Phosphorylation: A molecular switch in opioid tolerance

Biasing Gβγ Downstream Signaling with Gallein Inhibits Development of Morphine Tolerance and Potentiates Morphine-Induced Nociception in a Tolerant State

Opioid analgesics are widely used as a treatment option for pain management and relief. However, the misuse of opioid analgesics has contributed to the current opioid epidemic in the United States. Prescribed opioids such as morphine, codeine, oxycodone, and fentanyl are mu-opioid receptor (MOR) agonists primarily used in the clinic to treat pain or during medical procedures, but development of tolerance limits their utility for treatment of chronic pain. Here we explored the effects of biasing Gβγ signaling on tolerance development after chronic morphine treatment in vivo. We hypothesized that biasing Gβγ signaling with gallein could prevent activation of regulatory signaling pathways that result in tolerance to antinociceptive effects of MOR agonists. Gallein has been shown to bind to Gβγ and inhibit interactions of Gβγ with phospholipase-Cβ3 (PLCβ3) or G-protein-coupled receptor kinase 2 (GRK2) but not G-protein inwardly rectifying potassium (GIRK) channels. In mice, morphine-induced antinociception was evaluated in the 55°C warm water tail withdrawal assay. We used two paradigms for gallein treatment: administration during and after three times-daily morphine administration. Our results show that gallein cotreatment during repeated administration of morphine decreased opioid tolerance development and that gallein treatment in an opioid-tolerant state enhanced the potency of morphine. Mechanistically, our data suggest that PLCβ3 is necessary for potentiating effects of gallein in an opioid-tolerant state but not in preventing the development of tolerance. These studies demonstrate that small molecules that target Gβγ signaling could reduce the need for large doses of opioid analgesics to treat pain by producing an opioid-sparing effect. SIGNIFICANCE STATEMENT: Biasing Gβγ signaling prevents tolerance to repeated morphine administration in vivo and potentiates the antinociceptive effects of morphine in an opioid-tolerant state. Mechanistically, phospholipase-Cβ is necessary for potentiating effects of gallein in an opioid-tolerant state but not in preventing the development of tolerance. This study identifies a novel treatment strategy to decrease the development of tolerance to the analgesic effects of mu-opioid receptor agonists, which are necessary to improve pain treatment and decrease the incidence of opioid use disorder.

Paroxetine effects on morphine analgesic tolerance in rats

OBJECTIVES

To alleviate different pain intensities, morphine administration has been extensively used. However, prolonged administration of morphine leads to a progressive decline of its analgesic effect which limits their overall utility. Morphine tolerance is considered as a challenging issue for the treatment of both acute and chronic pain. We conducted this study in rats to investigate the effect of paroxetine on morphine tolerance when used preemptively or after morphine tolerance had developed.

METHODS

Male Wistar rats (weight 250-300 g, n=10) were used to evaluate the effects of paroxetine on tolerance to morphine. In order to induce tolerance, daily intraperitoneal injection of morphine (7 mg/kg) was done. After tolerance induction, a group of animals received intraperitoneal injection of 10 mg/kg paroxetine 30 min prior to each morphine dose. In another trial, to investigate the potential of paroxetine to prevent tolerance to morphine, animals were pretreated with 10 mg/kg paroxetine 30 min before morphine administration. In the control groups, 10 mL/kg of saline was injected. The behavioral test (tail-flick test) was done for all groups.

RESULTS

Our data showed that paroxetine significantly reversed tolerance to morphine when used after tolerance induction (p<0.001). However, administration of paroxetine before occurrence of tolerance had no effect.

CONCLUSIONS

We conclude that paroxetine could decrease tolerance to morphine when used after the occurrence of morphine tolerance, while it was not able to prevent morphine tolerance when administered preemptively.

ETHICAL COMMITTEE NUMBER

IRIB.SBMU.MSP.REC.1394.098.

Donepezil prevents morphine tolerance by regulating N-methyl-d-aspartate receptor, protein kinase C and CaM-dependent kinase II expression in rats

Current studies have indicated that donepezil as a cholinesterase inhibitor can attenuate morphine-induced tolerance. The present study aimed to evaluate the possible role of N-methyl-d-aspartate receptors (NMDARs), protein kinase C (PKC) and CaM-dependent kinase II (CaMKII) pathways in this effect. Female Wistar rats received daily morphine (10 mg/kg, i.p.) alone or in combination with donepezil (1.5 or 2 mg/kg, gavaged) for 14 days. The analgesic effect was assessed by Von-frey, hotplate and tail flick test. On the 15th day, the periaqueductal gray (PAG) and lumbar spinal cord of rats were dissected. Then, protein levels of NMDAR-NR1, NR2B, PKCγ and CaMKIIα were tested using Western blot method. The results showed that morphine tolerance was seen after 8-10 days of injection compared with control group, while daily co-administration of donepezil with morphine prolonged the occurrence of analgesic tolerance. Western blot showed that morphine significantly increased NR1, PKCγ and CaMKIIα expressions in PAG, and significantly increased PKCγ and CaMKIIα in spinal cord. In contrast, donepezil downregulated NR1 and PKCγ in PAG, and downregulated PKCγ and CaMKIIα in spinal cord. Moreover, donepezil alone activates NR1 and NR2B in spinal cord, which needs to be further studied. Thus, the present results suggest that the attenuation effects of donepezil on morphine tolerance are possibly mediated by preventing morphine-induced upregulations in NR1, PKCγ and CaMKIIα expressions.

