Attenuation of morphine tolerance, withdrawal-induced hyperalgesia, and associated spinal inflammatory immune responses by propentofylline in rats

Emerging evidence of a link between inflammation and the neuropathology of prenatal opioid exposure

Opioid use continues to increase, particularly among women of reproductive age. As a result, increasing numbers of infants are born with prenatal exposure to opioids, suffering both acute and long-term negative consequences. Studies performed across the past 5 years have highlighted both peripheral and central inflammation as a consistent feature of prenatal opioid exposure. Dysregulated innate and adaptive immunity have been detected in human and rodent studies, highlighting a likely role of inflammation in the neuropathology associated with opioid exposure. Identifying immune changes occurring following prenatal opioid exposure will be critical for developing new therapeutic approaches in this field.

Comparison of Morphine and Endomorphin Analog ZH853 for Tolerance and Immunomodulation in a Rat Model of Neuropathic Pain

µ-Opioid receptor agonists, the gold standard for analgesia, come with significant side effects when used chronically. Tolerance, defined as the decrease in analgesic activity after repeated use, remains a vital therapeutic obstacle as it increases the likelihood of dose escalation and potentially lethal side effects like respiratory depression. Previous experiments have shown that the endomorphin-1 analog, ZH853, is a specific µ-opioid receptor agonist with reduced side effects like tolerance and glial activation following chronic central administration in pain-naive animals. Here, we investigated the effects of chronic, peripheral administration of µ-opioid receptor agonists following neuropathic injury. Though µ-opioids are effective at reducing neuropathic pain, they are not recommended for first-line treatment due to negative side effects. Compared with chronic morphine, chronic ZH853 treatment led to decreased tolerance and reduced glial activation. Following twice-daily intravenous injections, morphine was less potent and had a shorter duration of antinociception compared with ZH853. Chronic morphine, but not chronic ZH853, elevated markers of activation/inflammation of astrocytes (glial fibrillary acidic protein), microglia (ionized calcium-binding adapter molecule 1), the proinflammatory cytokine tumor necrosis factor-α, and phosphorylated mitogen-activated protein (MAP) kinase p38 (pp38). By contrast, chronic ZH853 reduced ionized calcium-binding adapter molecule 1 and tumor necrosis factor-α relative to both morphine and vehicle, suggesting anti-inflammatory properties with respect to these markers. Glial fibrillary acidic protein and pp38 were not significantly different from vehicle but were significantly lower than morphine. This study demonstrates the effectiveness of chronic ZH853 for providing analgesia in a neuropathic pain state with reduced tolerance compared with morphine, potentially due to reductions in spinal glial activation. PERSPECTIVE: Neuropathic pain is generally undertreated and resistant to medication, and side-effects limit opioid treatment. Here, we show that, compared with an equiantinociceptive dose of morphine, chronic intravenous administration of endomorphin analog ZH853 led to prolonged antiallodynia, reduced tolerance, and inhibition of spinal cord neuroinflammation in male spared nerve-injured rats.

PPARβ/δ Agonist GW0742 Modulates Microglial and Astroglial Gene Expression in a Rat Model of Temporal Lobe Epilepsy

The role of astroglial and microglial cells in the pathogenesis of epilepsy is currently under active investigation. It has been proposed that the activity of these cells may be regulated by the agonists of peroxisome proliferator-activated nuclear receptors (PPARs). This study investigated the effects of a seven-day treatment with the PPAR β/δ agonist GW0742 (Fitorine, 5 mg/kg/day) on the behavior and gene expression of the astroglial and microglial proteins involved in the regulation of epileptogenesis in the rat brain within a lithium-pilocarpine model of temporal lobe epilepsy (TLE). TLE resulted in decreased social and increased locomotor activity in the rats, increased expression of astro- and microglial activation marker genes (Gfap, Aif1), pro- and anti-inflammatory cytokine genes (Tnfa, Il1b, Il1rn), and altered expression of other microglial (Nlrp3, Arg1) and astroglial (Lcn2, S100a10) genes in the dorsal hippocampus and cerebral cortex. GW0742 attenuated, but did not completely block, some of these impairments. Specifically, the treatment affected Gfap gene expression in the dorsal hippocampus and Aif1 gene expression in the cortex. The GW0742 injections attenuated the TLE-specific enhancement of Nlrp3 and Il1rn gene expression in the cortex. These results suggest that GW0742 may affect the expression of some genes involved in the regulation of epileptogenesis.

