Agonist-selective endocytosis of mu opioid receptor by neurons in vivo

Phospholipase D2 modulates agonist-induced µ-opioid receptor desensitization and resensitization

Receptor phosphorylation, arrestin binding, uncoupling from G protein and subsequent endocytosis have been implicated in G protein-coupled receptor desensitization after chronic agonist exposure. In search of proteins regulating the mu-opioid receptor endocytosis, we have recently established that activation of phospholipase D (PLD)2 is required for agonist-induced mu-opioid receptor endocytosis. In this study, we determined the effect of PLD2 activity on the desensitization and resensitization rate of the mu-opioid receptor. We clearly demonstrated that inhibition of PLD2-mediated phosphatidic acid formation by alcohol (1-butanol or ethanol) or overexpression of a dominant negative mutant of PLD2 prevented agonist-mediated endocytosis and resulted in a faster desensitization rate of the mu-opioid receptor after chronic (D-Ala2, Me Phe4, Glyol5)enkephalin treatment in human embryonic kidney 293 cells. Moreover, inhibition of PLD2 activity led to an impairment of the resensitization rate of the mu-opioid receptor. In summary, our data strongly suggest that PLD2 is a modulator of agonist-induced endocytosis, desensitization and resensitization of the mu-opioid receptor.

Ligand-induced mu opioid receptor endocytosis and recycling in enteric neurons

Immunohistochemistry and confocal microscopy were used to investigate endocytosis and recycling of the native mu opioid receptor (muOR) in enteric neurons. Isolated segments of the guinea-pig ileum were exposed to increasing concentrations of muOR agonists at 4 degrees C to allow ligand binding and warming to 37 degrees C for 0 min (baseline) to 6 h in ligand-free medium to allow receptor internalization and recycling. The endogenous ligand, [Met]enkephalin, and [D-Ala(2),MePhe(4),Gly-ol(5)] enkephalin (DAMGO), an opioid analog, and the alkaloids, etorphine and fentanyl, induced rapid internalization of muOR immunoreactivity in enteric neurons, whereas morphine did not. muOR internalization was prevented by muOR antagonists. Basal levels of muOR immunoreactivity in the cytoplasm were 10.52+/-2.05%. DAMGO (1 nM-100 microM) induced a concentration-dependent increase of muOR immunofluorescence density in the cytoplasm to a maximum of 84.37+/-2.26%. Translocation of muOR immunoreactivity in the cytoplasm was detected at 2 min, reached the maximum at 15-30 min, remained at similar levels for 2 h, began decreasing at 4 h, and was at baseline values at 6 h. A second exposure to DAMGO (100 nM) following recovery of internalized muOR immunoreactivity at the cell surface induced a translocation of muOR immunoreactivity in the cytoplasm comparable to the one observed following the first exposure (46.89+/-3.11% versus 43.31+/-3.80%). muOR internalization was prevented by hyperosmolar sucrose, phenylarsine oxide or potassium depletion, which inhibit clathrin-mediated endocytosis. muOR recycling was prevented by pre-treatment with bafilomycin A1, an acidotropic agent that inhibits endosomal acidification, but not by the protein synthesis inhibitor, cycloheximide. This study shows that native muOR in enteric neurons undergoes ligand-selective endocytosis, which is primarily clathrin-mediated, and recycles following endosomal acidification. Following recycling, muOR is activated and internalized by DAMGO indicating that recycled receptors are functional.

Morphine acutely regulates opioid receptor trafficking selectively in dendrites of nucleus accumbens neurons

Morphine stimulates the internalization of mu-opioid receptors (MORs) in transfected cell models to a lesser degree than opioid peptides and other analgesic drugs, such as methadone, and previous studies have reported that morphine does not produce a detectable redistribution of MORs in neural tissue after either acute or chronic administration. Nevertheless, morphine produces profound physiological effects, raising the question of whether receptor trafficking plays any role in the in vivo actions of morphine. We investigated the effects of opiate drugs on recombinant and native opioid receptors in the nucleus accumbens, which plays an important role in mediating the behavioral effects of opiate drugs. Morphine and methadone differed in their effects on the internalization of epitope-tagged MORs in cell bodies, introduced by viral gene transfer and imaged by fluorescence microscopy. A mutation of the cytoplasmic tail that confers morphine-induced internalization in cultured cells had a similar effect on receptor trafficking in nucleus accumbens cell bodies. Surprisingly, in contrast to its failure to affect MOR distribution detectably in cell bodies, acute morphine administration produced a pronounced change in MOR distribution visualized in the processes of the same neurons. A similar effect of acute morphine administration was observed for endogenously expressed MORs by immunoelectron microscopy; the acute administration of morphine increased the density of MORs associated with internal membrane structures specifically in dendrites. These results provide the first evidence that morphine regulates the distribution of MORs in neuronal processes, suggesting that "compartment-selective" membrane trafficking represents a previously unanticipated type of opioid receptor regulation contributing to the in vivo effects of opiate drugs on a physiologically relevant population of CNS neurons.

Anti-inflammatory properties of the mu opioid receptor support its use in the treatment of colon inflammation

The physiologic role of the mu opioid receptor (MOR) in gut nociception, motility, and secretion is well established. To evaluate whether MOR may also be involved in controlling gut inflammation, we first showed that subcutaneous administration of selective peripheral MOR agonists, named DALDA and DAMGO, significantly reduces inflammation in two experimental models of colitis induced by administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) or peripheral expansion of CD4(+) T cells in mice. This therapeutic effect was almost completely abolished by concomitant administration of the opioid antagonist naloxone. Evidence of a genetic role for MOR in the control of gut inflammation was provided by showing that MOR-deficient mice were highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after TNBS administration. The mechanistic basis of these observations suggests that the anti-inflammatory effects of MOR in the colon are mediated through the regulation of cytokine production and T cell proliferation, two important immunologic events required for the development of colon inflammation in mice and patients with inflammatory bowel disease (IBD). These data provide evidence that MOR plays a role in the control of gut inflammation and suggest that MOR agonists might be new therapeutic molecules in IBD.

Regulated endocytosis of opioid receptors: cellular mechanisms and proposed roles in physiological adaptation to opiate drugs

Opiate drugs such as morphine and heroin are among the most effective analgesics known. Prolonged or repeated administration of opiates produces adaptive changes in the nervous system that lead to reduced drug potency or efficacy (tolerance), as well as physiological withdrawal symptoms and behavioral manifestations such as craving when drug use is terminated (dependence). These adaptations limit the therapeutic utility of opiate drugs, particularly in the treatment of chronically painful conditions, and are thought to contribute to the highly addictive nature of opiates. For many years it has been proposed that physiological tolerance to opiate drugs is associated with a modification of the number or functional activity of opioid receptors in specific neurons. We now understand certain mechanisms of opioid receptor desensitization and endocytosis in considerable detail. However, the functional roles that these mechanisms play in the complex physiological adaptation of the intact nervous system to opiates are only beginning to be explored.

Internalization and recycling of human mu opioid receptors expressed in Sf9 insect cells

Internalization and recycling of G protein-coupled receptors (GPCRs), such as the mu-opioid receptor, largely depend on agonist stimulation. Agonist-promoted internalization of some GPCRs has been shown to mediate receptor desensitization, resensitization, and down-regulation. In this study, we investigated whether different mu opioid agonists displayed different effects in receptor internalization and recycling, the potential mechanisms involved in ohmefentanyl-induced internalization process. In transfected Sf9 insect cells expressing 6His-tagged wild type mu opioid receptor, exposure to 100 nM ohmefentanyl caused a maximum internalization of the receptor at 30 min and receptors seemed to reappear at the cell membrane after 60 min as determined by radioligand binding assay. Ohmefentanyl-induced human mu opioid receptor internalization was concentration-dependent, with about 40% of the receptors internalized following a 30-min exposure to 1 microM ohmefentanyl. 10 microM morphine and 1 microM DAMGO could also induce about 40% internalization. The antagonist naloxone and pretreatment with pertussis toxin both blocked ohmefentanyl-induced internalization without affecting internalization themselves. Incubation with sucrose 0.45 M significantly inhibited ohmefentanyl-induced internalization of the mu receptor. The removal of agonists ohmefentanyl and morphine resulted in the receptors gradually returning to the cell surface over a 60 min period, while the removal of agonist DAMGO only partly resulted in the receptor recycling. The results of this study suggest that ohmefentanyl-induced internalization of human mu opioid receptor in Sf9 insect cells occurs via Gi/o protein-dependent process that likely involves clathrin-coated pits. In addition, the recycling process displays the differential modes of action of different agonists.

