphin A-(2-17)), several other opioids, morphine and benzo morphans failed to compete for binding at 1-10 mu M. Gpp(NH)p plus NaC l markedly decreased binding, consistent with involvement of a G prote in-linked receptor. It is concluded that rat retina contains nocicepti n receptors similar in concentration to those present in striatum. Pro perties of both the retinal and the striatal receptors are similar to those previously found for rat hypothalamus. (C) 1997 Elsevier Science B.V.