Diversity of microglial transcriptional responses during opioid exposure and neuropathic pain

ABSTRACT

Microglia take on an altered morphology during chronic opioid treatment. This morphological change is broadly used to identify the activated microglial state associated with opioid side effects, including tolerance and opioid-induced hyperalgesia (OIH). Microglia display similar morphological responses in the spinal cord after peripheral nerve injury (PNI). Consistent with this observation, functional studies have suggested that microglia activated by opioids or PNI engage common molecular mechanisms to induce hypersensitivity. In this article, we conducted deep RNA sequencing (RNA-seq) and morphological analysis of spinal cord microglia in male mice to comprehensively interrogate transcriptional states and mechanistic commonality between multiple models of OIH and PNI. After PNI, we identify an early proliferative transcriptional event across models that precedes the upregulation of histological markers of microglial activation. However, we found no proliferative transcriptional response associated with opioid-induced microglial activation, consistent with histological data, indicating that the number of microglia remains stable during morphine treatment, whereas their morphological response differs from PNI models. Collectively, these results establish the diversity of pain-associated microglial transcriptomic responses and point towards the targeting of distinct insult-specific microglial responses to treat OIH, PNI, or other central nervous system pathologies.

The interplay between the microbiota and opioid in the treatment of neuropathic pain

Neuropathic pain (NP) is characterized by its complex and multifactorial nature and limited responses to opioid therapy; NP is associated with risks of drug resistance, addiction, difficulty in treatment cessation, and psychological disorders. Emerging research on gut microbiota and their metabolites has demonstrated their effectiveness in alleviating NP and augmenting opioid-based pain management, concurrently mitigating the adverse effects of opioids. This review addresses the following key points: (1) the current advances in gut microbiota research and the challenges in using opioids to treat NP, (2) the reciprocal effects and benefits of gut microbiota on NP, and (3) the interaction between opioids with gut microbiota, as well as the benefits of gut microbiota in opioid-based treatment of NP. Through various intricate mechanisms, gut microbiota influences the onset and progression of NP, ultimately enhancing the efficacy of opioids in the management of NP. These insights pave the way for further pragmatic clinical research, ultimately enhancing the efficacy of opioid-based pain management.

Zingerone Alleviates Morphine Tolerance and Dependence in Mice by Reducing Oxidative Stress-Mediated NLRP3 Inflammasome Activation

Morphine (MPH) is widely used for pain management; however, long-term MPH therapy results in antinociceptive tolerance and physical dependence, limiting its clinical use. Zingerone (ZIN) is a natural phenolic compound with neuroprotective effects. We investigated the effects of single and repeated doses of ZIN on MPH-induced tolerance, dependence, and underlying biochemical mechanisms. After a dose-response experiment, tolerance was developed to MPH (10 mg/kg, i.p.) for seven days. In the single-dose study, ZIN was administered on day seven. In the repeated-dose study, ZIN was administered for seven days. Naloxone (5 mg/kg, i.p., 120 min after MPH) was injected to assess withdrawal signs on day seven. The levels of thiobarbituric acid reactive substances (TBARS), nitric oxide (NO), total thiol (TT), and glutathione peroxidase (GPx) were measured in the prefrontal cortex. The protein levels of interleukin-1 beta (IL-1β) and NLRP3-ASC-Caspase-1 axis were assessed by ELISA and Western blotting, respectively. Results showed that ZIN (100 mg/kg) had no antinociceptive activity, and subsequent experiments were performed at this dose. Repeated ZIN reversed MPH antinociceptive tolerance, whereas single ZIN did not. Single and repeated ZIN attenuated naloxone-induced jumping. In addition, repeated ZIN significantly inhibited weight loss. Repeated ZIN suppressed the MPH-induced increase in TBARS, NO, IL-1β, NLRP3, ASC, and Caspase-1. It also inhibited MPH-induced TT and GPx reduction. In contrast, single ZIN had no effect. Findings suggest that ZIN reduces MPH-induced tolerance and dependence by suppressing oxidative stress and NLRP3 inflammasome activation. This study provides a novel therapeutic approach to reduce the side effects of MPH.