Opioid agonists have different efficacy profiles for G protein activation, rapid desensitization, and endocytosis of mu-opioid receptors

The differential ability of various mu-opioid receptor (MOP) agonists to induce rapid receptor desensitization and endocytosis of MOP could arise simply from differences in their efficacy to activate G proteins or, alternatively, be due to differential capacity for activation of other signaling processes. We used AtT20 cells stably expressing a low density of FLAG-tagged MOP to compare the efficacies of a range of agonists to 1) activate G proteins using inhibition of calcium channel currents (ICa) as a reporter before and after inactivation of a fraction of receptors by beta-chlornaltrexamine, 2) produce rapid, homologous desensitization of ICa inhibition, and 3) internalize receptors. Relative efficacies determined for G protein coupling were [Tyr-D-Ala-Gly-MePhe-Glyol]enkephalin (DAMGO) (1) > or = methadone (0.98) > morphine (0.58) > pentazocine (0.15). The same rank order of efficacies for rapid desensitization of MOP was observed, but greater concentrations of agonist were required than for G protein activation. By contrast, relative efficacies for promoting endocytosis of MOP were DAMGO (1) > methadone (0.59) >> morphine (0.07) > or = pentazocine (0.03). These results indicate that the efficacy of opioids to produce activation of G proteins and rapid desensitization is distinct from their capacity to internalize mu-opioid receptors but that, contrary to some previous reports, morphine can produce rapid, homologous desensitization of MOP.

Morphine acutely regulates opioid receptor trafficking selectively in dendrites of nucleus accumbens neurons

Morphine stimulates the internalization of mu-opioid receptors (MORs) in transfected cell models to a lesser degree than opioid peptides and other analgesic drugs, such as methadone, and previous studies have reported that morphine does not produce a detectable redistribution of MORs in neural tissue after either acute or chronic administration. Nevertheless, morphine produces profound physiological effects, raising the question of whether receptor trafficking plays any role in the in vivo actions of morphine. We investigated the effects of opiate drugs on recombinant and native opioid receptors in the nucleus accumbens, which plays an important role in mediating the behavioral effects of opiate drugs. Morphine and methadone differed in their effects on the internalization of epitope-tagged MORs in cell bodies, introduced by viral gene transfer and imaged by fluorescence microscopy. A mutation of the cytoplasmic tail that confers morphine-induced internalization in cultured cells had a similar effect on receptor trafficking in nucleus accumbens cell bodies. Surprisingly, in contrast to its failure to affect MOR distribution detectably in cell bodies, acute morphine administration produced a pronounced change in MOR distribution visualized in the processes of the same neurons. A similar effect of acute morphine administration was observed for endogenously expressed MORs by immunoelectron microscopy; the acute administration of morphine increased the density of MORs associated with internal membrane structures specifically in dendrites. These results provide the first evidence that morphine regulates the distribution of MORs in neuronal processes, suggesting that "compartment-selective" membrane trafficking represents a previously unanticipated type of opioid receptor regulation contributing to the in vivo effects of opiate drugs on a physiologically relevant population of CNS neurons.

Dramatic depletion of cell surface m2 muscarinic receptor due to limited delivery from intracytoplasmic stores in neurons of acetylcholinesterase-deficient mice

We have studied the consequences of the constitutive acetylcholinesterase (AChE) deficiency in knockout mice for the AChE gene on the subcellular localization of the m2 receptor (m2R) and the regulation of its intraneuronal compartmentalization by the cholinergic environment, using immunohistochemistry at light and electron microscopic levels. (1) In AChE +/+ mice in vivo, m2R is mainly located at the neuronal membrane in striatum, hippocampus, and cortex. In AChE -/- mice, m2R is almost absent at the membrane but is accumulated in the endoplasmic reticulum and Golgi complex. (2) In vivo and in vitro (organotypic culture) dynamic studies demonstrate that the balance between membrane and intracytoplasmic m2R can be regulated by the cholinergic influence: In AChE -/- mice, m2R is translocated from the cytoplasm to the cell surface after (1) blockade of muscarinic receptors by atropine, (2) supplementation of AChE -/- neurons with AChE in vitro, and (3) disruption of the cortical and hippocampal cholinergic afferents in vitro. Our results suggest that the neurochemical environment may contribute to the control of the abundance and availability of cell surface receptors, and consequently to the control of neuronal sensitivity to neurotransmitters or drugs, by regulating their delivery from the endoplasmic reticulum and Golgi complex.

Agonist-dependent trafficking of alpha2-adrenoceptor subtypes: dependence on receptor subtype and employed agonist

Many G protein-coupled receptors (GPCRs) are internalized from the plasma membrane after agonist exposure. Previously, marked agonist-induced internalization of human alpha2A- and alpha2B-adrenergic receptors (AR) was observed in transfected neuronal rat pheochromocytoma (PC12) cells; alpha2A- and alpha2B-AR were internalized into partly distinct intracellular vesicles (Olli-Lähdesmäki et al., J. Neurosci. 19, 9281-9288, 1999). In this paper, the extent of alpha2-AR internalization was quantitated in human embryonic kidney (HEK-293) and PC12 cells by combined application of cell surface biotinylation and ELISA methods, which allow measurement of protein trafficking in intact, differentiated and undifferentiated cells. Significant subtype-specific (but not cell type-dependent) trafficking of human alpha2-AR was observed by quantitation and immunocytochemistry. Agonist-induced sequestration of alpha2B-AR was markedly reduced after blocking the formation of clathrin-coated vesicles by hyperosmotic sucrose pretreatment. The sequestration of alpha2A-AR was partly inhibited after sucrose pretreatment but could be further reduced after inhibiting the formation of both clathrin-coated and caveolin vesicles by combined pretreatment with hyperosmotic sucrose and filipin. Differences were also observed in the recycling of alpha2A- and alpha2B-AR. The extent of maximal agonist-induced sequestration in PC12 cells was not directly dependent on relative agonist efficacy.

Differential regulation of the human kappa opioid receptor by agonists: etorphine and levorphanol reduced dynorphin A- and U50,488H-induced internalization and phosphorylation

We previously observed that (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50,488H) promoted internalization and phosphorylation of the FLAG-tagged human kappa opioid receptor (FLAG-hkor) stably expressed in Chinese hamster ovary (CHO) cells. In this study, we compared regulation of the FLAG-hkor expressed in CHO cells by U50,488H, dynorphin A, etorphine, and levorphanol, which were potent full agonists as determined by stimulation of guanosine 5'-O-(3-[(35)S]thio)triphosphate binding. Using fluorescence flow cytometry, we found that dynorphin A(1-17), like U50,488H, promoted internalization of the FLAG-hkor in a time- and dose-dependent manner. The antagonists naloxone and norbinaltorphimine, having no effect on FLAG-hkor internalization, effectively blocked dynorphin A(1-17)- and U50,488H-induced internalization. Interestingly, the full agonists etorphine and levorphanol did not cause internalization of the FLAG-hkor but significantly reduced dynorphin A(1-17)- and U50,488H-induced internalization in a dose-dependent manner. Immunofluorescence staining of FLAG-hkor yielded similar results. Dynorphin A(1-17) and U50,488H enhanced phosphorylation of FLAG-hkor to a greater extent than etorphine, but levorphanol did not increase FLAG-hkor phosphorylation. Etorphine or levorphanol decreased dynorphin- or U50,488H-induced phosphorylation. It is likely that conformations of the hkor required for phosphorylation and initiation of internalization are different from those for activation of G proteins. We also examined whether the four agonists had differential effects on superactivation of adenylate cyclase. Pretreatment with U50,488H, dynorphin A(1-17), or etorphine enhanced forskolin-stimulated adenylate cyclase activity to approximately 200 to 250% of the control, whereas levorphanol pretreatment did not result in significant adenylate cyclase superactivation. Thus, the degree of superactivation caused by an agonist is unrelated to its ability to promote internalization of the hkor.

