Presence of nociceptin (orphanin FQ) receptors in rat retina: comparison with receptors in striatum.

Nociceptin (orphanin FQ), a heptadecapeptide with some sequence homology to dynorphin A, has been proposed as an endogenous ligand for a previously cloned orphan receptor with significant homology to opioid receptors. Utilizing [(125)I][Tyr14]nociceptin as ligand, saturable and high affinity nociceptin binding sites were detected and characterized in rat retina and striatum. For retina, Bmax = 44.0 +/- 4.5 fmol/mg and Kd = 32.4 +/- 2.7 pM; for striatum, Bmax = 51.6 +/- 7.7 fmol/mg and Kd = 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-Tyr1]dynorphin (dynorphin A-(2-17)), several other opioids, morphine and benzomorphans failed to compete for binding at 1-10 microM. Gpp(NH)p plus NaCl markedly decreased binding, consistent with involvement of a G protein-linked receptor. It is concluded that rat retina contains nociceptin receptors similar in concentration to those present in striatum. Properties of both the retinal and the striatal receptors are similar to those previously found for rat hypothalamus.