The present study was undertaken to characterize simultaneously [
3H]nociceptin binding to opioid receptor-like 1 (ORL1) receptors in the rat brain and spinal cord. Specific binding of [
3H]nociceptin to crude membranes from the rat brain and spinal cord at 25°C was saturable, reversible and of high affinity, and it also exhibited a pharmacological specificity involving the ORL1 receptor. The K
d and B
max values for [
3H]nociceptin in the spinal cord were significantly lower than those in the brain. At 4°C, there was a significant increase in the dissociation constant (K
d) for [
3H]nociceptin in the brain and spinal cord with little change in the maximal number of binding sites (B
max) compared with that at 25°C. Nociceptin and its analogue, [Phe
1Ψ(CH
2-NH)-Gly
2]nociceptin(1—13)NH
2 were found to be potent inhibitors of [
3H]nociceptin binding to crude membranes from the brain and spinal cord, while opioid ligands such as naloxone-benzoylhydrazone, naltrindole and nor-binaltorphimine, exhibited an inhibitory effect only at high concentrations. The K
i values for nociceptin, its analogue and opioid ligands in the spinal cord were significantly lower than those in the brain. There were regional variations in the specific [
3H]nociceptin binding to crude membranes from the rat brain: a relatively high density of [
3H]nociceptin binding in the cerebral cortex, hippocampus, thalamus and midbrain, moderately dense binding in the corpus striatum and pons/medulla oblongata, and the lowest density of binding in the cerebellum. In conclusion, the present study has shown that [
3H]nociceptin binds selectively to ORL1 receptors in the rat brain and spinal cord.
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