In vitro and ex vivo effects of a selective nociceptin/orphanin FQ (N/OFQ) peptide receptor antagonist, CompB, on specific binding of [ 3 H]N/OFQ and [ 35 S]GTP γ S in rat brain and spinal cord

A novel selective nociceptin/orphanin FQ (N/OFQ) peptide receptor antagonist, 1‐[(3 R ,4 R )‐1‐cyclooctylmethyl]‐3‐hydroxymethyl‐4‐piperidyl)‐3‐ethyl‐1,3‐dihydro‐2 H ‐benzimidazol‐2‐one (CompB), inhibited specific binding of [ 3 H]N/OFQ to crude membranes from the rat brain and spinal cord in a concentration‐dependent manner and their K i values were 7.11 and 4.02 n M , respectively. Rosenthal analysis indicated that there was a significant increase in the K d value for [ 3 H]N/OFQ binding in the brain and spinal cord in the presence of CompB (10 n M ). There was a dose‐dependent increase in K d values for [ 3 H]N/OFQ binding in the brain and spinal cord following i.v. injection of CompB at relatively low doses (0.69–6.88 μ mol kg −1 ), compared with the control values. In the spinal cord, enhancement with each dose was constantly greater and the duration of enhancement (6.88 μ mol kg −1 ) was significantly longer. The degree of increase in K d values for [ 3 H]N/OFQ binding after i.v. injection of CompB (6.88 μ mol kg −1 ) was significantly larger in the lumbar region of the spinal cord compared to other regions. CompB (0.1, 0.3 μ M ) shifted the concentration–effect curves of N/OFQ‐stimulated [ 35 S]GTP γ S binding in the brain and spinal cord to the right. The i.v. injection of CompB (6.88 μ mol kg −1 ) significantly suppressed the N/OFQ‐stimulated [ 35 S]GTP γ S binding in the rat spinal cord and shifted the concentration–effect curve to the right, while it produced little inhibitory effect in the brain. The present study has shown that CompB may exhibit pharmacological effects through a predominant blockade of N/OFQ peptide receptors in the spinal cord under in vivo conditions.British Journal of Pharmacology (2003) 139 , 1462–1468. doi: 10.1038/sj.bjp.0705371