Opioid modulation of extracellular signal-regulated protein kinase activity is ras-dependent and involves Gbetagamma subunits

Kappa opioid activation changes protein profiles in different regions of the brain relevant to depression

Depression is a widespread disorder with a significant burden on individuals and society. There are various available treatments for patients with depression. However, not all patients respond adequately to their treatment. Recently, the opioid system has regained interest in depression studies. Research in animals and humans suggest that blocking the kappa opioid receptor (KOR) may potentially alleviate the symptoms of depression. The mechanism behind this effect is not fully understood. Stress and alterations in hypothalamic-pituitary-adrenal axis (HPA-axis) activity are thought to play a crucial role in depression. This study aimed to characterize stress hormones and stress-related protein expression following activation of KOR using a selective agonist. The longitudinal effect was investigated 24 h after KOR activation using the selective agonist U50,488 in Sprague Dawley rats. Stress-related hormones and protein expression patterns were explored using multiplex bead-based assays and western blotting. We found that KOR activation caused an increase in both adrenocorticotropic hormone (ACTH) and corticosterone (CORT) in serum. Regarding protein assays in different brain regions, phosphorylated glucocorticoid receptors also increased significantly in thalamus (THL), hypothalamus (HTH), and striatum (STR). C-Fos increased time-dependently in THL following KOR activation, extracellular signal-regulated kinases 1/2 (ERK1/2) increased significantly in STR and amygdala (AMG), while phosphorylated ERK1/2 decreased during the first 2 h and then increased again in AMG and prefrontal cortex (PFC). This study shows that KOR activation alters the HPA axis and ERK signaling which may cause to develop mood disorders.

Development of a novel cell-based, In-Cell Western/ERK assay system for the high-throughput screening of agonists acting on the delta-opioid receptor

Background: Extracellular signal-regulated kinases (ERKs) are important signaling mediators in mammalian cells and, as a result, one of the major areas of research focus. The detection and quantification of ERK phosphorylation as an index of activation is normally conducted using immunoblotting, which does not allow high-throughput drug screening. Plate-based immunocytochemical assays provide a cheaper and relatively high-throughput alternative method for quantifying ERK phosphorylation. Here, we present optimization steps aimed to increase assay sensitivity and reduce variance and cost using the LI-COR In-Cell Western (I-CW) system in a recombinant CHO-K1 cell line, over-expressing the human delta-opioid receptor (hDOPr) as a model.

Methods: Cells cultured in 96-well microassay plates were stimulated with three standard/selective DOPr agonists (SNC80, ADL5859, and DADLE) and a novel selective DOPr agonist (PN6047) to elicit a phospho-ERK response as an index of activation. A number of experimental conditions were investigated during the assay development.

Key results: Preliminary experiments revealed a clearly visible edge-effect which significantly increased assay variance across the plate and which was reduced by pre-incubation for 30 min at room temperature. ERK phosphorylation was detectable as early as 1 min after agonist addition, with a distinct peak at 3–5 min. Optimization of the cell seeding densities showed that 25,000 cells per well have the lowest basal phospho-ERK response and an optimal agonist ERK1/2 signal. Pre-incubation with apyrase (an ATPase) did not reduce the basal or agonist responses. All agonists produced concentration-dependent increases in phospho-ERK activation, and pertussis toxin was able to attenuate these ERK responses. Naltrindole, which is a selective DOPr antagonist, was able to antagonize the DOPr-mediated ERK activation of the ligands.

Conclusion: We have developed an optimization protocol and highlighted a number of considerations when performing this high-throughput fluorescence immunocytochemical (ICC) assay measuring ERK phosphorylation in the human DOPr. The optimized protocol was found to be a more conducive option for the screening of delta agonists. This provides a basis for additional assay development to investigate opioid pharmacology. This protocol should be widely applicable for measuring ERK phosphorylation in any cell line and investigating other protein targets in GPCR drug discovery.

Erythroxylum cuneatum prevented cellular adaptation in morphine-induced neuroblastoma cells

BACKGROUND

Chronic morphine stimulates prolonged stimulation of opioid receptors, especially µ-opioid subtype (MOR), which in turn signals cellular adaptation. However, the sudden termination of morphine after chronic intake causes withdrawal syndrome.

OBJECTIVES

Hence, this study was designed to find an alternative treatment for the morphine withdrawal using the alkaloid leaf extract of Erythroxylum cuneatum (E. cuneatum), done on morphine-exposed neuroblastoma cell lines.

METHODS

SK-N-SH, a commercialised neuroblastoma cell line, was used in two separate study designs; the antagonistic and pre-treatment of morphine. The antagonistic treatment was conducted through concurrent exposure of the cells to morphine and E. cuneatum or morphine and methadone for 24 h. The pre-treatment design was carried out by exposing the cells to morphine for 24 h, followed by 24 h exposures to E. cuneatum or methadone. The cytosolic fraction was collected and run for protein expression involved in cellular adaptation; mitogen-activated protein (MAP)/extracellular signal-regulated (ERK) kinase 1/2 (MEK 1/2), extracellular signal-regulated kinase 2 (ERK 2), cAMP-dependent protein kinase (PKA) and protein kinases C (PKC).

RESULTS

The antagonistic treatment showed the normal level of MEK 1/2, ERK 2, PKA and PKC by the combination treatment of morphine and E. cuneatum, comparable to the combination of morphine and methadone. Neuroblastoma cells exposed to morphine pre-treatment expressed a high level of MEK 1/2, ERK 2, PKA and PKC, while the treatments with E. cuneatum and methadone normalised the expression of the cellular adaptation proteins.

CONCLUSION

E. cuneatum exerted anti-addiction properties by lowering the levels of cellular adaptation proteins, and its effects are comparable to that of methadone (an established anti-addiction drug).

Cross-Talk of Toll-Like Receptor 5 and Mu-Opioid Receptor Attenuates Chronic Constriction Injury-Induced Mechanical Hyperalgesia through a Protein Kinase C Alpha-Dependent Signaling

Recently, Toll-like receptors (TLRs), a family of pattern recognition receptors, are reported as potential modulators for neuropathic pain; however, the desired mechanism is still unexplained. Here, we operated on the sciatic nerve to establish a pre-clinical rodent model of chronic constriction injury (CCI) in Sprague-Dawley rats, which were assigned into CCI and Decompression groups randomly. In Decompression group, the rats were performed with nerve decompression at post-operative week 4. Mechanical hyperalgesia and mechanical allodynia were obviously attenuated after a month. Toll-like receptor 5 (TLR5)-immunoreactive (ir) expression increased in dorsal horn, particularly in the inner part of lamina II. Additionally, substance P (SP) and isolectin B4 (IB4)-ir expressions, rather than calcitonin-gene-related peptide (CGRP)-ir expression, increased in their distinct laminae. Double immunofluorescence proved that increased TLR5-ir expression was co-expressed mainly with IB4-ir expression. Through an intrathecal administration with FLA-ST Ultrapure (a TLR5 agonist, purified flagellin from Salmonella Typhimurium, only the CCI-induced mechanical hyperalgesia was attenuated dose-dependently. Moreover, we confirmed that mu-opioid receptor (MOR) and phospho-protein kinase Cα (pPKCα)-ir expressions but not phospho-protein kinase A RII (pPKA RII)-ir expression, increased in lamina II, where they mostly co-expressed with IB4-ir expression. Go 6976, a potent protein kinase C inhibitor, effectively reversed the FLA-ST Ultrapure- or DAMGO-mediated attenuated trend towards mechanical hyperalgesia by an intrathecal administration in CCI rats. In summary, our current findings suggest that nerve decompression improves CCI-induced mechanical hyperalgesia that might be through the cross-talk of TLR5 and MOR in a PKCα-dependent manner, which opens a novel opportunity for the development of analgesic therapeutics in neuropathic pain.

κ-Opioid Signaling in the Lateral Hypothalamic Area Modulates Nicotine-Induced Negative Energy Balance

Several studies have reported that nicotine, the main bioactive component of tobacco, exerts a marked negative energy balance. Apart from its anorectic action, nicotine also modulates energy expenditure, by regulating brown adipose tissue (BAT) thermogenesis and white adipose tissue (WAT) browning. These effects are mainly controlled at the central level by modulation of hypothalamic neuropeptide systems and energy sensors, such as AMP-activated protein kinase (AMPK). In this study, we aimed to investigate the kappa opioid receptor (κOR)/dynorphin signaling in the modulation of nicotine’s effects on energy balance. We found that body weight loss after nicotine treatment is associated with a down-regulation of the κOR endogenous ligand dynorphin precursor and with a marked reduction in κOR signaling and the p70 S6 kinase/ribosomal protein S6 (S6K/rpS6) pathway in the lateral hypothalamic area (LHA). The inhibition of these pathways by nicotine was completely blunted in κOR deficient mice, after central pharmacological blockade of κOR, and in rodents where κOR was genetically knocked down specifically in the LHA. Moreover, κOR-mediated nicotine effects on body weight do not depend on orexin. These data unravel a new central regulatory pathway modulating nicotine’s effects on energy balance.

