Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor, the fourth member of opioid receptor family, shares 60-70% homology with traditional opioid receptors but displays little affinity for opioids. This receptor was implicated in many neurological functions and its functional heterogeneity has been proposed. Therefore, it is imperative to develop and characterize new ligands for NOP receptors. 1-Benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl]pyrrolidine-2-carboxamide (Compound 24) is a new non-peptide ligand of NOP receptor having antagonistic actions in cloned and peripheral NOP receptors. In this study, we quantitatively characterized its effect on the native NOP receptors in the midbrain slices containing ventrolateral periaqueductal gray (vlPAG), a region with dense NOP receptors and involved in pain regulation. In vlPAG neurons, N/OFQ induced G-protein-coupled inwardly rectifying potassium (GIRK) current through NOP receptors. Compound 24, at 0.3-10 microM, attenuated N/OFQ-induced GIRK current concentration-dependently. The antagonistic potency of Compound 24 in vlPAG neurons (IC(50): 2.6+/-0.6 microM) was, however, lower than that obtained in mouse vas deferens preparations or expressed human NOP receptors. The action kinetic of Compound 24 was slower than [Nphe(1), Arg(14), Lys(15)]N/OFQ-NH(2) (UFP-101), a peptide antagonist, in the same preparation. Compound 24 had no intrinsic agonistic activity at NOP receptors at the concentration up to 10 microM. However, at concentrations higher than 3 microM, it also attenuated the GIRK current induced by [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin, a mu-opioid receptor agonist. It is concluded that Compound 24 acts as a pure antagonist at the native NOP receptors in the vlPAG with moderate potency and selectivity.