Anti-inflammatory properties of the mu opioid receptor support its use in the treatment of colon inflammation

The physiologic role of the mu opioid receptor (MOR) in gut nociception, motility, and secretion is well established. To evaluate whether MOR may also be involved in controlling gut inflammation, we first showed that subcutaneous administration of selective peripheral MOR agonists, named DALDA and DAMGO, significantly reduces inflammation in two experimental models of colitis induced by administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) or peripheral expansion of CD4(+) T cells in mice. This therapeutic effect was almost completely abolished by concomitant administration of the opioid antagonist naloxone. Evidence of a genetic role for MOR in the control of gut inflammation was provided by showing that MOR-deficient mice were highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after TNBS administration. The mechanistic basis of these observations suggests that the anti-inflammatory effects of MOR in the colon are mediated through the regulation of cytokine production and T cell proliferation, two important immunologic events required for the development of colon inflammation in mice and patients with inflammatory bowel disease (IBD). These data provide evidence that MOR plays a role in the control of gut inflammation and suggest that MOR agonists might be new therapeutic molecules in IBD.

Neurotrophin-regulated sorting of opioid receptors in the biosynthetic pathway of neurosecretory cells

Neurotrophins modulate the endogenous opioid system, but the underlying mechanisms are poorly understood. We observed an unexpected effect of neurotrophin signaling on the membrane trafficking of recombinant opioid receptors expressed in neurosecretory cells. Epitope-tagged delta opioid receptor (DOR) and mu opioid receptor (MOR) were differentially localized between surface and internal membrane pools, respectively, when expressed in primary cultured hippocampal neurons, consistent with previous studies by others of natively expressing neurons. Selective intracellular targeting of DOR was observed in nerve growth factor (NGF)-differentiated PC12 neurosecretory cells but not in PC12 cells cultured in the absence of NGF, where both DOR and MOR were localized in the plasma membrane. Surprisingly, NGF initiated intracellular targeting of DOR in PC12 cells acutely, within 60 min after initial activation of TrkA. The NGF-induced intracellular pool of DOR originated from a late stage of the biosynthetic pathway after exit from the endoplasmic reticulum and processing of N-linked glycans in the Golgi, resulting in the accumulation in cells of a biochemically mature "reserve" pool of intracellular DOR that exhibited depolarization-dependent insertion into the plasma membrane. The C-terminal cytoplasmic tail of DOR contains a signal determining the specificity of NGF-regulated intracellular targeting. These results indicate that cloned opioid receptors are differentially targeted when expressed heterologously in neurosecretory cells, establish a model system that facilitates mechanistic study of this process, and suggest a novel function of neurotrophins in modulating the anterograde membrane trafficking of opioid receptors.

Neurochemically distinct classes of myenteric neurons express the mu-opioid receptor in the guinea pig ileum

The mu-opioid receptor (muOR), which mediates many of the opioid effects in the nervous system, is expressed by enteric neurons. The aims of this study were to determine whether 1) different classes of myenteric neurons in the guinea pig ileum contain muOR immunoreactivity by using double- and triple-labeling immunofluorescence and confocal microscopy, 2) muOR immunoreactivity is localized to enteric neurons immunoreactive for the endogenous opioid enkephalin, and 3) muOR immunoreactivity is localized to interstitial cells of Cajal visualized by c-kit. In the myenteric plexus, 50% of muOR-immunoreactive neurons contained choline acetyltransferase (ChAT) immunoreactivity, whereas about 43% of ChAT-immunoreactive neurons were muOR immunoreactive. Approximately 46% of muOR myenteric neurons were immunoreactive for vasoactive intestinal polypeptide (VIP), and about 31% were immunoreactive for nitric oxide synthase (NOS). MuOR immunoreactivity was found in about 68% of VIP-containing neurons and 60% of NOS-immunoreactive neurons. Triple labeling showed that about 32% of muOR neurons contained VIP and ChAT immunoreactivities. The endogenous opioid enkephalin (ENK) was observed in about 30% of muOR neurons; conversely, 48% of ENK neurons contained muOR immunoreactivity. MuOR was not detected in neurons containing calbindin, nor in interstitial cells of Cajal. MuOR-immunoreactive fibers formed a dense network around interstitial cells of Cajal in the deep muscular plexus. This study demonstrates that muOR is expressed by neurochemically distinct classes of myenteric neurons that are likely to differ functionally, is colocalized with the endogenous opioid ENK, and is not expressed by interstitial cells of Cajal.

Visualizing activation of opioid circuits by internalization of G protein-coupled receptors

Mu-opioid receptor (MOR) and opioid receptor-like receptor (ORL-1) circuits in the limbic hypothalamic system are important for the regulation of sexual receptivity in the female rat. Sexual receptivity is tightly regulated by the sequential release of estrogen and progesterone from the ovary suggesting ovarian steroids regulate the activity of these neuropeptide systems. Both MOR and ORL-1 distributions overlap with the distribution of estrogen and progesterone receptors in the hypothalamus and limbic system providing a morphological substrate for interaction between steroids and the opioid circuits in the brain. Both MOR and ORL-1 are receptors that respond to activation by endogenous ligands with internalization into early endosomes. This internalization is part of the mechanism of receptor desensitization or down regulation. Although receptor activation and internalization are separate events, internalization can be used as a temporal measure of circuit activation by endogenous ligands. This review focuses on the estrogen and progesterone regulation of MOR and ORL-1 circuits in the medial preoptic nucleus and ventromedial nucleus of the hypothalamus that are central to modulating sexual receptivity.

Neurotrophin-regulated sorting of opioid receptors in the biosynthetic pathway of neurosecretory cells

Neurotrophins modulate the endogenous opioid system, but the underlying mechanisms are poorly understood. We observed an unexpected effect of neurotrophin signaling on the membrane trafficking of recombinant opioid receptors expressed in neurosecretory cells. Epitope-tagged delta opioid receptor (DOR) and mu opioid receptor (MOR) were differentially localized between surface and internal membrane pools, respectively, when expressed in primary cultured hippocampal neurons, consistent with previous studies by others of natively expressing neurons. Selective intracellular targeting of DOR was observed in nerve growth factor (NGF)-differentiated PC12 neurosecretory cells but not in PC12 cells cultured in the absence of NGF, where both DOR and MOR were localized in the plasma membrane. Surprisingly, NGF initiated intracellular targeting of DOR in PC12 cells acutely, within 60 min after initial activation of TrkA. The NGF-induced intracellular pool of DOR originated from a late stage of the biosynthetic pathway after exit from the endoplasmic reticulum and processing of N-linked glycans in the Golgi, resulting in the accumulation in cells of a biochemically mature "reserve" pool of intracellular DOR that exhibited depolarization-dependent insertion into the plasma membrane. The C-terminal cytoplasmic tail of DOR contains a signal determining the specificity of NGF-regulated intracellular targeting. These results indicate that cloned opioid receptors are differentially targeted when expressed heterologously in neurosecretory cells, establish a model system that facilitates mechanistic study of this process, and suggest a novel function of neurotrophins in modulating the anterograde membrane trafficking of opioid receptors.

Discrimination of a single dose of morphine followed by naltrexone: substitution of other agonists for morphine and other antagonists for naltrexone in a rat model of acute dependence

Rats were trained to discriminate 4-h pretreatment with 10 mg/kg morphine and 15-min pretreatment with 0.3 mg/kg naltrexone (morphine-->naltrexone) from pretreatment with saline and 0.3 mg/kg naltrexone (saline-->naltrexone). The discrimination seems to derive from interoceptive stimuli from antagonist-precipitated withdrawal from acute morphine dependence. The purpose of this study was to extend pharmacological characterization of the discrimination by testing opioid agonists other than morphine and antagonists other than naltrexone. Of seven mu-opioid agonists tested in place of morphine, only two (heroin and levorphanol) substituted completely for it; trials completed on the morphine-->naltrexone-appropriate lever increased as a function of agonist and naltrexone dose. Agonists with intrinsic efficacy higher (etorphine, fentanyl, and methadone) or lower (buprenorphine and meperidine) than that of morphine substituted only partially. However, when naltrexone was administered during continuous infusion of fentanyl or methadone via s.c. osmotic pump, rats responded as if they had received morphine-->naltrexone; discriminative responding correlated with global withdrawal scores. Rats responded primarily on the saline-->naltrexone-appropriate lever when naltrexone was administered after pretreatment with dextrorphan, the dextrorotatory isomer of levorphanol, or kappa-opioid agonists (5-alpha,7-alpha,8-beta)-(+)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl]-benzeneacetamide (U69,593) and spiradoline. Antagonists with no intrinsic efficacy at mu-opioid receptors (naloxone and diprenorphine) substituted completely for naltrexone, whereas those with some efficacy (nalorphine and levallorphan) substituted partially. Thus, morphine-->naltrexone-like stimulus control of behavior by drugs administered acutely requires pretreatment with certain mu-opioid agonists and a pure antagonist, is independent of agonist efficacy, and is stereoselective. Interoceptive stimuli from naltrexone-precipitated opioid withdrawal are more similar across morphine-like agonists during chronic dependence than they are during acute dependence.

