Chronic morphine induces the concomitant phosphorylation and altered association of multiple signaling proteins: a novel mechanism for modulating cell signaling

Morphine Preconditioning Alleviates Ischemia/Reperfusion-induced Caspase-8-dependent Neuronal Apoptosis through cPKCγ-NF-κB-cFLIPL Pathway

BACKGROUND

Perioperative cerebral ischemia/reperfusion injury is a major contributor to postoperative death and cognitive dysfunction in patients. It was reported that morphine preconditioning (MP) can mimic ischemia/hypoxia preconditioning to protect against ischemia/reperfusion injury. However, the mechanism of MP on the ischemia/reperfusion-induced neuronal apoptosis has not been fully clarified.

METHODS

The middle cerebral artery occlusion/reperfusion (MCAO/R) model of mice and the oxygen-glucose deprivation/reoxygenation (OGD/R) model in primary cortical neurons were used to mimic ischemic stroke. In vivo, the infarct size was measured by using TTC staining; NDSS, Longa score system, and beam balance test were performed to evaluate the neurological deficits of mice; the expression of the protein was detected by using a western blot. In vitro, the viability of neurons was determined by using CCK-8 assay; the expression of protein and mRNA were assessed by using western blot, RT-qPCR, and immunofluorescent staining; the level of apoptosis was detected by using TUNEL staining.

RESULTS

MP can improve the neurological functions of mice following MCAO/R (P<0.001, n=10 per group). MP can decrease the infarct size (P<0.001, n=10 per group) and the level of cleaved-caspase-3 of mice following MCAO/R (P<0.01 or 0.001, n=6 per group). MP can increase the levels of cPKCγ membrane translocation, p-p65, and cFLIPL, and decrease the levels of cleaved-caspase-8, 3 in neurons after OGD/R or MCAO/R 1 d (P<0.05, 0.01 or 0.001, n=6 per group). In addition, MP could alleviate OGD/R-induced cell apoptosis (P<0.001, n=6 per group).

CONCLUSION

MP alleviates ischemia/reperfusion-induced Caspase 8-dependent neuronal apoptosis through the cPKCγ-NF-κB-cFLIPL pathway.

Morphine attenuates neuroinflammation and blood-brain barrier disruption following traumatic brain injury through the opioidergic system

Endogenous opiates are suggested to have a role in the pathophysiology of traumatic brain injury (TBI). Furthermore, administration of opioidergic agents in TBI injured animals have been shown to affect the brain injury and provide neuroprotection post-TBI. This study aims to investigate the potential neuroprotective effects of morphine through inhibition of neuroinflammatory pathways in acute severe TBI. Male Wistar rats were divided into seven groups (24 rats per group): Sham, Vehicle (TBI + intraperitoneal (i.p) injection of normal saline), TBI + i.p injection of morphine in 1, 5 and 10 mg/kg doses (MOR 1, MOR 5 and MOR 10 groups), TBI + morphine (5 mg/kg i.p) + Naloxone (NAL + MOR), and TBI + morphine (5 mg/kg i.p) + Naltrindole (NALT + MOR). A severe diffuse TBI model (weight dropping Marmarou model) was used to induce TBI in rats. The veterinary coma scale (VCS), beam-walk, and beam-balance tasks were used to assess short-term neurological deficits. Histolopathological changes of brain tissue was evaluated using light microscopy and hematoxilin and eosin staining. Blood-Brain barrier (BBB) disruption was evaluated by the Evans Blue method 6 h post-injury. Brain water content and cerebrospinal fluid (CSF) content of IL-1β and IL-10 were assessed by the wet-dry method and enzyme-linked immunosorbent assay (ELISA), respectively. Morphine (1 and 5 mg/kg doses) attenuated BBB leakage, improved VCS score, pathological changes of brain tissue, and vestibulomotor function compared to the vehicle group (p < 0.0001). Only 5 mg/kg morphine attenuated brain edema (p < 0.0001). Furthermore, 1 and 5 mg/kg morphine significantly changed CSF concentration of IL-1β and IL-10 compared to the vehicle group (p < 0.0001). Inhibition of opioid receptors by naloxone and naltrindole abolished morphine neuroprotective effects (p < 0.0001 vs. MOR 5 group). This study suggests that morphine administration inhibits TBI-mediated neuroinflammation via opioid receptors and improves neurobehavioral function following TBI, which provides a potential therapeutic opportunity in the treatment of traumatic brain injury.

Relevance of Mu-Opioid Receptor Splice Variants and Plasticity of Their Signaling Sequelae to Opioid Analgesic Tolerance

Opioid dose escalation to effectively control pain is often linked to the current prescription opioid abuse epidemic. This creates social as well as medical imperatives to better understand the mechanistic underpinnings of opioid tolerance to develop interventions that minimize it, thereby maximizing the analgesic effectiveness of opioids. Profound opioid analgesic tolerance can be observed in the absence of mu-opioid receptor (MOR) downregulation, aggregate MOR G protein uncoupling, and MOR desensitization, in the absence of impaired G protein coupled receptor kinase phosphorylation, arrestin binding, or endocytosis. Thus, we have explored alternative biochemical sequelae that might better account for opioid analgesic tolerance. Our findings indicate that substantial plasticity among upstream and downstream components of opioid receptor signaling and the emergence of alternative signaling pathways are major contributors to opioid analgesic tolerance. An exemplar of this plasticity is our findings that chronic morphine upregulates the MOR variants MOR-1B2 and MOR-1C1 and phosphorylation of their C-terminal sites not present in MOR-1, events causally associated with the chronic morphine-induced shift in MOR G protein coupling from predominantly Gi/Go inhibitory to Gs-stimulatory adenylyl cyclase signaling. The unique feature(s) of these variants that underlies their susceptibility to adapting to chronic morphine by altering the nature of their G protein coupling reveals the richness and pliability of MOR signaling that is enabled by generating a wide diversity of MOR variants. Furthermore, given differential anatomical expression patterns of MOR variants, MOR splice variant-dependent adaptations to chronic morphine could enable mechanistic underpinnings of tolerance and dependence that are CNS region- and cell-specific.

Phosphorylation of unique C-terminal sites of the mu-opioid receptor variants 1B2 and 1C1 influences their Gs association following chronic morphine

We recently demonstrated in rat spinal cord that a regimen of escalating doses of systemic morphine, analogous to regimens used clinically for chronic pain management, selectively up-regulates the mu-opioid receptor (MOR) splice variants MOR-1B2 and MOR-1C1 mRNA and functional protein. This study investigated the potential relevance of up-regulating MOR-1B2 and MOR-1C1 to the ability of chronic morphine to shift MOR signaling from predominantly G_i /G_o inhibitory to G_s stimulatory. Specifically, we tested the hypotheses that chronic morphine induces phosphorylation of carboxyl terminal sites unique to MOR-1B2 and MOR-1C1, and that this phosphorylation is causally related to augmented association of these variants with G_s α. Hypotheses were validated by (i) abolition of the chronic morphine-induced increment in MOR-1C1 and MOR-1B2 association with G_s α by inhibitors of protein kinase A and Casein kinase 2, respectively; (ii) failure of chronic morphine to augment MOR variant G_s α interactions in Chinese hamster ovary cells transiently transfected with either rat MOR-1C1 or MOR-1B2 in which targeted protein kinase A and Casein kinase 2 serine phosphorylation sites, respectively, were mutated to alanine; (iii) abrogation of chronic morphine-induced augmented MOR G_s α association in spinal cord of male rats following intrathecal administration of dicer substrate small interfering RNAs targeting MOR-1B2/MOR-1C1 mRNA. The ability of chronic morphine to not only up-regulate-specific MOR variants but also their carboxyl terminal phosphorylation and consequent augmented association with G_s α may represent a novel component of opioid tolerance mechanisms, suggesting novel potential targets for tolerance abatement.

Acute ethanol exposure reduces serotonin receptor 1A internalization by increasing ubiquitination and degradation of β-arrestin2

Acute alcohol exposure alters the trafficking and function of many G-protein-coupled receptors (GPCRs) that are associated with aberrant behavioral responses to alcohol. However, the molecular mechanisms underlying alcohol-induced changes in GPCR function remain unclear. β-Arrestin is a key player involved in the regulation of GPCR internalization and thus controls the magnitude and duration of GPCR signaling. Although β-arrestin levels are influenced by various drugs of abuse, the effect of alcohol exposure on β-arrestin expression and β-arrestin-mediated GPCR trafficking is poorly understood. Here, we found that acute ethanol exposure increases β-arrestin2 degradation via its increased ubiquitination in neuroblastoma-2a (N2A) cells and rat prefrontal cortex (PFC). β-Arrestin2 ubiquitination was likely mediated by the E3 ligase MDM2 homolog (MDM2), indicated by an increased coupling between β-arrestin2 and MDM2 in response to acute ethanol exposure in both N2A cells and rat PFC homogenates. Importantly, ethanol-induced β-arrestin2 reduction was reversed by siRNA-mediated MDM2 knockdown or proteasome inhibition in N2A cells, suggesting β-arrestin2 degradation is mediated by MDM2 through the proteasomal pathway. Using serotonin 5-HT1A receptors (5-HT1ARs) as a model receptor system, we found that ethanol dose-dependently inhibits 5-HT1AR internalization and that MDM2 knockdown reverses this effect. Moreover, ethanol both reduced β-arrestin2 levels and delayed agonist-induced β-arrestin2 recruitment to the membrane. We conclude that β-arrestin2 dysregulation by ethanol impairs 5-HT1AR trafficking. Our findings reveal a critical molecular mechanism underlying ethanol-induced alterations in GPCR internalization and implicate β-arrestin as a potential player mediating behavioral responses to acute alcohol exposure.