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Retracted Article: Upregulation of miR-26b alleviates morphine tolerance by inhibiting BDNF via Wnt/β-catenin pathway in rats

Background: Morphine is a commonly used analgesic drug. However, long-term use of morphine will cause tolerance which limits its clinical application in pain treatment. MicroRNAs (miRNAs) have been reported to be involved in the morphine tolerance, but the underlying mechanism is still poorly understood. Methods: Tail flick test was used to measure the maximum possible effect (MPE). Quantitative real-time PCR was employed to detect miR-26b, BDNF, and Wnt5a expression in rat dorsal root ganglia (DRG). Luciferase report assay was introduced to verify the binding relationship between miR-26b and Wnt5a. BDNF, Wnt5a and β-catenin protein level were tested by western blotting. Results: MiR-26b was down-regulated during the development of morphine tolerance while BDNF was upregulated. Overexpression of miR-26b or BDNF inhibition alleviated morphine tolerance. Wnt5a was directly targeted and inhibited by miR-26b via binding to the 3'-UTR of Wnt5a. The Wnt/β-catenin pathway was active in morphine tolerant rats. Moreover, overexpression of Wnt5a could partially enhance miR-26 mimic-mediated morphine tolerance, while a Wnt5a inhibitor could attenuate the tolerance. Conclusion: The present study demonstrated that miR-26b overexpression alleviated morphine tolerance by inhibiting BDNF via the Wnt/β-catenin pathway in rats, highlighting a promising target for the treatment of morphine tolerance.

Spinal and afferent PKC signaling mechanisms that mediate chronic pain in sickle cell disease

Pain is the most characteristic feature of sickle cell disease (SCD). Patients with SCD live with unpredictable, recurrent episodes of acute painful crisis, as well as chronic unremitting pain throughout their lifetime. While most of the research and medical efforts have focused on treating vaso-occlusion crisis and acute pain, chronic pain remains a significant challenge faced by patients and physicians. Emerging evidence from human and animal studies has suggested the presence of a neuropathic component in SCD pain. New knowledge on the neurobiology of chronic pain in SCD has significant implications in unraveling the underlying mechanisms. This review focuses on the recent advances on the role of protein kinase C or PKC in promoting and maintaining chronic pain conditions. With a highlight of a specific PKC isoform, PKCδ, we aim to propose PKC as an essential regulator of chronic pain in SCD, which may ultimately lead to innovative therapeutic strategies for treating this devastating life-long problem in patients with SCD.

TRPV1 modulates morphine self-administration via activation of the CaMKII-CREB pathway in the nucleus accumbens

Opioid addiction is a growing problem for public health, and opioids have correspondingly become more heavily regulated over time. We have previously shown that TRPV1 plays a critical role in morphine addiction using a self-administration paradigm in rats, and the current study evaluates the effects of the TRPV1 signaling pathway on morphine self-administration (SA). We found that treatment with a selective TRPV1 antagonist, SB366791, significantly decreased the morphine SA-induced activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII), Akt and the cAMP response element binding protein (CREB) in the nucleus accumbens (NAc). In addition, phospho-PKA and phospho-PKC expression levels were significantly increased in the NAc of the morphine-SA groups, regardless of SB366791 treatment. Finally, local microinjection of SB366791 into the NAc significantly suppressed the maintenance of morphine SA. Taken together, our findings highlight that TRPV1 plays an important role in morphine addiction, likely via activation of the CaMKII-CREB pathway in the NAc.

Aquaporin 4 Forms a Macromolecular Complex with Glutamate Transporter 1 and Mu Opioid Receptor in Astrocytes and Participates in Morphine Dependence

The water channel aquaporin 4 (AQP4) is abundantly expressed in astrocytes and provides a mechanism by which water permeability of the plasma membrane can be regulated. Evidence suggests that AQP4 is associated with glutamate transporter-1 (GLT-1) for glutamate clearance and contributes to morphine dependence. Previous studies show that AQP4 deficiency changed the mu opioid receptor expression and opioid receptors' characteristics as well. In this study, we focused on whether AQP4 could form macromolecular complex with GLT-1 and mu opioid receptor (MOR) and participates in morphine dependence. By using immunofluorescence staining, fluorescence resonance energy transfer, and co-immunoprecipitation, we demonstrated that AQP4 forms protein complexes with GLT-1 and MOR in both brain tissue and primary cultured astrocytes. We then showed that the C-terminus of AQP4 containing the amino acid residues 252 to 323 is the site of interaction with GLT-1. Protein kinase C, activated by morphine, played an important role in regulating the expression of these proteins. These findings may help to reveal the mechanism that AQP4, GLT-1, and MOR form protein complex and participate in morphine dependence, and deeply understand the reason that AQP4 deficiency maintains extracellular glutamate homeostasis and attenuates morphine dependence, moreover emphasizes the function of astrocyte in morphine dependence.

miR-365 targets β-arrestin 2 to reverse morphine tolerance in rats

Morphine tolerance is a challenging clinical problem that limits its clinical application in pain treatment. Non-coding microRNAs (miRNAs) modulate gene expression in a post transcriptional manner, and their dysregulation causes various diseases. However, the significance of miRNAs in morphine tolerance is still poorly understood. In the present study, we hypothesized that microRNA-365 (miR-365) is a key functional small RNA that reverses morphine tolerance through regulation of β-arrestin 2 in rats. Here, microarray analysis and quantitative real-time PCR showed that miR-365 was robustly decreased in the spinal cord after chronic morphine administration. In situ hybridization and immunochemistry double staining showed that miR-365 was expressed in neurons of the spinal cord. We identified β-arrestin 2 as the target gene of miR-365 by bioinformatics analysis and luciferase reporter assay. The data showed that overexpression of miR-365 prevented and reversed established morphine tolerance, and increased expression of miR-365 caused a decrease in expression of β-arrestin 2 protein. miR-365 downregulation is involved in the development and maintenance of morphine tolerance through regulation of β-arrestin 2, and miR-365 upregulation provides a promising and novel approach for treatment of morphine tolerance.