Genetic dissociation of opiate tolerance and physical dependence in delta-opioid receptor-1 and preproenkephalin knock-out mice

Previous experiments have shown that mice lacking a functional delta-opioid receptor (DOR-1) gene do not develop analgesic tolerance to morphine. Here we report that mice lacking a functional gene for the endogenous ligand preproenkephalin (ppENK) show a similar tolerance deficit. In addition, we found that the DOR-1 and ppENK knock-outs as well as the NMDA receptor-deficient 129S6 inbred mouse strain, which also lacks tolerance, exhibit antagonist-induced opioid withdrawal. These data demonstrate that although signaling pathways involving ppENK, DOR, and NMDA receptor are necessary for the expression of morphine tolerance, other pathways independent of these factors can mediate physical dependence. Moreover, these studies illustrate that morphine tolerance can be genetically dissociated from physical dependence, and thus provide a genetic framework to assess more precisely the contribution of various cellular and molecular changes that accompany morphine administration to these processes.

Neurokinin-1 versus mu-opioid receptor binding in rat nucleus tractus solitarius after single and recurrent intermittent hypoxia

G protein-coupled excitatory neurokinin-1 and inhibitory mu-opioid receptors exist in respiratory brainstem with their peptides and influence breathing. To assess their putative role in respiratory responses to hypoxia, neurokinin-1, and mu-opioid receptor binding was determined in the respiratory nucleus tractus solitarius of the mature rat after single and recurrent intermittent hypoxia versus normoxia. Hypoxia comprised six 5-min bouts of 8% O(2)-92% N(2) interceded by 5-min bouts in 21% O(2)-79% N(2) (normoxia), either on 6 consecutive days (recurrent intermittent hypoxia) or on the 6th day only (single intermittent hypoxia). Controls comprised six daily sessions in normoxia. To examine the plasticity in receptor response, brains were collected 5min, 2h, or 24h after the last gaseous exposure. Sections from each brainstem underwent quantitative film autoradiography with iodinated substance P and DAMGO for neurokinin-1 and mu-opioid receptors, respectively. Neurokinin-1 receptor binding decreased 5min after single and recurrent hypoxia and 2h after recurrent hypoxia, whereas mu-opioid binding remained unchanged. The binding of both receptors increased 24h after recurrent intermittent hypoxia. Neurokinin versus mu-opioid binding differences immediately posthypoxia might affect physiological responses to episodic hypoxia.

Agonist-specific down-regulation of the human delta-opioid receptor

Down-regulation of the delta-opioid receptor contributes to the development of tolerance to delta-opioid receptor agonists. The involvement of the carboxy terminus of the mouse delta-opioid receptor in peptide agonist-mediated down-regulation has been established. In the present study, we examined the down-regulation of the truncated human delta-opioid receptor by structurally distinct delta-opioid receptor agonists. Chinese hamster ovary (CHO) cells, expressing the full-length or truncated epitope-tagged human delta-opioid receptors were incubated with various delta-opioid receptor agonists (100 nM, 24 h), and membrane receptor levels were determined by [(3)H]naltrindole saturation binding. Each delta-opioid receptor agonist tested down-regulated the full-length receptor. Truncation of the carboxy terminus abolished down-regulation by all delta-opioid receptor agonists, except SNC80 ((+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]N,N-diethylbenzamide). In addition, truncation of the C-terminus completely attenuated [D-Pen(2)-D-Pen(5)]enkephalin (DPDPE), but not SNC80-mediated [32P] incorporation into the protein immunoreactive with an anti-epitope-tagged antibody. These findings suggest that SNC80-mediated phosphorylation and down-regulation of the human delta-opioid receptor involves other receptor domains in addition to the carboxy terminus. Pertussis toxin treatment did not block SNC80-mediated down-regulation of the truncated Et-hDOR, indicating that the down-regulation is independent of G(i/o) protein activation and subsequent downstream signaling.

Dexmedetomidine fails to cause hyperalgesia after cessation of chronic administration

Hyperalgesia occurring after the cessation of chronic opioid administration occurs in humans and has been modeled in rodents with chronic systemic and intrathecal administration paradigms. It is, however, unclear if this type of postanalgesic hyperalgesia is unique to opioids. The alpha(2)-adrenergic receptor agonist, dexmedetomidine (Dex), is similar to opioids in that it is an analgesic that interacts with cell-surface receptors linked to the inhibition of adenylate cyclase and the modulation of ion channel activity. In these studies, we first constructed antinociceptive dose-response curves for Dex and morphine (MSO4). The 50% effective doses for Dex and MSO4 administered intraperitoneally to C57Bl/6 mice were 75 micro g/kg and 5.2 mg/kg, respectively. Using equally effective doses, we treated separate groups of mice with twice-daily injections of Dex or MSO4 for 5 days. Tolerance to these drugs was documented after this period. In the 16-72 h after cessation of administration, MSO4-treated mice demonstrated both thermal hyperalgesia and mechanical allodynia. However, the Dex-treated mice showed no changes in their thermal or mechanical withdrawal thresholds. We conclude that using this experimental paradigm, opioids but not an alpha(2)-adrenergic agonist, cause hyperalgesia and allodynia after cessation of chronic administration.

IMPLICATIONS

The cessation of the administration of opioids is associated with hyperalgesia in both humans and other animals. However, antinociceptive dexmedetomidine does not seem to be associated with this type of hyperalgesia syndrome during periods of abstinence.

Chronic morphine treatment inhibits opioid receptor desensitization and internalization

Chronic opioid receptor (OR) activation by morphine causes distinct cellular adaptations responsible for the development of tolerance. The present study examines the effect of chronic morphine exposure on the ability of high-efficacy agonists to mediate delta-OR (DOR) and mu-OR (MOR) uncoupling and internalization, two regulatory mechanisms contributing to rapid desensitization of OR function. Chronic morphine treatment (1 microm; 72 hr) of DOR carrying neuroblastoma x glioma (NG108-15) hybrid cells, a prototypical model system frequently used to study cellular aspects of opioid tolerance, completely blocked the capacity of [d-Ala2, d-Leu5]enkephalin (DADLE) and etorphine to desensitize opioid-stimulated [35S]GTPgammaS binding and to mediate DOR internalization. Similar findings were obtained on stably DOR- and MOR-transfected human embryonic kidney (HEK) 293 cells. Chronic morphine treatment also heterologously impaired agonist regulation of non-opioid G-protein-coupled receptors, such as the m(4)-muscarinic acetylcholine receptor and the brain-type cannabinoid receptor. As a possible underlying mechanism, we found that chronic morphine treatment completely blocked agonist-induced redistribution of beta-arrestin1 in both NG108-15 and stably MOR-transfected HEK293 cells. Moreover, attenuation of beta-arrestin1 function appears to depend on persistent stimulation of MAP kinase activity during the course of chronic morphine treatment, because coincubation of the cells together with the MAP kinase blocker PD98059 fully restored beta-arrestin1 translocation and receptor internalization. These results demonstrate that chronic morphine treatment produces adaptational changes at the beta-arrestin1 level, which in turn attenuates agonist-mediated desensitization and internalization of G-protein-coupled receptors.

Genetic dissociation of opiate tolerance and physical dependence in delta-opioid receptor-1 and preproenkephalin knock-out mice

Previous experiments have shown that mice lacking a functional delta-opioid receptor (DOR-1) gene do not develop analgesic tolerance to morphine. Here we report that mice lacking a functional gene for the endogenous ligand preproenkephalin (ppENK) show a similar tolerance deficit. In addition, we found that the DOR-1 and ppENK knock-outs as well as the NMDA receptor-deficient 129S6 inbred mouse strain, which also lacks tolerance, exhibit antagonist-induced opioid withdrawal. These data demonstrate that although signaling pathways involving ppENK, DOR, and NMDA receptor are necessary for the expression of morphine tolerance, other pathways independent of these factors can mediate physical dependence. Moreover, these studies illustrate that morphine tolerance can be genetically dissociated from physical dependence, and thus provide a genetic framework to assess more precisely the contribution of various cellular and molecular changes that accompany morphine administration to these processes.

