Pioglitazone prevents morphine antinociception tolerance and withdrawal symptoms in rats

Molecular mechanisms of morphine tolerance and dependence; novel insights and future perspectives

Drug addiction is a devastating condition that poses a serious burden on the society. The use of some drugs like morphine for their tremendous analgesic properties is also accompanied with developing tolerance, dependence and the withdrawal symptoms. These symptoms are frequently severe enough to reinforce the person in recovery to start over the use of drug again and hinder the clinical use of drugs like morphine for chronic pain. Research into opioid receptors and related molecular pathways has seen resurgence in the wake of the growing opioid epidemic. The current study provides a comprehensive scientific exploration of the molecular mechanisms and underlying signalling in morphine tolerance and dependence. It also critically evaluates current therapeutic approaches, shedding light on their efficacy and limitations, and future prospects.

Neuroimmune modulators as novel pharmacotherapies for substance use disorders

One promising avenue of research is the use of neuroimmune modulators to treat substance use disorders (SUDs). Neuroimmune modulators target the interactions between the nervous system and immune system, which have been found to play a crucial role in the development and maintenance of SUDs. Multiple classes of substances produce alterations to neuroimmune signaling and peripheral immune function, including alcohol, opioids, and psychostimulants Preclinical studies have shown that neuroimmune modulators can reduce drug-seeking behavior and prevent relapse in animal models of SUDs. Additionally, early-phase clinical trials have demonstrated the safety and feasibility of using neuroimmune modulators as a treatment for SUDs in humans. These therapeutics can be used as stand-alone treatments or as adjunctive. This review summarizes the current state of the field and provides future directions with a specific focus on personalized medicine.

Peroxisome proliferator activated receptor-gamma (PPAR-γ) ligand, pioglitazone, increases analgesic and anti-inflammatory effects of naproxen

The aim of this study was the investigation of analgesic and anti-inflammatory activity of naproxen and pioglitazone following intra-plantar injection of carrageenan and assessment of the PPAR-γ receptor involvement in these effects. Rats were intra-plantarly injected with carrageenan (1%, 100 μl) to induce thermal hyperalgesia and paw inflammation. Different groups of rats were pre-treated intraperitoneally with naproxen (1 and 10 mg/kg) or pioglitazone (3 and 10 mg/kg) or GW9662 (a selective PPAR-γ antagonist, 100 μl/paw). The volume of the paw was evaluated using a plethysmometer, and the hot plate test was employed to assess the pain threshold in the animals. Finally, TNF-α, IL-1ß, IL-6, and myeloperoxidase (MPO) activity status were evaluated in the hind paw tissue. Naproxen and pioglitazone demonstrated analgesic and anti-inflammatory activity. Concurrent injection of an ineffective dose of naproxen (1 mg/kg) with an ineffective dose of pioglitazone (3 mg/kg) caused augmented analgesic and anti-inflammatory activity, significantly (p≤0.001 and p≤0.01, respectively). Additionally, intra-plantar injection of GW-9662 before naproxen or pioglitazone significantly suppressed their analgesic (p≤0.001) and anti-inflammatory activity (p≤0.01). Also, naproxen and pioglitazone (10 mg/kg) significantly (p≤0.001) reduced carrageenan-induced MPO activity and TNF-α, IL-6, and IL-1ß releasing. Furthermore, PPAR-γ blockade significantly prevented suppressive effects of naproxen and pioglitazone on the MPO activity and inflammatory cytokines. Pioglitazone significantly increased analgesic and anti-inflammatory effects of naproxen. This study proposes that concurrent treatment with naproxen and pioglitazone may be a substitute for overcome pain and inflammation clinically, in the future, particularly in patients with cardiovascular disorders and diabetes.

Angiotensin II receptor type 1 blocker candesartan improves morphine tolerance by reducing morphine‑induced inflammatory response and cellular activation of BV2 cells via the PPARγ/AMPK signaling pathway