Discovery of a novel series of potent carbonic anhydrase inhibitors with selective affinity for μ Opioid receptor for Safer and long-lasting analgesia

In this study, we investigated the development of dual-targeted ligands that bind to both μ-opioid receptor (MOR) and carbonic anhydrase (CA) enzymes, using fentanyl structure as a template. We synthesized and evaluated 21 novel compounds with dual-targeted affinity identifying the lead candidate compound 8, showing selective affinity for MOR and potent inhibition of several cytosolic CA isoforms. By means of repeated treatment of 3 daily administrations for 17 days, fentanyl (0.1 mg/kg, subcutaneously) led to tolerance development, pain threshold alterations and withdrawal symptoms in CD-1 mice, as well as astrocyte and microglia activation in the dorsal horn of the lumbar spinal cord. In contrast, compound 8 (0.32 mg/kg s.c.) maintained stable during days its analgesic effect at the higher dose tested with fewer withdrawal symptoms, allodynia development and glial cells activation. Our results suggest that targeting both MOR and CA enzymes can lead to the development of new class of potent analgesic agents with fewer side effects and reduced tolerance development. Further studies are needed to explore the potential mechanisms underlying these effects and to further optimize the therapeutic potential of these compounds.

Epigenetic regulation in opioid induced hyperalgesia

About 25 million American adults experience pain daily and one of the most commonly prescribed drugs to treat pain are opioids. Prolonged opioid usage and dose escalations can cause a paradoxical response where patients experience enhanced pain sensitivity. This opioid induced hyperalgesia (OIH) is a major hurdle when treating pain in the clinic because its underlying mechanisms are still not fully understood. OIH is also commonly overlooked and lacks guidelines to prevent its onset. Research on pain disorders and opioid usage have recognized potential epigenetic drivers of disease including DNA methylation, histone modifications, miRNA regulation, but their involvement in OIH has not been well studied. This article discusses epigenetic changes that may contribute to pathogenesis, with an emphasis on miRNA alterations in OIH. There is a crucial gap in knowledge including how multiple epigenetic modulators contribute to OIH. Elucidating the epigenetic changes underlying OIH and the crosstalk among these mechanisms could lead to the development of novel targets for the prevention and treatment of this painful phenomena.

Morphine Withdrawal-Induced Hyperalgesia in Models of Acute and Extended Withdrawal Is Attenuated by l-Tetrahydropalmatine

Effective pain control is an underappreciated aspect of managing opioid withdrawal, and its absence presents a significant barrier to successful opioid detoxification. Accordingly, there is an urgent need for effective non-opioid treatments to facilitate opioid detoxification. l-Tetrahydropalmatine (l-THP) possesses powerful analgesic properties and is an active ingredient in botanical formulations used in Vietnam for the treatment of opioid withdrawal syndrome. In this study, rats receiving morphine (15 mg/kg, i.p.) for 5 days per week displayed a progressive increase in pain thresholds during acute 23 h withdrawal as assessed by an automated Von Frey test. A single dose of l-THP (5 or 7.5 mg/kg, p.o.) administered during the 4th and 5th weeks of morphine treatment significantly improves pain tolerance scores. A 7-day course of l-THP treatment in animals experiencing extended withdrawal significantly attenuates hyperalgesia and reduces the number of days to recovery to baseline pain thresholds by 61% when compared to vehicle-treated controls. This indicates that the efficacy of l-THP on pain perception extends beyond its half-life. As a non-opioid treatment for reversing a significant hyperalgesic state during withdrawal, l-THP may be a valuable addition to the currently limited arsenal of opioid detoxification treatments.

Beneficial Effects of Probiotic Bifidobacterium longum in a Lithium-Pilocarpine Model of Temporal Lobe Epilepsy in Rats

Epilepsy is a challenging brain disorder that is often difficult to treat with conventional therapies. The gut microbiota has been shown to play an important role in the development of neuropsychiatric disorders, including epilepsy. In this study, the effects of Bifidobacterium longum, a probiotic, on inflammation, neuronal degeneration, and behavior are evaluated in a lithium-pilocarpine model of temporal lobe epilepsy (TLE) induced in young adult rats. B. longum was administered orally at a dose of 10⁹ CFU/rat for 30 days after pilocarpine injection. The results show that B. longum treatment has beneficial effects on the TLE-induced changes in anxiety levels, neuronal death in the amygdala, and body weight recovery. In addition, B. longum increased the expression of anti-inflammatory and neuroprotective genes, such as Il1rn and Pparg. However, the probiotic had little effect on TLE-induced astrogliosis and microgliosis and did not reduce neuronal death in the hippocampus and temporal cortex. The study suggests that B. longum may have a beneficial effect on TLE and may provide valuable insights into the role of gut bacteria in epileptogenesis. In addition, the results show that B. longum may be a promising drug for the comprehensive treatment of epilepsy.