Effect of short-term prescription opioids on DNA methylation of the OPRM1 promoter

Background

A long-term opioid use has been associated with hypermethylation of the opioid receptor mu 1 (OPRM1) promoter. Very little is currently known about the early epigenetic response to therapeutic opioids. Here, we examine whether we can detect DNA methylation changes associated with a few days’ use of prescribed opioids. Genome-wide DNA methylation was assayed in a cohort of 33 opioid-naïve participants who underwent standard dental surgery followed by opioid self-administration. Saliva samples were collected before surgery (visit 1), and at two postsurgery visits at 2.7 ± 1.5 days (visit 2), and 39 ± 10 days (visit 3) after the discontinuation of opioid analgesics.

Results

The perioperative methylome underwent significant changes over the three visits that were primarily due to postoperative inflammatory response and cell heterogeneity. To specifically examine the effect of opioids, we started with a candidate gene approach and evaluated 10 CpGs located in the OPRM1 promoter. There was a significant cross-sectional variability in opioid use, and for participants who self-administered the prescribed drugs, the total dosage ranged from 5–210 morphine milligram equivalent (MME). Participants were categorized by cumulative dosage into three groups: < 25 MME, 25–90 MME, and ≥ 90 MME. Using mixed-effects modeling, 4 CpGs had significant positive associations with opioid dose at two-tailed p value < 0.05, and overall, 9 of the 10 OPRM1 promoter CpGs showed the predicted higher methylation in the higher dose groups relative to the lowest dose group. After adjustment for age, cellular heterogeneity, and past tobacco use, the promoter mean methylation also had positive associations with cumulative MME (regression coefficient = 0.0002, one-tailed p value = 0.02) and duration of opioid use (regression coefficient = 0.003, one-tailed p value = 0.001), but this effect was significant only for visit 3. A preliminary epigenome-wide association study identified a significant CpG in the promoter of the RAS-related signaling gene, RASL10A, that may be predictive of opioid dosage.

Conclusion

The present study provides evidence that the hypermethylation of the OPRM1 promoter is in response to opioid use and that epigenetic differences in OPRM1 and other sites are associated with a short-term use of therapeutic opioids.

Estrogen Regulation of GRK2 Inactivates Kappa Opioid Receptor Signaling Mediating Analgesia, But Not Aversion

Activation of κ opioid receptors (KORs) produces analgesia and aversion via distinct intracellular signaling pathways, but whether G protein-biased KOR agonists can be designed to have clinical utility will depend on a better understanding of the signaling mechanisms involved. We found that KOR activation produced conditioned place aversion and potentiated CPP for cocaine in male and female C57BL/6N mice. Consistent with this, males and females both showed arrestin-mediated increases in phospho-p38 MAPK following KOR activation. Unlike in males, however, KOR activation had inconsistent analgesic effects in females and KOR increased Gβγ-mediated ERK phosphorylation in males, but not females. KOR desensitization was not responsible for the lack of response in females because neither Grk3 nor Pdyn gene knock-out enhanced analgesia. Instead, responsiveness was estrous cycle dependent because KOR analgesia was evident during low estrogen phases of the cycle and in ovariectomized (OVX) females. Estradiol treatment of OVX females suppressed KOR-mediated analgesia, demonstrating that estradiol was sufficient to blunt Gβγ-mediated KOR signals. G protein-coupled receptor kinase 2 (GRK2) is known to regulate ERK activation, and we found that the inhibitory, phosphorylated form of GRK2 was significantly higher in intact females. GRK2/3 inhibition by CMPD101 increased KOR stimulation of phospho-ERK in females, decreased sex differences in KOR-mediated inhibition of dopamine release, and enhanced mu opioid receptor and KOR-mediated analgesia in females. In OVX females, estradiol increased the association between GRK2 and Gβγ. These studies suggest that estradiol, through increased phosphorylation of GRK2 and possible sequestration of Gβγ by GRK2, blunts G protein-mediated signals.SIGNIFICANCE STATEMENT Chronic pain disorders are more prevalent in females than males, but opioid receptor agonists show inconsistent analgesic efficacy in females. κ opioid receptor (KOR) agonists have been tested in clinical trials for treating pain disorders based on their analgesic properties and low addictive potential. However, the molecular mechanisms underlying sex differences in KOR actions were previously unknown. Our studies identify an intracellular mechanism involving estradiol regulation of G protein-coupled receptor kinase 2 that is responsible for sexually dimorphic analgesic responses following opioid receptor activation. Understanding this mechanism will be critical for developing effective nonaddictive opioid analgesics for use in women and characterizing sexually dimorphic effects in other inhibitory G protein-coupled receptor signaling responses.

ERK Activation Pathways Downstream of GPCRs

GPCRs, the 7-TM receptors, represent a class of cell surface receptors which modulate a variety of physiological responses. The serpentine structure in addition to contributing the diversity of stimuli these receptors can sense also provides flexibility to the extracellular and intracellular regions where other proteins can interact with and can form functionally active multimeric entities. The range in signaling and physiological responses generated by these receptors can be attributed to a large repertoire of the receptor subtypes as well as their differential coupling to various classes of G-protein subunits and other proteins which facilitate multistate activation. A multistate GPCR can engage diverse signaling molecules, thereby modulating not only the canonical cellular responses but also noncanonical responses typically associated with activation of other cascades such as RTK and MAPK/ERK signaling. Given the crucial involvement of MAP kinase/ERK signaling in cell fate determination specially with respect to regulating cell proliferation, cellular apoptosis, and survival, GPCR-mediated cross-activation of MAPK has been explored in various systems and shown to involve functional integration of multiple pathways. This review describes the present knowledge of the different mechanisms of ERK activation downstream of GPCRs and our present understanding of receptor-dependent and -independent MAPK activation cascades.

Opioid and hypocretin neuromodulation of ventral tegmental area neuronal subpopulations

The current view of the midbrain dopaminergic system is evolving towards a complex system of subpopulations of neurons with distinct afferent and efferent connections and, importantly, functionally different intrinsic characteristics. Recent literature on the phenotypic diversity of dopaminergic neurons has outlined that in the ventral tegmental area dopaminergic neurons are not as anatomically or electrophysiologically homogeneous as they were once thought to be. Instead, the midbrain dopaminergic system is now understood to be composed of anatomically and functionally heterogeneous dopaminergic subpopulations receiving specific afferent inputs and with different axonal projections. An additional layer of complexity is the neuromodulation of each of these dopaminergic circuits. This review will examine the distinguishing electrophysiological and neuromodulatory characteristics of the afferent and efferent connections of midbrain dopaminergic neurons.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Agonist-Dependent and -Independent κ Opioid Receptor Phosphorylation: Distinct Phosphorylation Patterns and Different Cellular Outcomes

We reported previously that the selective agonist U50,488H promoted phosphorylation of the mouse κ opioid receptor (KOPR) at residues S356, T357, T363, and S369. Here, we found that agonist (U50,488H)-dependent KOPR phosphorylation at all the residues was mediated by Gi/o _α proteins and multiple protein kinases [GRK2, GRK3, GRK5, GRK6 and protein kinase C (PKC)]. In addition, PKC activation by phorbol ester induced agonist-independent KOPR phosphorylation. Compared with U50,488H, PKC activation promoted much higher S356/T357 phosphorylation, much lower T363 phosphorylation, and similar levels of S369 phosphorylation. After U50,488H treatment, GRKs, but not PKC, were involved in agonist-induced KOPR internalization. In contrast, PKC activation caused a lower level of agonist-independent KOPR internalization, compared with U50,488H. U50,488H-induced activation of extracellular signal-regulated kinase 1/2 (ERK1/2) was G protein-, but not β-arrestin-, dependent. After U50,488H treatment, GRK-mediated, but not PKC-mediated, KOPR phosphorylation followed by β-arrestin recruitment desensitized U50,488H-induced ERK1/2 response. Therefore, agonist-dependent (GRK- and PKC-mediated) and agonist-independent (PKC-promoted) KOPR phosphorylations show distinct phosphorylation patterns, leading to diverse cellular outcomes.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Antagonizing the different stages of kappa opioid receptor activation selectively and independently attenuates acquisition and consolidation of associative memories

Previous work from our laboratory has shown that nonspecific kappa opioid receptor (KOR) antagonism in primary somatosensory cortex (S1) can inhibit acquisition for the forebrain-dependent associative task, Whisker-Trace Eyeblink conditioning (WTEB). Although studies have demonstrated that KOR activation can alter stimuli salience, our studies controlled for these factors, demonstrating that KOR also plays a role in facilitating learning. KOR has two distinct phases of activation followed by internalization/downregulation, that each independently activate kinases and transcription factors known to mediate task acquisition and memory consolidation respectively. The current study demonstrated that antagonism of the initial phase of KOR activation in S1 via local injections of the g-protein inhibitor, pertussis toxin (PTX), blocked initial WTEB acquisition without affecting retention of the association. In contrast, KOR late phase antagonism in S1 via local injections of the GRK3-specific antagonist, guanidinonaltrindole (GNTI), blocked retention of the WTEB association without affecting task acquisition. Consistent with the known mechanism for KOR activation, KOR protein expression in S1 was found to be decreased following WTEB training, further supporting the involvement of neocortical KOR activation with learning. Prior studies have shown that task acquisition and memory consolidation are mediated by distinct molecular processes; however, little is known regarding a potential mechanism driving these processes. The current study suggests that neocortical KOR activation mediates activation of these processes with learning. This study provides the first evidence for a time- and learning-dependent property of neocortical KOR in facilitating acquisition and consolidation of associative memories, while elucidating an unexplored neocortical learning mechanism.

