Opioid-related (ORL1) receptors are enriched in a subpopulation of sensory neurons and prolonged activation produces no functional loss of surface N-type calcium channels.

Key points  •  The nociceptin/ORL1 receptor neuropeptide system is related to opioid systems and thought to be involved in pain modulation. •  A major mechanism of action of this system is inhibition of calcium channels that control excitability of sensory nerves. •  To understand the potential for drugs acting on this system to modulate pain it is important to identify the types of sensory nerve cells functionally expressing the ORL1 receptor and how they are modulated. •  Here we identified a subpopulation of small, presumably pain sensing sensory nerves that are highly responsive to this neuropeptide both in their cell bodies and nerve terminals. •  We then established that nociceptin/ORL1 stimulation inhibits calcium channels only while the peptide is present on the cells or their nerve terminals but does not produce long-term down-regulation of calcium channel function as had been previously proposed. Abstract  The opioid-related receptor, ORL1, is activated by the neuropeptide nociceptin/orphanin FQ (N/OFQ) and inhibits high-voltage-activated (HVA) calcium channel currents (ICa) via a G-protein-coupled mechanism. Endocytosis of ORL1 receptor during prolonged N/OFQ exposure was proposed to cause N-type voltage-gated calcium channel (VGCC) internalization via physical interaction between ORL1 and the N-type channel. However, there is no direct electrophysiological evidence for this mechanism in dorsal root ganglion (DRG) neurons or their central nerve terminals. The present study tested this using whole-cell patch-clamp recordings of HVA ICa in rat DRG neurons and primary afferent excitatory synaptic currents (eEPSCs) in spinal cord slices. DRG neurons were classified on the basis of diameter, isolectin-B4 (IB4) binding and responses to capsaicin, N/OFQ and a μ-opioid agonist, DAMGO. IB4-negative neurons less than 20 μm diameter were selectively responsive to N/OFQ as well as DAMGO. In these neurons, ORL1 desensitization by a supramaximal concentration of N/OFQ was not followed by a decrease in HVA ICa current density or proportion of whole-cell HVA ICa contributed by N-type VGCC as determined using the N-type channel selective blocker, ω-conotoxin CVID. There was also no decrease in the proportion of N-type ICa when neurons were incubated at 37°C with N/OFQ for 30 min prior to recording. In spinal cord slices, N/OFQ consistently inhibited eEPSCs onto dorsal horn neurons. As observed in DRG neurons, preincubation of slices in N/OFQ for 30 min produced no decrease in the proportion of eEPSCs inhibited by CVID. In conclusion, no internalization of the N-type VGCC occurs in either the soma or central nerve terminals of DRG neurons following prolonged exposure to high, desensitizing concentrations of N/OFQ.