Abstract

μ-Opioid receptors have been shown to contribute to orphanin FQ/nociceptin (OFQ/N)-mediated analgesia and hyperalgesia, indicating that both pro- and antinociceptive actions of OFQ/N are influenced by μ-opioid receptors. A 60-min activation of μ-or opioid receptor-like 1 (ORL1) opioid receptors natively expressed in BE(2)-C human neuroblastoma cells desensitized both μ- and ORL1 receptor-mediated inhibition of cAMP accumulation. The mechanism(s) of OFQ/N-mediated μ- and ORL1 cross talk involves the conventional protein kinase C isozyme, PKC-α, and G protein-coupled receptor kinases (GRKs) 2 and 3. Unlike OFQ/N-mediated desensitization of ORL1 and μ-opioid receptors, [d-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin (DAMGO)-mediated ORL1 desensitization in BE(2)-C cells is PKC-independent. However, DAMGO (1 μM) pretreatment increased membrane levels of GRK2 and GRK3, indicating their translocation to the membrane upon activation. This suggests that DAMGO activation of μ-opioid receptors results in GRK2 and GRK3 inactivation of ORL1 upon challenge with OFQ/N. Antisense, but not sense, DNA selectively targeting GRK2 or GRK3 blocks DAMGO-mediated μ- and ORL1 desensitization, respectively. However, in SH-SY5Y neuroblastoma cells, DAMGO failed to desensitize ORL1 or alter membrane PKC-α or GRK levels. Instead, DAMGO stimulated PKC-ϵ translocation to the cell membrane and produced μ-receptor desensitization. These results indicate that acute exposure to μ-receptor agonists can regulate ORL1 function, but the ability to do so varies from cell type to cell type. These results also confirm the existence of multiple signaling mechanisms for μ-opioid receptors and the importance of these mechanisms for μ-receptor-mediated-heterologous effects.