Morphine is the most common drug of choice in clinical pain management; however, morphine tolerance presents a significant clinical challenge. The pathogenesis of morphine tolerance is known to be closely associated with angiotensin II receptor type 1 (AT1R) in microglia. As an AT1R antagonist, candesartan may serve an important role in regulating morphine tolerance. Therefore, the present study aimed to investigate the role of candesartan in morphine tolerance, and to explore the underlying mechanism. To meet this aim, BV2 microglial cells were treated with morphine or candesartan alone, or as a combination, and the expression levels of AT1R in BV2 cells were detected by reverse transcription‑quantitative PCR (RT‑qPCR) and western blotting. The levels of the inflammatory cytokines tumor necrosis factor‑α, interleukin (IL)‑1β and IL‑6 were subsequently detected by ELISA and western blotting. In addition, immunofluorescence analysis, western blotting and RT‑qPCR were used to detect the expression levels of the BV2 cell activation marker, ionized calcium‑binding adaptor molecule 1 (IBA‑1). Western blotting was also used to detect the expression levels of peroxisome proliferator‑activated receptor‑γ/AMP‑activated protein kinase (PPARγ/AMPK) signaling pathway‑associated proteins. Finally, the cells were treated with the PPARγ antagonist GW9662 and the AMPK inhibitor compound C to further explore the mechanism underlying the effects of candesartan on improving morphine tolerance. The expression levels of AT1R were revealed to be significantly increased following morphine induction; however, candesartan treatment inhibited the expression levels of AT1R, the levels of inflammatory cytokines and the protein expression levels of IBA‑1 in morphine‑induced BV2 cells in a dose‑dependent manner. These processes may be associated with activation of the PPARγ/AMPK signaling pathway. Taken together, the present study revealed that treatment with candesartan reduced morphine‑induced inflammatory response and cellular activation of BV2 cells via PPARγ/AMPK signaling.

The PPARγ agonist pioglitazone produces a female-predominant inhibition of hyperalgesia associated with surgical incision, peripheral nerve injury, and painful diabetic neuropathy

Pioglitazone, an agonist at peroxisome proliferator-activated receptor gamma, is FDA-approved for the treatment of insulin resistance in type 2 diabetes. Numerous studies in male rodents suggest that pioglitazone inhibits inflammatory and neuropathic pain, but few included female subjects. To address this gap, we compared the effects of pioglitazone in both sexes in the intraplantar methylglyoxal model (MG) model of chemical pain and painful diabetic neuropathy (PDN), the plantar incision model (PIM) of postoperative pain, the spared nerve injury (SNI) model of traumatic nerve injury, and the ZDF rat and db/db mouse models of PDN. We administered pioglitazone by one-time intrathecal or intraperitoneal injection or by adding it to chow for 6 weeks, followed by measurement of hypersensitivity to non-noxious mechanical, noxious mechanical, heat, and/or cold stimuli. In all mouse models, injection of pioglitazone decreased pain-like behaviors with greater potency and/or efficacy in females as compared to males: heat and mechanical hypersensitivity in the MG model (0.1-10 mg/kg); mechanical hypersensitivity in the PIM model (10 μg); mechanical and cold hypersensitivity in the SNI model (100 mg/kg); and heat hypersensitivity in the db/db model (100 mg/kg). Furthermore, co-administration of low doses of morphine (1 mg/kg) and pioglitazone (10 mg/kg) decreased SNI-induced mechanical and cold hypersensitivity in female but not male mice. In the ZDF rat, pioglitazone (100 mg/kg) decreased heat and mechanical hypersensitivity with no sex difference. In the db/db model, pioglitazone had no effect when given into chow for 6 weeks at 0.3, 3 or 30 mg/kg doses. We conclude that females exhibit greater anti-hyperalgesic responses to pioglitazone in mouse models of chemical-induced nociception, postsurgical pain, neuropathic pain, and PDN. These findings set the stage for clinical trials to determine whether pioglitazone has analgesic properties across a broad spectrum of chronic pain conditions, particularly in women.

Potential of Glial Cell Modulators in the Management of Substance Use Disorders

The pervasive and devastating nature of substance use disorders underlies the need for the continued development of novel pharmacotherapies. We now know that glia play a much greater role in neuronal processes than once believed.  The various types of glial cells (e.g., astrocytes, microglial, oligodendrocytes) participate in numerous functions that are crucial to healthy central nervous system function. Drugs of abuse have been shown to interact with glia in ways that directly contribute to the pharmacodynamic effects responsible for their abuse potential. Through their effect upon glia, drugs of abuse also alter brain function resulting in behavioral changes associated with substance use disorders. Therefore, drug-induced changes in glia and inflammation within the central nervous system (neuroinflammation) have been investigated to treat various aspects of drug abuse and dependence. This article presents a brief overview of the effects of each of the major classes of addictive drugs on glia. Next, the paper reviews the pre-clinical and clinical studies assessing the effects that glial modulators have on abuse-related behavioral effects, such as pleasure, withdrawal, and motivation. There is a strong body of pre-clinical literature demonstrating the general effectiveness of several glia-modulating drugs in models of reward and relapse. Clinical studies have also yielded promising results, though not as robust. There is still much to disentangle regarding the integration between addictive drugs and glial cells. Improved understanding of the relationship between glia and the pathophysiology of drug abuse should allow for more precise exploration in the development and testing of glial-directed treatments for substance use disorders.