Decrease of inhibitory synaptic currents of locus coeruleus neurons via orexin type 1 receptors in the context of naloxone-induced morphine withdrawal

Acute opioid withdrawal syndrome is a series of neurological symptoms caused by the abrupt cessation of the chronic administration of opioids such as morphine. The locus coeruleus (LC) in the brain stem receives a dense projection of orexinergic fibers from the hypothalamus and is a candidate site for the expression of the somatic aspects of morphine withdrawal. Previous studies have shown that orexin-A contributes to the behavioral symptoms of naloxone-induced morphine withdrawal, partly by reducing the activity of GABAergic neurons, suggesting that orexin-A may negatively modulate fast GABAergic neurotransmission during morphine withdrawal. We used whole-cell patch-clamp recordings of LC neurons in brainstem slices to investigate the effect of orexin-A on bicuculline-sensitive GABAergic inhibitory postsynaptic currents (IPSCs) during naloxone-induced morphine withdrawal. Male Wistar rats (P14-P21) were given morphine (20 mg/kg, i.p.) daily for seven consecutive days to create dependency on the drug. The application of naloxone (1 µM) to brain slices of morphine-treated rats reduced the amplitude of evoked IPSCs (eIPSCs) as well as spontaneous IPSCs (sIPSCs) frequency but did not change sIPSCs amplitude. Orexin-A (100 nM) significantly enhanced the suppressive effect of naloxone on eIPSCs amplitude and sIPSCs frequency but had no effect on the presence of the orexin type 1 receptor (OX1R) antagonist, SB-334867. Orexin-A alone had no significant effect on eIPSCs and sIPSCs in the absence of naloxone. In summary, our results show that orexin-A, via OX1R, potentiates the suppressive effect of naloxone on GABAergic IPSCs of LC neurons in morphine-treated rats. We conclude that orexins may have a critical role in regulating GABAergic neurotransmission to LC neurons during naloxone-induced morphine withdrawal.

Dynamic Opioid Receptor Regulation in the Periphery

Opioids serve a vital role in the current analgesic array of treatment options. They are useful in acute instances involving severe pain associated with trauma, surgery, and terminal diseases such as cancer. In the past three decades, multiple receptor isoforms and conformations have been reported throughout literature. Most of these studies conducted systemic analyses of opioid receptor function, often generalizing findings from receptor systems in central nervous tissue or exogenously expressing immortalized cell lines as common mechanisms throughout physiology. However, a culmination of innovative experimental data indicates that opioid receptor systems are differentially modulated depending on their anatomic expression profile. Importantly, opioid receptors expressed in the peripheral nervous system undergo regulation uncommon to similar receptors expressed in central nervous system tissues. This distinctive characteristic begs one to question whether peripheral opioid receptors maintain anatomically unique roles, and whether they may serve an analgesic advantage in providing pain relief without promoting addiction.

G protein subunit phosphorylation as a regulatory mechanism in heterotrimeric G protein signaling in mammals, yeast, and plants

Heterotrimeric G proteins composed of Gα, Gβ, and Gγ subunits are vital eukaryotic signaling elements that convey information from ligand-regulated G protein-coupled receptors (GPCRs) to cellular effectors. Heterotrimeric G protein-based signaling pathways are fundamental to human health [Biochimica et Biophysica Acta (2007) 1768, 994-1005] and are the target of >30% of pharmaceuticals in clinical use [Biotechnology Advances (2013) 31, 1676-1694; Nature Reviews Drug Discovery (2017) 16, 829-842]. This review focuses on phosphorylation of G protein subunits as a regulatory mechanism in mammals, budding yeast, and plants. This is a re-emerging field, as evidence for phosphoregulation of mammalian G protein subunits from biochemical studies in the early 1990s can now be complemented with contemporary phosphoproteomics and genetic approaches applied to a diversity of model systems. In addition, new evidence implicates a family of plant kinases, the receptor-like kinases, which are monophyletic with the interleukin-1 receptor-associated kinase/Pelle kinases of metazoans, as possible GPCRs that signal via subunit phosphorylation. We describe early and modern observations on G protein subunit phosphorylation and its functional consequences in these three classes of organisms, and suggest future research directions.

Selective up-regulation of functional mu-opioid receptor splice variants by chronic opioids

We recently reported (Verzillo, et al. J. Neurochem: 130, 790-796, 2014) that chronic systemic morphine selectively up-regulates mRNA encoding two C-terminal μ-opioid receptor (MOR) splice variants, MOR-1C1 and MOR-1B2 (MOR-1B2/-1C1). Given the known disconnects between changes in levels of mRNA and corresponding protein, it is essential to directly demonstrate that chronic opioid treatment elevates functional MOR-1B2/-1C1 protein prior to inferring relevance of these MOR variants to spinal opioid tolerance mechanisms. Accordingly, we investigated the ability of chronic opioid exposure to up-regulate MOR protein in Chinese hamster ovary cells stably transfected with rat MOR variants MOR-1B2, MOR-1C1, or MOR-1 (considered to be the predominant MOR). Findings revealed that chronic treatment with the clinically relevant opioids morphine, oxycodone and hydrocodone substantially up-regulated membrane MOR-1B2/-1C1 protein. This up-regulation was abolished by the protein synthesis inhibitor cycloheximide, eliminating contributions from receptor redistribution. The increment in MOR-1B2/-1C1 protein was paralleled by a significant increment in opioid agonist-stimulated GTPγS-binding (reflective of increased aggregate MOR G protein coupling) indicating that up-regulated MOR-1B2/-1C1 represented functional receptors. Strikingly, these tolerance-associated adaptations of MOR-1B2/-1C1 differed considerably from those of MOR-1. Antithetical regulation of MOR-1B2/-1C1 and MOR-1 by chronic opioids has significant implications for the design of new therapeutic agents to counteract opioid analgesic tolerance and accompanying addiction. Since chronic opioids induce MOR-1B2/-1C1 up-regulation in spinal cord of males, but not females, elucidating cellular compartments and intracellular pathways mediating MOR-1B2/-1C1 up-regulation and defining their unique signaling attributes would enable a precision medicinal approach to pain management and addiction therapy. In the spinal cord of males, but not females, chronic morphine up-regulates mRNA encoding two mu-opioid receptor (MOR) variants, MOR-1B2 and MOR-1C1 (MOR-1B2/-1C1). We now demonstrate that chronic treatment with the clinically relevant opioids morphine, hydrocodone or oxycodone up-regulates MOR-1B2/-1C1 functional protein, which is dependent on de novo protein synthesis. Findings underscore the importance of unique signaling attributes of MOR variants to sexually dimorphic tolerance mechanisms.

Acute morphine alters GABAergic transmission in the central amygdala during naloxone-precipitated morphine withdrawal: role of cyclic AMP

The central amygdala (CeA) plays an important role in opioid addiction. Therefore, we examined the effects of naloxone-precipitated morphine withdrawal (WD) on GABAergic transmission in rat CeA neurons using whole-cell recordings with naloxone in the bath. The basal frequency of miniature inhibitory postsynaptic currents (mIPSCs) increased in CeA neurons from WD compared to placebo rats. Acute morphine (10 μ M) had mixed effects (≥20% change from baseline) on mIPSCs in placebo and WD rats. In most CeA neurons (64%) from placebo rats, morphine significantly decreased mIPSC frequency and amplitude. In 32% of placebo neurons, morphine significantly increased mIPSC amplitudes but had no effect on mIPSC frequency. In WD rats, acute morphine significantly increased mIPSC frequency but had no effect on mIPSC amplitude in 41% of CeA neurons. In 45% of cells, acute morphine significantly decreased mIPSC frequency and amplitude. Pre-treatment with the cyclic AMP inhibitor (R)-adenosine, cyclic 3',5'-(hydrogenphosphorothioate) triethylammonium (RP), prevented acute morphine-induced potentiation of mIPSCs. Pre-treatment of slices with the Gi/o G-protein subunit inhibitor pertussis toxin (PTX) did not prevent the acute morphine-induced enhancement or inhibition of mIPSCs. PTX and RP decreased basal mIPSC frequencies and amplitudes only in WD rats. The results suggest that inhibition of GABAergic transmission in the CeA by acute morphine is mediated by PTX-insensitive mechanisms, although PTX-sensitive mechanisms cannot be ruled out for non-morphine responsive cells; by contrast, potentiation of GABAergic transmission is mediated by activated cAMP signaling that also mediates the increased basal GABAergic transmission in WD rats. Our data indicate that during the acute phase of WD, the CeA opioid and GABAergic systems undergo neuroadaptative changes conditioned by a previous chronic morphine exposure and dependence.