Chronic Daily Headache: Mechanisms and Principles of Management

Primary headache is a common malady that is often under-recognized and frequently inadequately managed in spite of the fact that it affects up to 95 % of the population in a lifetime. Many forms of headache, including episodic tension and migraine headaches, if properly diagnosed, are reasonably amenable to treatment, but a smaller, though not insignificant, percent of the population suffer daily from a chronic, intractable form of headache that destroys one's productivity and quality of life. These patients are frequently seen in neurological practices at a point when treatment options are limited and largely ineffective. In the following review, we will discuss mechanisms drawn from recent studies that address the transition from acute to chronic pain that may apply to the transformation from episodic to chronic daily headaches which may offer opportunities for preempting headache transformation.

siRNA capsulated brain-targeted nanoparticles specifically knock down OATP2B1 in mice: a mechanism for acute morphine tolerance suppression

Regulating main brain-uptake transporter of morphine may restrict its tolerance generation, then modify its antinociception. In this study, more than 2 fold higher intracellular uptake concentrations for morphine and morphine-6-glucuronide (M6G) were observed in stable expression cells, HEK293-hOATP2B1 than HEK293-MOCK. Specifically, the K_m value of morphine to OATP2B1 (57.58 ± 8.90 μM) is 1.4-time more than that of M6G (80.31 ± 21.75 μM); Cyclosporine A (CsA), an inhibitor of OATP2B1, can inhibit their intracellular accumulations with IC₅₀ = 3.90 ± 0.50 μM for morphine and IC₅₀ = 6.04 ± 0.86 μM for M6G, respectively. To further investigate the role of OATP2B1 in morphine brain transport and tolerance, the novel nanoparticles of DGL-PEG/dermorphin capsulated siRNA (OATP2B1) were applied to deliver siRNA into mouse brain. Along with OATP2B1 depressed, a main reduction was found for each of morphine or M6G in cerebrums or epencephalons of acute morphine tolerance mice. Furthermore, calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) in mouse prefrontal cortex (mPFC) underwent dephosphorylation at Thr286. In conclusion, OATP2B1 downregulation in mouse brain can suppress tolerance via blocking morphine and M6G brain transport. These findings might help to improve the pharmacological effects of morphine.

Curcumin attenuates opioid tolerance and dependence by inhibiting Ca2+/calmodulin-dependent protein kinase II α activity

Chronic use of opioid analgesics has been hindered by the development of opioid addiction and tolerance. We have reported that curcumin, a natural flavonoid from the rhizome of Curcuma longa, attenuated opioid tolerance, although the underlying mechanism remains unclear. In this study, we tested the hypothesis that curcumin may inhibit Ca(2+)/calmodulin-dependent protein kinase II α (CaMKIIα), a protein kinase that has been previously proposed to be critical for opioid tolerance and dependence. In this study, we used state-of-the-art polymeric formulation technology to produce poly(lactic-co-glycolic acid) (PLGA)-curcumin nanoparticles (nanocurcumin) to overcome the drug's poor solubility and bioavailability, which has made it extremely difficult for studying in vivo pharmacological actions of curcumin. We found that PLGA-curcumin nanoparticles reduced the dose requirement by 11- to 33-fold. Pretreatment with PLGA-curcumin (by mouth) prevented the development of opioid tolerance and dependence in a dose-dependent manner, with ED50 values of 3.9 and 3.2 mg/kg, respectively. PLGA-curcumin dose-dependently attenuated already-established opioid tolerance (ED50 = 12.6 mg/kg p.o.) and dependence (ED50 = 3.1 mg/kg p.o.). Curcumin or PLGA-curcumin did not produce antinociception by itself or affect morphine (1-10 mg/kg) antinociception. Moreover, we found that the behavioral effects of curcumin on opioid tolerance and dependence correlated with its inhibition of morphine-induced CaMKIIα activation in the brain. These results suggest that curcumin may attenuate opioid tolerance and dependence by suppressing CaMKIIα activity.

Intra-nucleus accumbens administration of the calcium/calmodulin-dependent protein kinase II inhibitor KN93 induced antinociception in rats with mononeuropathy

The present study was conducted on rats with mononeuropathy induced by left common sciatic nerve ligation. Unilateral sciatic nerve loose ligation produced decreases of the hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation. Intra-nucleus accumbens (NAc) injection of 1μg, 3μg and 6μg of KN93, the calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitor, dose-dependently increased the HWL in mononeuropathic rats. Furthermore, intra-NAc administration of morphine, the HWL to noxious thermal and mechanical stimulation increased markedly, and there were no significant differences between morphine group and KN93 group. The results demonstrated that intra-NAc injection of KN93 induced significant antinociceptive effects in rats with mononeuropathy, indicating CaMKII may play important roles in transmission of nociceptive information in the NAc of mononeuropathic rats.

Large conductance voltage- and Ca2+-gated potassium (BK) channel β4 subunit influences sensitivity and tolerance to alcohol by altering its response to kinases

Tolerance is a well described component of alcohol abuse and addiction. The large conductance voltage- and Ca(2+)-gated potassium channel (BK) has been very useful for studying molecular tolerance. The influence of association with the β4 subunit can be observed at the level of individual channels, action potentials in brain slices, and finally, drinking behavior in the mouse. Previously, we showed that 50 mm alcohol increases both α and αβ4 BK channel open probability, but only α BK develops acute tolerance to this effect. Currently, we explore the possibility that the influence of the β4 subunit on tolerance may result from a striking effect of β4 on kinase modulation of the BK channel. We examine the influence of the β4 subunit on PKA, CaMKII, and phosphatase modulation of channel activity, and on molecular tolerance to alcohol. We record from human BK channels heterologously expressed in HEK 293 cells composed of its core subunit, α alone (Insertless), or co-expressed with the β4 BK auxiliary subunit, as well as, acutely dissociated nucleus accumbens neurons using the cell-attached patch clamp configuration. Our results indicate that BK channels are strongly modulated by activation of specific kinases (PKA and CaMKII) and phosphatases. The presence of the β4 subunit greatly influences this modulation, allowing a variety of outcomes for BK channel activity in response to acute alcohol.