Regulatory mechanisms and therapeutic potential of microglial inhibitors in neuropathic pain and morphine tolerance

Microglia are important cells involved in the regulation of neuropathic pain (NPP) and morphine tolerance. Information on their plasticity and polarity has been elucidated after determining their physiological structure, but there is still much to learn about the role of this type of cell in NPP and morphine tolerance. Microglia mediate multiple functions in health and disease by controlling damage in the central nervous system (CNS) and endogenous immune responses to disease. Microglial activation can result in altered opioid system activity, and NPP is characterized by resistance to morphine. Here we investigate the regulatory mechanisms of microglia and review the potential of microglial inhibitors for modulating NPP and morphine tolerance. Targeted inhibition of glial activation is a clinically promising approach to the treatment of NPP and the prevention of morphine tolerance. Finally, we suggest directions for future research on microglial inhibitors.

Clinical Trials for Opioid Use Disorder

This chapter describes recent clinical trials for opioid use disorder (OUD), an area that has rapidly accelerated in response to the opioid overdose crisis in the USA and newly appropriated funding. Trials involve a wide range of compounds including cannabinoids and psychedelics, new and existing compounds targeting domains emerging from addiction neuroscience, agents repurposed from other indications, and novel strategies including vaccines, enzymes, and other biologicals. In parallel, new formulations of existing compounds offer immediate promise, as do a variety of web-based interventions and smartphone-delivered apps. Trials focused on implementing existing effective interventions in mainstream healthcare settings, and others focused on special populations, e.g., adolescents, criminal justice, pregnant women, native Americans, etc., have the potential to vastly expand treatment in the near term. Given the range of ongoing and recent trials, this chapter is not intended to be an exhaustive review but rather to present an overview of approaches within the framework of the opioid treatment cascade and the context of current OUD pharmacotherapies.

Evaluating the Effects of Chronic Administration of Natural Honey on the Development of Dependence on Morphine in the Male Rats

Background: According to the previous studies, the exact mechanism of dependence on opioids and withdrawal syndrome has not been fully understood but one of the most important mechanisms is the increase of pro-inflammatory cytokines in CNS. On the other way, previous studies showed that natural honey (NHO) has anti-inflammatory properties. This study was aimed to evaluate the effects of chronic administration of natural honey on the development of morphine dependence in male rats. Methods: Honey was prepared from Tarom Oliya region in Zanjan province. Experiments were performed on male Wistar rats weighing 225-275 g, randomly divided into 6 groups (n=8). The study groups included morphine group, the three doses of morphine plus honey group (at doses of 200,400 and 800 mg/kg, i.p.), the morphine plus vehicle group, and the saline group. The subcutaneous injections of additive doses of morphine were used for 9 days to create morphine dependency. On the 9th day, one hour after the morning dose of morphine, naloxone (4 mg/kg, i.p.) was injected, and symptoms of withdrawal syndrome were assessed for 60 minutes. Then, blood samples were taken to measure TNF-α. One-way ANOVA and Tukey tests were used to compare the results. P- Value of <0.05 was considered as statistically significant. Results: The results of this study showed that intraperitoneal injection of honey at 3 doses (200, 400 and 800 mg/kg with p <0.001) could significantly decrease the total score of the symptoms compared to the morphine-vehicle control group. Natural honey (NHO) could significantly decrease TNF-α at dose of 400 mg/kg. Conclusion: The results indicated that chronic administration of NHO had beneficial effects in reducing symptoms of morphine withdrawal syndrome, and this effect is probably due to the anti-inflammatory effect caused by the polyphenolic compounds in honey.<br />

Evaluation of the Effects of Chronic Administration of Citrus aurantium Essential Oil on the Development of Tolerance and Dependence to Morphine

