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

TitleInduction of delta opioid receptor function by up-regulation of membrane receptors in mouse primary afferent neurons.
Publication TypeJournal Article
Year of Publication2005
AuthorsWalwyn, W, Maidment NT, Sanders M, Evans CJ, Kieffer BL, Hales TG
JournalMolecular pharmacology
Date Published2005 Dec
KeywordsAnimals, Calcium Channels, Ganglia, Spinal, Ligands, Mice, Mice, Knockout, Neurons, Afferent, Receptors, Cell Surface, Receptors, Opioid, Receptors, Opioid, delta, Receptors, Opioid, mu, Up-Regulation

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.

Alternate JournalMol. Pharmacol.