Role of D₁/D₂ dopamin receptors antagonist perphenazine in morphine analgesia and tolerance in rats

While opioid receptors have been implicated in the development of tolerance, the subsequent mechanisms involved in these phenomena have not been completely understood. The purpose of this study was to investigate effects of D1/D2 dopamine receptors antagonist perphenazine on morphine analgesia and tolerance in rats. Male Wistar albino rats weighing 190-205 g were used in these experiments. To constitute of morphine tolerance, animals received morphine (50 mg/kg) once daily for 3 days. After last dose of morphine was injected on day 4, morphine tolerance was evaluated by the analgesia tests. The analgesic effects of perphenazine (1, 5, and 10 mg/kg ), D1-dopamine receptor antagonist SCH 23390 (1 mg/kg), D2-dopamine receptor antagonist eticlopride (1 mg/kg), and morphine were considered at 30-min intervals (0, 30, 60, 90, and 120 min) by tail-flick and hot-plate analgesia tests. Obtained data suggested that D1/D2 dopamine receptors antagonist perphenazine was capable of suppressing opioid tolerance, possibly by the mechanism of inhibiting D2-dopamine receptor. Because the data indicated that D2-dopamine receptor antagonist eticloride, but not D1-dopamine receptor antagonist SCH 23390, significantly decreased morphine tolerance in analgesia tests. In addition, administration of perphenazine with morphine increased morphine analgesia. Results from the present study suggested that dopamine receptors play a significant role in the morphine analgesic tolerance. In particular, D2-dopamine receptor has an important role rather than D1-dopamine receptor in development tolerance to morphine.

Opiate exposure and withdrawal induces a molecular memory switch in the basolateral amygdala between ERK1/2 and CaMKIIα-dependent signaling substrates

Opiate reward memories are powerful triggers for compulsive opiate-seeking behaviors. The basolateral amygdala (BLA) is an important structure for the processing of opiate-related associative memories and is functionally linked to the mesolimbic dopamine (DA) pathway. Transmission through intra-BLA DA D1-like and D2-like receptors independently modulates the formation of opiate reward memories as a function of opiate-exposure state. Thus, in the opiate-naive state, intra-BLA D1 transmission is required for opiate-related memory formation. Once opiate dependence and withdrawal has developed, a functional switch to a DA D2-mediated memory mechanism takes place. However, the downstream molecular signaling events that control this functional switch between intra-BLA DA D1 versus D2 receptor transmission are not currently understood. Using an unbiased place conditioning procedure in rats combined with molecular analyses, we report that opiate reward memory acquisition requires intra-BLA ERK1/2 signaling only in the previously opiate-naive state. However, following chronic opiate exposure and withdrawal, intra-BLA reward memory processing switches to a CaMKIIα-dependent memory substrate. Furthermore, the ability of intra-BLA DA D1 or D2 receptor transmission to modulate the motivational salience of opiates similarly operates through a D1-mediated ERK-dependent mechanism in the opiate-naive state, but switches to a D2-mediated CaMKIIα-dependent mechanism in the dependent/withdrawn state. Protein analysis of BLA tissue revealed a downregulation of ERK1/2 phosphorylation and a dramatic reduction in both total and phosphorylated CaMKIIα signaling, specifically in the opiate-dependent/withdrawn state, demonstrating functional control of ERK1/2-dependent versus CaMKIIα-dependent memory mechanisms within the BLA, controlled by opiate-exposure state.

Screening of kinase inhibitors targeting BRAF for regulating autophagy based on kinase pathways

The aim of this study was to identify agents that regulate autophagy. A total of 544 differentially expressed genes were screened from the intersection set of GSE2435 and GSE31040, which was obtained from the Gene Expression Omnibus database and 19 differentially expressed kinases were selected according to a 'protein kinase database'. Gene ontology‑biological process (GO-BP) enrichment analysis revealed that the 19 kinases were mainly associated with phosphorylation. The protein-protein interaction network exhibited 30 differentially expressed genes that interacted with BRAF, and GO-BP enrichment analysis showed the function of these genes were mainly involved in cell death and apoptosis. The kinase-kinase inhibitor regulatory network identified16 kinase inhibitors that specifically inhibited BRAF. Previous studies indicated that sorafenib is capable of regulating autophagy and regorafenib has also been reported; however, there have been no studies regarding the regulation of autophagy by afatinib, selumetinib, PD318088, axitinib, TAK-733, GDC-0980, GSK2126458, PLX-4720, AS703026, trametinib, GDC-0941 and PF-04217903. Thus, these kinase inhibitors are potential targets for further study on the regulation of autophagy in the future.

Botulinum toxin A increases analgesic effects of morphine, counters development of morphine tolerance and modulates glia activation and μ opioid receptor expression in neuropathic mice

The use of botulinum neurotoxin type A (BoNT/A) against pain, with emphasis for its possible use in alleviating chronic pain, still represents an outstanding challenge for experimental research. In this study, we examined the effects of BoNT/A on morphine-induced tolerance during chronic morphine treatment in neuropathic CD1 mice subjected to sciatic nerve lesion according to the Chronic Constriction Injury (CCI) model of neuropathic pain. We measured the effects of BoNT/A on CCI-induced allodynia and hyperalgesia and on the expression of glial fibrillary acidic protein (GFAP, marker of astrocytes), complement receptor 3/cluster of differentiation 11b (CD11b, marker of microglia), and neuronal nuclei (NeuN) at the spinal cord level. We also analyzed the colocalized expression of GFAP, CD11b and NeuN with phosphorylated p-38 mitogen-activated protein kinase and with μ-opioid receptor (MOR). A single intraplantar injection of BoNT/A (15 pg/paw) into the injured hindpaw, the day before the beginning of chronic morphine treatment (9 days of twice daily injections of 40 mg/kg morphine), was able to counteract allodynia and enhancement of astrocytes expression/activation induced by CCI. In addition, BoNT/A increased the analgesic effect of morphine and countered morphine-induced tolerance during chronic morphine treatment. These effects were accompanied, in neurons, by re-expression of MORs that had been reduced by repeated morphine administration. The combinatory effects of BoNT/A and morphine could have relevant therapeutic implications for sufferers of chronic pain who could benefit of pain relief reducing tolerance due to repeated treatment with opiates.