Inhibitory effect of spinal mGlu(5) receptor antisense oligonucleotide on the up-regulated expression of spinal G protein associated with chronic morphine treatment

Knockdown of spinal metabotropic glutamate 5 (mGlu5) receptor was shown to inhibit the development of intrathecal morphine antinociceptive tolerance. The present work was designed to evaluate the expression of spinal G-protein during morphine tolerance and knockdown of spinal mGlu5 receptor with antisense oligonucleotide (ODN). Rats were treated with saline, morphine, mGlu5 receptor antisense or mismatch ODN intrathecally. Behavioral tests were employed to test the thermal and mechanical pain thresholds. Five days later, rats were scarified and spinal expression of spinal Gαi, Gαo, Gαq and Gβ were detected. Consistent with the previous results, knockdown of spinal mGlu5 receptor could inhibit spinal morphine antinociceptive tolerance in behavioral tests (P<0.05). The mGlu5 receptor antisense ODN produced a significant reduction in mGlu5 receptor protein of about 56.6% compared with the control group (P<0.05). Expression of spinal Gαi, Gαo, Gαq and Gβ were up-regulated while morphine tolerance developed (P<0.05). Antisense ODN of spinal mGlu5 receptor, but not mismatched ODN, reduced the spinal dorsal horn levels of Gαi, Gαo, Gαs, Gαq and Gβ (P<0.05). We conclude that expression of spinal G (αi, αo, αs, αq and β) protein may be up-regulated after chronic morphine treatment which could be attenuated by knockdown of spinal mGlu5 receptor with antisense ODN.

A novel noncanonical signaling pathway for the μ-opioid receptor

The µ-opioid receptor (OPRM1) signals as a classic G protein-coupled receptor by activating heterotrimeric Gi/Go proteins resulting in adenylyl cyclase (AC) inhibition. Such AC inhibition is desensitized after prolonged agonist treatment. However, after receptor desensitization, the intracellular cAMP level remains regulated by OPRM1, as demonstrated by the intracellular cAMP level increase or AC superactivation upon removal of an agonist or addition of an antagonist. We now demonstrate that such intracellular cAMP regulation is mediated by a novel noncanonical signaling pathway resulting from OPRM1 being converted to a receptor tyrosine kinase (RTK)-like entity. This noncanonical OPRM1 signaling is initiated by the receptor recruiting and activating Src kinase within the receptor complex, leading to phosphorylation of the OPRM1 Tyr(336) residue. Phospho-Tyr(336) serves as the docking site for growth factor receptor-bound protein/son of sevenless, leading to the recruitment and activation of the Ras/Raf-1 and subsequent phosphorylation and activation of AC5/6 by Raf-1. Such sequence of events was established by the absence of Ras/Raf1 recruitment and activation by the OPRM1-Y336F mutant, by the presence of Src kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) or the absence of Src activity, by the presence of specific Raf-1 inhibitor GW5074 (5-iodo-3-[(3,5-dibromo-4-hydroxyphenyl) methylene]-2-indolinone) or the absence of Raf-1, or by the dominant negative RasN17 mutant. Src together with Ras activates Raf1 which was established by the inability of the Raf1-Tyr(340/341) mutant to activate AC. Hence, the phosphorylation of OPRM1 at Tyr(336) by Src serves as the trigger for the conversion of a classic Gi/Go-coupled receptor into an RTK-like entity, resulting in a noncanonical pathway even after the original Gi/Go signals are blunted.

Role of neuroinflammation in morphine tolerance: effect of tumor necrosis factor-α

Opioids have been used as potent analgesics in clinics for decades; however, their long-term administration leads to tolerance. Two possible mechanisms for drug tolerance are postulated as within-system and between-systems adaptation. The within-system tolerance is involved in the signal transduction of opioid receptors, including downregulation of opioid receptors, uncoupling of G-protein from opioid receptors, and β-arrestin recruitment to opioid receptors, which causes receptor desensitization and internalization/endocytosis. The between-systems tolerance comprehends the glutamatergic receptor system and glial activation with the release of proinflammatory cytokines, and thus the analgesic effect of morphine is reduced. Tumor necrosis factor-α (TNF-α) is a vital proinflammatory cytokine and exerts either a neurotoxic or neuroprotective effect on different diseases of the central nervous system. TNF-α has also been demonstrated to correlate with neuronal plasticity via activation of spinal glial cells and enhancement of glutamatergic transmission. Previous studies had revealed an increased expression of TNF-α in morphine tolerance. This review article focuses on the role of TNF-α in neuroinflammation and the glutamatergic receptor system in morphine tolerance. It may provide another adjuvant therapy for morphine tolerance, which extends the effectiveness of opioids in clinical pain management.

Pleiotropic opioid regulation of spinal endomorphin 2 release and its adaptations to opioid withdrawal are sexually dimorphic

We studied adaptations to acute precipitated opioid withdrawal of spinal μ-opioid receptor (MOR)-coupled regulation of the release of endomorphin 2 (EM2). The release of this highly MOR-selective endogenous opioid from opioid-naive spinal tissue of male rats is subjected to MOR-coupled positive as well as negative modulation via cholera toxin-sensitive G(s) and pertussis toxin-sensitive G(i)/G(o), respectively. The net effect of this concomitant bidirectional modulation is inhibitory. MOR-coupled pleiotropic regulation of EM2 release is retained in opioid-withdrawn spinal tissue of male rats, but the balance of MOR-coupled inhibitory and facilitatory regulation shifted such that facilitatory regulation predominates. Augmented coupling of MOR to G(s) is causally associated with this change. Strikingly, pleiotropic characteristics of MOR-coupled regulation of spinal EM2 release and adaptations thereof to opioid withdrawal are male-specific. In females, MOR-coupled regulation of EM2 release from opioid-naive and -withdrawn spinal tissue does not have a significant G(s)-coupled facilitatory component, and MOR-coupled inhibition of EM2 release persists unabated in withdrawn preparations. The male-specific adaptations to chronic morphine that shift the relative predominance of opposing dual G protein-coupled MOR pathways provides a mechanism for mitigating inhibitory MOR signaling without losing MOR-coupled feedback regulation. These adaptations enable using endogenous EM2 as a substitute for morphine that had been precipitously removed. The sexually dimorphic functionality and regulation of spinal EM2/MOR-coupled signaling suggest the clinical utility of using sex-specific treatments for addiction that harness the activity of endogenous opioids.

Etanercept restores the antinociceptive effect of morphine and suppresses spinal neuroinflammation in morphine-tolerant rats

BACKGROUND

In the present study we examined the effect of the tumor necrosis factor (TNF)-α antagonist etanercept on the antinociceptive effect of morphine in morphine-tolerant rats.

METHODS

Male Wistar rats were implanted with 2 intrathecal catheters, and 1 was connected to a mini-osmotic pump for either morphine (15 μg/h) or saline (1 μL/h) infusion for 5 days. On day 5, either etanercept (5 μg, 25 μg, and 50 μg/10 μL) or saline (10 μL) was injected via the other catheter after morphine infusion was discontinued. Three hours later, morphine (15 μg/10 μL, intrathecally) was given and tail-flick latency was measured to evaluate the antinociceptive effect of morphine. Rats were then killed and their spinal cords were removed for quantitative real-time polymerase chain reaction and immunohistochemistry to measure proinflammatory cytokines expression.

RESULTS

We found that acute etanercept (50 μg) treatment preserved a significant antinociceptive effect of morphine in morphine-tolerant rats. In addition, the expression of TNFα mRNA was increased by 2.5-fold, interleukin (IL)-1β mRNA increased by 13-fold and IL-6 mRNA by 111-fold in the dorsal spinal cord of morphine-tolerant rats. The increase in TNFα, IL-1β, and IL-6 mRNA expression was blocked by 50 μg etanercept pretreatment. The immunohistochemistry analysis revealed that 50 μg etanercept suppressed proinflammatory cytokines expression and neuroinflammation in the microglia.

CONCLUSIONS

The present study demonstrates that etanercept restores the antinociceptive effect of morphine in morphine-tolerant rats by inhibition of proinflammatory cytokine TNF-α, IL-1β, and IL-6 expression and spinal neuroinflammation. The results suggest that etanercept could also be an adjuvant therapy for morphine tolerance, which extends the effectiveness of opioids in clinical pain management.

Down-regulation of c-Cbl by morphine accounts for persistent ERK1/2 signaling in delta-opioid receptor-expressing HEK293 cells

Opioids display ligand-specific differences in the time course of ERK1/2 signaling. Whereas full agonists, like etorphine, induce only transient activation of ERK1/2, the partial agonist morphine mediates persistent stimulation of mitogenic signaling. Here we report that in stably delta-opioid receptor (DOR)-expressing HEK293 (HEK/DOR) cells, the transient nature of etorphine-induced ERK1/2 signaling is due to desensitization of epidermal growth factor (EGF) receptor-mediated activation of the Ras/Raf-1/ERK1/2 cascade. Desensitization of ERK1/2 activity by etorphine is associated with down-regulation of EGF receptors, an effect mediated by the ubiquitin ligase c-Cbl. In contrast, chronic morphine treatment failed to desensitize EGF receptors, resulting in unimpeded ERK1/2 signaling. The failure of morphine to desensitize ERK1/2 signaling is mediated by persistent activation of c-Src, which induces degradation of c-Cbl. The role of c-Src in opioid-specific ERK1/2 signaling is further demonstrated by pretreatment of the cells with PP2 and SKI-I as well as overexpression of a dominant negative c-Src mutant (c-Src(dn)) or a c-Src-resistant c-Cbl mutant (CblY3F), both of which facilitate desensitization of ERK1/2 signaling by morphine. Conversely, overexpression of c-Src as well as down-regulation of c-Cbl by small interfering RNA results in persistent etorphine-induced stimulation of ERK1/2 activity. Subcellular fractionation experiments finally attributed the ability of morphine to persistently activate c-Src to its redistribution from Triton X-100-insensitive membrane rafts to DOR and EGF receptor containing high density membrane compartments implicated in ERK1/2 signaling. These results demonstrate that agonist-specific differences in the temporal and spatial pattern of c-Src activation determine the kinetics of DOR-mediated regulation of ERK1/2 signaling.