Hypothalamic neuropeptide signaling in alcohol addiction

The hypothalamus is now known to regulate alcohol intake in addition to its established role in food intake, in part through neuromodulatory neurochemicals termed neuropeptides. Certain orexigenic neuropeptides act in the hypothalamus to promote alcohol drinking, although they affect different aspects of the drinking response. These neuropeptides, which include galanin, the endogenous opioid enkephalin, and orexin/hypocretin, appear to stimulate alcohol intake not only through mechanisms that promote food intake but also by enhancing reward and reinforcement from alcohol. Moreover, these neuropeptides participate in a positive feedback relationship with alcohol, whereby they are upregulated by alcohol intake to promote even further consumption. They contrast with other orexigenic neuropeptides, such as melanin-concentrating hormone and neuropeptide Y, which promote alcohol intake under limited circumstances, are not consistently stimulated by alcohol, and do not enhance reward. They also contrast with neuropeptides that can be anorexigenic, including the endogenous opioid dynorphin, corticotropin-releasing factor, and melanocortins, which act in the hypothalamus to inhibit alcohol drinking as well as reward and therefore counter the ingestive drive promoted by orexigenic neuropeptides. Thus, while multiple hypothalamic neuropeptides may work together to regulate different aspects of the alcohol drinking response, excessive signaling from orexigenic neuropeptides or inadequate signaling from anorexigenic neuropeptides can therefore allow alcohol drinking to become dysregulated.

Role for engagement of β-arrestin2 by the transactivated EGFR in agonist-specific regulation of δ receptor activation of ERK1/2

BACKGROUND AND PURPOSE

β-Arrestins function as signal transducers linking GPCRs to ERK1/2 signalling either by scaffolding members of ERK1/2s cascades or by transactivating receptor tyrosine kinases through Src-mediated release of transactivating factor. Recruitment of β-arrestins to the activated GPCRs is required for ERK1/2 activation. Our previous studies showed that δ receptors activate ERK1/2 through a β-arrestin-dependent mechanism without inducing β-arrestin binding to the δ receptors. However, the precise mechanisms involved remain to be established.

EXPERIMENTAL APPROACH

ERK1/2 activation by δ receptor ligands was assessed using HEK293 cells in vitro and male Sprague Dawley rats in vivo. Immunoprecipitation, immunoblotting, siRNA transfection, intracerebroventricular injection and immunohistochemistry were used to elucidate the underlying mechanism.

KEY RESULTS

We identified a new signalling pathway in which recruitment of β-arrestin2 to the EGFR rather than δ receptor was required for its role in δ receptor-mediated ERK1/2 activation in response to H-Tyr-Tic-Phe-Phe-OH (TIPP) or morphine stimulation. Stimulation of the δ receptor with ligands leads to the phosphorylation of PKCδ, which acts upstream of EGFR transactivation and is needed for the release of the EGFR-activating factor, whereas β-arrestin2 was found to act downstream of the EGFR transactivation. Moreover, we demonstrated that coupling of the PKCδ/EGFR/β-arrestin2 transactivation pathway to δ receptor-mediated ERK1/2 activation was ligand-specific and the Ser(363) of δ receptors was crucial for ligand-specific implementation of this ERK1/2 activation pathway.

CONCLUSIONS AND IMPLICATIONS

The δ receptor-mediated activation of ERK1/2 is via ligand-specific transactivation of EGFR. This study adds new insights into the mechanism by which δ receptors activate ERK1/2.

Signaling pathways relevant to cognition-enhancing drug targets

Aging is generally associated with a certain cognitive decline. However, individual differences exist. While age-related memory deficits can be observed in humans and rodents in the absence of pathological conditions, some individuals maintain intact cognitive functions up to an advanced age. The mechanisms underlying learning and memory processes involve the recruitment of multiple signaling pathways and gene expression, leading to adaptative neuronal plasticity and long-lasting changes in brain circuitry. This chapter summarizes the current understanding of how these signaling cascades could be modulated by cognition-enhancing agents favoring memory formation and successful aging. It focuses on data obtained in rodents, particularly in the rat as it is the most common animal model studied in this field. First, we will discuss the role of the excitatory neurotransmitter glutamate and its receptors, downstream signaling effectors [e.g., calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC), extracellular signal-regulated kinases (ERK), mammalian target of rapamycin (mTOR), cAMP response element-binding protein (CREB)], associated immediate early gene (e.g., Homer 1a, Arc and Zif268), and growth factors [insulin-like growth factors (IGFs) and brain-derived neurotrophic factor (BDNF)] in synaptic plasticity and memory formation. Second, the impact of the cholinergic system and related modulators on memory will be briefly reviewed. Finally, since dynorphin neuropeptides have recently been associated with memory impairments in aging, it is proposed as an attractive target to develop novel cognition-enhancing agents.

The δ-opioid receptor affects epidermal homeostasis via ERK-dependent inhibition of transcription factor POU2F3

Neuropeptides and their receptors are present in human skin, and their importance for cutaneous homeostasis and during wound healing is increasingly appreciated. However, there is currently a lack of understanding of the molecular mechanisms by which their signaling modulates keratinocyte function. Here, we show that δ-opioid receptor (DOPr) activation inhibits proliferation of human keratinocytes, resulting in decreased epidermal thickness in an organotypic skin model. DOPr signaling markedly delayed induction of keratin intermediate filament (KRT10) during in vitro differentiation and abolished its induction in the organotypic skin model. This was accompanied by deregulation of involucrin (IVL), loricrin, and filaggrin. Analysis of the transcription factor POU2F3, which is involved in regulation of KRT10, IVL, and profilaggrin expression, revealed a DOPr-mediated extracellular signal-regulated kinase (ERK)-dependent downregulation of this factor. We propose that DOPr signaling specifically activates the ERK 1/2 mitogen-activated protein kinase pathway to regulate keratinocyte functions. Complementing our earlier studies in DOPr-deficient mice, these data suggest that DOPr activation in human keratinocytes profoundly influences epidermal morphogenesis and homeostasis.

Homogeneous time-resolved fluorescence-based assay to monitor extracellular signal-regulated kinase signaling in a high-throughput format

The extracellular signal-regulated kinases (ERKs) are key components of multiple important cell signaling pathways regulating diverse biological responses. This signaling is characterized by phosphorylation cascades leading to ERK1/2 activation and promoted by various cell surface receptors including G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). We report the development of a new cell-based Phospho-ERK1/2 assay (designated Phospho-ERK), which is a sandwich proximity-based assay using the homogeneous time-resolved fluorescence technology. We have validated the assay on endogenously expressed ERK1/2 activated by the epidermal growth factor as a prototypical RTK, as well as various GPCRs belonging to different classes and coupling to different heterotrimeric G proteins. The assay was successfully miniaturized in 384-well plates using various cell lines endogenously, transiently, or stably expressing the different receptors. The validation was performed for agonists, antagonists, and inhibitors in dose-response as well as kinetic analysis, and the signaling and pharmacological properties of the different receptors were reproduced. Furthermore, the determination of a Z'-factor value of 0.7 indicates the potential of the Phospho-ERK assay for high-throughput screening of compounds that may modulate ERK1/2 signaling. Finally, our study is of great interest in the current context of investigating ERK1/2 signaling with respect to the emerging concepts of biased ligands, G protein-dependent/independent ERK1/2 activation, and functional transactivation between GPCRs and RTKs, illustrating the importance of considering the ERK1/2 pathway in cell signaling.

Label-free monitoring of μ-opioid receptor-mediated signaling

In this study, we used a combination of traditional signaling investigation approaches, bioluminescence resonance energy transfer (BRET) biosensors, and the label-free approach surface plasmon resonance (SPR) spectroscopy to monitor the signaling cascades of the μ-opioid receptor (MOP). In human embryonic kidney cells stably expressing a Flag-tagged version of human MOP, we compared the signals triggered by the noninternalizing and internalizing MOP agonists morphine and DAMGO (Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol), respectively. We studied three major and well described components of MOP signaling: receptor internalization, G protein coupling, and activation of extracellular signal-regulated kinase ERK1/ERK2. Our results show that morphine and DAMGO display different profiles of receptor internalization and a similar ability to trigger the phosphorylation of ERK1/ERK2. Our SPR analyses revealed that morphine and DAMGO evoke similar SPR signatures and that Gαi, cAMP-dependent pathways, and ERK1/ERK2 have key roles in morphine- and DAMGO-mediated signaling. Most interestingly, we found that the so-called MOP neutral antagonists CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2)), naloxone, and naltrexone behave like partial agonists. Even more intriguing, BRET experiments indicate that CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) induces similar conformational changes as naltrexone at the Gαi-βγ interface, whereas it appears as an inverse agonist based on its SPR response thus indicating distinct signaling mechanisms for the two ligands. Taken together, our results support the usefulness of label-free methods such as SPR to study whole-cell responses and signaling cascades triggered by G protein-coupled receptors and complement the conventional approaches by revealing cellular responses that would have been otherwise undetectable.