Sigma-1 receptor antagonism as opioid adjuvant strategy: enhancement of opioid antinociception without increasing adverse effects

While opioids are potent analgesics widely used in the management of pain, a number of well-known adverse effects limit their use. The sigma-1 receptor is a ligand-regulated molecular chaperone involved in pain processing, including modulation of opioid antinociception. However, data supporting the potential use of sigma-1 receptor ligands as suitable opioid adjuvants are based on studies that use non selective ligands. Also, safety issues derived from combination therapy are poorly addressed. In this study we used the new selective sigma-1 receptor antagonist S1RA (E-52862) to characterize the effect of selective sigma-1 receptor blockade on opioid-induced efficacy- and safety-related outcomes in mice. S1RA (40 mg/kg) had no effect in the tail-flick test but did enhance the antinociceptive potency of several opioids by a factor between 2 and 3.3. The potentiating effect of S1RA on morphine antinociception did not occur in sigma-1 receptor knockout mice, which supports the selective involvement of the sigma-1 receptor. Interestingly, S1RA co-administration restored morphine antinociception in tolerant mice and reverted the reward effects of morphine in the conditioned place preference paradigm. In addition, enhancement of antinociception was not accompanied by potentiation of other opioid-induced effects, such as the development of morphine analgesic tolerance, physical dependence, inhibition of gastrointestinal transit, or mydriasis. The use of sigma-1 receptor antagonists as opioid adjuvants could represent a promising pharmacological strategy to enhance opioid potency and, most importantly, to increase the safety margin of opioids. S1RA is currently in phase II clinical trials for the treatment of several pain conditions.

The influences of reproductive status and acute stress on the levels of phosphorylated delta opioid receptor immunoreactivity in rat hippocampus

In the hippocampus, ovarian hormones and sex can alter the trafficking of delta opioid receptors (DORs) and the proportion of DORs that colocalize with the stress hormone, corticotropin releasing factor. Here, we assessed the effects of acute immobilization stress (AIS) and sex on the phosphorylation of DORs in the rat hippocampus. We first localized an antibody to phosphorylated DOR (pDOR) at the SER363 carboxy-terminal residue, and demonstrated its response to an opioid agonist. By light microscopy, pDOR-immunoreactivity (ir) was located predominantly in CA2/CA3a pyramidal cell apical dendrites and in interneurons in CA1-3 stratum oriens and the dentate hilus. By electron microscopy, pDOR-ir primarily was located in somata and dendrites, associated with endomembranes, or in dendritic spines. pDOR-ir was less frequently found in mossy fibers terminals. Quantitative light microscopy revealed a significant increase in pDOR-ir in the CA2/CA3a region of male rats 1h following an injection of the opioid agonist morphine (20mg/kg, I.P). To look at the effects of stress on pDOR, we compared pDOR-ir in males and cycling females after AIS. The level of pDOR-ir in stratum radiatum of CA2/CA3a was increased in control estrus (elevated estrogen and progesterone) females compared to proestrus and diestrus females and males. However, immediately following 30min of AIS, no significant differences in pDOR levels were seen across estrous cycle phase or sex. These findings suggest that hippocampal levels of phosphorylated DORs vary with estrous cycle phase and that acute stress may dampen the differential effects of hormones on DOR activation in females.

Phosphorylated CaMKII levels increase in rat central nervous system after large-dose intravenous remifentanil

Background

Postoperative remifentanil-induced pain sensitization is common, but its molecular mechanism remains unclear. Calcium/calmodulin-dependent protein kinase II (CaMKII) has been shown to have a critical role in morphine-induced hyperalgesia. This study was designed to determine how CaMKII phosphorylation and protein expression levels change in the central nervous system of rats with remifentanil-induced hyperalgesia.

Material/Methods

Male Sprague-Dawley^® rats were exposed to large-dose (bolus of 6.0 μg/kg and 2.5 μg/kg/min for 2 hours) intravenous remifentanil to induce post-transfusion hyperalgesia. Levels of phosphorylated CaMKII (P-CaMKII) and total protein of CaMKII (T-CaMKII) were determined at different post-transfusion times by Western blot and immunostaining and were compared with controls.

Results

P-CaMKII increased significantly (P<0.05) at 0, 0.5, and 2 hours. However, P-CaMKII at 5 to 24 hours and T-CaMKII at 0 to 24 hours post-transfusion did not change significantly in rats’ spinal dorsal horn, hippocampus, or primary somatosensory (S1) cortex (n=6 per group). Similarly, immunostaining showed stronger P-CaMKII immunoreactants (P<0.05) and more P-CaMKII- positive cells (P<0.05) in the spinal dorsal horn, CA1 region of the hippocampus, and S1 cortex of rats 0.5 hours post-transfusion compared with the control group treated with 0.9% sodium chloride (n=3 per group).

Conclusions

These results suggest that a temporary rise in the P-CaMKII level in the central nervous system may correlate with remifentanil-induced pain sensitization in the postoperative period.

Let-7 microRNAs and Opioid Tolerance

This chapter will focus on the role of microRNAs (miRs) in regulating the actions of opioid drugs through the opioid receptors. Opioids, such as morphine, are analgesics that are used for treating many forms of acute and chronic pain. However, their chronic use is limited by undesirable effects such as opioid tolerance. The μ opioid receptor (MOR) is the primary receptor responsible for opioids' analgesia and antinociceptive tolerance. The long 3'-untranslated region (3'-UTR) of MOR mRNA is of great interest since this region may contain elements for the post-transcriptional regulation of receptor expression, such as altering the stability of mRNA, influencing translational efficiency, and controlling mRNA transport. MicroRNAs are small non-coding RNA molecules that exert their functions through base-pairing with partially complementary sequences in the 3'-UTR of target mRNAs, resulting in decreased polypeptide formation from those mRNAs. Since the discovery of the first miR, lin-4 in Caenorhabditis elegans, hundreds of miRs have been identified from humans to viruses, which have provided a crucial and pervasive layer of post-transcriptional gene regulation. The nervous system is a rich source of miR expression, with a diversity of miR functions in fundamental neurobiological processes including neuronal development, plasticity, metabolism, and apoptosis. Recently, the let-7 family of miRs is found to be a critical regulator of MOR function in opioid tolerance. Let-7 is the first identified human miR. Its family members are highly conserved across species in sequence and function. In the review, we will present a brief review of the opioid receptors, their regulation, and opioid tolerance as well as an overview of miRs and a perspective how miRs may interact with MOR and serve as a regulator of opioid tolerance.

