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

Abstract

1. Whole-cell patch clamp recordings were made from rat rostral ventromedial medulla (RVM) neurons in vitro to investigate the cellular actions of the opioid-like receptor ORL1 (NOP), ligand nociceptin/orphanin FQ and other putative prepronociceptin products.

2. Primary and secondary RVM neurons were identified as responding to the κ-opioid receptor agonist U-69593 (300 nm to 1 μm) and the μ- and δ-opioid receptor agonist met-enkephalin (10 μm), respectively. Both primary and secondary RVM neurons responded to nociceptin (3 nm to 1 μm) with 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 μm) produced no change in membrane current and abolished the outward current produced by nociceptin (100 nm). In contrast, Phe¹ψ(CH₂-NH)Gly²]-nociceptin-(1-13)NH₂ (300 nm to 1 μm) alone produced an outward current and partially reduced the outward 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 μm), but not nociceptin (300 nm), reduced the rate of 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 mm, Mg²⁺ 1.2 mm) but not in the absence of external calcium (Ca²⁺ 0 mm, Mg²⁺ 10 mm, Cd²⁺ 10 μm). Nociceptin and met-enkephalin had no effect on the amplitude of miniature IPSCs.

6. The putative nociceptin precursor products nocistatin (rat prepronociceptin_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 directly inhibit both primary and secondary RVM neurons by activating a potassium conductance and by inhibiting calcium conductances. In addition, nociceptin inhibits GABA release within the RVM via a presynaptic Ca²⁺-dependent mechanism. Thus, nociceptin has the potential to exert both disinhibitory and inhibitory effects on neuronal action potential firing within the RVM.