Opioid hedonic hotspot in nucleus accumbens shell: mu, delta, and kappa maps for enhancement of sweetness "liking" and "wanting"

A specialized cubic-millimeter hotspot in the rostrodorsal quadrant of medial shell in nucleus accumbens (NAc) of rats may mediate opioid enhancement of gustatory hedonic impact or "liking". Here, we selectively stimulated the three major subtypes of opioid receptors via agonist microinjections [mu (DAMGO), delta (DPDPE), or kappa (U50488H)] and constructed anatomical maps for functional localizations of consequent changes in hedonic "liking" (assessed by affective orofacial reactions to sucrose taste) versus "wanting" (assessed by changes in food intake). Results indicated that the NAc rostrodorsal quadrant contains a shared opioid hedonic hotspot that similarly mediates enhancements of sucrose "liking" for mu, delta, and kappa stimulations. Within the rostrodorsal hotspot boundaries each type of stimulation generated at least a doubling or higher enhancement of hedonic reactions, with comparable intensities for all three types of opioid stimulation. By contrast, a negative hedonic coldspot was mapped in the caudal half of medial shell, where all three types of opioid stimulation suppressed "liking" reactions to approximately one-half normal levels. Different anatomical patterns were produced for stimulation of food "wanting", reflected in food intake. Altogether, these results indicate that the rostrodorsal hotspot in medial shell is unique for generating opioid-induced hedonic enhancement, and add delta and kappa signals to mu as hedonic generators within the hotspot. Also, the identification of a separable NAc caudal coldspot for hedonic suppression, and separate NAc opioid mechanisms for controlling food "liking" versus "wanting" further highlights NAc anatomical heterogeneity and localizations of function within subregions of medial shell.

Peripheral sensitization increases opioid receptor expression and activation by crotalphine in rats

Inflammation enhances the peripheral analgesic efficacy of opioid drugs, but the mechanisms involved in this phenomenon have not been fully elucidated. Crotalphine (CRP), a peptide that was first isolated from South American rattlesnake C.d. terrificus venom, induces a potent and long-lasting anti-nociceptive effect that is mediated by the activation of peripheral opioid receptors. Because the high efficacy of CRP is only observed in the presence of inflammation, we aimed to elucidate the mechanisms involved in the CRP anti-nociceptive effect induced by inflammation. Using real-time RT-PCR, western blot analysis and ELISA assays, we demonstrate that the intraplantar injection of prostaglandin E2 (PGE2) increases the mRNA and protein levels of the µ- and κ-opioid receptors in the dorsal root ganglia (DRG) and paw tissue of rats within 3 h of the injection. Using conformation state-sensitive antibodies that recognize activated opioid receptors, we show that PGE2, alone does not increase the activation of these opioid receptors but that in the presence of PGE2, the activation of specific opioid receptors by CRP and selective µ- and κ-opioid receptor agonists (positive controls) increases. Furthermore, PGE2 down-regulated the expression and activation of the δ-opioid receptor. CRP increased the level of activated mitogen-activated protein kinases in cultured DRG neurons, and this increase was dependent on the activation of protein kinase Cζ. This CRP effect was much more prominent when the cells were pretreated with PGE2. These results indicate that the expression and activation of peripheral opioid receptors by opioid-like drugs can be up- or down-regulated in the presence of an acute injury and that acute tissue injury enhances the efficacy of peripheral opioids.

Role of kappa-opioid receptors in stress and anxiety-related behavior

RATIONALE

Accumulating evidence indicates that brain kappa-opioid receptors (KORs) and dynorphin, the endogenous ligand that binds at these receptors, are involved in regulating states of motivation and emotion. These findings have stimulated interest in the development of KOR-targeted ligands as therapeutic agents. As one example, it has been suggested that KOR antagonists might have a wide range of indications, including the treatment of depressive, anxiety, and addictive disorders, as well as conditions characterized by co-morbidity of these disorders (e.g., post-traumatic stress disorder) A general effect of reducing the impact of stress may explain how KOR antagonists can have efficacy in such a variety of animal models that would appear to represent different disease states.

OBJECTIVE

Here, we review evidence that disruption of KOR function attenuates prominent effects of stress. We will describe behavioral and molecular endpoints including those from studies that characterize the effects of KOR antagonists and KOR ablation on the effects of stress itself, as well as on the effects of exogenously delivered corticotropin-releasing factor, a brain peptide that mediates key effects of stress.

CONCLUSION

Collectively, available data suggest that KOR disruption produces anti-stress effects and under some conditions can prevent the development of stress-induced adaptations. As such, KOR antagonists may have unique potential as therapeutic agents for the treatment and even prevention of stress-related psychiatric illness, a therapeutic niche that is currently unfilled.

Remifentanil produces cross-desensitization and tolerance with morphine on the mu-opioid receptor

Remifentanil is a powerful mu-opioid (MOP) receptor agonist used in anaesthesia with a very short half-life. However, per-operative perfusion of remifentanil was shown to increase morphine consumption during post-operative period to relieve pain. In the present study, we aimed to describe the cellular mechanisms responsible for this apparent reduction of morphine efficacy. For this purpose, we first examined the pharmacological properties of both remifentanil and morphine at the MOP receptor, endogenously expressed in the human neuroblastoma SH-SY5Y cell line, to regulate adenylyl cyclase and the MAP kinase ERK1/2 pathway, their potency to promote MOP receptor phosphorylation, arrestin 3-CFP (cyan fluorescent protein) recruitment and receptor trafficking during acute and sustained exposure. In the second part of this work, we studied the effects of a prior exposure of remifentanil on morphine-induced inhibition of cAMP accumulation, activation of ERK1/2 and analgesia. We showed that sustained exposure to remifentanil promoted a rapid desensitization of opioid receptors on both signalling pathways and a pretreatment with this agonist reduced signal transduction produced by a second challenge with morphine. While both opioid agonists promoted Ser(375) phosphorylation on MOP receptor, remifentanil induced a rapid internalization of opioid receptors compared to morphine but without detectable arrestin 3-CFP translocation to the plasma membrane in our experimental conditions. Lastly, a cross-tolerance between remifentanil and morphine was observed in mice using the hot plate test. Our in vitro and in vivo data thus demonstrated that remifentanil produced a rapid desensitization and internalization of the MOP receptor that would reduce the anti-nociceptive effects of morphine.

In vivo and in vitro characterization of naltrindole-derived ligands at the κ-opioid receptor

Accumulating evidence supports a role for κ-opioid receptor antagonists in the treatment of mood disorders. Standard κ-antagonists have an unusual pharmacodynamic action, with a single injection blocking receptor signaling for several weeks. Here, we have characterized the κ-selective properties of two ligands, 5'-(2-aminomethyl) naltrindole (5'-AMN) and N-((Naltrindol-5-yl) methyl) pentanimidamide (5'-MABN), to identify whether modifications of the naltrindole side chain produces short-acting κ-antagonists. Opioid receptor binding affinity and activity were assessed using [(3)H]-diprenorphine binding, guanosine-5'-O-(3-[35S]-thio) triphosphate ([(35)S]-GTPγS) binding and isolated guinea-pig ileum. Pharmacodynamic profiles of 5'-AMN and 5'-MABN (1-10 mg/kg) were investigated using the tail-withdrawal assay and diuresis. Efficacy was also determined in depression- and anxiety-related behavioral paradigms in CD-1 mice. Both 5'-AMN and 5'-MABN had high affinity for κ-receptors (K (i) 1.36 ± 0.98 and 0.27 ± 0.08, respectively) and were revealed as potent κ-antagonists (pA(2) 7.43 and 8.18, respectively) and μ-receptor antagonists (pA(2) 7.62 and 7.85, respectively) in the ileum. Contrary to our hypothesis, in vivo, 5'-AMN and 5'-MABN displayed long-lasting antagonist effects in mice, reducing the antinociceptive actions of U50,488 (10 mg/kg) at 28 and 21 days post-injection, respectively. Interestingly, while 5'-AMN and 5'-MABN were not κ-selective, both compounds did show significant antidepressant- and anxiolytic-like effects at 7-14 days post-injection in mice.

Targeting dynorphin/kappa opioid receptor systems to treat alcohol abuse and dependence

This review represents the focus of a symposium that was presented at the "Alcoholism and Stress: A Framework for Future Treatment Strategies" conference in Volterra, Italy on May 3-6, 2011 and organized/chaired by Dr. Brendan M. Walker. The primary goal of the symposium was to evaluate and disseminate contemporary findings regarding the emerging role of kappa-opioid receptors (KORs) and their endogenous ligands dynorphins (DYNs) in the regulation of escalated alcohol consumption, negative affect and cognitive dysfunction associated with alcohol dependence, as well as DYN/KOR mediation of the effects of chronic stress on alcohol reward and seeking behaviors. Dr. Glenn Valdez described a role for KORs in the anxiogenic effects of alcohol withdrawal. Dr. Jay McLaughlin focused on the role of KORs in repeated stress-induced potentiation of alcohol reward and increased alcohol consumption. Dr. Brendan Walker presented data characterizing the effects of KOR antagonism within the extended amygdala on withdrawal-induced escalation of alcohol self-administration in dependent animals. Dr. Georgy Bakalkin concluded with data indicative of altered DYNs and KORs in the prefrontal cortex of alcohol dependent humans that could underlie diminished cognitive performance. Collectively, the data presented within this symposium identified the multifaceted contribution of KORs to the characteristics of acute and chronic alcohol-induced behavioral dysregulation and provided a foundation for the development of pharmacotherapeutic strategies to treat certain aspects of alcohol use disorders.