Opiate antagonist prevents μ- and δ-opiate receptor dimerization to facilitate ability of agonist to control ethanol-altered natural killer cell functions and mammary tumor growth

In the natural killer (NK) cells, δ-opiate receptor (DOR) and μ-opioid receptor (MOR) interact in a feedback manner to regulate cytolytic function with an unknown mechanism. Using RNK16 cells, a rat NK cell line, we show that MOR and DOR monomer and dimer proteins existed in these cells and that chronic treatment with a receptor antagonist reduced protein levels of the targeted receptor but increased levels of opposing receptor monomer and homodimer. The opposing receptor-enhancing effects of MOR and DOR antagonists were abolished following receptor gene knockdown by siRNA. Ethanol treatment increased MOR and DOR heterodimers while it decreased the cellular levels of MOR and DOR monomers and homodimers. The opioid receptor homodimerization was associated with an increased receptor binding, and heterodimerization was associated with a decreased receptor binding and the production of cytotoxic factors. Similarly, in vivo, opioid receptor dimerization, ligand binding of receptors, and cell function in immune cells were promoted by chronic treatment with an opiate antagonist but suppressed by chronic ethanol feeding. Additionally, a combined treatment of an MOR antagonist and a DOR agonist was able to reverse the immune suppressive effect of ethanol and reduce the growth and progression of mammary tumors in rats. These data identify a role of receptor dimerization in the mechanism of DOR and MOR feedback interaction in NK cells, and they further elucidate the potential for the use of a combined opioid antagonist and agonist therapy for the treatment of immune incompetence and cancer and alcohol-related diseases.

Botulinum neurotoxin A enhances the analgesic effects on inflammatory pain and antagonizes tolerance induced by morphine in mice

Over the recent years compelling evidence has accumulated indicating that botulinum neurotoxin serotype A (BoNT/A) results in analgesic effects on neuropathic as well as inflammatory pain, both in humans and in animal models. In the present study, the pharmacological interaction of BoNT/A with morphine in fighting inflammatory pain was investigated in mice using the formalin test. Moreover, the effects of BoNT/A on the tolerance-induced by chronic administration of morphine were tested and the behavioral effects were correlated with immunofluorescence staining of glial fibrillary acidic protein, the specific marker of astrocytes, at the spinal cord level. An ineffective dose of BoNT/A (2 pg/paw) combined with an ineffective dose of morphine (1 mg/kg) exerted a significant analgesic action both during the early and the late phases of formalin test. A single intraplantar injection of BoNT/A (15 pg/paw; i.pl.), administered the day before the beginning of chronic morphine treatment (7 days of s.c. injections of 20 mg/kg), was able to counteract the occurrence of tolerance to morphine. Moreover, BoNT/A reduces the enhancement of the expression of astrocytes induced by inflammatory formalin pain. Side effects of opiates, including the development of tolerance during repeated use, may limit their therapeutic use, the possibility of using BoNT/A for lowering the effective dose of morphine and preventing the development of opioid tolerance would have relevant implications in terms of potential therapeutic perspectives.

The role of cyclo-oxygenase inhibitors in attenuating opioid-induced tolerance, hyperalgesia, and dependence

There is no denying that opioids are the most important analgesic drugs which are widely used in clinical situations. Still, prolonged administration of these drugs can cause to reduce their analgesic efficacy due to the development of tolerance. These drugs can also cause induction of hyperalgesia. In addition, long-term administration of opioids through reinforcing- and rewarding pathways of limbic system can result in expression of opioid dependence with the unintended consequences of opioid abuse/misuse and finally opioid addiction. As studies show, over-activity in cyclo-oxygenase pathways and production of prostaglandins due to long-term exposures of opioid have a critical role in the development of tolerance to antinociceptive effect of opioid, hyperalgesia, and opioid dependence. The present study aims at suggesting the hypothesis that through blending a non-steroid anti-inflammatory drug with opioid actively causes reduction in unwanted effects of opioid i.e. by inhibition of opioid-induced cyclo-oxygenase overactivity whereas it is well-known that the combination therapy via reducing opioid dosage reduces the unwanted effects.

Haloperidol disrupts opioid-antinociceptive tolerance and physical dependence

Previous studies from our laboratory and others have implicated a critical role of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in opioid tolerance and dependence. Translational research targeting the CaMKII pathway is challenging, if not impossible, because of a lack of selective inhibitors. We discovered in a preliminary study that haloperidol, a butyrophenone antipsychotic drug, inhibited CaMKII, which led us to hypothesize that haloperidol can attenuate opioid tolerance and dependence by inhibiting CaMKII. The hypothesis was tested in two rodent models of opioid tolerance and dependence. Pretreatment with haloperidol (0.2-1.0 mg/kg i.p.) prevented the development of morphine tolerance and dependence in a dose-dependent manner. Short-term treatment with haloperidol (0.06-0.60 mg/kg i.p.) dose-dependently reversed the established morphine-antinociceptive tolerance and physical dependence. Correlating with behavioral effects, pretreatment or short-term treatment with haloperidol dose-dependently inhibited morphine-induced up-regulation of supraspinal and spinal CaMKIIα activity. Moreover, haloperidol given orally was also effective in attenuating morphine-induced CaMKIIα activity, antinociceptive tolerance, and physical dependence. Taken together, these data suggest that haloperidol attenuates opioid tolerance and dependence by suppressing CaMKII activity. Because haloperidol is a clinically used drug that can be taken orally, we propose that the drug may be of use in attenuating opioid tolerance and dependence.