Effect of KEPI (Ppp1r14c) deletion on morphine analgesia and tolerance in mice of different genetic backgrounds: when a knockout is near a relevant quantitative trait locus

We previously identified KEPI as a morphine-regulated gene using subtractive hybridization and differential display PCR. Upon phosphorylation by protein kinase C, KEPI becomes a powerful inhibitor of protein phosphatase 1. To gain insights into KEPI functions, we created KEPI knockout (KO) mice on mixed 129S6xC57BL/6 genetic backgrounds. KEPI maps onto mouse chromosome 10 close to the locus that contains the mu-opioid receptor (Oprm1) and provides a major quantitative trait locus for morphine effects. Analysis of single nucleotide polymorphisms in and near the Oprm1 locus identified a doubly-recombinant mouse with C57BL/6 markers within 1 Mb on either side of the KEPI deletion. This strategy minimized the amount of 129S6 DNA surrounding the transgene and documented the C57BL/6 origin of the Oprm1 gene in this founder and its offspring. Recombinant KEPIKO mice displayed (a) normal analgesic responses and normal locomotion after initial morphine treatments, (b) accelerated development of tolerance to analgesic effects of morphine, (c) elevated activity of protein phosphatase 1 in thalamus, (d) attenuated morphine reward as assessed by conditioned place preference. These data support roles for KEPI action in adaptive responses to repeated administration of morphine that include analgesic tolerance and drug reward.

Mu-opioid receptor redistribution in the locus coeruleus upon precipitation of withdrawal in opiate-dependent rats

Administration of mu-opioid receptor (MOR) agonists is known to produce adaptive changes within noradrenergic neurons of the rat locus coeruleus (LC). Alterations in the subcellular distribution of MOR have been shown to occur in the LC in response to full agonists and endogenous peptides; however, there is considerable debate in the literature whether trafficking of MOR occurs after chronic exposure to the partial-agonist morphine. In the present study, we examined adaptations in MOR after chronic opioid exposure using immunofluorescence and electron microscopy (EM), using receptor internalization as a functional endpoint. MOR trafficking in LC neurons was characterized in morphine-dependent rats that were given naltrexone at a dose known to precipitate withdrawal. After chronic morphine exposure, a subtle redistribution of MOR immunoreactivity from the membrane to the cytosol was detected within dendrites of LC neurons. Interestingly, an acute injection of naltrexone in rats exposed to chronic morphine produced a robust internalization of MOR, whereas administration of naltrexone failed to do so in naïve animals. These findings provide anatomical evidence for modified regulation of MOR trafficking after chronic morphine treatment in brain noradrenergic neurons. Adaptations in the MOR signaling pathways that regulate internalization may occur as a consequence of chronic treatment and precipitation of withdrawal. Mechanisms underlying this effect might include differential MOR regulation in the LC, or downstream effects of withdrawal-induced enkephalin (ENK) release from afferents to the LC.

Src phosphorylation of micro-receptor is responsible for the receptor switching from an inhibitory to a stimulatory signal

Recent studies have revealed that in G protein-coupled receptor signalings switching between G protein- and beta-arrestin (betaArr)-dependent pathways occurs. In the case of opioid receptors, the signal is switched from the initial inhibition of adenylyl cyclase (AC) to an increase in AC activity (AC activation) during prolonged agonist treatment. The mechanism of such AC activation has been suggested to involve the switching of G proteins activated by the receptor, phosphorylation of signaling molecules, or receptor-dependent recruitment of cellular proteins. Using protein kinase inhibitors, dominant negative mutant studies and mouse embryonic fibroblast cells isolated from Src kinase knock-out mice, we demonstrated that mu-opioid receptor (OPRM1)-mediated AC activation requires direct association and activation of Src kinase by lipid raft-located OPRM1. Such Src activation was independent of betaArr as indicated by the ability of OPRM1 to activate Src and AC after prolonged agonist treatment in mouse embryonic fibroblast cells lacking both betaArr-1 and -2. Instead the switching of OPRM1 signals was dependent on the heterotrimeric G protein, specifically Gi2 alpha-subunit. Among the Src kinase substrates, OPRM1 was phosphorylated at Tyr336 within NPXXY motif by Src during AC activation. Mutation of this Tyr residue, together with mutation of Tyr166 within the DRY motif to Phe, resulted in the complete blunting of AC activation. Thus, the recruitment and activation of Src kinase by OPRM1 during chronic agonist treatment, which eventually results in the receptor tyrosine phosphorylation, is the key for switching the opioid receptor signals from its initial AC inhibition to subsequent AC activation.

Physiological roles for G protein-regulated adenylyl cyclase isoforms: insights from knockout and overexpression studies

Cyclic AMP is a universal second messenger, produced by a family of adenylyl cyclase (AC) enzymes. The last three decades have brought a wealth of new information about the regulation of cyclic AMP production by ACs. Nine hormone-sensitive, membrane-bound AC isoforms have been identified in addition to a tenth isoform that lacks membrane spans and more closely resembles the cyanobacterial AC enzymes. New model systems for purifying and characterizing the catalytic domains of AC have led to the crystal structure of these domains and the mapping of numerous interaction sites. However, big hurdles remain in unraveling the roles of individual AC isoforms and their regulation in physiological systems. In this review we explore the latest on AC knockout and overexpression studies to better understand the roles of G protein regulation of ACs in the brain, olfactory bulb, and heart.

Oxycodone plus ultra-low-dose naltrexone attenuates neuropathic pain and associated mu-opioid receptor-Gs coupling

Both peripheral nerve injury and chronic opioid treatment can result in hyperalgesia associated with enhanced excitatory neurotransmission at the level of the spinal cord. Chronic opioid administration leads to a shift in mu-opioid receptor (MOR)-G protein coupling from G(i/o) to G(s) that can be prevented by cotreatment with an ultra-low-dose opioid antagonist. In this study, using lumbar spinal cord tissue from rats with L(5)/L(6) spinal nerve ligation (SNL), we demonstrated that SNL injury induces MOR linkage to G(s) in the damaged (ipsilateral) spinal dorsal horn. This MOR-G(s) coupling occurred without changing G(i/o) coupling levels and without changing the expression of MOR or Galpha proteins. Repeated administration of oxycodone alone or in combination with ultra-low-dose naltrexone (NTX) was assessed on the SNL-induced MOR-G(s) coupling as well as on neuropathic pain behavior. Repeated spinal oxycodone exacerbated the SNL-induced MOR-G(s) coupling, whereas ultra-low-dose NTX cotreatment slightly but significantly attenuated this G(s) coupling. Either spinal or oral administration of oxycodone plus ultra-low-dose NTX markedly enhanced the reductions in allodynia and thermal hyperalgesia produced by oxycodone alone and minimized tolerance to these effects. The MOR-G(s) coupling observed in response to SNL may in part contribute to the excitatory neurotransmission in spinal dorsal horn in neuropathic pain states. The antihyperalgesic and antiallodynic effects of oxycodone plus ultra-low-dose NTX (Oxytrex, Pain Therapeutics, Inc., San Mateo, CA) suggest a promising new treatment for neuropathic pain.

PERSPECTIVE

The current study investigates whether Oxytrex (oxycodone with an ultra-low dose of naltrexone) alleviates mechanical and thermal hypersensitivities in an animal model of neuropathic pain over a period of 7 days, given locally or systemically. In this report, we first describe an injury-induced shift in mu-opioid receptor coupling from G(i/o) to G(s), suggesting why a mu-opioid agonist may have reduced efficacy in the nerve-injured state. These data present a novel approach to neuropathic pain therapy.