Molecular mechanisms of opioid receptor-dependent signaling and behavior

Opioid receptors have been targeted for the treatment of pain and related disorders for thousands of years and remain the most widely used analgesics in the clinic. Mu (μ), kappa (κ), and delta (δ) opioid receptors represent the originally classified receptor subtypes, with opioid receptor like-1 (ORL1) being the least characterized. All four receptors are G-protein coupled and activate inhibitory G proteins. These receptors form homo- and heterodimeric complexes and signal to kinase cascades and scaffold a variety of proteins.The authors discuss classic mechanisms and developments in understanding opioid tolerance and opioid receptor signaling and highlight advances in opioid molecular pharmacology, behavioral pharmacology, and human genetics. The authors put into context how opioid receptor signaling leads to the modulation of behavior with the potential for therapeutic intervention. Finally, the authors conclude there is a continued need for more translational work on opioid receptors in vivo.

Ser/ Thr residues at α3/β5 loop of Gαs are important in morphine-induced adenylyl cyclase sensitization but not mitogen-activated protein kinase phosphorylation

The signaling switch of β2-adrenergic and μ(1) -opioid receptors from stimulatory G-protein (G(αs) ) to inhibitory G-protein (G(αi) ) (and vice versa) influences adenylyl cyclase (AC) and extracellular-regulated kinase (ERK)1/2 activation. Post-translational modifications, including dephosphorylation of G(αs) , enhance opioid receptor coupling to G(αs) . In the present study, we substituted the Ser/Thr residues of G(αs) at the α3/β5 and α4/β6 loops aiming to study the role of G(αs) lacking Ser/Thr phosphorylation with respect to AC sensitization and mitogen-activated protein kinase activation. Isoproterenol increased the cAMP concentration (EC(50) = 22.8 ± 3.4 μm) in G(αs) -transfected S49 cyc- cells but not in nontransfected cells. However, there was no significant difference between the G(αs) -wild-type (wt) and mutants. Morphine (10 μm) inhibited AC activity more efficiently in cyc- compared to G(αs) -wt introduced cells (P < 0.05); however, we did not find a notable difference between G(αs) -wt and mutants. Interestingly, G(αs) -wt transfected cells showed more sensitization with respect to AC after chronic morphine compared to nontransfected cells (101 ± 12% versus 34 ± 6%; P < 0.001); μ1-opioid receptor interacted with G(αs) , and both co-immunoprecipitated after chronic morphine exposure. Furthermore, mutation of T270A and S272A (P < 0.01), as well as T270A, S272A and S261A (P < 0.05), in α3/β5, resulted in a higher level of AC supersensitization. ERK1/2 phosphorylation was rapidly induced by isoproterenol (by 9.5 ± 2.4-fold) and morphine (22 ± 2.2-fold) in G(αs) -transfected cells; mutations of α3/β5 and α4/β6 did not affect the pattern or extent of mitogen-activated protein kinase activation. The findings of the present study show that G(αs) interacts with the μ1-opioid receptor, and the Ser/Thr mutation to Ala at the α3/β5 loop of G(αs) enhances morphine-induced AC sensitization. In addition, G(αs) was required for the rapid phosphorylation of ERK1/2 by isoproterenol but not morphine.

Serine 363 of the {delta}-opioid receptor is crucial for adopting distinct pathways to activate ERK1/2 in response to stimulation with different ligands

Distinct opioid receptor agonists have been proved to induce differential patterns of ERK activation, but the underlying mechanisms remain unclear. Here, we report that Ser363 in the δ-opioid receptor (δOR) determines the different abilities of the δOR agonists DPDPE and TIPP to activate ERK by G-protein- or β-arrestin-dependent pathways. Although both DPDPE and TIPP activated ERK1/2, they showed different temporal, spatial and desensitization patterns of ERK activation. We show that that DPDPE employed G protein as the primary mediator to activate the ERK cascade in an Src-dependent manner, whereas TIPP mainly adopted a β-arrestin1/2-mediated pathway. Moreover, we found that DPDPE gained the capacity to adopt the β-arrestin1/2-mediated pathway upon Ser363 mutation, accompanied by the same pattern of ERK activation as that induced by TIPP. Additionally, we found that TIPP- but not DPDPE-activated ERK could phosphorylate G-protein-coupled receptor kinase-2 and β-arrestin1. However, such functional differences of ERK disappeared with the mutation of Ser363. Therefore, the present study reveals a crucial role for Ser363 in agonist-specific regulation of ERK activation patterns and functions.

Kinase cascades and ligand-directed signaling at the kappa opioid receptor

BACKGROUND AND RATIONALE

The dynorphin/kappa opioid receptor (KOR) system has been implicated as a critical component of the stress response. Stress-induced activation of dynorphin-KOR is well known to produce analgesia, and more recently, it has been implicated as a mediator of stress-induced responses including anxiety, depression, and reinstatement of drug seeking.

OBJECTIVE

Drugs selectively targeting specific KOR signaling pathways may prove potentially useful as therapeutic treatments for mood and addiction disorders.

RESULTS

KOR is a member of the seven transmembrane spanning (7TM) G-protein coupled receptor (GPCR) superfamily. KOR activation of pertussis toxin-sensitive G proteins leads to Galphai/o inhibition of adenylyl cyclase production of cAMP and releases Gbetagamma, which modulates the conductances of Ca(+2) and K(+) channels. In addition, KOR agonists activate kinase cascades including G-protein coupled Receptor Kinases (GRK) and members of the mitogen-activated protein kinase (MAPK) family: ERK1/2, p38 and JNK. Recent pharmacological data suggests that GPCRs exist as dynamic, multi-conformational protein complexes that can be directed by specific ligands towards distinct signaling pathways. Ligand-induced conformations of KOR that evoke beta-arrestin-dependent p38 MAPK activation result in aversion; whereas ligand-induced conformations that activate JNK without activating arrestin produce long-lasting inactivation of KOR signaling.

CONCLUSIONS

In this review, we discuss the current status of KOR signal transduction research and the data that support two novel hypotheses: (1) KOR selective partial agonists that do not efficiently activate p38 MAPK may be useful analgesics without producing the dysphoric or hallucinogenic effects of selective, highly efficacious KOR agonists and (2) KOR antagonists that do not activate JNK may be effective short-acting drugs that may promote stress-resilience.

Single-nucleotide polymorphism (A118G) in exon 1 of OPRM1 gene causes alteration in downstream signaling by mu-opioid receptor and may contribute to the genetic risk for addiction

The opioid receptor mu1 (OPRM1) mediates the action of morphine. Although genetic background plays an important role in the susceptibility toward abuse of drugs as evident from familial, adoption and twin studies, association of specific single-nucleotide polymorphisms of OPRM1 gene with narcotic addiction is to be established. Here, we demonstrate the involvement of A118G polymorphism of exon1 of human OPRM1 gene (hOPRM1), with heroin and alcohol addiction, in a population in eastern India. Statistical analysis exhibited a significant association of G allele with both heroin and alcohol addiction with a risk factor of P(trend) < 0.05. The functional significance of G allele in A118G single-nucleotide polymorphisms was evaluated by studying the regulation of protein kinase A (PKA), pCREB, and pERK1/2 by morphine in Neuro 2A cells, stably transfected with either wild type or A118G mutant hOPRM1. Unlike acute morphine treatment, both chronic morphine exposure and withdrawal precipitated by naloxone were differentially regulated by A118 and G118 receptor isoforms when both PKA and pERK1/2 activities were compared. Results suggest that the association of A118G polymorphism to heroin and alcohol addiction may be because of the altered regulation of PKA and pERK1/2 during opioid and alcohol exposures.

Activation of the kappa opioid receptor in the dorsal raphe nucleus mediates the aversive effects of stress and reinstates drug seeking

Although stress has profound effects on motivated behavior, the underlying mechanisms responsible are incompletely understood. In this study we elucidate a functional pathway in mouse brain that encodes the aversive effects of stress and mediates stress-induced reinstatement of cocaine place preference (CPP). Activation of the dynorphin/kappa opioid receptor (KOR) system by either repeated stress or agonist produces conditioned place aversion (CPA). Because KOR inhibition of dopamine release in the mesolimbic pathway has been proposed to mediate the dysphoria underlying this response, we tested dopamine-deficient mice in this study and found that KOR agonist in these mice still produced CPA. However, inactivation of serotonergic KORs by injection of the KOR antagonist norBNI into the dorsal raphe nucleus (DRN), blocked aversive responses to the KOR agonist U50,488 and blocked stress-induced reinstatement of CPP. KOR knockout (KO) mice did not develop CPA to U50,488; however, lentiviral re-expression of KOR in the DRN of KOR KO mice restored place aversion. In contrast, lentiviral expression in DRN of a mutated form of KOR that fails to activate p38 MAPK required for KOR-dependent aversion, did not restore place aversion. DRN serotonergic neurons project broadly throughout the brain, but the inactivation of KOR in the nucleus accumbens (NAc) coupled with viral re-expression in the DRN of KOR KO mice demonstrated that aversion was encoded by a DRN to NAc projection. These results suggest that the adverse effects of stress may converge on the serotonergic system and offers an approach to controlling stress-induced dysphoria and relapse.