Endomorphin analog ZH853 shows low reward, tolerance, and affective-motivational signs of withdrawal, while inhibiting opioid withdrawal and seeking

Currently available μ-opioid receptor agonist pharmacotherapies for opioid use disorder possess adverse effects limiting their use and, despite treatment, rates of relapse remain high. We previously showed that endomorphin analog ZH853 had no effect in rodent models that predict abuse liability in humans. Here we extended these findings by examining dependence liability and reinforcing properties in female rats and male rats with previous opioid exposure. The potential use of ZH853 in managing opioid use disorder was evaluated by examining its effect on opioid-seeking behavior and withdrawal. We found that ZH853 did not induce locomotor activation in male and female mice and was not self-administered by female rats. Relative to morphine, ZH853 led to similar somatic signs of withdrawal, but low affective-motivational signs of withdrawal, and absent changes in ventral tegmental area K(+)-Cl(-) co-transporter expression associated with reward dysregulation. The low abuse liability of ZH853 was further supported in oxycodone self-administering male rats, where ZH853 substitution extinguished opioid-seeking behavior. ZH853 priming also did not reinstate morphine conditioned place preference. Lastly, ZH853 inhibited oxycodone-seeking behavior during relapse after forced abstinence and decreased the expression of morphine withdrawal. These findings suggest the potential use of ZH853 as a safer opioid medication for long-term treatment of pain and opioid use disorder.

Curcumin attenuates morphine dependence by modulating μ-opioid receptors and glial cell-activated neuroinflammation in rat

In recent years, the association between neuroinflammation and opioid dependence has attracted considerable attention. Curcumin, a component of the Curcuma longa, has been shown to act as a suppressor of glial cells and inflammatory cytokines. The main goal of this study was to explore the attenuating effects of curcumin on morphine dependence with a focus on neuroinflammation and μ-opioid receptors in the rat prefrontal cortex. To induce morphine dependence in male Wistar rats, morphine was administered i.p. once daily for 18 days in an escalating dose of 10, 20, and 40 mg/kg. Curcumin (2.5, 5, and 10 mg/kg, i.p.) was given from the days 10th to 18th. Immunofluorescence staining and ELISA methods were used to evaluate glial cells activity and inflammatory cytokines levels, respectively. Western blotting was used to evaluate the expression of μ-opioid receptors. The administration of curcumin (2.5, 5, and 10 mg/kg) for 9 days significantly attenuated the symptoms of morphine withdrawal syndrome. The prefrontal cortex concentration of TNF-α and IL-6 was also reduced by curcumin (2.5, 5, and 10 mg/kg) significantly. Furthermore, curcumin decreased the number of Iba1 and GFAP positive cells in morphine-dependent rats. Moreover, the expression of μ-opioid receptors was significantly reduced by curcumin (10 mg/kg). The results of this study demonstrate that curcumin attenuates morphine dependence in rats through an inhibitory effect on neuroinflammation and a decrease in the expression of μ-opioid receptors in the prefrontal cortex.

Crosstalk between Mu-Opioid receptors and neuroinflammation: Consequences for drug addiction and pain

Mu-Opioid Receptors (MORs) are well-known for participating in analgesia, sedation, drug addiction, and other physiological functions. Although MORs have been related to neuroinflammation their biological mechanism remains unclear. It is suggested that MORs work alongside Toll-Like Receptors to enhance the release of pro-inflammatory mediators and cytokines during pathological conditions. Some cytokines, including TNF-α, IL-1β and IL-6, have been postulated to regulate MORs levels by both avoiding MOR recycling and enhancing its production. In addition, Neurokinin-1 Receptor, also affected during neuroinflammation, could be regulating MOR trafficking. Therefore, inflammation in the central nervous system seems to be associated with altered/increased MORs expression, which might regulate harmful processes, such as drug addiction and pain. Here, we provide a critical evaluation on MORs' role during neuroinflammation and its implication for these conditions. Understanding MORs' functioning, their regulation and implications on drug addiction and pain may help elucidate their potential therapeutic use against these pathological conditions and associated disorders.