Plasticity of adenylyl cyclase-related signaling sequelae after long-term morphine treatment

Adaptations to long-term morphine treatment resulting in tolerance are protective by counteracting the consequences of sustained opioid receptor activation. Consequently, the manifestation of specific adenylyl cyclase (AC)-related neurochemical sequelae of long-term morphine treatment should depend on the consequences of short-term mu-opioid receptor (MOR) activation. We tested this by comparing complementary chemical sequelae of long-term morphine treatment among cells in which short-term MOR activation inhibited instead of stimulated AC activity. Short-term activation of MOR in Chinese hamster ovary (CHO) cells stably transfected with MOR (MOR-CHO) inhibits AC activity. Long-term morphine treatment of these cells increased AC and Gbeta phosphorylation, membrane protein kinase Cgamma (PKCgamma) translocation, and MOR G(s) association. All converge, shifting the consequences of short-term MOR activation from Galpha(i)/Galpha(o) inhibitory to AC stimulatory signaling. In contrast, overexpression of the Gbetagamma-stimulated AC isoform AC2 (which converted MOR-coupled inhibition to stimulation of AC) eliminated or reversed these adaptations to long-term morphine treatment; it negated the increase in Gbeta phosphorylation and PKCgamma translocation while reversing the increase in AC phosphorylation and MOR G(s) association. These adaptations greatly attenuated MOR-coupled stimulation of AC activity. Altered overexpression of AC protein per se was not a confounding factor because MOR-CHO overexpressing AC1, which is inhibited by short-term MOR activation, manifested adaptations to long-term morphine treatment qualitatively identical with those of MOR-CHO. These results reveal that adaptations elicited by long-term morphine treatment depend on the effects of short-term MOR activation. This dynamic and pliable nature of tolerance mechanisms could represent a new paradigm for pharmacotherapeutics.

Phosphorylation of Galphas influences its association with the micro-opioid receptor and is modulated by long-term morphine exposure

The recent biochemical demonstration of the association of the mu-opioid receptor (MOR) with Galpha(s) that increases after long-term morphine treatment (Mol Brain Res 135:217-224, 2005) provides a new imperative for studying MOR-Galpha(s) interactions and the mechanisms that modulate it. A persisting challenge is to elucidate those neurochemical parameters modulated by long-term morphine treatment that facilitate MOR-Galpha(s) association. This study demonstrates that 1) Galpha(s) exists as a phosphoprotein, 2) the stoichiometry of Galpha(s) phosphorylation decreases after long-term morphine treatment, and 3) in vitro dephosphorylation of Galpha(s) increases its association with MOR. Furthermore, our data suggest that increased association of Galpha(s) with protein phosphatase 2A is functionally linked to the long-term morphine treatment-induced reduction in Galpha(s) phosphorylation. These findings are observed in MOR-Chinese hamster ovary and F11 cells as well as spinal cord, indicating that they are not idiosyncratic to the particular cell line used or a "culture" phenomenon and generalize to complex neural tissue. Taken together, these results indicate that the phosphorylation state of Galpha(s) is a critical determinant of its interaction with MOR. Long-term morphine treatment decreases Galpha(s) phosphorylation, which is a key mechanism underlying the previously demonstrated increased association of MOR and Galpha(s) in opioid tolerant tissue.

Augmentation of spinal morphine analgesia and inhibition of tolerance by low doses of mu- and delta-opioid receptor antagonists

BACKGROUND AND PURPOSE

Ultralow doses of naltrexone, a non-selective opioid antagonist, have previously been found to augment acute morphine analgesia and block the development of tolerance to this effect. Since morphine tolerance is dependent on the activity of micro and delta receptors, the present study investigated the effects of ultralow doses of antagonists selective for these receptor types on morphine analgesia and tolerance in tests of thermal and mechanical nociception.

EXPERIMENTAL APPROACH

Effects of intrathecal administration of mu-receptor antagonists, CTOP (0.01 ng) or CTAP (0.001 ng), or a delta-receptor antagonist, naltrindole (0.01 ng), on spinal morphine analgesia and tolerance were evaluated using the tail-flick and paw-pressure tests in rats.

KEY RESULTS

Both micro and delta antagonists augmented analgesia produced by a sub-maximal (5 microg) or maximal (15 microg) dose of morphine. Administration of the antagonists with morphine (15 microg) for 5 days inhibited the progressive decline of analgesia and prevented the loss of morphine potency. In animals exhibiting tolerance to morphine, administration of the antagonists with morphine produced a recovery of the analgesic response and restored morphine potency.

CONCLUSIONS AND IMPLICATIONS

Combining ultralow doses of micro- or delta-receptor antagonists with spinal morphine augmented the acute analgesic effects, inhibited the induction of chronic tolerance and reversed established tolerance. The remarkably similar effects of micro- and delta-opioid receptor antagonists on morphine analgesia and tolerance are interpreted in terms of blockade of the latent excitatory effects of the agonist that limit expression of its full activity.

Different effects of opioid antagonists on mu-, delta-, and kappa-opioid receptors with and without agonist pretreatment

Opioid receptors display basal signaling (constitutive, agonist-independent activity), which seems to be regulated by agonist exposure. Whereas agonist pretreatment desensitizes receptors to subsequent agonist stimulation, basal signaling of mu-opioid receptor (MOR) was shown to increase. Moreover, agonist pretreatment converts the neutral antagonists naloxone and naltrexone into inverse agonists, suppressing basal signaling, whereas analogs with reduced C6-position, e.g., 6beta-naltrexol, remain neutral antagonists at MOR under any condition. This study compares the regulation of basal signaling of MOR, delta-(DOR), and kappa-(KOR) opioid receptors after pretreatment with morphine or receptor-selective agonists, in transfected human embryonic kidney 293 cell membranes. Moreover, naloxone, naltrexone, and related antagonists were compared for binding potency and effect on basal and agonist-stimulated receptor signaling, measuring guanosine 5'-O-(3-[35S]thio)triphosphate binding. The results demonstrate basal activity for each opioid receptor, which is modulated by pretreatment with agonists. Even closely related opioid antagonists display distinct patterns of neutral and inverse effects before and after agonist pretreatment, including distinct efficacies between naloxone and naltrexone at agonist-pretreated DOR and KOR. Pretreatment with different agonists has varying effects on inverse and neutral activities of some analogs tested. These results demonstrate that antagonist efficacy is context-dependent, possibly accounting for paradoxical pharmacological effects. Activity profiles at the three opioid receptors under different conditions could lead to antagonists with optimal clinical properties in treatment of addiction and adverse opioid effects.

Functional interaction between α2-adrenoceptors, μ- and κ-opioid receptors in the guinea pig myenteric plexus: Effect of chronic desipramine treatment

The existence of a functional interplay between alpha(2)-adrenoceptor and opioid receptor inhibitory pathways modulating neurotransmitter release has been demonstrated in the enteric nervous system by development of sensitivity changes to alpha(2)-adrenoceptor, mu- and kappa-opioid receptor agents on enteric cholinergic neurons after chronic sympathetic denervation. In the present study, to further examine this hypothesis we evaluated whether manipulation of alpha(2)-adrenoceptor pathways by chronic treatment with the antidepressant drug, desipramine (10 mg/kg i.p. daily, for 21 days), could entail changes in enteric mu- and kappa-opioid receptor pathways in the myenteric plexus of the guinea pig distal colon. In this region, subsensitivity to the inhibitory effect of both UK14,304 and U69,593, respectively alpha(2A)-adrenoceptor and kappa-opioid receptor agonist, on the peristaltic reflex developed after chronic desipramine treatment. On opposite, in these experimental conditions, supersensitivity developed to the inhibitory effect of [D-Ala, N-Me-Phe4-Gly-ol5]-enkephalin (DAMGO), mu-opioid receptor agonist, on propulsion velocity. Immunoreactive expression levels of alpha(2A)-adrenoceptors, mu- and kappa-opioid receptors significantly decreased in the myenteric plexus of the guinea pig colon after chronic desipramine treatment. In these experimental conditions, mRNA levels of alpha(2A)-adrenoceptors, mu- and kappa-opioid receptors significantly increased, excluding a direct involvement of transcription mechanisms in the regulation of receptor expression. Levels of G protein-coupled receptor kinase 2/3 and of inhibitory G(i/o) proteins were significantly reduced in the myenteric plexus after chronic treatment with desipramine. Such changes might represent possible molecular mechanisms involved in the development of subsensitivity to UK14,304 and U69,593 on the efficiency of peristalsis. Alternative molecular mechanisms, including a higher efficiency in the coupling between receptor activation and downstream intracellular effector systems, possibly independent from inhibitory G(i/o) proteins, may be accounted for the development of supersensitivity to DAMGO. Increased sensitivity to the mu-opioid agonist might compensate for the development of alpha(2A)-adrenoceptor and kappa-opioid receptor subsensitivity. On the whole, the present data further strengthen the concept that, manipulation of alpha(2)-adrenergic inhibitory receptor pathways in the enteric nervous system entails changes in opioid inhibitory receptor pathways, which might be involved in maintaining homeostasis as suggested for mu-opioid, but not for kappa-opioid receptors.

G-protein coupled receptor kinase-like immunoreactivity in the snail, Helix pomatia, neurons

GPRCs are regulated via phosphorylation by different protein kinases including GRKs and PKA and PKC. The purpose of this study was to determine the presence and physiological role of GRKs in the tissues of the snail, Helix pomatia. Here we report that immunoblotting of brain homogenate with anti-GRK2/3 antibody prepared from mammalian tissue can be detected in snail GRK-like immunoreactivity. The GRK2/3 immunoreactivity was found at approximately 80 kDa in a variety of cells, including salivary duct, salivary gland and eye. Intracellular injection of the anti-GRK2/3 prevented the neuron from desensitization and agonist-induced activation augmented the phosphorylated GRKs in the membrane fraction suggesting that GRKs may have a functional role in the neuropeptide receptor desensitization in snail.