The dynorphin/kappa opioid system as a modulator of stress-induced and pro-addictive behaviors

Stress is a complex experience that carries both aversive and motivating properties. Chronic stress causes an increase in the risk of depression, is well known to increase relapse of drug seeking behavior, and can adversely impact health. Several brain systems have been demonstrated to be critical in mediating the negative affect associated with stress, and recent evidence directly links the actions of the endogenous opioid neuropeptide dynorphin in modulating mood and increasing the rewarding effects of abused drugs. These results suggest that activation of the dynorphin/kappa opioid receptor (KOR) system is likely to play a major role in the pro-addictive effects of stress. This review explores the relationship between dynorphin and corticotropin-releasing factor (CRF) in the induction of dysphoria, the potentiation of drug seeking, and stress-induced reinstatement. We also provide an overview of the signal transduction events responsible for CRF and dynorphin/KOR-dependent behaviors. Understanding the recent work linking activation of CRF and dynorphin/KOR systems and their specific roles in brain stress systems and behavioral models of addiction provides novel insight to neuropeptide systems that regulate affective state.

Extracellular signal-regulated kinase 1/2 activation counteracts morphine tolerance in the periaqueductal gray of the rat

Repeated administration of opioids produces long-lasting changes in micro-opioid receptor (MOR) signaling that underlie behavioral changes such as tolerance. Mitogen-activated protein kinase (MAPK) pathways, including MAPK extracellular signal-regulated kinases (ERK1/2), are modulated by opioids and are known to produce long-lasting changes in cell signaling. Thus, we tested the hypothesis that ERK1/2 activation contributes to the development and/or expression of morphine tolerance mediated by the periaqueductal gray (PAG). Changes in phosphorylated ERK1/2 expression were assessed with confocal microscopy and compared to behavioral measures of tolerance to the antinociceptive effects of chronic morphine administration. Repeated microinjection of morphine into the PAG produced tolerance and caused a significant increase in ERK1/2 phosphorylation, an effect not evident with acute morphine microinjection. Microinjection of the MAPK/ERK kinase inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene ethanolate (U0126), into the PAG had no effect on antinociception produced by acute morphine administration. However, repeated coadministration of U0126 and morphine into the PAG blocked ERK1/2 phosphorylation and enhanced the development of morphine tolerance. Coadministration of U0126 with morphine only on the test day also enhanced the expression of morphine tolerance. Administration of the irreversible opioid receptor antagonist beta-chlornaltrexamine blocked the activation of ERK1/2 caused by repeated morphine microinjections, demonstrating that ERK1/2 activation was a MOR-mediated event. In summary, these studies show that chronic morphine administration alters ERK1/2 signaling and that disruption of ERK1/2 signaling enhances both the development and expression of morphine tolerance. Contrary to expectations, these data indicate that ERK1/2 activation opposes the development of morphine tolerance.

30 years of dynorphins--new insights on their functions in neuropsychiatric diseases

Since the first description of their opioid properties three decades ago, dynorphins have increasingly been thought to play a regulatory role in numerous functional pathways of the brain. Dynorphins are members of the opioid peptide family and preferentially bind to kappa opioid receptors. In line with their localization in the hippocampus, amygdala, hypothalamus, striatum and spinal cord, their functions are related to learning and memory, emotional control, stress response and pain. Pathophysiological mechanisms that may involve dynorphins/kappa opioid receptors include epilepsy, addiction, depression and schizophrenia. Most of these functions were proposed in the 1980s and 1990s following histochemical, pharmacological and electrophysiological experiments using kappa receptor-specific or general opioid receptor agonists and antagonists in animal models. However, at that time, we had little information on the functional relevance of endogenous dynorphins. This was mainly due to the complexity of the opioid system. Besides actions of peptides from all three classical opioid precursors (proenkephalin, prodynorphin, proopiomelanocortin) on the three classical opioid receptors (delta, mu and kappa), dynorphins were also shown to exert non-opioid effects mainly through direct effects on NMDA receptors. Moreover, discrepancies between the distribution of opioid receptor binding sites and dynorphin immunoreactivity contributed to the difficulties in interpretation. In recent years, the generation of prodynorphin- and opioid receptor-deficient mice has provided the tools to investigate open questions on network effects of endogenous dynorphins. This article examines the physiological, pathophysiological and pharmacological implications of dynorphins in the light of new insights in part obtained from genetically modified animals.

Kappa opioids promote the proliferation of astrocytes via Gbetagamma and beta-arrestin 2-dependent MAPK-mediated pathways

GTP binding regulatory protein (G protein)-coupled receptors can activate MAPK pathways via G protein-dependent and -independent mechanisms. However, the physiological outcomes correlated with the cellular signaling events are not as well characterized. In this study, we examine the involvement of G protein and beta-arrestin 2 pathways in kappa opioid receptor-induced, extracellular signal-regulated kinase 1/2 (ERK1/2)-mediated proliferation of both immortalized and primary astrocyte cultures. As different agonists induce different cellular signaling pathways, we tested the prototypic kappa agonist, U69593 as well as the structurally distinct, non-nitrogenous agonist, C(2)-methoxymethyl salvinorin B (MOM-Sal-B). In immortalized astrocytes, U69593, activated ERK1/2 by a rapid (min) initial stimulation that was sustained over 2 h and increased proliferation. Sequestration of activated Gbetagamma subunits attenuated U69593 stimulation of ERK1/2 and suppressed proliferation in these cells. Furthermore, small interfering RNA silencing of beta-arrestin 2 diminished sustained ERK activation induced by U69593. In contrast, MOM-Sal-B induced only the early phase of ERK1/2 phosphorylation and did not affect proliferation of immortalized astrocytes. In primary astrocytes, U69593 produced the same effects as seen in immortalized astrocytes. MOM-Sal-B elicited sustained ERK1/2 activation which was correlated with increased primary astrocyte proliferation. Proliferative actions of both agonists were abolished by either inhibition of ERK1/2, Gbetagamma subunits or beta-arrestin 2, suggesting that both G protein-dependent and -independent ERK pathways are required for this outcome.

Delta-opioid receptors activate ERK/MAP kinase via integrin-stimulated receptor tyrosine kinases

Integrin-mediated cell adherence to extracellular matrix proteins results in stimulation of ERK1/2 activity, a mechanism involving focal adhesion tyrosine kinases (pp125FAK, Pyk-2) and epidermal growth factor receptors (EGFRs). G protein-coupled receptors (GPCRs) may also mediate ERK1/2 activation in an integrin-dependent manner, the underlying signaling mechanism of which still remains unclear. Here we demonstrate that the delta-opioid receptor (DOR), a typical GPCR, stimulates ERK1/2 activity in HEK293 cells via integrin-mediated transactivation of EGFR function. Inhibition of integrin signaling by RGDT peptides, cytochalasin, and by keeping the cells in suspension culture both blocked [D-Ala(2), D-Leu(5)]enkephalin (DADLE)- and etorphine-stimulated ERK1/2 activity. Integrin-dependent ERK1/2 activation does not involve FAK/Pyk-2, because over-expression of the FAK/Pyk-2 inhibitor SOCS-3 failed to attenuate DOR signaling. Exposure of the cells to the EGFR inhibitors AG1478 and BPIQ-I blocked DOR-mediated ERK1/2 activation. Because RGDT peptides also prevented DOR-mediated EGFR activation, the present findings indicate that in HEK293 cells DOR-stimulated ERK1/2 activity is mediated by integrin-stimulated EGFRs. Further studies with the phospholipase C (PLC) inhibitors U73122 and ET-18-OCH(3) revealed that opioid-stimulated integrin activation is sensitive to PLC. In contrast, integrin-mediated transactivation of EGFR function appears to be dependent on PKC-delta, as indicated by studies with rottlerin and siRNA knock-down. A similar ERK1/2 signaling pathway was observed for NG108-15 cells, a neuronal cell line endogenously expressing the DOR. In these cells, the nerve growth factor TrkA receptor replaces the EGFR in connecting DOR-activated integrins to the Ras/Raf/ERK1/2 pathway. Together, these data describe an alternative ERK1/2 signaling pathway in which the DOR transactivates the growth factor receptor associated mitogen-activated protein kinase cascade in an integrin-dependent manner.

