Mu-opioid-induced desensitization of opioid receptor-like 1 and mu-opioid receptors: differential intracellular signaling determines receptor sensitivity.
Mu-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 mu-opioid receptors. A 60-min activation of mu-or opioid receptor-like 1 (ORL1) opioid receptors natively expressed in BE(2)-C human neuroblastoma cells desensitized both mu- and ORL1 receptor-mediated inhibition of cAMP accumulation. The mechanism(s) of OFQ/N-mediated mu and ORL1 cross talk involves the conventional protein kinase C isozyme, PKC-alpha, and G protein-coupled receptor kinases (GRKs) 2 and 3. Unlike OFQ/N-mediated desensitization of ORL1 and mu-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 microM) pretreatment increased membrane levels of GRK2 and GRK3, indicating their translocation to the membrane upon activation. This suggests that DAMGO activation of mu-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 mu- and ORL1 desensitization, respectively. However, in SH-SY5Y neuroblastoma cells, DAMGO failed to desensitize ORL1 or alter membrane PKC-alpha or GRK levels. Instead, DAMGO stimulated PKC-epsilon translocation to the cell membrane and produced micro-receptor desensitization. These results indicate that acute exposure to mu-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 mu-opioid receptors and the importance of these mechanisms for mu-receptor-mediated-heterologous effects.
Mandyam
Chitra D
CD
Department of Pharmacological and Pharmaceutical Sciences, University of Houston, Houston, TX 77204-5037, USA.
Thakker
Deepak R
DR
Standifer
Kelly M
KM
D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology*; D006801:Humans; D018847:Opioid Peptides / Q000494:pharmacology; D020239:Receptor Cross-Talk / Q000502:physiology; D011957:Receptors, Opioid / Q000378:metabolism*; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*; D014407:Tumor Cells, Cultured; D000094942:Nociceptin Receptor
D018847:Opioid Peptides; D011957:Receptors, Opioid; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D000094942:Nociceptin Receptor; C000722283:OPRL1 protein, human