title: 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.
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 (I(Ca)) 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 I(Ca) 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 I(Ca) current density or proportion of whole-cell HVA I(Ca) 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 I(Ca) 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.
authors.affiliation: Discipline of Pharmacology, University of Sydney, Sydney, Australia.
authors.forename: Swetha S
authors.initials: SS
authors.lastname: Murali
authors.forename: Ian A
authors.initials: IA
authors.lastname: Napier
authors.forename: Beth K
authors.initials: BK
authors.lastname: Rycroft
authors.forename: MacDonald J
authors.initials: MJ
authors.lastname: Christie
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D020864:Calcium Channels, N-Type / Q000502:physiology*; D002211:Capsaicin / Q000494:pharmacology; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D005260:Female; D005727:Ganglia, Spinal / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D018847:Opioid Peptides / Q000494:pharmacology; D037121:Plant Lectins / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011957:Receptors, Opioid / Q000819:agonists / Q000502:physiology*; D011984:Sensory Receptor Cells / Q000502:physiology*; D013116:Spinal Cord / Q000187:drug effects / Q000502:physiology*; D000094942:Nociceptin Receptor; D000097629:Nociceptin
chemical_list: D000701:Analgesics, Opioid; D020864:Calcium Channels, N-Type; D002211:Capsaicin; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D018847:Opioid Peptides; D037121:Plant Lectins; D011957:Receptors, Opioid; D000094942:Nociceptin Receptor; D000097629:Nociceptin; C045475:Griffonia simplicifolia lectins; C000722285:Oprl1 protein, rat