Received for publication May 16, 2005.
Revised August 9, 2005.
Accepted for publication August 31, 2005.

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

Abstract

It is not clear whether primary afferent neurons express functional cell surface opioid receptors. We examined receptor coupling to Ca²⁺ channels in mouse dorsal root ganglion neurons under basal conditions and after receptor up-regulation. DAMGO, DADLE, U-50,488H (1 µM), and baclofen (50 µM) inhibited Ca²⁺ currents, the -selective ligands DPDPE and deltorphin II (1 µM) did not. The effect of DADLE (1 µM) was blocked by the µ-antagonist CTAP (300 nM) but not by the -antagonist TIPP (300 nM) implicating µ receptors. Despite a lack of functional receptors, flow cytometry revealed cell surface receptors. We used this approach to identify conditions that up-regulate receptors including µ receptor gene deletion in dorsal root ganglion neurons of µ -/- mice and 18 h incubation of µ +/+ neurons with CTAP followed by brief (10 min) DPDPE exposure. Under these conditions the expression of cell surface receptors was up-regulated by 149 ± 9% and 139 ± 5%, respectively, furthermore DPDPE and deltorphin II (1 µM) inhibited Ca²⁺ currents in both cases. Viral replacement of µ receptors in µ -/- neurons reduced receptor expression to µ +/+ 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 µ receptors predominate. However up-regulation of cell surface receptors induces their coupling to Ca²⁺ channels. Pharmacological approaches that increase functional receptor expression may reveal a novel target for analgesic therapy.

Key words: GABAB, Opioid, Gi family, G protein regulation, Receptor synthesis/trafficking, Opioids

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