Phosphorylation: A molecular switch in opioid tolerance

Protein phosphorylation is a key posttranslational modification mechanism controlling the conformation and activity of many proteins. Increasing evidence has implicated an essential role of phosphorylation by several major protein kinases in promoting and maintaining opioid tolerance. We review some of the most recent studies on protein kinase C (PKC), cyclic AMP dependent protein kinase A (PKA), calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase G (PKG), and G protein receptor kinase (GRK). These kinases act as the molecular switches to modulate opioid tolerance. Pharmacological interventions at one or more of the protein kinases and phosphatases may provide valuable strategies to improve opioid analgesia by attenuating tolerance to these drugs.

Post-opioid receptor adaptations to chronic morphine; Altered functionality and associations of signaling molecules

Opioid desensitization/tolerance mechanisms have largely focused on adaptations that occur on the level of the mu-opioid receptor (MOR) itself. These include opioid receptor phosphorylation and ensuing trafficking events. Recent research, however, has revealed additional adaptations that occur downstream from the opioid receptor, which involve covalent modification of signaling molecules and altered associations among them. These include augmented isoform-specific synthesis of adenylyl cyclase (AC) and their phosphorylation as well as augmented phosphorylation of the G(beta) subunit of G(beta gamma). The aggregate effect of these changes is to shift mu-opioid receptor-coupled signaling from predominantly G(i alpha) inhibitory to (G(i)-derived) G(beta gamma) stimulatory AC signaling. Most recently, chronic morphine has been shown to enhance the association (interaction) between MOR and G(s), which should provide an additional avenue for offsetting inhibitory MOR signaling sequelae. The unfolding complexity of chronic morphine-induced sequelae demands an evolving broader and more encompassing perspective on opioid tolerance-producing mechanisms. This should facilitate understanding tolerance within the context of physiological plasticity that is activated by chronic exposure to drugs of abuse. Additional research is required to integrate the various tolerance-producing adaptations that have been elucidated to date. Specifically, the relative contribution to opioid tolerance of identified adaptations is still unknown as is the extent to which they vary among different regions of the central nervous system.

Adenylyl cyclase type II activity is regulated by two different mechanisms: implications for acute and chronic opioid exposure

Acute and chronic activation of opioid receptors differentially regulate the activity of the various adenylyl cyclase (AC) isoforms. In several AC isoforms (I, V, VI and VIII) acute opioid activation (by agonists such as morphine) leads to AC inhibition, while prolonged opioid activation leads to increase in AC activity, a phenomenon known as AC sensitization or superactivation. In several other AC isoforms (II, IV and VII), acute opioid activation leads to AC stimulation, while chronic opioid exposure inhibits AC activity, in a process, which in analogy to the term "superactivation" is referred to as "superinhibition". AC-II is highly regulated by multiple and independent biochemical stimuli, including Gbetagamma, Galphas and PKC activation. We investigated the regulation of AC-II by Galphas and by PKC under conditions of acute and chronic exposure to opioid agonists in COS-7 transfected cells. We found that acute opioid exposure led to an increase in AC-II activity by either Galphas or PKC stimulation. This effect seems to be regulated by Gbetagamma subunits, in both activation pathways, as the increase in AC-II activity was abolished by pertussis toxin treatment and by Gbetagamma scavengers. On the other hand, while chronic opioid exposure led to a decrease in AC-II activity ("superinhibition") upon stimulation of the Galphas pathway, this superinhibition was not observed when the opioid treated cells were stimulated via PKC activation.

Opposing effects of phorbol-12-myristate-13-acetate, an activator of protein kinase C, on the signaling of structurally related human dopamine D1 and D5 receptors

The 'cross-talk' between different types of neurotransmitters through second messenger pathways represents a major regulatory mechanism in neuronal function. We investigated the effects of activation of protein kinase C (PKC) on cAMP-dependent signaling by structurally related human D1-like dopaminergic receptors. Human embryonic kidney 293 (HEK293) cells expressing D1 or D5 receptors were pretreated with phorbol-12-myristate-13-acetate (PMA), a potent activator of PKC, followed by analysis of dopamine-mediated receptor activation using whole cell cAMP assays. Unpredictably, PKC activation had completely opposite effects on D1 and D5 receptor signaling. PMA dramatically augmented agonist-evoked D1 receptor signaling, whereas constitutive and dopamine-mediated D5 receptor activation were rapidly blunted. RT-PCR and immunoblotting analyses showed that phorbol ester-regulated PKC isozymes (conventional: alpha, betaI, betaII, gamma; novel: delta, epsilon, eta, theta) and protein kinase D (PKCmicro) are expressed in HEK293 cells. PMA appears to mediate these contrasting effects through the activation of Ca2+-independent novel PKC isoforms as revealed by specific inhibitors, bisindolylmaleimide I, Gö6976, and Gö6983. The finding that cross-talk between PKC and cAMP pathways can produce such opposite outcomes following the activation of structurally similar D1-like receptor subtypes is novel and further strengthens the view that D1 and D5 receptors serve distinct functions in the mammalian nervous and endocrine systems.

Adenylyl cyclase superactivation induced by long-term treatment with opioid agonist is dependent on receptor localized within lipid rafts and is independent of receptor internalization

Long-term opioid agonist treatment results in adenylyl cyclase superactivation. A recent "RAVE" theory implicates a direct correlation between the ability of agonist to induce receptor internalization and the magnitude of adenylyl cyclase superactivation. We decided to test such a theory by examining the adenylyl cyclase superactivation after long-term activation of mu-opioid receptor (MOR) in an EcR293 cell model. We examined the magnitudes of adenylyl cyclase superactivation in the presence of naloxone after long-term treatment with morphine, etorphine, and methadone, three agonists reported to have differential activities in promoting MOR internalization. It can be shown that the magnitudes of adenylyl cyclase superactivation after treating with these three agonists, although different, were dependent on MOR density. Blunting MOR internalization with the dominant-negative mutant of dynamin, K44E, did not alter the magnitude of either morphine- or etorphine-induced adenylyl cyclase superactivation. In the presence of diprenorphine, the magnitude of adenylyl cyclase superactivation after etorphine treatment was identical to that observed with morphine. It could be demonstrated further that adenylyl cyclase superactivation is dependent on the cell surface-located MOR. Sucrose gradient fractionation demonstrated the colocalization of MOR and adenylyl cyclase V/VI with caveolin-1, a marker for lipid rafts. After long-term agonist treatment, the majority of MOR remained at the lipid rafts. Methyl-beta-cyclodextrin (MbetaCD) completely blunted the adenylyl cyclase superactivation and agonist-induced receptor internalization. These MbetaCD actions were reversed by incubating the cells with cholesterol. Thus, the adenylyl cyclase superactivation is not dependent on agonist-induced receptor internalization. Rather, the location of MOR at lipid rafts is an absolute requirement for the observed adenylyl cyclase superactivation.

Morphine tolerance does not develop in mice treated with endothelin-A receptor antagonists

Long-term use of morphine leads to development of antinociceptive tolerance. We provide evidence that central endothelin (ET) mechanisms are involved in development of morphine tolerance. In the present study, we investigated the effect of ET(A) receptor antagonists, BQ123 and BMS182874, on morphine antinociception and tolerance in mice. Mechanism of interaction of ET(A) receptor antagonists with morphine was investigated. BQ123 (3 microg, i.c.v.) and BMS182874 (50 microg, i.c.v.) significantly enhanced antinociceptive effect of morphine (P < 0.05), through an opioid-mediated effect. Treatment with a single dose of BQ123 (3 microg, i.c.v.) reversed tolerance to morphine antinociception in morphine-tolerant mice. BQ123 or BMS182874 did not affect naloxone binding in the brain. Therefore, ET(A) receptor antagonists did not bind directly to opioid receptors. [35S]GTPgammaS binding was stimulated by morphine and ET-1 in non-tolerant mice. Morphine- and ET-1-induced GTP stimulation was significantly lower (P < 0.05) in morphine-tolerant group (33% and 42%, respectively) compared to control group. BQ123 and BMS182874 did not activate binding in non-tolerant mice. BQ123 and BMS182874 significantly increased G protein activation in morphine-tolerant mice (96% and 86%, respectively; P < 0.05). These results provide evidence that uncoupling of G protein occurs in morphine-tolerant mice, and ET(A) antagonists promote coupling of G protein to its receptors, thereby restoring antinociceptive effect. These findings indicate that ET(A) receptor antagonists potentiate morphine antinociception and reverse antinociceptive tolerance in mice, through their ability to couple G proteins to opioid receptors.