Background: Long-term exposure to opioids may lead to physical dependence and tolerance. The purpose of this study was to investigate the effects of Citrus aurantium essential oil (CEO) on the morphine-induced tolerance and dependence. Methods: To evaluate morphine tolerance, the experiments were carried out in 6 rat groups (n=8) in the weight range of 225-275 g. The control group received morphine (10 mg/kg/day) and the test groups received morphine with the different doses of essential oil (CEO 20, 40 and 80 mg/kg/day) or 4 mL/kg of essential oil vehicle (KolliphorÒ HS15 30% in normal saline that adjusted in pH=7.4 with phosphate buffer) intraperitoneally. The hot-plate test was carried out every other day, 90 minutes after the injections. To examine morphine withdrawal, male Wistar rats were divided into seven groups (n=8) randomly, including: morphine sulphate, CEO (20, 40 and 80 mg/kg) + morphine, vehicle of CEO + morphine. The rats were rendered morphine-dependent by injection of additive doses of morphine subcutaneously for 9 days. The procedure of the morphine administration was as following protocol: day1: 5 mg/kg/12h, day 2 and 3: 10 mg/kg/12h, day 4 and 5: 15 mg/kg/12h, day 6 and 7: 20 mg/kg/12h and day 8 and 9: 25 mg/kg/12h. In the 9th day, 2 hours after the last dose of morphine, naloxone (4 mg/kg) was injected intraperitoneally. Some withdrawal behaviors were counted for 60 minutes. Results: Morphine tolerance was completed after 5 days in the control group. The vehicle group showed tolerance on the 9th day (p-value=0.991), 20mg group in the 13th day (p-value to control=0.010, to vehicle=0.049), 40 mg group on the 15th day (p-value to control and vehicle<0.001) and 80 mg group on the 13th day (p-value to control= 0.001, to vehicle= 0.007). The results showed that CEO could reduce the morphine withdrawal syndrome and total withdrawal score (TWS). Intraperitoneally injection of CEO in two doses (40 mg/kg with p<0.001 and 80 mg/kg with p<0.01) significantly reduced the TWS in comparison to the morphine+vehicle treated group. Conclusion: The results indicated that chronic administration of C. aurantium essential oil extracted had beneficial effects in reducing morphine withdrawal syndrome and could significantly delay tolerance to morphine.

Fluoxetine increases analgesic effects of morphine, prevents development of morphine tolerance and dependence through the modulation of L-type calcium channels expression in mice

Here, we aimed to investigate the effects of fluoxetine on morphine-induced analgesia, as well as preventive effects of it on morphine induced tolerance and dependence in mice. We also elucidate the involvement of L-type Ca²⁺ channels in these phenomena. To induce morphine tolerance, mice were treated with morphine (50 mg/kg) for 3 consecutive days. To evaluate the involvement of the calcium channel in the effects of fluoxetine (5, 20 mg/kg), combination ineffective doses of the two L-type calcium channel blockers, nimodipine (5 mg/kg) or diltiazem (20 mg/kg) with flouxetine were used with each morphine dose. Nociceptive behavior was evaluated using hot-plate test, while physical dependence assessed by naloxone-precipitated withdrawal on the fourth day of experiment. The expression of Cav1.2 and Cav1.3 subunits of the L-type calcium channels in cortex and mesolimbic tissues were measured using western immunoassay. Results showed that co-administration of fluoxetine (20 mg/kg) with morphine increased its acute analgesia effect and prevented the induction of morphine antinociceptive tolerance and physical dependence in mice. Moreover, these effects was potentiated by pre-treatment with diltiazem or nimodipine. Results also showed up-regulation of the Cav1.3 and Cav1.2 expression in the cerebral cortex and mesolimbic regions through the development of morphine dependence. Moreover, chronic administration of fluoxetine with morphine reduced the observed up-regulation of Cav1.3 and Cav1.2 expression in cortex and mesolimbic tissues. Our data indicated that co-administering of fluoxetine with morphine could potentiate the antinociceptive effect of morphine, prevent morphine analgesia tolerance and attenuated the morphine withdrawal signs during induction phases. Moreover, we also pointed out for the first time the role of L-type Ca²⁺ channel channels in the modulatory effects of fluoxetine on the morphine-related effects.

Assessment of pioglitazone and proinflammatory cytokines during buprenorphine taper in patients with opioid use disorder

BACKGROUND

Preliminary evidence suggested that the PPARγ agonist pioglitazone reduces opioid-withdrawal symptoms, possibly by inhibiting increases in proinflammatory cytokines.

