Induction of delta opioid receptor function by up-regulation of membrane receptors in mouse primary afferent neurons

• Published in: Molecular pharmacology Vol. 68; no. 6; pp. 1688 - 1698
• Main Authors: Walwyn, Wendy, Maidment, Nigel T, Sanders, Matthew, Evans, Christopher J, Kieffer, Brigitte L, Hales, Tim G
• Format: Journal Article
• Language: English
• Published: United States American Society for Pharmacology and Experimental Therapeutics 01.12.2005
• Subjects: Animals; Biochemistry, Molecular Biology; Calcium Channels; Calcium Channels - metabolism; Ganglia, Spinal; Ganglia, Spinal - cytology; Life Sciences; Ligands; Mice; Mice, Knockout; Molecular biology; Neurons, Afferent; Neurons, Afferent - metabolism; Receptors, Cell Surface; Receptors, Cell Surface - analysis; Receptors, Cell Surface - genetics; Receptors, Opioid; Receptors, Opioid - analysis; Receptors, Opioid - genetics; Receptors, Opioid, delta; Receptors, Opioid, delta - analysis; Receptors, Opioid, delta - genetics; Receptors, Opioid, delta - physiology; Receptors, Opioid, mu; Receptors, Opioid, mu - analysis; Receptors, Opioid, mu - metabolism; Up-Regulation
• ISSN: 0026-895X; 1521-0111
• DOI: 10.1124/mol.105.014829

It is not clear whether primary afferent neurons express functional cell-surface opioid receptors. We examined delta receptor coupling to Ca2+ channels in mouse dorsal root ganglion neurons under basal conditions and after receptor up-regulation. [D-Ala2,Phe4,Gly5-ol]-enkephalin (DAMGO), [D-Ala2,D-Leu5]-enkephalin (DADLE), trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) benzene-acetamide methanesulfonate (U-50,488H; 1 microM), and baclofen (50 microM) inhibited Ca2+ currents, whereas the -selective ligands [D-Pen2,Pen5]-enkephalin (DPDPE) and deltorphin II (1 microM) did not. The effect of DADLE (1 microM) was blocked by the mu-antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 300 nM) but not by the -antagonist Tyr-1,2,3,4-tetrahydroisoquinoline-Phe-Phe-OH (300 nM), implicating mu receptors. Despite a lack of functional delta receptors, flow cytometry revealed cell-surface receptors. We used this approach to identify conditions that up-regulate receptors, including mu receptor gene deletion in dorsal root ganglion neurons of mu-/- mice and 18-h incubation of mu+/+ neurons with CTAP followed by brief (10-min) DPDPE exposure. Under these conditions, the expression of cell-surface delta receptors was up-regulated to 149 +/- 9 and 139 +/- 5%, respectively; furthermore, DPDPE and deltorphin II (1 microM) inhibited Ca2+ currents in both cases. Viral replacement of mu receptors in mu-/- neurons reduced delta receptor expression to mu+/+ levels, restored the inhibition of Ca2+ currents by DAMGO, and abolished receptor coupling. Our observations suggest that receptor-Ca2+ channel coupling in primary afferent fibers may have little functional significance under basal conditions in which mu receptors predominate. However, up-regulation of cell-surface delta receptors induces their coupling to Ca2+ channels. Pharmacological approaches that increase functional delta receptor expression may reveal a novel target for analgesic therapy.