Endogenous expression of opioid receptor-like receptor (ORL1), in human neuroblastoma SK-N-SH cells was demonstrated by binding with nociceptin/ orphanin FQ (N/OFQ). Scatchard analysis of [3H]N/ OFQ saturation binding data gave Kd = 1.3 +/- 0.1 nM and Bmax = 1.58 +/- 2.5 fmol/mg protein. N/OFQ stimulation increased [35S]GTP gamma S binding to cell membranes and attenuated forskolin-induced cAMP accumulation in a concentration-dependent manner. The effects of N/OFQ were eliminated by the pretreatment of pertussis toxin (PTX) but not by the antagonists of opioid receptors, revealing mediation of N/OFQ signal transduction by ORL1 receptor and PTX sensitive G protein(s). The ability of N/OFQ to inhibit cAMP production was greatly reduced after prechallenging with N/OFQ, indicating that ORL1 undergoes homologous desensitization in neuronal cells.