Chronic morphine treatment inhibits opioid receptor desensitization and internalization

Chronic opioid receptor (OR) activation by morphine causes distinct cellular adaptations responsible for the development of tolerance. The present study examines the effect of chronic morphine exposure on the ability of high-efficacy agonists to mediate delta-OR (DOR) and mu-OR (MOR) uncoupling and internalization, two regulatory mechanisms contributing to rapid desensitization of OR function. Chronic morphine treatment (1 microm; 72 hr) of DOR carrying neuroblastoma x glioma (NG108-15) hybrid cells, a prototypical model system frequently used to study cellular aspects of opioid tolerance, completely blocked the capacity of [d-Ala2, d-Leu5]enkephalin (DADLE) and etorphine to desensitize opioid-stimulated [35S]GTPgammaS binding and to mediate DOR internalization. Similar findings were obtained on stably DOR- and MOR-transfected human embryonic kidney (HEK) 293 cells. Chronic morphine treatment also heterologously impaired agonist regulation of non-opioid G-protein-coupled receptors, such as the m(4)-muscarinic acetylcholine receptor and the brain-type cannabinoid receptor. As a possible underlying mechanism, we found that chronic morphine treatment completely blocked agonist-induced redistribution of beta-arrestin1 in both NG108-15 and stably MOR-transfected HEK293 cells. Moreover, attenuation of beta-arrestin1 function appears to depend on persistent stimulation of MAP kinase activity during the course of chronic morphine treatment, because coincubation of the cells together with the MAP kinase blocker PD98059 fully restored beta-arrestin1 translocation and receptor internalization. These results demonstrate that chronic morphine treatment produces adaptational changes at the beta-arrestin1 level, which in turn attenuates agonist-mediated desensitization and internalization of G-protein-coupled receptors.

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors. Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.

Internalization and recycling of 5-HT2A receptors activated by serotonin and protein kinase C-mediated mechanisms

Serotonin (5-HT), a major neurotransmitter, has a large number of G protein-coupled receptors in mammals. On activation by exposure to their ligand, 5-HT(2) receptor subtypes increase IP(3) levels and undergo desensitization and internalization. To visualize the receptor in cells during these processes, we have constructed a 5-HT(2A)-enhanced GFP (SR2-GFP) fusion receptor. We show that this fusion receptor undergoes internalization on exposure to its natural ligand, 5-HT. Because 5-HT(2A) receptors activate the phospholipase C pathway, we studied the effect of protein kinase C (PKC) on the internalization process and found that activation of PKC by its specific activator phorbol 12-myristate 13-acetate, in the absence of 5-HT, leads to internalization of the receptor. Moreover, inhibition of PKC by its inhibitor sphingosine in the presence of 5-HT prevents the internalization process, suggesting that activation of PKC is sufficient and necessary for the internalization of 5-HT(2A) receptors. We also show that SR2-GFP recycles back to the plasma membrane after 5-HT-dependent internalization, suggesting a mechanism for resensitization. In addition, receptors that have been internalized on addition of phorbol 12-myristate 13-acetate in the absence of 5-HT also recycle to the surface, with a time course similar to that seen after activation of the receptors by 5-HT. Our study suggests that 5-HT(2A) receptors internalize and return to the surface after both serotonin- and PKC-mediated processes. This study reveals a role for PKC in receptor internalization and also shows that 5-HT(2A) receptors are recycled.

Chronic morphine-induced changes in mu-opioid receptors and G proteins of different subcellular loci in rat brain

Prolonged exposure to opioid agonists can induce adaptive changes resulting in tolerance and dependence. Here, rats were rendered tolerant by subcutaneous injections of increasing doses of morphine from 10 to 60 mg/kg for 3, 5, or 10 consecutive days. Binding parameters of the mu-opioid receptor in subcellular fractions were measured with [(3)H]DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin). Although the density of surface mu-sites did not change after the 5-day morphine treatment, up-regulation of synaptic plasma membrane binding was detected after the 10-day drug administration. In contrast, the number of mu-binding sites in a light vesicle or microsomal fraction (MI) was elevated by 68 and 30% after 5 and 10 days of morphine exposure, respectively. The up-regulated MI mu-sites displayed enhanced coupling to G proteins compared with those detected in saline-treated controls. Pertussis toxin catalyzed ADP ribosylation, and Western blotting with specific antisera was used to quantitate chronic morphine-induced changes in levels of various G protein alpha-subunits. Morphine treatment of 5 days and longer induced significant increases in levels of Galpha(o), Galpha(i1), and Galpha(i2) in MI fractions that are part of an adaptation process. Up-regulation of intracellular mu-sites may be the result of post-translational changes and in part de novo synthesis. The results provide the first evidence that distinct regulation of intracellular mu-opioid receptor G protein coupling and G protein levels may accompany the development of morphine tolerance.

mu-Opioid receptors: Ligand-dependent activation of potassium conductance, desensitization, and internalization

micro-Opioid receptor (MOR) desensitization and endocytosis have been implicated in tolerance and dependence to opioids. The efficiency of each process is known to be agonist dependent; however, it is not known what determines the relative efficiency of various agonists at either process. In the present study, homologous MOR desensitization in locus ceruleus (LC) neurons and MOR internalization in HEK293 cells were examined using a series of agonists. The results show that the rank order of this series of agonists was different when comparing the magnitude of hyperpolarization and the ability to cause desensitization in LC neurons. Endocytosis of MOR was also examined in HEK293 cells using the same agonists. The relative ability to cause endocytosis in HEK293 cells correlated with the degree of desensitization in LC cells. This strong correlation suggests that the two processes are closely linked. The results also suggest that agonist efficacy is not necessarily a predictor of the ability to cause MOR desensitization or endocytosis. Identification and characterization of the biophysical properties of agonists that favor desensitization and internalization of receptors will lead to a better understanding of opioid signaling.

Profound spinal tolerance after repeated exposure to a highly selective mu-opioid peptide agonist: role of delta-opioid receptors

Recent studies suggest that delta-opioid receptors play a role in the development of opioid tolerance and led us to hypothesize that highly selective mu-opioid agonists may produce less tolerance. H-2',6'-dimethyltyrosine-D-Arg-Phe-Lys-NH(2) ([Dmt(1)]DALDA) has extraordinary selectivity for mu-receptors (K(i)(delta)/K(i)(mu) > 14,000). Daily administration of [Dmt(1)]DALDA (5 times ED(50); s.c.) for 7 days increased ED(50) 3.6-fold from 0.16 to 0.58 micromol/kg. A higher dose of [Dmt(1)]DALDA (10 times ED(50), every 12 h) for 2.5 days resulted in a 11.7 times increase in the ED(50) (1.9 micromol/kg). Complete cross-tolerance to morphine was observed, with a 3.4- and 15.1-fold shift in the morphine ED(50), respectively. We also compared the extent of spinal versus supraspinal tolerance after repeated s.c. [Dmt(1)]DALDA administration. Five doses of [Dmt(1)]DALDA (10 times ED(50), every 12 h) resulted in a 3.4 times shift in the i.c.v. ED(50) (15.4 versus 4.6 pmol/mouse) but a 44 times shift in the i.t. ED(50) (52.9 versus 1.2 pmol/mouse). Tolerance to [Dmt(1)]DALDA was associated with 30 to 35% reduction in [(3)H][Dmt(1)]DALDA binding in brain and spinal cord. Coadministration of [Dmt(1)]DALDA with delta-antagonist naltriben (NTB) reduced spinal tolerance by 50%. Even after spinal tolerance had been established, addition of a delta-antagonist (NTB or H-Tyr-TicPsi[CH(2)NH]Phe-Phe-OH) significantly enhanced the potency of i.t. [Dmt(1)]DALDA 2- to 4-fold. These results suggest that agonist activation of delta-receptors is not necessary for the development of opioid tolerance; however, delta-receptors play a modulatory role in the maintenance of the tolerant state.

Acute and chronic acetylcholinesterase inhibition regulates in vivo the localization and abundance of muscarinic receptors m2 and m4 at the cell surface and in the cytoplasm of striatal neurons

Acetylcholinesterase inhibitors (AChE-I) of various pharmacological classes have been used to provoke acute and chronic hypercholinergy in brain. Each condition induces a dramatic decrease of the abundance of muscarinic receptors at the membrane of neurons with simultaneous increase of these receptors in the cytoplasm in association with different subcellular organelles with characteristics depending on the duration of the treatment (short-term versus long term treatment). Each condition also induces a dramatic increase of cytoplasmic receptors associated with endosomes and multivesicular bodies. Chronic treatment with MTF induces a general decrease of m4R in the striatum without modification of the mRNA level but with an exaggerated abundance of muscarinic receptors in the cytoplasm at the sites of synthesis and maturation, i.e., endoplasmic reticulum, nuclear membrane and Golgi apparatus. These results suggest that the membrane abundance and intraneuronal distribution of neurotransmitter receptors are modified following drug treatment with specificity depending on the nature and the duration of treatment.

