Abstract

Recombinant μ and δ opioid receptors expressed in cell lines can form heterodimers with distinctive properties and trafficking. However, a role for opioid receptor heterodimerization in neurons has yet to be identified. The inhibitory coupling of opioid receptors to voltage-dependent Ca²⁺ channels (VDCCs) is a relatively inefficient process and therefore provides a sensitive assay of altered opioid receptor function and expression. We examined μ-receptor coupling to VDCCs in dorsal root ganglion neurons of δ(+/+), δ(+/-), and δ(-/-) mice. Neurons deficient in δ receptors exhibited reduced inhibition of VDCCs by morphine and [d-Ala²,Phe⁴,Gly⁵-ol]-enkephalin (DAMGO). An absence of δ receptors caused reduced efficacy of DAMGO without affecting potency. An absence of δ receptors reduced neither the density of VDCCs nor their inhibition by either the GABA_B receptor agonist baclofen or intracellular guanosine 5′-O-(3-thio)triphosphate. Flow cytometry revealed a reduction in μ-receptor surface expression in δ(-/-) neurons without altered DAMGO-induced internalization. There was no change in μ-receptor mRNA levels. d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH₂-sensitive μ-receptor-coupling efficacy was fully restored to δ(+/+) levels in δ(-/-) neurons by expression of recombinant δ receptors. However, the dimerization-deficient δ-15 construct expressed in δ(-/-) neurons failed to fully restore the inhibitory coupling of μ receptors compared with that seen in δ(+/+) neurons, suggesting that, although not essential for μ-receptor function, μ-δ receptor dimerization contributes to full μ-agonist efficacy. Because DAMGO exhibited a similar potency in δ(+/+) and δ(-/-) neurons and caused similar levels of internalization, the role for heterodimerization is probably at the level of receptor biosynthesis.