Chronic morphine acts via a protein kinase Cgamma-G(beta)-adenylyl cyclase complex to augment phosphorylation of G(beta) and G(betagamma) stimulatory adenylyl cyclase signaling

Chronic morphine augments protein kinase C (PKC) phosphorylation of G(beta), which enhances the potency of G(betagamma) to stimulate adenylyl cyclase II (ACII) activity. The present study demonstrates an in vivo association between phosphorylated G(beta) and a specific PKC isoform, PKCgamma. We investigated the association of G(beta) and PKCgamma by assessing the ability of anti-PKCgamma antibodies to co-immunoprecipitate G(beta) from (32)P-radiolabeled Chinese Hamster Ovary cells stably transfected with a mu-opioid receptor (MOR-CHO). PKCgamma immunoprecipitate (IP) obtained from MOR-CHO membranes contained radiolabeled signals of approximately equals 33 and 36--38 kDa that were subsequently identified as G(beta)(s). Chronic morphine significantly increased ( approximately equals 75%) the magnitude of (32)P incorporated into G(beta) present in PKCgamma IP. This suggests that G(beta) is an in vivo substrate for PKCgamma, which mediates the chronic morphine-induced increment in G(beta) phosphorylation. In order to evaluate AC as a putative effector for phosphorylated G(betagamma), its presence in IP obtained using anti-AC antibodies was evaluated. Autoradiographic analyses of AC IP also revealed the presence of phosphorylated G(beta)(s), the magnitude of which was significantly enhanced ( approximately equals 60%) following chronic morphine treatment. This indicates that phosphorylated G(betagamma) associates and presumably interacts in vivo with AC, indicating that it is a target for the enhanced phosphorylated G(betagamma) that is generated following chronic morphine treatment. This would contribute to the previously observed shift from predominantly G(ialpha) inhibitory to G(betagamma) stimulatory AC signaling following chronic morphine. The PKCgamma-G(beta)-AC complex identified in this study provides an organizational framework for understanding the well-documented participation of PKCgamma in opioid tolerance-producing mechanisms.

Engineered G protein coupled receptors reveal independent regulation of internalization, desensitization and acute signaling

BACKGROUND

The physiological regulation of G protein-coupled receptors, through desensitization and internalization, modulates the length of the receptor signal and may influence the development of tolerance and dependence in response to chronic drug treatment. To explore the importance of receptor regulation, we engineered a series of Gi-coupled receptors that differ in signal length, degree of agonist-induced internalization, and ability to induce adenylyl cyclase superactivation. All of these receptors, based on the kappa opioid receptor, were modified to be receptors activated solely by synthetic ligands (RASSLs). This modification allows us to compare receptors that have the same ligands and effectors, but differ only in desensitization and internalization.

RESULTS

Removal of phosphorylation sites in the C-terminus of the RASSL resulted in a mutant that was resistant to internalization and less prone to desensitization. Replacement of the C-terminus of the RASSL with the corresponding portion of the mu opioid receptor eliminated the induction of AC superactivation, without disrupting agonist-induced desensitization or internalization. Surprisingly, removal of phosphorylation sites from this chimera resulted in a receptor that is constitutively internalized, even in the absence of agonist. However, the receptor still signals and desensitizes in response to agonist, indicating normal G-protein coupling and partial membrane expression.

CONCLUSIONS

These studies reveal that internalization, desensitization and adenylyl cyclase superactivation, all processes that decrease chronic Gi-receptor signals, are independently regulated. Furthermore, specific mutations can radically alter superactivation or internalization without affecting the efficacy of acute Gi signaling. These mutant RASSLs will be useful for further elucidating the temporal dynamics of the signaling of G protein-coupled receptors in vitro and in vivo.

Chronic morphine-induced loss of the facilitative interaction between vasoactive intestinal polypeptide and delta-opioid: involvement of protein kinase C and phospholipase Cbetas

This laboratory recently demonstrated a multiplicative interaction between the pelvic visceral afferent transmitter vasoactive intestinal polypeptide (VIP) and the delta-opioid receptor (DOR)-selective agonist [D-Pen2,5] enkephalin (DPDPE) to regulate cAMP levels in spinal cord [Brain Res. 959 (2003) 103]. Although DOR activation is required for the manifestation of the VIP-DPDPE facilitative interaction, its relevance to opioid antinociception remains unclear. The current study investigates whether or not the VIP-DPDPE facilitation of cAMP formation is subject to tolerance formation, a hallmark characteristic of opioid antinociception. Chronic morphine exposure abolishes the VIP-DPDPE facilitative interaction, consistent with its relevance to DOR antinociception. However, acute in vitro inhibition of protein kinase C (PKC) reinstates the VIP-DPDPE multiplicative interaction characteristic of opioid naïve spinal tissue. This suggests that its chronic morphine-induced loss requires a PKC phosphorylation. PKC phosphorylation negatively modulates phospholipase C (PLC)beta, enzymes intimately associated with phosphoinositide turnover and calcium trafficking. These are essential determinants of acute and chronic opioid effects. Accordingly, the effect of chronic morphine on their state of phosphorylation was also investigated. Central nervous system opioid tolerance is associated with the reciprocal phosphorylation (regulation) of two PLCbeta isoforms, PLCbeta1 and PLCbeta3. However, although chelerythrine reinstates the chronic morphine-induced loss of the multiplicative VIP-DPDPE interaction, it does not alter the associated changes in PLCbeta phosphorylation, possibly indicating different time courses of restitution of function and/or involvement of different kinases for different components of tolerance. These results could provide a mechanistic rubric for understanding positive modulation of opioid antinociception by afferent transmission.

Chronic exposure to morphine produces a marked cardioprotective phenotype in aged mouse hearts

Aging is often associated with decreased myocardial ischemic tolerance. We recently reported that chronic preconditioning produced by continuous exposure to morphine affords a profound cardioprotective phenotype in young mice. In this study, we determined if chronic exposure to morphine retained its ability to precondition the myocardium in the young or aged heart. Young (10-14 weeks) or aged (24-26 months) C57/BL6 mice were untreated, administered morphine acutely (30 microM), or implanted with a morphine pellet (75 mg) for 5 days prior to heart isolation and perfusion. Following equilibration, perfused hearts were subjected to 25 min ischemia and 45 min reperfusion. Untreated hearts from both young and aged mice displayed marked contractile dysfunction and LDH release following reperfusion. Acute infusion of morphine improved recovery of end-diastolic pressure and developed pressure in young (P < 0.05 vs. untreated) but not senescent hearts. Hearts from mice exposed to morphine for 5 days displayed a further improvement in post-ischemic contractile function (P < 0.05 vs. acute treatment), and a marked reduction in post-ischemic LDH efflux (P < 0.05 vs. untreated) in both young and senescent hearts. These data demonstrate that aged hearts maintain the ability to be preconditioned by chronic exposure to morphine in the absence of acute protection.

Decreased immunodensities of micro-opioid receptors, receptor kinases GRK 2/6 and beta-arrestin-2 in postmortem brains of opiate addicts

The homologous regulation of opioid receptors, through G protein-coupled receptor kinases (GRKs) and beta-arrestins, is an initial step in the complex molecular mechanisms leading to opiate tolerance and dependence. This study was designed to evaluate in parallel the contents of immunolabeled micro-opioid receptors (glycosylated proteins), two representative GRKs (GRK 2 and GRK 6) and beta-arrestin-2 in brains of opiate addicts who had died of an opiate overdose (heroin or methadone). The immunodensities of micro-opioid receptors were decreased (66 kDa protein: 24%, n=24, P<0.0001; 85 kDa protein: 16%, n=24, P<0.05) in the prefrontal cortex of opiate addicts compared with sex-, age-, and PMD-matched controls. This down-regulation of brain micro-opioid receptors was more pronounced in opiate addicts dying of a heroin overdose (27-30%, n=13) than in those who died of a methadone overdose (5-16%, n=11). In the same brains, significant decreases in the immunodensities of GRK 2 (19%, n=24, P<0.05), GRK 6 (25%, n=24, P<0.002) and beta-arrestin-2 (22%, n=24, P< 0.0005) were also quantitated. In contrast, the content of alpha-internexin (a neuronal marker used as a negative control) was not changed in brains of opiate addicts. In these subjects, there was a significant correlation between the densities of GRK 6 and beta-arrestin-2 (r=0.63, n=24, P=0.001), suggesting that both proteins are regulated in a coordinated manner by opiate drugs in the brain. The results indicate that opiate addiction in humans (tolerant state) is associated with down-regulation of brain micro-opioid receptors and regulatory GRK 2/6 and beta-arrestin-2 proteins. These molecular adaptations may be relevant mechanisms for the induction of opiate tolerance in brains of opiate addicts.

Reciprocal modulation of phospholipase Cbeta isoforms: adaptation to chronic morphine

Phosphoinositide turnover and calcium mobilization are fundamental determinants of acute and chronic opioid effects. Phosphoinositide-specific phospholipase C (PLC) are key signaling enzymes that play a pivotal role in mediating opioid modulation of inositol trisphosphate production and cytosolic calcium distribution, substrates for many acute and chronic opioid effects. Notably, phosphorylation of the beta isoforms of PLC, by kinases that are up-regulated after chronic morphine, is a potent modality for their regulation. Direct assessment of PLCbeta1 and PLCbeta3 phosphorylation in the guinea pig longitudinal muscle myenteric plexus tissue revealed substantial alterations after the induction of opioid tolerance. Notably, the direction of this modulation is isoform-specific. Phosphorylation of PLCbeta1 is significantly reduced, whereas that of PLCbeta3 is substantially augmented, changes not accompanied by altered content of PLCbeta1 or PLCbeta3 protein. In contrast to chronic morphine, acute morphine treatment of opioid naïve longitudinal muscle myenteric plexus tissue attenuates PLCbeta3 phosphorylation, an effect also manifested by endogenous opioids that is reflected by the ability of acute naloxone to substantially augment PLCbeta3 phosphorylation. This indicates that PLCbeta phosphorylation is dynamically regulated. PLCbeta1 and PLCbeta3 activities are negatively modulated by phosphorylation. Thus, their concomitant reciprocal phosphorylation would alter the relative contribution of these isoforms to PLC/Ca2+ signaling, a significant shift in light of their differential regulatory characteristics. Reciprocal modulation of the phosphorylation (activity) of two isoforms within the same subclass of signaling enzyme, proteins that have a high degree of structural similarity and subserve the same biological function, represents an adaptation modality to chronic morphine that has heretofore not been recognized.

