Modulation of Silent and Constitutively Active Nociceptin/Orphanin FQ Receptors by Potent Receptor Antagonists and Na+ Ions in Rat Sympathetic Neurons

The pharmacology of G protein-coupled receptors can be influenced by factors such as constitutive receptor activation and Na(+) ions. In this study, we examined the coupling of natively and heterologously expressed nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors with voltage-dependent Ca(2+) channels following exposure to four high affinity NOP receptor blockers (UFP-101, Trap-101, Comp 24 and JTC-801) in sympathetic neurons. The enhanced tonic inhibition of Ca(2+) currents in the absence of agonists, indicative of constitutively active NOP receptors in transfected neurons, was abolished following pretreatment with pertussis toxin. In control neurons, the four antagonists did not exert any effects when applied alone, but significantly blocked the N/OFQ-mediated Ca(2+) current inhibition. Exposure of transfected neurons to UFP-101 resulted in partial agonist effects. In contrast, Trap-101, Comp 24 and JTC-801 exerted inverse agonism as measured by the loss of tonic Ca(2+) current inhibition. In experiments designed to measure the N/OFQ concentration-response relationship under varying Na(+) concentrations, a leftward shift of IC(50) values was observed after Na(+) exposure. Although similar N/OFQ efficacies were measured with all solutions, a significant decrease of Hill coefficient values was obtained with increasing Na(+) concentrations. Examination of the allosteric effects of Na(+) on heterologously overexpressed NOP receptors showed that the tonic Ca(2+) current inhibition was abolished in the presence of the monovalent cation. These results demonstrate that constitutively active NOP receptors exhibit differential blocker pharmacology and allosteric regulation by Na(+). Data are also presented demonstrating that heterologously expressed mu opioid receptors in sympathetic neurons are similarly modulated.