Quantitative study of [(pF)Phe4,Arg14,Lys15]nociceptin/orphanin FQ-NH2 (UFP-102) at NOP receptors in rat periaqueductal gray slices

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a novel member of the opioid receptor family with little affinity for traditional opioids. This receptor and its endogenous ligand, N/OFQ, are widely distributed in the brain and are implicated in many physiological functions including pain regulation. [(pF)Phe4,Arg14,Lys15]N/OFQ-NH2 (UFP-102) is a newly developed peptide agonist of NOP receptors. In this study, we quantitatively investigated the effect of UFP-102 at native NOP receptors of the ventrolateral periaqueductal gray (PAG), a crucial midbrain area involved in pain regulation and enriched with NOP receptors, using blind patch-clamp whole-cell recording technique in rat brain slices. UFP-102, like N/OFQ, induced an outward current in ventrolateral PAG neurons and increased the membrane current elicited by a hyperpolarization ramp from − 60 to − 140 mV. The current induced by UFP-102 was characterized with inward rectification and had a reversal potential near the equilibrium potential of K+ ions, indicating that UFP-102 activates G-protein coupled inwardly rectifying K+ channels. The effect of UFP-102 was concentration-dependent with the maximal effect similar to that of N/OFQ. The EC50 value was 11 ± 2 nM, which is 5 fold lower than that of N/OFQ. The effect of UFP-102 was not affected by naloxone while competitively antagonized by UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ-NH2), a potent NOP receptor antagonist, with a pA2 value of 6.7. These results suggest that UFP-102 is a full agonist at the postsynaptic NOP receptors of the midbrain of rats and is 5 fold more potent than N/OFQ.