Ligand stabilization of CXCR4/delta-opioid receptor heterodimers reveals a mechanism for immune response regulation

The CXCR4 chemokine receptor and the delta opioid receptor (DOR) are pertussis toxin-sensitive G protein-coupled receptors (GPCR). Both are widely distributed in brain tissues and immune cells, and have key roles in inflammation processes and in pain sensation on proximal nerve endings. We show that in immune cells expressing CXCR4 and DOR, simultaneous addition of their ligands CXCL12 and [D-Pen2, D-Pen5]enkephalin does not trigger receptor function. This treatment does not affect ligand binding or receptor expression, nor does it promote heterologous desensitization. Our data indicate that CXCR4 and DOR form heterodimeric complexes that are dynamically regulated by the ligands. This is compatible with a model in which GPCR oligomerization leads to suppression of signaling, promoting a dominant negative effect. Knockdown of CXCR4 and DOR signaling by heterodimerization might have repercussions on physiological and pathological processes such as inflammation, pain sensation and HIV-1 infection.

In vitro and in vivo pharmacological profile of UFP-512, a novel selective delta-opioid receptor agonist; correlations between desensitization and tolerance

BACKGROUND AND PURPOSE

Delta-opioid receptors (DOP receptors) could represent a novel target in the treatment of depressive disorders. To explore this new field of interest, the development of highly selective DOP receptor agonists is essential. UFP-512 [H-Dmt-Tic-NH-CH(CH2-COOH)-Bid], was recently shown to behave in vitro as a selective and potent DOP receptor agonist and to promote antidepressant- and anxiolytic-like effects in vivo (Vergura et al., 2007). Here, we have characterized the pharmacological properties of UFP-512 and established a link between desensitization and tolerance.

EXPERIMENTAL APPROACH

Studies were performed in the human neuroblastoma SK-N-BE cells to establish i) binding parameters for UFP-512 ii) signalling pathways activated after acute and chronic treatment iii) regulation (phosphorylation and trafficking) of human DOP (hDOP) receptors after sustained activation by UFP-512. In vivo, we studied UFP-512-induced antidepressant-like effects after acute or chronic treatment in the mouse forced swimming test.

KEY RESULTS

In vitro, UFP-512 was a high affinity agonist for DOP receptors. While UFP-512 induced marked phosphorylation of DOP receptors on Ser363, we observed a low desensitization of the cAMP pathway, associated with receptor endocytosis and recycling without any reduction on extracellular signal-regulated protein kinase 1/2 activation. In vivo, acute administration of UFP-512 produced an antidepressant-like effect, without any sign of tolerance after chronic administration.

CONCLUSIONS AND IMPLICATIONS

There was a correlation between weak desensitization, significant internalization and recycling of the human DOP receptors and lack of tolerance to UFP-512. This suggests that this compound would be a promising drug prototype for exploring innovative treatments for mood disorders.

Long-acting kappa opioid antagonists disrupt receptor signaling and produce noncompetitive effects by activating c-Jun N-terminal kinase

Norbinaltorphimine (NorBNI), guanidinonaltrindole, and atrans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl) piperidine (JDTic) are selective kappa opioid receptor (KOR) antagonists having very long durations of action in vivo despite binding non-covalently in vitro and having only moderately high affinities. Consistent with this, we found that antagonist treatment significantly reduced the subsequent analgesic response of mice to the KOR agonist U50,488 in the tail-withdrawal assay for 14-21 days. Receptor protection assays were designed to distinguish between possible explanations for this anomalous effect, and we found that mice pretreated with the readily reversible opioid antagonists naloxone or buprenorphine before norBNI responded strongly in the tail-flick analgesia assay to a subsequent challenge with U50,488 1 week later. Protection by a rapidly cleared reagent indicates that norBNI did not persist at the site of action. In vitro binding of [(3)H]U69,593 to KOR showed that K(d) and Bmax values were not significantly affected by prior in vivo norBNI exposure, indicating that the agonist binding site was intact. Consistent with the concept that the long-lasting effects might be caused by a functional disruption of KOR signaling, both norBNI and JDTic were found to stimulate c-Jun N-terminal kinase (JNK) phosphorylation in HEK293 cells expressing KOR-GFP but not in untransfected cells. Similarly, norBNI increased phospho-JNK in both the striatum and spinal cord in wild type mice but not in KOR knock-out mice. Pretreatment of mice with the JNK inhibitor SP600125 before norBNI attenuated the long acting antagonism. Together, these results suggest that the long duration KOR antagonists disrupt KOR signaling by activating JNK.

An opioid agonist that does not induce mu-opioid receptor--arrestin interactions or receptor internalization

G protein-coupled receptor desensitization and trafficking are important regulators of opioid receptor signaling that can dictate overall drug responsiveness in vivo. Furthermore, different mu-opioid receptor (muOR) ligands can lead to varying degrees of receptor regulation, presumably because of distinct structural conformations conferred by agonist binding. For example, morphine binding produces a muOR with low affinity for beta-arrestin proteins and limited receptor internalization, whereas enkephalin analogs promote robust trafficking of both beta-arrestins and the receptors. Here, we evaluate muOR trafficking in response to activation by a novel mu-selective agonist derived from the naturally occurring plant product, salvinorin A. It is interesting that this compound, termed herkinorin, does not promote the recruitment of beta-arrestin-2 to the muOR and does not lead to receptor internalization. Moreover, whereas G protein-coupled receptor kinase overexpression can promote morphine-induced beta-arrestin interactions and muOR internalization, such manipulations do not promote herkinorin-induced trafficking. Studies in mice have shown that beta-arrestin-2 plays an important role in the development of morphine-induced tolerance, constipation, and respiratory depression. Therefore, drugs that can activate the receptor without recruiting the arrestins may be a promising step in the development of opiate analgesics that distinguish between agonist activity and receptor regulation and may ultimately lead to therapeutics designed to provide pain relief without the adverse side effects normally associated with the opiate narcotics.

Tracking the opioid receptors on the way of desensitization

Opioid receptors belong to the super family of G-protein coupled receptors (GPCRs) and are the targets of numerous opioid analgesic drugs. Prolonged use of these drugs results in a reduction of their effectiveness in pain relief also called tolerance, a phenomenon well known by physicians. Opioid receptor desensitization is thought to play a major role in tolerance and a lot of work has been dedicated to elucidate the molecular basis of desensitization. As described for most of GPCRs, opioid receptor desensitization involves their phosphorylation by kinases and their uncoupling from G-proteins realized by arrestins. More recently, opioid receptor trafficking was shown to contribute to desensitization. In this review, our knowledge on the molecular mechanisms of desensitization and recent progress on the role of opioid receptor internalization, recycling or degradation in desensitization will be reported. A better understanding of these regulatory mechanisms would be helpful to develop new analgesic drugs or new strategies for pain treatment by limiting opioid receptor desensitization and tolerance.

Mu opioid receptor activation of ERK1/2 is GRK3 and arrestin dependent in striatal neurons

In this study we investigated the mechanisms responsible for MAP kinase ERK1/2 activation following agonist activation of endogenous mu opioid receptors (MOR) normally expressed in cultured striatal neurons. Treatment with the MOR agonist fentanyl caused significant activation of ERK1/2 in neurons derived from wild type mice. Fentanyl effects were blocked by the opioid antagonist naloxone and were not evident in neurons derived from MOR knock-out (-/-) mice. In contrast, ERK1/2 activation by fentanyl was not evident in neurons from GRK3-/- mice or neurons pretreated with small inhibitory RNA for arrestin3. Consistent with this observation, treatment with the opiate morphine (which is less able to activate arrestin) did not elicit ERK1/2 activation in wild type neurons; however, transfection of arrestin3-(R170E) (a dominant positive form of arrestin that does not require receptor phosphorylation for activation) enabled morphine activation of ERK1/2. In addition, activation of ERK1/2 by fentanyl and morphine was rescued in GRK3-/- neurons following transfection with dominant positive arrestin3-(R170E). The activation of ERK1/2 appeared to be selective as p38 MAP kinase activation was not increased by either fentanyl or morphine treatment in neurons from wild type, MOR-/-, or GRK3-/- mice. In addition, U0126 (a selective inhibitor of MEK kinase responsible for ERK phosphorylation) blocked ERK1/2 activation by fentanyl. These results support the hypothesis that MOR activation of ERK1/2 requires opioid receptor phosphorylation by GRK3 and association of arrestin3 to initiate the cascade resulting in ERK1/2 phosphorylation in striatal neurons.

Cross talk between nitric oxide and ERK1/2 signaling pathway in the spinal cord mediates naloxone-precipitated withdrawal in morphine-dependent rats

Our recent study has shown activation of spinal extracellular signal-regulated kinase-1 and -2 (ERK1/2), a member of the mitogen-activated protein kinase (MAPK) family, contributes to naloxone-precipitated withdrawal and withdrawal-induced spinal neuronal sensitization in morphine-dependent rats. However, the mechanism and significance of the spinal ERK1/2 activation during morphine dependence and withdrawal remain unknown. In this study, we reported that intrathecal (i.t.) pretreatment with either the non-selective nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), neuronal NOS (nNOS) inhibitor 7-nitro indazole (7-NI), or the inducible NOS (iNOS) inhibitor aminoguanidine (AG), could reduce morphine withdrawal-induced increase of phospho-ERK1/2 (pERK1/2) expression in the rat spinal cord. On the other hand, attenuation of the spinal ERK phosphorylation by the MAPK kinase (MEK) inhibitor U0126 also could inhibit the increase of nNOS and iNOS expression in the spinal cord of morphine withdrawal rats. Inhibitory expression of pERK1/2 by i.t. NOS inhibitor L-NAME, 7-NI or AG and of nNOS and iNOS by i.t. U0126 in the spinal cord were accompanied by decreased scores of morphine withdrawal and the inhibited spinal Fos protein (a maker for neuronal excitation or activation) expression induced by morphine withdrawal. These findings suggest cross talk between nitric oxide (NO) and the ERK1/2 signaling pathway mediates morphine withdrawal and withdrawal-induced spinal neuronal sensitization in morphine-dependent rats.