Receptor-mediated internalization of [3H]-neurotensin in synaptosomal preparations from rat neostriatum

Following its binding to somatodendritic receptors, the neuropeptide neurotensin (NT) internalizes via a clathrin-mediated process. In the present study, we investigated whether NT also internalizes presynaptically using synaptosomes from rat neostriatum, a region in which NT1 receptors are virtually all presynaptic. Binding of [(3)H]-NT to striatal synaptosomes in the presence of levocabastine to block NT2 receptors is specific, saturable, and has NT1 binding properties. A significant fraction of the bound radioactivity is resistant to hypertonic acid wash indicating that it is internalized. Internalization of [(3)H]-NT, like that of [(125)I]-transferrin, is blocked by sucrose and low temperature, consistent with endocytosis occurring via a clathrin-dependent pathway. However, contrary to what was reported at the somatodendritic level, neither [(3)H]-NT nor [(125)I]-transferrin internalization in synaptosomes is sensitive to the endocytosis inhibitor phenylarsine oxide. Moreover, treatment of synaptosomes with monensin, which prevents internalized receptors from recycling to the plasma membrane, reduces [(3)H]-NT binding and internalization, suggesting that presynaptic NT1 receptors, in contrast to somatodendritic ones, are recycled back to the plasma membrane. Taken together, these results suggest that NT internalizes in nerve terminals via an endocytic pathway that is related to, but is mechanistically distinct from that responsible for NT internalization in nerve cell bodies.

Glycosylated phosducin-like protein long regulates opioid receptor function in mouse brain

Phosducin (Phd), a protein that in retina regulates rhodopsin desensitization by controlling the activity of Gt beta gamma-dependent G-protein-coupled receptor kinases (GRKs), is present in very low levels in the CNS of mammals. However, this tissue contains proteins of related sequence and function. This paper reports the presence of N-glycosylated phosducin-like protein long (PhLP(L)) in all structures of mouse CNS, mainly in synaptic plasma membranes and associated with G beta subunits and 14-3-3 proteins. To analyze the role PhLP(L) in opioid receptor desensitization, its expression was reduced by the use of antisense oligodeoxynucleotides (ODNs). The antinociception induced by morphine, [D-Ala(2), N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO), beta-endorphin, [D-Ala(2)]deltorphin II, [D-Pen(2,5)]-enkephalin (DPDPE) or clonidine in the tail-flick test was reduced in PhLP(L)-knock-down mice. A single intracerebroventricular (icv)-ED(80) analgesic dose of morphine gave rise to acute tolerance that lasted for 4 days, but which was prevented or reversed by icv-injection of myristoylated (myr(+)) G(i2)alpha subunits. PhLP(L) knock-down brought about a myr(+)-G(i2)alpha subunit-insensitive acute tolerance to morphine that was still present after 8 days. It also diminished the specific binding of (125)I-Tyr(27)-beta-endorphin-(1-31) (human) to mouse periaqueductal gray matter membranes. After being exposed to chronic morphine treatment, post-dependent mice required about 10 days for complete recovery of morphine antinociception. The impairment of PhLP(L) extended this period beyond 17 days. It is concluded that PhLP(L) knock-down facilitates desensitization and uncoupling of opioid receptors.

Internalization of mu-opioid receptors produced by etorphine in the rat locus coeruleus

Chronic administration of mu-opioid receptor agonists is known to produce adaptive changes within noradrenergic neurons of the locus coeruleus. Although mu-opioid receptors are densely expressed by locus coeruleus neurons, the effects of acute and chronic administration of agonists on the subcellular distribution of mu-opioid receptors remain poorly understood. Therefore, we examined the ultrastructural distribution of mu-opioid receptor immunoreactivity in the locus coeruleus of rats subjected to either acute morphine, or etorphine, or chronic morphine treatment. In the locus coeruleus of control rats receiving acute saline injections or placebo pellet implants, immunogold-silver labeling for mu-opioid receptors was localized to parasynaptic and extrasynaptic portions of the plasma membranes of perikarya and dendrites. Only 8% of the gold-silver particles analyzed were distributed within the cytoplasm of dendrites and perikarya in vehicle-treated rats. Immunolabeling for mu-opioid receptors was distributed along portions of the plasma membrane that were often apposed by astroglial sheaths. After acute injections of etorphine, there was a dramatic internalization of mu-opioid receptors to intracellular compartments. Quantitative analysis of gold-silver particles indicative of mu-opioid receptors showed that a substantial number of gold particles shifted from the plasma membrane to early endosomes in dendrites from etorphine-treated rats. In dendrites sampled from etorphine-treated rats, 85% of the gold-silver grains indicative of mu-opioid receptor labeling were located in intracellular compartments as compared to 15% that were distributed along the plasma membrane. In animals that received either acute morphine injections or chronic morphine via pellet implantation, no change in the subcellular distribution of immunogold particles indicative of mu-opioid receptors was detected when compared to matched control animals. These results provide the first ultrastructural evidence that mu-opioid receptors are internalized by agonists such as etorphine, but not the partial agonist morphine, in the locus coeruleus.

Involvement of mu- and kappa-, but not delta-, opioid receptors in the peristaltic motor depression caused by endogenous and exogenous opioids in the guinea-pig intestine

Opiates inhibit gastrointestinal propulsion, but it is not clear which opioid receptor types are involved in this action. For this reason, the effect of opioid receptor - selective agonists and antagonists on intestinal peristalsis was studied. Peristalsis in isolated segments of the guinea-pig small intestine was triggered by a rise of the intraluminal pressure and recorded via the intraluminal pressure changes associated with the peristaltic waves. Mu-opioid receptor agonists (DAMGO, morphine), kappa-opioid receptor agonists (ICI-204,448 and BRL-52,537) and a delta-opioid receptor agonist (SNC-80) inhibited peristalsis in a concentration-related manner as deduced from a rise of the peristaltic pressure threshold (PPT) and a diminution of peristaltic effectiveness. Experiments with the delta-opioid receptor antagonists naltrindole (30 nM) and HS-378 (1 microM), the kappa-opioid receptor antagonist nor-binaltorphimine (30 nM) and the mu-opioid receptor antagonist cyprodime (10 microM) revealed that the antiperistaltic effect of ICI-204,448 and BRL-52,537 was mediated by kappa-opioid receptors and that of morphine and DAMGO by mu-opioid receptors. In contrast, the peristaltic motor inhibition caused by SNC-80 was unrelated to delta-opioid receptor activation. Cyprodime and nor-binaltorphimine, but not naltrindole and HS-378, were per se able to stimulate intestinal peristalsis as deduced from a decrease in PPT. The results show that the neural circuits controlling peristalsis in the guinea-pig small intestine are inhibited by endogenous and exogenous opioids acting via mu- and kappa-, but not delta-, opioid receptors.

Internalization and trafficking of opioid receptor ligands in rat cortical neurons

The binding, internalization, and trafficking of the fluorescently labeled opioid peptides Fluo-dermorphin and Fluo-deltorphin were quantitatively studied by confocal microscopy in primary cortical neurons in culture. Specific binding of these selective ligands to neurons naturally expressing mu (mu) and delta (delta) opioid receptors (OR), respectively, resulted in their internalization into neuronal somas and processes, as indicated by the persistence of fluorescent labeling following removal of cell surface binding by hypertonic acid wash. This internalization was receptor-specific, as the fluorescent signal was completely abolished when the cells were concomitantly incubated with the opioid receptor antagonist naloxone. It also was clathrin-dependent, as it was totally prevented by the endocytosis inhibitor phenylarsine oxide. Accordingly, internalized ligands were detected inside small, endosome-like vesicles. These labeled vesicles accumulated within nerve cell bodies between 5-30 min of incubation with the fluorescent ligands. This accumulation was abolished after treatment with the antitubular agent nocodazole, suggesting that it was due to a microtubule-dependent, retrograde transport of the internalized ligands from processes to the soma. By contrast, there was no change in the compartmentalization of internalized (mu)OR or deltaOR, as assessed by immunocytochemistry, suggesting that the latter were recycled locally. The present results provide the first demonstration of receptor-mediated internalization of opioid peptides in cultured neurons. It is proposed that their retrograde transport into target cells might be involved in mediating some of the long-term, transcriptional effects of opioids.

