μ Opioid Receptor Phosphorylation, Desensitization, and Ligand Efficacy

Abstract

Opioid receptors are G-protein coupled receptors that mediate the potent analgesic actions and addictive properties of morphine-derived compounds. Under physiological conditions, these receptors interact with endogenous opioid peptides to modulate pain-controlling pathways and circuits that modulate behaviors including mood and reward (2). opioid receptors interact with rapidly acting opioid drugs, such as heroin, to produce marked euphoria and behavioral reward. They are also primary targets of the slower and longer acting opioids, such as methadone and LAAM, 1 that represent the best current substitution therapeutics for opiate addiction (3).receptors desensitize after repeated stimulation by opioid agonists, in fashions that display similarities to desensitizing events noted for other G-protein coupled receptors. Agonistinduced receptor desensitization can be correlated with receptor phosphorylation. receptors display naloxone-reversible phosphorylation and desensitize after morphine or DAMGO treatments (1). Both of these agonist-induced events are insensitive to pretreatments with the protein kinase C inhibitor staurosporine, which inhibits phorbol ester-induced receptor phosphorylation and desensitization.Many opiates and opioid ligands can recognize receptors with high affinities. Plant-derived alkaloids, synthetic compounds of several classes, and endogenous opioid peptides can function as agonists, partial agonists, or antagonists with a range of abilities to induce or block induction of analgesia and euphoria. However, mutagenesis studies and studies with receptor chimeras support the idea that different receptor features could be involved in recognition of these different ligand classes (4 -6). Mutations that change naloxone from a full antagonist to a partial agonist can leave the intrinsic activity of opioid peptides unchanged, for example see Claude et al. (7). These differences raise the possibility that receptor occupancies by opioid drugs of different classes could alter the conformation of the receptor in distinct fashions. Some of these differences could render the receptor an improved or a worse substrate for kinases and phosphatases and thus directly confer different susceptibilities to phosphorylation or dephosphorylation. Alternatively, selective activation of different G-protein classes by different agonists could trigger different receptor phosphorylation and desensitization events.The current study thus investigates the effects of opioid ligands of various classes and varying intrinsic activities on receptor activation, phosphorylation, and desensitization using human receptors expressed in CHO cells (hCHO), and the receptors coexpressed in Xenopus oocytes with a G-protein linked K ϩ channel. The results support striking parallels between opioid efficacies in opening ion channels and inhibiting

AP-1 activity is negatively regulated by cannabinol through inhibition of its protein components, c-fos and c-jun

“…early gene expression after treatment with cannabinoid agonists, which was attributed to cannabinoid-induced activation of MAP kinase activity [24,25]. This result; however, is in direct opposition to our results in which we demonstrated an inhibition of ERK activation after cellular activation for 240 min.…”

“…This resulted in cells that expressed approximately 152,000 CB2 receptors/cell [24]; whereas determination of number of total receptors on spleen cells was approximately 1000 receptors/cell [14]. Second, MAP kinase activity was measured after treatment with CP-55940, a cannabinoid agonist, at various intervals up to 30 min in CHO cells transfected with either the central CB1 receptor [25] or the peripheral CB2 receptor [24]. MAP kinase activity in cells transfected with both CB1 and CB2 peaked at 10 min, with indications of the activity decreasing with further agonist treatment.…”

Phosphorylation of Ser363, Thr370, and Ser375 Residues within the Carboxyl Tail Differentially Regulates μ-Opioid Receptor Internalization

“…desensitization, internalization, and down-regulation, molecular events related to receptor phosphorylation/regulation are still poorly understood. In addition, differential phosphorylation/regulation of opioid receptors by opioid agonists has been observed (6,11,14,33). These reports suggest that different agonist-activated receptor conformations may exist and that specific phosphorylation site(s) may participate in regulating receptor activity and trafficking.…”

“…Interestingly, in addition to the subtype-specific regulation of opioid receptors (9 -12), individual opioid receptors are differentially regulated by distinct opioid agonists (5-8, 13, 14). In the case of MOR, opioid agonists demonstrating equivalent ability to activate receptor signaling exhibit remarkable differences in their abilities to functionally desensitize (5,6) and induce internalization of the receptor in both transfected cells and neurons (7,8,(13)(14)(15). However, the detailed molecular events underlying this differential regulation of MOR by distinct agonists remain unclear.…”

Primary Afferent NMDA Receptors Increase Dorsal Horn Excitation and Mediate Opiate Tolerance in Neonatal Rats

“…The mechanisms mediating opiate tolerance are multifaceted, including both pharmacological and physiological adaptations (Hsu and Wong, 2000;Liu and Anand, 2001). For example, like other G-protein-coupled receptors, opiate receptors reconstituted in expression systems show a reduction in agonist effect with prolonged agonist exposure with phosphorylation by G-protein-coupled receptor kinases (GRKs) (Ferguson et al, 1996;Kovoor et al, 1997;Yu et al, 1997;Lowe et al, 2002). We recently have implicated GRKs in mediating tolerance to the behavioral and electrophysiological effects of the highly efficacious opiate fentanyl (Terman et al, 2004), but not to the same effects of another opiate, morphine (cf.…”