Morphine Desensitization, Internalization, and Down-Regulation of the μ Opioid Receptor Is Facilitated by Serotonin 5-Hydroxytryptamine2A Receptor Coactivation

Analysis of the distribution of mRNA encoding the serotonin (5-hydroxytryptamine) 5-HT2A receptor and the mu opioid peptide receptor in rat brain demonstrated their coexpression in neurons in several distinct regions. These regions included the periaqueductal gray, an area that plays an important role in morphine-induced analgesia but also in the development of tolerance to morphine. To explore potential cross-regulation between these G protein-coupled receptors, the human mu opioid peptide receptor was expressed stably and constitutively in Flp-In T-REx human embryonic kidney 293 cells that harbored the human 5-HT2A receptor at the inducible Flp-In locus. In the absence of the 5-HT2A receptor, pretreatment with the enkephalin agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin but not with the alkaloid agonist morphine produced desensitization, internalization, and down-regulation of the mu opioid peptide receptor. Induction of 5-HT2A receptor expression in these cells resulted in up-regulation of mu opioid peptide receptor levels that was blocked by both a 5-HT2A receptor inverse agonist and selective inhibition of signaling via G alpha(q)/G alpha(11) G proteins. After induction of the 5-HT2A receptor, coaddition of 5-HT with morphine now also resulted in desensitization, receptor internalization, and down-regulation of the mu opioid peptide receptor. It has been argued that enhancement of mu opioid peptide receptor internalization in response to morphine would limit the development of tolerance without limiting analgesia. These data suggest that selective activation of the 5-HT2A receptor in concert with treatment with morphine might achieve this aim.