Regulation of opioid receptor trafficking and morphine tolerance by receptor oligomerization

The utility of morphine for the treatment of chronic pain is hindered by the development of tolerance to the analgesic effects of the drug. Morphine is unique among opiates in its ability to activate the mu opioid receptor (MOR) without promoting its desensitization and endocytosis. Here we demonstrate that [D-Ala(2)-MePhe(4)-Gly(5)-ol] enkephalin (DAMGO) can facilitate the ability of morphine to stimulate MOR endocytosis. As a consequence, rats treated chronically with both drugs show reduced analgesic tolerance compared to rats treated with morphine alone. These results demonstrate that endocytosis of the MOR can reduce the development of tolerance, and hence suggest an approach for the development of opiate analogs with enhanced efficacy for the treatment of chronic pain.

Endocytosis of the mu opioid receptor reduces tolerance and a cellular hallmark of opiate withdrawal

Morphine is unusual in its failure to promote robust desensitization and endocytosis of the mu opioid receptor (MOR), processes that for many receptors contribute directly to tolerance. This apparent paradox has led us to revise the idea that receptor desensitization and endocytosis are solely responsible for tolerance and withdrawal to morphine, and instead test the hypothesis that these side effects occur due to abnormally prolonged MOR signaling. We report here that MOR mutations that facilitate endocytosis reduce the development of cellular tolerance and cAMP superactivation, a cellular hallmark of withdrawal. Moreover, mutant receptors with reduced endocytosis produce exacerbated superactivation. These data demonstrate a critical role for receptor endocytosis in the development of adverse side effects associated with prolonged opiate use.

Agonist-induced internalization of serotonin-1a receptors in the dorsal raphe nucleus (autoreceptors) but not hippocampus (heteroreceptors)

Serotonin-1A (5-HT(1A)) receptors in the CNS are a major target for psychotropic drugs. In nucleus raphe dorsalis (NRD) and hippocampus (CA3), the selective 5-HT(1A) agonist (+)-8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) reduces the firing activity of serotoninergic (5-HT) and pyramidal neurons, respectively. When located on 5-HT (autoreceptors), but not on non-5-HT (heteroreceptors) neurons, 5-HT(1A) receptors are known to be subject to desensitization. Using quantitative electron microscopy after pre-embedding immunogold labeling with specific antibodies, we examined the subcellular distribution of these receptors after acute administration of 8-OH-DPAT (0.5 mg/kg, i.v.). Silver-intensified immunogold particles associated with the plasma membrane or the cytoplasm were counted in somata and dendrites within the NRD, 15 min, 1 hr and 24 hr after 8-OH-DPAT injection, and in hippocampal dendrites 1 hr after the same treatment. Significant decrease in the density of membrane labeling and concomitant increase of cytoplasmic labeling were demonstrated in the NRD, 15 min and 1 hr after 8-OH-DPAT administration, with a return to baseline level at 24 hr. Internalization was blocked by previous administration of the 5-HT(1A) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane-carboxamide (WAY 100635), which, by itself, was without apparent effect. In hippocampus (CA3), there were no apparent changes in the distribution of the receptor after 8-OH-DPAT administration. These findings are in line with earlier results showing a desensitization of 5-HT(1A) autoreceptors but not heteroreceptors after treatment with 5-HT(1A) receptor agonist. They suggest that this desensitization is the result of autoreceptor internalization.

Inverse agonist up-regulates the constitutively active D3.49(164)Q mutant of the rat mu-opioid receptor by stabilizing the structure and blocking constitutive internalization and down-regulation

We demonstrated previously that D3.49(164) mutations resulted in constitutive activation of the rat mu-opioid receptor and abolished receptor expression unless cells were pretreated with naloxone, an inverse agonist. In this study, we investigated the properties of the D3.49(164)Q mutant and the mechanisms underlying the effect of naloxone. Naloxone pretreatment up-regulated [(3)H]diprenorphine binding and protein expression of the D3.49(164)Q mutant in a time- and dose-dependent manner without affecting its mRNA level. After naloxone removal, binding and protein expression of the mutant declined with time with no effect on its mRNA level. Naloxone methiodide (a quaternary ammonium analog) caused a maximal up-regulation about 50% of the naloxone effect, indicating that naloxone acts extracellularly and intracellularly. Expression of the mutant was enhanced by inverse agonists, a neutral antagonist, and agonists, with inverse agonists being most effective. In membranes, the mutant was structurally less stable than the wild type upon incubation at 37 degrees C, and naloxone and [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin stabilized the mutant. Coexpression of the dominant-negative mutants GRK2-K220R, arrestin-2(319-418), dynamin I-K44A, rab5A-N133I or rab7-N125I partially prevented the decline in binding of the mutant after naloxone removal. Chloroquine or proteasome inhibitor I reduced the down-regulation of the mutant. These results indicate that the D3.49(164)Q mutant is constitutively internalized via G protein coupled-receptor kinase-, arrestin-2-, dynamin-, rab5-, and rab7-dependent pathways and probably trafficked through early and late endosomes into lysosomes and degraded by lysosomes and proteasomes. Naloxone up-regulates the D3.49(164)Q mutant by stabilizing the mutant protein and blocking its constitutive internalization and down-regulation. To the best of our knowledge, this represents the first comprehensive analysis of the mechanisms involved in up-regulation of constitutively active mutants by an inverse agonist.

Agonist-induced internalization of serotonin-1a receptors in the dorsal raphe nucleus (autoreceptors) but not hippocampus (heteroreceptors)

Serotonin-1A (5-HT(1A)) receptors in the CNS are a major target for psychotropic drugs. In nucleus raphe dorsalis (NRD) and hippocampus (CA3), the selective 5-HT(1A) agonist (+)-8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) reduces the firing activity of serotoninergic (5-HT) and pyramidal neurons, respectively. When located on 5-HT (autoreceptors), but not on non-5-HT (heteroreceptors) neurons, 5-HT(1A) receptors are known to be subject to desensitization. Using quantitative electron microscopy after pre-embedding immunogold labeling with specific antibodies, we examined the subcellular distribution of these receptors after acute administration of 8-OH-DPAT (0.5 mg/kg, i.v.). Silver-intensified immunogold particles associated with the plasma membrane or the cytoplasm were counted in somata and dendrites within the NRD, 15 min, 1 hr and 24 hr after 8-OH-DPAT injection, and in hippocampal dendrites 1 hr after the same treatment. Significant decrease in the density of membrane labeling and concomitant increase of cytoplasmic labeling were demonstrated in the NRD, 15 min and 1 hr after 8-OH-DPAT administration, with a return to baseline level at 24 hr. Internalization was blocked by previous administration of the 5-HT(1A) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane-carboxamide (WAY 100635), which, by itself, was without apparent effect. In hippocampus (CA3), there were no apparent changes in the distribution of the receptor after 8-OH-DPAT administration. These findings are in line with earlier results showing a desensitization of 5-HT(1A) autoreceptors but not heteroreceptors after treatment with 5-HT(1A) receptor agonist. They suggest that this desensitization is the result of autoreceptor internalization.

Differential in vivo internalization of MOR-1 and MOR-1C by morphine

The mu opioid receptor MOR-1 is internalized by many mu agonists, but not morphine. To see whether differences in the intracellular carboxy terminus influences internalization, we examined internalization of a splice variant of the mu opioid receptor, MOR-1C, in the lateral septum of the mouse in vivo. Following intracerebroventricular (i.c.v.) saline treatment, MOR-1C-like immunoreactivity (LI) within neurons in naive mice was found predominantly in clusters close to the plasma membrane. Following either intracerebroventricular [d-Ala2, MePhe4,Gly(ol)5]enkephalin (DAMGO) or morphine, MOR-1C-LI clustered into endosomes in the cytoplasm. This effect was suppressed by prior administration of the opioid antagonist naloxone. In contrast, only DAMGO, and not morphine, internalized MOR-1-LI. These results illustrate differences in internalization between two MOR-1 variants that have alternative splicing at the COOH terminus.

