Morphine Desensitization, Internalization, and Down-Regulation of the μ Opioid Receptor Is Facilitated by Serotonin 5-Hydroxytryptamine_2AReceptor Coactivation

Abstract

Analysis of the distribution of mRNA encoding the serotonin (5-hydroxytryptamine) 5-HT 2A receptor and the 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 opioid peptide receptor was expressed stably and constitutively in Flp-In T-REx human embryonic kidney 293 cells that harbored the human 5-HT 2A receptor at the inducible Flp-In locus. In the absence of the 5-HT 2A 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 opioid peptide receptor. Induction of 5-HT 2A receptor expression in these cells resulted in up-regulation of opioid peptide receptor levels that was blocked by both a 5-HT 2A receptor inverse agonist and selective inhibition of signaling via G␣ q /G␣ 11 G proteins. After induction of the 5-HT 2A receptor, coaddition of 5-HT with morphine now also resulted in desensitization, receptor internalization, and down-regulation of the opioid peptide receptor. It has been argued that enhancement of 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-HT 2A receptor in concert with treatment with morphine might achieve this aim.

Opioid-induced Down-Regulation of RGS4

“…This would occur by increasing signaling via other co-expressed G␣ i/o -coupled receptors but also, given the promiscuity of RGS4 action, by increasing signaling of G␣ q -linked GPCRs, several of which, for example, CCK 2 (48) and 5-HT 2A (49), are co-expressed with MOR in certain brain regions. Moreover, because activation of the heterologously expressed 5-HT 1A receptors has been shown to cause degradation of RGS20 via proteasome degradation in COS-7 cells and Neuro2A cells (50), agonist-induced alterations in RGS protein levels may provide a more general mechanism of cellular adaptation.…”

Opioid Regulation of Mu Receptor Internalisation: Relevance to the Development of Tolerance and Dependence

“…The first demonstration of a receptor facilitating mu receptor internalisation reflected a potential role for mu opioid receptor dimerisation or oligomerisation [50]. These authors demonstrated, firstly in HEK293 cells, that the chimeric mu-delta receptor construct previously described predominantly in recycling endocytic vesicles [53]. Another recent report has described the regulation of mu receptor endocytosis and desensitisation by NK1 neurokinin receptors [54].…”

Allostery at G Protein-Coupled Receptor Homo- and Heteromers: Uncharted Pharmacological Landscapes

“…For example, activation of a serotonin 5-HT 2A receptor results in enhanced efficacy of morphine to promote internalization of a coexpressed -opioid receptor (Lopez-Gimenez et al, 2008). However, unlike ligand coregulation of internalization and recycling of coexpressed orexin OX 1 and cannabinoid CB 1 receptors, which does reflect heteromerization of these two GPCRs (Ellis et al, 2006), detailed studies demonstrated that the 5-HT 2A receptor and -opioid receptor were trafficking independently, rather than within a heteromeric complex, and that the enhanced efficacy of morphine required downstream signals generated by the 5-HT 2A receptor (Lopez-Gimenez et al, 2008).…”