METHODS

A randomized, placebo-controlled clinical trial was conducted utilizing two different study designs (entirely outpatient, and a combination of inpatient and outpatient) to evaluate the safety and efficacy of pioglitazone as an adjunct medication for people with opioid physical dependence undergoing a buprenorphine taper. Participants were stabilized on buprenorphine/naloxone (sublingual, up to 16/4 mg/day), then randomized to receive oral pioglitazone (up to 45 mg/day) or placebo before, during, and after buprenorphine taper. Outcome measures included the Subjective Opiate Withdrawal Scale (SOWS) and Clinical Opiate Withdrawal Scale, use of rescue medications to alleviate opioid withdrawal symptoms, and opioid-positive urine specimens. Cerebrospinal fluid (CSF) and plasma were collected during the taper in a subset of participants for measurement of proinflammatory cytokines.

RESULTS

The clinical trial was prematurely terminated due to slow enrollment; 40 participants per group were required for adequate statistical power to test study hypotheses. Twenty-four participants enrolled; 17 received at least one dose of study medication (6 pioglitazone, 11 placebo). SOWS scores were higher in the pioglitazone arm than in the placebo arm after adjusting for use of rescue medications; participants in the pioglitazone arm needed more rescue medications than the placebo arm during the post-taper phase. SOWS scores were positively correlated with monocyte chemoattractant protein-1 (MCP-1) in CSF (r = 0.70, p = 0.038) and plasma (r = 0.77, p = 0.015). Participants having higher levels of plasma MCP-1 reported higher SOWS, most notably after the buprenorphine taper ended.

CONCLUSIONS

Results from this study provide no evidence that pioglitazone reduces opioid withdrawal symptoms during buprenorphine taper. High correlations between MCP-1 and opioid withdrawal symptoms support a role of proinflammatory processes in opioid withdrawal.

TRIAL REGISTRATION

clinicaltrials.gov identifier: NCT01517165.

PPARs and pain

Chronic pain is a common cause of disability worldwide and remains a global health and socio-economic challenge. Current analgesics are either ineffective in a significant proportion of patients with chronic pain or associated with significant adverse side effects. The PPARs, a family of nuclear hormone transcription factors, have emerged as important modulators of pain in preclinical studies and therefore a potential therapeutic target for the treatment of pain. Modulation of nociceptive processing by PPARs is likely to involve both transcription-dependent and transcription-independent mechanisms. This review presents a comprehensive overview of preclinical studies investigating the contribution of PPAR signalling to nociceptive processing in animal models of inflammatory and neuropathic pain. We examine current evidence from anatomical, molecular and pharmacological studies demonstrating a role for PPARs in pain control. We also discuss the limited evidence available from relevant clinical studies and identify areas that warrant further research. LINKED ARTICLES: This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Morphine-Mediated Brain Region-Specific Astrocytosis Involves the ER Stress-Autophagy Axis

A recent study from our lab has revealed a link between morphine-mediated autophagy and synaptic impairment. The current study was aimed at investigating whether morphine-mediated activation of astrocytes involved the ER stress/autophagy axis. Our in vitro findings demonstrated upregulation of GFAP indicating astrocyte activation with a concomitant increase in the production of proinflammatory cytokines in morphine-exposed human astrocytes. Using both pharmacological and gene-silencing approaches, it was demonstrated that morphine-mediated defective autophagy involved upstream activation of ER stress with subsequent downstream astrocyte activation via the μ-opioid receptor (MOR). In vivo validation demonstrated preferential activation of ER stress/autophagy axis in the areas of the brain not associated with pain such as the basal ganglia, frontal cortex, occipital cortex, and the cerebellum of morphine-dependent rhesus macaques, and this correlated with increased astrocyte activation and neuroinflammation. Interventions aimed at blocking either the MOR or ER stress could thus likely be developed as promising therapeutic targets for abrogating morphine-mediated astrocytosis.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Protective effect of pioglitazone on morphine-induced neuroinflammation in the rat lumbar spinal cord

Background

Morphine-induced tolerance is associated with the spinal neuroinflammation. The aim of this study was to explore the effects of oral administration of the pioglitazone, the peroxisome proliferator activated receptor gamma (PPAR-γ) agonist, on the morphine-induced neuroinflammation in the lumbar region of the male Wistar rat spinal cord.