Morphine- and CaMKII-dependent enhancement of GIRK channel signaling in hippocampal neurons

G-protein-gated inwardly rectifying potassium (GIRK) channels, which help control neuronal excitability, are important for the response to drugs of abuse. Here, we describe a novel pathway for morphine-dependent enhancement of GIRK channel signaling in hippocampal neurons. Morphine treatment for ∼20 h increased the colocalization of GIRK2 with PSD95, a dendritic spine marker. Western blot analysis and quantitative immunoelectron microscopy revealed an increase in GIRK2 protein and targeting to dendritic spines. In vivo administration of morphine also produced an upregulation of GIRK2 protein in the hippocampus. The mechanism engaged by morphine required elevated intracellular Ca(2+) and was insensitive to pertussis toxin, implicating opioid receptors that may couple to Gq G-proteins. Met-enkephalin, but not the μ-selective (DAMGO) and δ-selective (DPDPE) opioid receptor agonists, mimicked the effect of morphine, suggesting involvement of a heterodimeric opioid receptor complex. Peptide (KN-93) inhibition of CaMKII prevented the morphine-dependent change in GIRK localization, whereas expression of a constitutively activated form of CaMKII mimicked the effects of morphine. Coincident with an increase in GIRK2 surface expression, functional analyses revealed that morphine treatment increased the size of serotonin-activated GIRK currents and Ba(2+)-sensitive basal K(+) currents in neurons. These results demonstrate plasticity in neuronal GIRK signaling that may contribute to the abusive effects of morphine.

Regulation of opioid tolerance by let-7 family microRNA targeting the mu opioid receptor

MicroRNA has emerged as a critical regulator of neuronal functions. This study aimed to test whether let-7 microRNAs can regulate the mu opioid receptor (MOR) and opioid tolerance. Employing bioinformatics, we identified a let-7 binding site in the 3'-untranslated region (UTR) of MOR mRNA, which was experimentally confirmed as a direct target of let-7. The repressive regulation of MOR by let-7 was revealed using a LNA-let-7 inhibitor to knockdown let-7 in SH-SY5Y cells. Conversely, morphine significantly upregulated let-7 expression in SH-SY5Y cells and in a mouse model of opioid tolerance. The LNA-let-7 inhibitor decreased brain let-7 levels and partially attenuated opioid antinociceptive tolerance in mice. Although chronic morphine treatment did not change overall MOR transcript, polysome-associated mRNA declined in a let-7-dependent manner. let-7 was identified as a mediator translocating and sequestering MOR mRNA to P-bodies, leading to translation repression. These results suggest that let-7 plays an integral role in opioid tolerance.

Neurobiological mechanisms of pain in sickle cell disease

Pain is a frequent complaint of people living with sickle cell disease (SCD); however, the neurobiology of pain in SCD remains poorly understood. Whereas this pain has been thought to be primarily related to visceral and somatic tissue injury subsequent to vaso-occlusion events, emerging evidence from human and animal studies has suggested that a component of SCD pain may be related to neuropathic processes. Significant knowledge has been obtained from studies of molecular and neurobiological mechanisms leading to and maintaining neuropathic pain. Some of the most promising evidence has implicated major roles of protein kinase C and Ca2+/calmodulin-dependent protein kinase II, and their interaction with the N-methyl-D-aspartate receptors and the transient receptor potential vanilloid 1 receptor in the development of neuropathic pain. The latest evidence from our studies suggests that these pathways are important for SCD pain as well. Coupled with emerging animal models of SCD pain, we can now start to elucidate neurobiological mechanisms underlying pain in SCD, which may lead to better understanding and effective therapies.

Identification of an additional supraspinal component to the analgesic mechanism of action of buprenorphine

BACKGROUND AND PURPOSE

Buprenorphine displays attributes of opioids, but also some features distinct from them. We examined spinal and supraspinal signal transduction of buprenorphine-induced anti-nociception in mice compared with morphine and fentanyl.

EXPERIMENTAL APPROACH

The opioid receptor antagonist naloxone, Pertussis toxin (PTX), G(z) protein antisense and nociceptin/orphanin-FQ receptor agonist nociceptin, and antagonist, JTC-801, were injected supraspinally (intracerebroventricular) and spinally (intrathecal). Also the cell-permeable Ser/Thr protein phosphatase inhibitor okadaic acid was given supraspinally.

KEY RESULTS

Spinal naloxone (20 microg) or PTX (1 microg) attenuated morphine, fentanyl and buprenorphine (s.c.) anti-nociception. Supraspinal naloxone or PTX attenuated morphine and fentanyl, but not buprenorphine anti-nociception. Spinal G(z) protein antisense did not alter buprenorphine, morphine or fentanyl anti-nociception and supraspinal G(z)-antisense did not alter morphine or fentanyl anti-nociception. However, supraspinal G(z)-antisense (not random sense) reduced buprenorphine anti-nociception. Peripheral JTC-801 (1 mgxkg(-1), i.p.) enhanced the ascending (3 mgxkg(-1)) and descending (30 mgxkg(-1)) portions of buprenorphine's dose-response curve, but only spinal, not supraspinal, nociceptin (10 nmolxL(-1)) enhanced buprenorphine anti-nociception. Intracereboventricular okadaic acid (0.001-10 pg) produced a biphasic low-dose attenuation, high-dose enhancement of buprenorphine(3 or 30 mgxkg(-1), s.c.) anti-nociception, but did not affect morphine or fentanyl anti-nociception.

CONCLUSIONS AND IMPLICATIONS

Buprenorphine has an opioid component to its supraspinal mechanism of analgesic action. Our present results reveal an additional supraspinal component insensitive to naloxone, PTX and nociceptin/orphanin-FQ, but involving G(z) protein and Ser/Thr protein phosphatase. These data might help explain the unique preclinical and clinical profiles of buprenorphine.

Receptor trafficking induced by mu-opioid-receptor phosphorylation

Opiates, including morphine, are widely used drugs for antinociception in clinics. Prolonged treatments of opioids induce both tolerance and dependence, which are the major side effects of opioid therapy. One of the mechanisms for the development of tolerance and dependence is implicated to be opioid-receptor trafficking. Here we review the current understandings of opioid-receptor phosphorylation, endocytosis and desensitization after repeated agonist treatments. Also, the role of G-protein coupled receptor kinases in opioid-receptor phosphorylation is discussed. How to associate these observations to physiological and behavioral changes in animal models and clinics is still under investigation.