Stimulation of mitogen-activated protein kinase kinases (MEK1/2) by mu-, delta- and kappa-opioid receptor agonists in the rat brain: regulation by chronic morphine and opioid withdrawal

Opioid addiction modulates the extracellular signal-regulated kinase (ERK) leading to synaptic plasticity in the brain. ERK1/2 are stimulated by mitogen-activated protein kinase kinases (MEK1/2), but little is known about the regulation of MEK activity by opioid drugs. This study was designed to assess the acute effects of selective mu-, delta-, and kappa-opioid receptor agonists, as well as those induced by chronic morphine and opioid withdrawal, on the content of phosphorylated MEK1/2 in the rat brain. Sufentanil (1-30 microg/kg, 30-120 min) induced dose- and time-dependent increases in MEK1/2 phosphorylation in the cerebral cortex and corpus striatum (30-177%) through a naloxone-sensitive mechanism. Morphine (100 mg/kg, 2 h) also augmented MEK1/2 phosphorylation in the both brain regions (50-70%). Similarly, the selective delta-opioid receptor agonist SNC-80 (10 mg/kg, 30 min) increased MEK1/2 activity in the cortex (60%) that was antagonized by naltrindole. In contrast, the selective kappa-opioid receptor agonist (-)-U50488H (10 mg/kg, 30-120 min) did not modify significantly MEK1/2 phosphorylation in the cortex. Chronic morphine (10-100 mg/kg, 5 days) was not associated with alterations in the content of phosphorylated MEK1/2 in the brain (induction of tachyphylaxis to the acute effects). In morphine-dependent rats, however, naloxone (2 mg/kg)-precipitated withdrawal (2-6 h) induced robust increases in MEK1/2 phosphorylation in cortex (27-49%) and striatum (83-123%). Spontaneous opioid withdrawal (24 h) in morphine-dependent rats did not alter MEK1/2 activity in the brain. The findings may be relevant in the context of the pivotal role played by the MEK/ERK pathway in various long-lasting forms of synaptic plasticity associated with opioid addiction.

Opioids trigger alpha 5 beta 1 integrin-mediated monocyte adhesion

Inflammatory reactions involve a network of chemical and molecular signals that initiate and maintain host response. In inflamed tissue, immune system cells generate opioid peptides that contribute to potent analgesia by acting on specific peripheral sensory neurons. In this study, we show that opioids also modulate immune cell function in vitro and in vivo. By binding to its specific receptor, the opioid receptor-specific ligand DPDPE triggers monocyte adhesion. Integrins have a key role in this process, as adhesion is abrogated in cells treated with specific neutralizing anti-alpha5beta1 integrin mAb. We found that DPDPE-triggered monocyte adhesion requires PI3Kgamma activation and involves Src kinases, the guanine nucleotide exchange factor Vav-1, and the small GTPase Rac1. DPDPE also induces adhesion of pertussis toxin-treated cells, indicating involvement of G proteins other than Gi. These data show that opioids have important implications in regulating leukocyte trafficking, adding a new function to their known effects as immune response modulators.

Molecular targets of opiate drug abuse in neuroAIDS

Opiate drug abuse, through selective actions at mu-opioid receptors (MOR), exacerbates the pathogenesis of human immunodeficiency virus-1 (HIV-1) in the CNS by disrupting glial homeostasis, increasing inflammation, and decreasing the threshold for pro-apoptotic events in neurons. Neurons are affected directly and indirectly by opiate-HIV interactions. Although most opiates drugs have some affinity for kappa (KOR) and/or delta (DOR) opioid receptors, their neurotoxic effects are largely mediated through MOR. Besides direct actions on the neurons themselves, opiates directly affect MOR-expressing astrocytes and microglia. Because of their broad-reaching actions in glia, opiate abuse causes widespread metabolic derangement, inflammation, and the disruption of neuron-glial relationships, which likely contribute to neuronal dysfunction, death, and HIV encephalitis. In addition to direct actions on neural cells, opioids modulate inflammation and disrupt normal intercellular interactions among immunocytes (macrophages and lymphocytes), which on balance further promote neuronal dysfunction and death. The neural pathways involved in opiate enhancement of HIV-induced inflammation and cell death, appear to involve MOR activation with downstream effects through PI3-kinase/Akt and/or MAPK signaling, which suggests possible targets for therapeutic intervention in neuroAIDS.

Mu and kappa opioid receptors activate ERK/MAPK via different protein kinase C isoforms and secondary messengers in astrocytes

Acute mu and kappa opioids activate the ERK/MAPK phosphorylation cascade that represents an integral part of the signaling pathway of growth factors in astrocytes. By this cross-talk, opioids may impact neural development and plasticity among other basic neurobiological processes in vivo. The mu agonist, [D-ala2,mephe4,glyol5]enkephalin (DAMGO), induces a transient stimulation of ERK phosphorylation, whereas kappa agonist, U69,593, engenders sustained ERK activation. Here we demonstrate that acute U69,593 and DAMGO stimulate ERK phosphorylation by utilization of different secondary messengers and protein kinase C (PKC) isoforms upstream of the growth factor pathway. Immortalized astrocytes transfected with either antisense calmodulin (CaM), a mutant mu opioid receptor that binds CaM poorly or a dominant negative mutant of PKCepsilon were used as a model system to study mu signaling. Evidence was gained to implicate CaM and PKCepsilon in DAMGO stimulation of ERK. DAMGO activation of PKCepsilon and/or ERK was insensitive to selective inhibitors of Ca2+ mobilization, but it was blocked upon phospholipase C inhibition. These results suggest a novel mechanism wherein, upon DAMGO binding, CaM is released from the mu receptor and activates phospholipase C. Subsequently, phospholipase C generates diacylglycerides that activate PKCepsilon. In contrast, U69,593 appears to act via phosphoinositide 3-kinase, PKCzeta, and Ca2+ mobilization. These signaling components were implicated based on studies with specific inhibitors and a dominant negative mutant of PKCzeta. Collectively, our findings on acute opioid effects suggest that differences in their mechanism of signaling may contribute to the distinct outcomes on ERK modulation induced by chronic mu and kappa opioids.

Opioid-induced preconditioning: recent advances and future perspectives

Opioids, named by Acheson for compounds with morphine-like actions despite chemically distinct structures, have received much research interest, particularly for their central nervous system (CNS) actions involved in pain management, resulting in thousands of scientific papers focusing on their effects on the CNS and other organ systems. A more recent area which may have great clinical importance concerns the role of opioids, either endogenous or exogenous compounds, in limiting the pathogenesis of ischemia-reperfusion injury in heart and brain. The role of endogenous opioids in hibernation provides tantalizing evidence for the protective potential of opioids against ischemia or hypoxia. Mammalian hibernation, a distinct energy-conserving state, is associated with depletion of energy stores, intracellular acidosis and hypoxia, similar to those which occur during ischemia. However, despite the potentially detrimental cellular state induced with hibernation, the myocardium remains resilient for many months. What accounts for the hypoxia-tolerant state is of great interest. During hibernation, circulating levels of opioid peptides are increased dramatically, and indeed, are considered a "trigger" of hibernation. Furthermore, administration of opioid antagonists can effectively reverse hibernation in mammals. Therefore, it is not surprising that activation of opioid receptors has been demonstrated to preserve cellular status following a hypoxic insult, such as ischemia-reperfusion in many model systems including the intestine [Zhang, Y., Wu, Y.X., Hao, Y.B., Dun, Y. Yang, S.P., 2001. Role of endogenous opioid peptides in protection of ischemic preconditioning in rat small intestine. Life Sci. 68, 1013-1019], skeletal muscle [Addison, P.D., Neligan, P.C., Ashrafpour, H., Khan, A., Zhong, A., Moses, M., Forrest, C.R., Pang, C.Y., 2003. Noninvasive remote ischemic preconditioning for global protection of skeletal muscle against infarction. Am. J. Physiol. Heart Circ. Physiol. 285, H1435-H1443], the CNS [Borlongan, C.V., Wang, Y., Su, T.P., 2005. Delta opioid peptide (d-ala 2, d-leu 5) enkephalin: linking hiberation and neuroprotection. Front Biosci. 9, 3392-3398] and the myocardium [Romano, M.A., Seymour, E.M., Berry, J.A., McNish, R.A., Bolling, S.F., 2004. Relative contribution of endogenous opioids to myocardial ischemic tolerance. J Surg Res. 118, 32-37; Peart, J.N., Gross, G.J., 2004a. Exogenous activation of delta- and kappa-opioid receptors affords cardioprotection in isolated murine heart. Basic Res Cardiol. 99(1), 29-37]. For the purpose of this review, we will focus primarily on the protective effects of opioids against post-reperfusion myocardial stunning and infarction.