C-terminal Splice Variants of the Mouse µ-Opioid Receptor Differ in Morphine-induced Internalization and Receptor Resensitization

The main analgesic effects of the opioid alkaloid morphine are mediated by the mu-opioid receptor. In contrast to endogenous opioid peptides, morphine activates the mu-opioid receptor without causing its rapid endocytosis. Recently, three novel C-terminal splice variants (MOR1C, MOR1D, and MOR1E) of the mouse mu-opioid receptor (MOR1) have been identified. In the present study, we show that these receptors differ substantially in their agonist-selective membrane trafficking. MOR1 and MOR1C stably expressed in human embryonic kidney 293 cells exhibited phosphorylation, internalization, and down-regulation in the presence of the opioid peptide [d-Ala(2),Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO) but not in response to morphine. In contrast, MOR1D and MOR1E exhibited robust phosphorylation, internalization, and down-regulation in response to both DAMGO and morphine. DAMGO elicited a similar desensitization (during an 8-h exposure) and resensitization (during a 50-min drug-free interval) of all four mu-receptor splice variants. After morphine treatment, however, MOR1 and MOR1C showed a faster desensitization and no resensitization as compared with MOR1D and MOR1E. These results strongly reinforce the hypothesis that receptor phosphorylation and internalization are required for opioid receptor reactivation thus counteracting agonist-induced desensitization. Our findings also suggest a mechanism by which cell- and tissue-specific C-terminal splicing of the mu-opioid receptor may significantly modulate the development of tolerance to the various effects of morphine.

Receptors and transmission in the brain-gut axis: potential for novel therapies. III. Mu-opioid receptors in the enteric nervous system

G protein-coupled receptors are cell surface signal-transducing proteins, which elicit a variety of biological functions by the activation of different intracellular effector systems. Many of these receptors, including the mu-opioid receptor (mu OR), have been localized in the gastrointestinal tract. mu OR is the target of opioids and alkaloids, potent analgesic drugs with high potential for abuse. mu OR is expressed by enteric neurons, and it undergoes ligand-selective endocytosis. It is of clinical importance because it mediates tolerance and other major side effects of opiate analgesics, including impairment of gastrointestinal propulsion. An important observation of mu OR is its differential trafficking and desensitization properties in response to individual agonists, which might have long-term physiological consequences and be involved in the development of opiate side effects. Receptor activation by agonists is the basis for signaling, and alterations of the mechanisms controlling cellular responses of G protein-coupled receptors to agonists might be the basis of several diseases, including gastrointestinal diseases. Therefore, understanding these basic cellular mechanisms is important for developing appropriate therapeutic agents.

Effects of beta-casomorphins on neuronal survival in culture of embryonic chick dorsal root ganglion neurons

We studied the effects of beta-casomorphins (beta-CMs, mu-acting opioid peptides from milk protein beta-casein) on survival of primary-cultured chick dorsal root ganglion neurons in the presence of nerve growth factor. Beta-CM-5 and beta-CM-7 had potent neuronal survival-promoting activities. Beta-CM-4 amide (morphiceptin) and des-Tyr'-beta-CM-7 also exhibited the similar promoting effects, although their effects were very weak. The promoting effect of beta-CM-5 was prevented by co-administration of naloxone, or pretreatment with pertussis toxin. These results suggest that the neuronal survival-promoting effects of beta-CMs might be mediated through opioid receptors coupled to G proteins.

Endocytosis and downregulation of G protein-coupled receptors

The number of G protein-coupled receptors (GPCRs) displayed at the cell surface is a critical determinant of physiological responsiveness to native ligands and drugs. Downregulation of the number of adrenergic and dopaminergic receptors present on specific neurons can be induced by receptor agonists or by drugs that increase extracellular concentrations of catecholamines such as dopamine. Thus agonist-induced downregulation of GPCRs is of potentially great importance to the treatment of Parkinson's Disease. However, little is known about biochemical mechanisms that mediate GPCR downregulation. Recent studies of cloned GPCRs have provided exciting insights into specific mechanisms that control endocytosis of receptors from the plasma membrane and regulate proteolytic degradation of receptors. In this review we briefly survey representative studies establishing that multiple mechanisms of GPCR membrane trafficking can function in downregulation function both in neural and non-neural cell types. Then we focus on our present view of mechanisms mediating regulated proteolysis of GPCRs, highlighting recent progress in understanding membrane trafficking of GPCRs from the cell surface to lysosomes. Finally we discuss emerging evidence regarding a specific mechanism that modulates sorting of certain GPCRs between recycling and degradative pathways.

Protein kinase C-mediated inhibition of mu-opioid receptor internalization and its involvement in the development of acute tolerance to peripheral mu-agonist analgesia

We investigated the role of protein kinase C (PKC) in cell mu-opioid receptor (MOR) internalization and MOR-mediated acute tolerance in vivo. When Chinese hamster ovary cells expressing MOR were exposed to [D-Ala(2),MePhe(4),Gly-ol(5)]-enkephalin (DAMGO), receptor internalization was observed at 30 min. Incubation with morphine failed to induce receptor internalization. When calphostin C, a PKC inhibitor, was added, receptor internalization was observed as early as 10 min after morphine stimulation. The MOR internalization induced by DAMGO or morphine in the presence of calphostin C was dynamin dependent, because it was abolished 2 d after pretreatment with recombinant adenovirus to express a dominant interfering dynamin mutant (K44A/dynamin adenovirus). On the other hand, in a peripheral nociception test in mice, the nociceptive flexor response after intraplantar injection (i.pl.) of bradykinin was markedly inhibited by DAMGO (i.pl.). DAMGO analgesia was not affected by 2 hr prior injection (i.pl.) of DAMGO. Marked acute tolerance was observed after pretreatment with dynamin antisense oligodeoxynucleotide or K44A/dynamin adenovirus. The DAMGO-induced acute tolerance under such pretreatments was inhibited by calphostin C. Together, these findings suggest that PKC desensitizes MOR or has a role in the development of acute tolerance through MOR by inhibiting internalization mechanisms as a resensitization process.

Protein kinase C-mediated inhibition of mu-opioid receptor internalization and its involvement in the development of acute tolerance to peripheral mu-agonist analgesia

We investigated the role of protein kinase C (PKC) in cell mu-opioid receptor (MOR) internalization and MOR-mediated acute tolerance in vivo. When Chinese hamster ovary cells expressing MOR were exposed to [D-Ala(2),MePhe(4),Gly-ol(5)]-enkephalin (DAMGO), receptor internalization was observed at 30 min. Incubation with morphine failed to induce receptor internalization. When calphostin C, a PKC inhibitor, was added, receptor internalization was observed as early as 10 min after morphine stimulation. The MOR internalization induced by DAMGO or morphine in the presence of calphostin C was dynamin dependent, because it was abolished 2 d after pretreatment with recombinant adenovirus to express a dominant interfering dynamin mutant (K44A/dynamin adenovirus). On the other hand, in a peripheral nociception test in mice, the nociceptive flexor response after intraplantar injection (i.pl.) of bradykinin was markedly inhibited by DAMGO (i.pl.). DAMGO analgesia was not affected by 2 hr prior injection (i.pl.) of DAMGO. Marked acute tolerance was observed after pretreatment with dynamin antisense oligodeoxynucleotide or K44A/dynamin adenovirus. The DAMGO-induced acute tolerance under such pretreatments was inhibited by calphostin C. Together, these findings suggest that PKC desensitizes MOR or has a role in the development of acute tolerance through MOR by inhibiting internalization mechanisms as a resensitization process.

Phosphorylation of Ser363, Thr370, and Ser375 residues within the carboxyl tail differentially regulates mu-opioid receptor internalization

Prolonged activation of opioid receptors leads to their phosphorylation, desensitization, internalization, and down-regulation. To elucidate the relationship between mu-opioid receptor (MOR) phosphorylation and the regulation of receptor activity, a series of receptor mutants was constructed in which the 12 Ser/Thr residues of the COOH-terminal portion of the receptor were substituted to Ala, either individually or in combination. All these mutant constructs were stably expressed in human embryonic kidney 293 cells and exhibited similar expression levels and ligand binding properties. Among those 12 Ser/Thr residues, Ser(363), Thr(370), and Ser(375) have been identified as phosphorylation sites. In the absence of the agonist, a basal phosphorylation of Ser(363) and Thr(370) was observed, whereas [d-Ala(2),Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO)-induced receptor phosphorylation occurs at Thr(370) and Ser(375) residues. Furthermore, the role of these phosphorylation sites in regulating the internalization of MOR was investigated. The mutation of Ser(375) to Ala reduced the rate and extent of receptor internalization, whereas mutation of Ser(363) and Thr(370) to Ala accelerated MOR internalization kinetics. The present data show that the basal phosphorylation of MOR could play a role in modulating agonist-induced receptor internalization kinetics. Furthermore, even though mu-receptors and delta-opioid receptors have the same motif encompassing agonist-induced phosphorylation sites, the different agonist-induced internalization properties controlled by these sites suggest differential cellular regulation of these two receptor subtypes.