RGS13 controls g protein-coupled receptor-evoked responses of human mast cells

IgE-mediated mast cell degranulation and release of vasoactive mediators induced by allergens elicits allergic responses. Although G protein-coupled receptor (GPCR)-induced signals may amplify IgE-dependent degranulation, how GPCR signaling in mast cells is regulated remains incompletely defined. We investigated the role of regulator of G protein signaling (RGS) proteins in the modulation of these pathways in human mast cells. Several RGS proteins were expressed in mast cells including RGS13, which we previously showed inhibited IgE-mediated mast cell degranulation and anaphylaxis in mice. To characterize how RGS13 affects GPCR-mediated functions of human mast cells, we analyzed human mast cell lines (HMC-1 and LAD2) depleted of RGS13 by specific small interfering RNA or short hairpin RNA and HMC-1 cells overexpressing RGS13. Transient RGS13 knockdown in LAD2 cells lead to increased degranulation to sphingosine-1-phosphate but not to IgE-Ag or C3a. Relative to control cells, HMC-1 cells stably expressing RGS13-targeted short hairpin RNA had greater Ca(2+) mobilization in response to several natural GPCR ligands such as adenosine, C5a, sphingosine-1-phosphate, and CXCL12 than wild-type cells. Akt phosphorylation, chemotaxis, and cytokine (IL-8) secretion induced by CXCL12 were also greater in short hairpin RGS13-HMC-1 cells compared with control. RGS13 overexpression inhibited CXCL12-evoked Ca(2+) mobilization, Akt phosphorylation and chemotaxis. These results suggest that RGS13 restricts certain GPCR-mediated biological responses of human mast cells.

Chronic ethanol exposure increases the association of hippocampal mu-opioid receptors with G-protein receptor kinase 2

Opioid receptors (ORs) have been shown to have a significant role in the central nervous system (CNS) effects of chronic ethanol consumption. The OR antagonist, naltrexone, is used clinically to reduce continued intake. We previously observed that chronic ethanol consumption, by adult male Sprague-Dawley rats, induced a reduction in functional coupling of mu- and delta-ORs to G-proteins in rat CNS regions, including the hippocampus. G-protein receptor kinase (GRK) 2 phosphorylates G-protein coupled receptors, including ORs, after agonist binding, as part of normal regulation and desensitization. We tested the hypothesis that chronic ethanol exposure affects the association of the GRK2 with the mu-OR. Co-immunoprecipitation methods were used to determine if mu-OR association with GRK2 is elevated in the hippocampus after chronic ethanol, when compared to controls. Hippocampal homogenates from chronic ethanol and pair-fed control rats were treated with affinity-purified rabbit polyclonal antibodies (ab) to mu-OR, and immune complexes were probed for GRK2 by immunoblotting techniques. Results demonstrate an association of GRK2 with mu-ORs in chronic ethanol-treated rats, but not in the controls. Possible changes in GRK2 association with ORs after chronic ethanol may be related to levels of phosphorylation and subsequent trafficking of the receptors.

Purification and mass spectrometric analysis of the kappa opioid receptor

A clonal human embryonic kidney (HEK) 293 cell line was established that stably expressed the rat kappa-opioid receptor (rKOR) with a FLAG epitope at the amino terminus. The Kd for [3H]diprenorphine was 1.1+/-0.2 nM, and the Bmax was 2.6+/-0.4 pmol/mg. Dynorphin A (1-13), U69,593 and naloxone competitively inhibited [3H]diprenorphine binding with Ki values of 2.0, 18 and 18 nM, respectively, in good agreement with previously reported affinities for the unmodified receptor. U69,593 stimulated [35S]GTPgammaS binding in a concentration-dependent manner and caused phosphorylation of mitogen-activated protein (MAP) kinase, indicating that the activated epitope-tagged receptor triggered appropriate signaling pathways. Immunoblot analysis demonstrated that two immunoreactive receptor species with apparent molecular masses of 42 and 52 kDa were expressed. Previous studies indicated that the 42 kDa protein was localized intracellularly and was a precursor of the 52 kDa receptor, which was present at the cell surface. rKOR was extracted from transfected HEK 293 cell membranes with n-dodecyl-beta-D-maltopyranoside. Sequential use of wheat germ agglutinin chromatography, Sephacryl S300 gel filtration chromatography, anti-FLAG immunoaffinity chromatography and SDS/PAGE permitted purification of the 52 kDa receptor. MALDI-TOF mass spectrometry was used to identify peptides derived from rKOR following sequential in-gel digestion with trypsin and cyanogen bromide. Eighteen rKOR peptides were detected, corresponding to 27.1% coverage of the receptor. Precursor-selective MS/MS confirmed the identity of most of these peptides. In addition, we have identified heat shock protein 70 (HSP70) as a rKOR-interacting protein.

Disruption of acute opioid dependence by antisense oligodeoxynucleotides targeting neurogranin

Neurogranin has been suggested to serve as a common regulator synchronizing the activities of PKC and CaMKII in acute opioid tolerance. To examine if a similar mechanism exists in acute opioid dependence, we directly targeted neurogranin using antisense oligodeoxynucleotides. Male ICR mice were pretreated with neurogranin antisense or mismatch oligodeoxynucleotides (2 microg/day, i.c.v.) for 3 consecutive days. On Day 4, morphine (100 mg/kg, s.c.) was used to induce dependence, as revealed by naloxone-precipitated withdrawal in saline or mismatch-pretreated mice. Antisense-pretreated mice showed decreased neurogranin expression, lack of morphine-induced phosphorylation of neurogranin and activation of CaMKII and CREB, and absence of naloxone-induced withdrawal jumping. Taken together, these data suggest that neurogranin plays an essential role in acute opioid dependence, possibly by affecting the CaMKII and CREB signaling pathway.