Actions of nociceptin/orphanin FQ and other prepronociceptin products on rat rostral ventromedial medulla neurons in vitro

1. Whole-cell patch clamp recordings were made from rat rostral ventromedia l medulla (RVM) neurons in vitro to investigate the cellular actions of the opioid-like receptor ORL1 (NOP), ligand nociceptin/orphanin FQ and other p utative prepronociceptin products. 2. Primary and secondary RVM neurons were identified as responding to the k appa -opioid receptor agonist U-69593 (300 nM to 1 muM) and the mu- and del ta -opioid receptor agonist met-enkephalin (10 muM), respectively. Both pri mary and secondary RVM neurons responded to nociceptin (3 nit to 1 muM) wit h an outward current that reversed polarity at -115 mV in brain slices and with inhibition of Ca2+ channel currents in acutely isolated cells. 3. The putative ORL1 antagonist J-113397 (1 muM) produced no change in memb rane current. and abolished the outward current produced by nociceptin (100 nM). In contrast, Phe(1)psi (CH2-NH)Gly(2)]-nociceptin-(1-13)NH2 (300 nM t o 1 muM) alone produced an outward current and partially reduced the outwar d current produced by nociceptin (300 nM) when co-applied. 4. In brain slices nociceptin (300 nM) reduced the amplitude of evoked GABA (A), receptor-mediated inhibitory postsynaptic currents (IPSCs) but not non -NMDA receptor-mediated excitatory postsynaptic currents (EPSCs). 5. Met-enkephalin (10 muM), but not nociceptin (300 nM), reduced the rate o f spontaneous miniature IPSCs in normal external potassium solution (K+ 2.5 mM). In high external potassium (K+ 17.5 mM), nociceptin reduced the rate of miniature IPSCs in the presence (Ca2+ 2.4 nM Mg2+ 1.2 mm) but not in the absence of external calcium (Ca2+ 0 nM, Mg2+ 10 nM, Cd2+ 10 muM). Nocicept in and met-enkephalin had no effect on the amplitude of miniature IPSCs. 6. The putative nociceptin precursor products nocistatin (rat prepronocicep tin(125 132)) and rat prepronociceptin(154-181) had no effect, on membrane currents, evoked IPSCs and evoked EPSCs. 7. These results indicate that nociceptin acts via the ORL1 receptor to dir ectly inhibit loth primary and secondary RVM neurons by activating a potass ium conductance and lip inhibiting calcium conductances. In addition, nocic eptin inhibits GABA release within the RVM via a presynaptic Ca2+-dependent mechanism. Thus, nociceptin has the potential to exert loth disinhibitory and inhibitory effects on neuronal action potential firing within the RVM.