Results

Co-administration of the pioglitazone with morphine not only attenuated morphine-induced tolerance, but also prevented the up-regulation of pro-inflammatory cytokines (tumor necrosis factor alpha, interleukin-1beta, and interleukin 6) and nuclear factor-kappa B activity. Administration of the GW-9662 antagonized the above mentioned effects of the pioglitazone.

Conclusions

It is concluded that oral administration of the pioglitazone attenuates morphine-induced tolerance and the neuroinflammation in the lumbar region of the rat spinal cord. This action of the pioglitazone may be, at least in part, due to an interaction with the spinal pro-inflammatory cytokine expression and the nuclear factor-kappa B activity.

Involvement of neuronal TGF-β activated kinase 1 in the development of tolerance to morphine-induced antinociception in rat spinal cord

BACKGROUND AND PURPOSE

Tolerance induced by morphine and other opiates remains a major unresolved problem in the clinical management of pain. There is now good evidence for the importance of MAPKs in morphine-induced antinociceptive tolerance. A member of the MAPK kinase kinase family, TGF-β activated kinase 1 (TAK1) is the common upstream kinase of MAPKs. Here, we have assessed the involvement of TAK1 in the development of tolerance to morphine-induced analgesia.

EXPERIMENTAL APPROACH

The effects of an antagonist of TAK1 on morphine tolerance were investigated in vivo using the Randall-Selitto test, and the mechanism was investigated using Western blot and immunohistochemistry. The expression of TAK1 after chronic morphine exposure was also evaluated in vitro by immunohistochemistry.

KEY RESULTS

Chronic intrathecal morphine exposure up-regulated protein levels and phosphorylation of spinal TAK1. TAK1 immunoreactivity was co-localized with the neuronal marker NeuN. Intrathecal administration of 5Z-7-oxozeaenol (OZ), a selective TAK1 inhibitor, attenuated the loss of morphine analgesic potency and morphine-induced TAK1 up-regulation. Furthermore, OZ decreased the up-regulated expression of spinal p38 and JNK after repeated morphine exposure. In vitro studies demonstrated that sustained morphine treatment induced TAK1 up-regulation, which was reversed by co-administration of OZ. A bolus injection of OZ showed some reversal of established morphine antinociceptive tolerance.

CONCLUSIONS AND IMPLICATIONS

TAK1 played a pivotal role in the development of morphine-induced antinociceptive tolerance. Modulation of TAK1 activation by the selective inhibitor OZ in the lumbar spinal cord may prove to be an attractive adjuvant therapy to attenuate such tolerance.

Pioglitazone prevents morphine antinociceptive tolerance via ameliorating neuroinflammation in rat cerebral cortex

BACKGROUND

Opioid induced neuroinflammation is shown to be implicated in opioid analgesic tolerance development. In the present study the effect of pioglitazone on morphine-induced tolerance and neuroinflammation in the cerebral cortex of the rat was investigated.

MATERIALS AND METHODS

Various groups of rats received morphine (10mg/kg; ip) and vehicle (po), or morphine (10mg/kg) and pioglitazone (20 or 40 mg/kg; po) once a day for 17 days. In order to determine the possible involvement of PPAR-γ in the pioglitazone effect, one group of rats received PPAR-γ antagonist, GW-9662 (2mg/kg; sc), and pioglitazone (40 mg/kg) and morphine once daily for 17 days. Nociception was assessed using a tail flick apparatus and the percentage of the maximal possible effect was calculated as well. On 18th day, 2h after the last morphine injection, the cerebral cortex of the animals were harvested and the tissue levels of tumour necrosis factor alpha, interleukin-1beta, interleukin-6, interleukin-10 and nuclear factor-kappa B activity were determined.

RESULTS

Co-administration of pioglitazone (40 mg/kg) with morphine not only attenuated morphine-induced tolerance, but also prevented the up-regulation of pro-inflammatory cytokines (tumour necrosis factor alpha, interleukin-1beta, interleukin-6) and nuclear factor-kappa B activity in the rat cerebral cortex. Moreover, GW-9662 (2mg/kg) administration 30 min before pioglitazone, antagonized the above mentioned pioglitazone-induced effects.

CONCLUSION

It is concluded that oral administration of pioglitazone attenuates morphine-induced tolerance. This effect of pioglitazone may be, at least in part, due to its anti-inflammatory property which suppressed the cortical pro-inflammatory cytokine and inhibited of nuclear factor-kappa B activity.