Phosphorylation of Gβ is augmented by chronic morphine and enhances Gβγ stimulation of adenylyl cyclase activity

We previously demonstrated (Chakrabarti, et al., 2001) that in vivo phosphorylation of the Gbeta subunit of G proteins, via protein kinase A (PKA) and protein kinase C (PKC), is dramatically increased following chronic morphine. The present study investigates the PKC isoform selectivity of Gbeta phosphorylation and the consequences thereof on the ability of Gbetagamma to stimulate adenylyl cyclase II (ACII). The catalytic subunit of PKC and PKA, as well as the conventional PKC isoform PKCgamma, was effective in phosphorylating Gbeta. In contrast, Gbeta was only minimally phosphorylated by another conventional isoform, PKCalpha or the atypical isoform PKCzeta. In the presence of activated recombinant Gsalpha, ACII activity was dose dependently stimulated by G(betagamma), the magnitude of which was dependent upon its phosphorylation state. The increment in ACII activity produced by Gbetagamma was increased approximately 2-fold following in vitro phosphorylation by the catalytic subunit of either PKA or PKC. In contrast, the concomitant or sequential phosphorylation of Gbetagamma by PKA and PKC catalytic subunits did not result in an additive enhancement of its ability to stimulate ACII and, in fact, negated the observed enhancing effect of each kinase, individually. Threonine phosphorylated G(beta) occurs naturally in the spinal cord, the levels of which are augmented (approximately 60%) by chronic morphine. The natural occurrence of phosphorylated Gbeta in spinal cord, its up-regulation following chronic morphine and the augmented ability of phosphorylated Gbetagamma to stimulate ACII activity, in the aggregate, indicate that phosphorylation of Gbeta could be a regulatory mechanism causally associated with altered cellular signaling.

Plastic changes of calcitonin gene-related peptide in morphine tolerance: Behavioral and immunohistochemical study in rats

The present study was undertaken to investigate the plasticity of calcitonin gene-related peptide (CGRP) in antinociception after morphine tolerance in rats. The hindpaw withdrawal latencies (HWLs) to both thermal and mechanical stimulation increased significantly after intracerebroventricular injection of 2.5 nmol of CGRP in opioid-naive rats, indicating that CGRP produces an antinociceptive effect in the brain. Furthermore, there was an antinociceptive effect after intracerebroventricular injection of 2.5 nmol of CGRP in morphine-tolerant rats. Interestingly, the antinociceptive effect induced by intracerebroventricular injection of CGRP was lower in morphine-tolerant rats than that in opioid-naive rats at the same dose. At the same time, there was downregulation of CGRP-like immunoreactivity in both lateral septal nucleus and central nucleus of amygdala tested by immunohistochemical methods, whereas no significant changes were observed in arcuate nucleus of hypothalamus and periaqueductal gray after morphine treatment in rats. The present study demonstrates plastic changes in both CGRP-induced antinociception and CGRP-like immunoreactivity in rat brain after morphine tolerance, suggesting that CGRP may play an important role in morphine tolerance.

Differential effects of agonists on adenylyl cyclase superactivation mediated by the kappa opioid receptors: adenylyl cyclase superactivation is independent of agonist-induced phosphorylation, desensitization, internalization, and down-regulation

Prolonged activation of opioid receptors followed by agonist removal leads to adenylyl cyclase (AC) superactivation. In this study, we examined in CHO cells stably expressing the human or rat kappa opioid receptor (hkor or rkor) whether agonists had differential abilities to induce AC superactivation and whether the hkor and rkor exhibited differential AC superactivation. Pretreatment of the hkor with (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methanesulfonate (U50,488H) induced AC superactivation in a time- and dose-dependent manner, reaching a plateau at 4 h and 0.1 microM. The extents of AC superactivation after a 4-h pretreatment of the hkor with saturating concentrations of agonists were in the order of the full agonists U50,488H, dynorphin A(1-17), (+/-)-ethylketocyclazocine, etorphine, and U69,593 > the high-efficacy partial agonist nalorphine > the low-efficacy partial agonists nalbuphine, morphine, and pentazocine. Interestingly, the full agonist levorphanol caused much lower AC superactivation than other full agonists and reduced the AC superactivation induced by U50,488H and dynorphin A(1-17) in a dose-dependent manner. The order of relative efficacies of agonists in causing AC superactivation mediated by the rkor was similar to that mediated by the hkor and the extents of AC superactivation were slightly lower. Because the rkor does not undergo U50,488H (1 microM)-induced phosphorylation, desensitization, internalization, and down-regulation in these cells, the degree of AC superactivation is independent of these processes. This is among the first reports to demonstrate that relative efficacies of agonists in causing AC superactivation generally correlated with those in activating G proteins and a full agonist reduced AC superactivation induced by another full agonist.

Involvement of protein kinase C in the adaptive changes of cholinergic neurons to sympathetic denervation in the guinea pig myenteric plexus

Supersensitivity to muscarinic, kappa- and mu-opioid agents modulating cholinergic neurons in the guinea pig colon develops after chronic sympathetic denervation. A possible role for protein kinase C (PKC) in contributing to development of these sensitivity changes was investigated. The PKC activator, phorbol-12-myristate-13-acetate (PMA), enhanced acetylcholine (ACh) overflow in preparations obtained from normal animals. The facilitatory effect of PMA was significantly reduced after prolonged exposure to the phorbol ester and by the PKC inhibitors, chelerythrine and calphostin C. Subsensitivity to the facilitatory effect of PMA developed after chronic sympathetic denervation. In this experimental condition, immunoblot analysis revealed reduced levels of PKC in myenteric plexus synaptosomes. The facilitatory effect of the muscarininc antagonist, scopolamine, on ACh overflow was significantly reduced by the phospolipase C (PLC) inhibitor, U73122, chelerythrine and calphostin C, both in normal and denervated animals. However, in both experimental groups, PLC antagonists and PKC antagonists did not affect the inhibitory effect of the muscarinic agonist, oxotremorine-M on ACh overflow. The inhibitory effects of U69593 (kappa-opioid receptor agonist) and DAMGO (mu-opioid receptor agonist) on ACh overflow significantly increased in the presence of U73122, chelerythrine and calphostin C in preparations obtained from normal animals, but not in those obtained from sympathetically denervated animals. These results indicate that activation of PKC enhances ACh release in the myenteric plexus of the guinea pig colon. At this level, chronic sympathetic denervation entails a reduced efficiency of the enzyme. In addition, PKC is involved in the inhibitory modulation of ACh release mediated by muscarinic-, kappa- and mu-opioid receptors, although with different modalities. Muscarinic receptors inhibit PKC activity, whereas kappa- and mu-opioid receptors increase PKC activity. Both the inhibitory and the facilitatory effect on PKC involve modulation of PLC activity. The possibility that the change in PKC activity represents one of the biochemical mechanisms at the basis of development of sensitivity changes to opioid and muscarinic agents after chronic sympathetic denervation is discussed.

Tolerance and inverse tolerance to the hyperalgesic and analgesic actions, respectively, of the novel analgesic, F 13640

5-HT(1A) receptor activation by the very-high-efficacy, selective 5-HT(1A) receptor agonist F 13640 [(3-Chloro-4-fluoro-phenyl)-[4-fluoro-4-([(5-methyl-pyridin-2-ylmethyl)-amino]-methyl)piperidin-1-yl]-methanone] was recently discovered to constitute a novel central mechanism of broad-spectrum analgesia that, remarkably, grows rather than decays with chronicity. However, in rodents not exposed to nociception, F 13640 induces its analgesic effect only after having initially induced hyperalgesia. Numerical simulations implementing a signal transduction theory here show that the progressive increase in the intensity of nociceptive stimulation which F 13640 presumably mimics should eventually produce a large analgesic effect without initially causing marked pain. In vivo studies examined the effects of progressively increasing doses of F 13640 on the threshold of mechanically induced vocalization and, also, on the 5-HT syndrome in rats. The infusion of increasing (0.04-0.63 mg/rat/day) doses of F 13640 over a 5-week period induced a large analgesia preceded by a hyperalgesic effect that was small and comparable to that induced by initial exposure to a low, 0.04 mg/rat/day dose. Furthermore, increasing the dose of F 13640 induced tachyphylaxis to the 5-HT syndrome. Producing the mirror opposite of morphine's neuroadaptive actions, F 13640 causes an analgesia that becomes more powerful with chronic administration, and this at the expense of the initial hyperalgesia which it may also produce.