Ultramicronized N-Palmitoylethanolamine Regulates Mast Cell-Astrocyte Crosstalk: A New Potential Mechanism Underlying the Inhibition of Morphine Tolerance

Persistent pain can be managed with opioids, but their use is limited by the onset of tolerance. Ultramicronized N-palmitoylethanolamine (PEA) in vivo delays morphine tolerance with mechanisms that are still unclear. Since glial cells are involved in opioid tolerance and mast cells (MCs) are pivotal targets of PEA, we hypothesized that a potential mechanism by which PEA delays opioid tolerance might depend on the control of the crosstalk between these cells. Morphine treatment (30 μM, 30 min) significantly increased MC degranulation of RBL-2H3 cells, which was prevented by pre-treatment with PEA (100 μM, 18 h), as evaluated by β-hexosaminidase assay and histamine quantification. The impact of RBL-2H3 secretome on glial cells was studied. Six-hour incubation of astrocytes with control RBL-2H3-conditioned medium, and even more so co-incubation with morphine, enhanced CCL2, IL-1β, IL-6, Serpina3n, EAAT2 and GFAP mRNA levels. The response was significantly prevented by the secretome from PEA pre-treated RBL-2H3, except for GFAP, which was further upregulated, suggesting a selective modulation of glial signaling. In conclusion, ultramicronized PEA down-modulated both morphine-induced MC degranulation and the expression of inflammatory and pain-related genes from astrocytes challenged with RBL-2H3 medium, suggesting that PEA may delay morphine tolerance, regulating MC-astrocyte crosstalk.

The endocannabinoid system and breathing

Recent changes in cannabis accessibility have provided adjunct therapies for patients across numerous disease states and highlights the urgency in understanding how cannabinoids and the endocannabinoid (EC) system interact with other physiological structures. The EC system plays a critical and modulatory role in respiratory homeostasis and pulmonary functionality. Respiratory control begins in the brainstem without peripheral input, and coordinates the preBötzinger complex, a component of the ventral respiratory group that interacts with the dorsal respiratory group to synchronize burstlet activity and drive inspiration. An additional rhythm generator: the retrotrapezoid nucleus/parafacial respiratory group drives active expiration during conditions of exercise or high CO₂. Combined with the feedback information from the periphery: through chemo- and baroreceptors including the carotid bodies, the cranial nerves, stretch of the diaphragm and intercostal muscles, lung tissue, and immune cells, and the cranial nerves, our respiratory system can fine tune motor outputs that ensure we have the oxygen necessary to survive and can expel the CO₂ waste we produce, and every aspect of this process can be influenced by the EC system. The expansion in cannabis access and potential therapeutic benefits, it is essential that investigations continue to uncover the underpinnings and mechanistic workings of the EC system. It is imperative to understand the impact cannabis, and exogenous cannabinoids have on these physiological systems, and how some of these compounds can mitigate respiratory depression when combined with opioids or other medicinal therapies. This review highlights the respiratory system from the perspective of central versus peripheral respiratory functionality and how these behaviors can be influenced by the EC system. This review will summarize the literature available on organic and synthetic cannabinoids in breathing and how that has shaped our understanding of the role of the EC system in respiratory homeostasis. Finally, we look at some potential future therapeutic applications the EC system has to offer for the treatment of respiratory diseases and a possible role in expanding the safety profile of opioid therapies while preventing future opioid overdose fatalities that result from respiratory arrest or persistent apnea.

Activation of neurons and satellite glial cells in the DRG produces morphine-induced hyperalgesia

Activation of neurons and glial cells in the dorsal root ganglion is one of the key mechanisms for the development of hyperalgesia. The aim of the present study was to examine the role of neuroglial activity in the development of opioid-induced hyperalgesia. Male rats were treated with morphine daily for 3 days. The resultant phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 in the dorsal root ganglion was analyzed by immunohistochemistry and Western blotting. Pain hypersensitivity was analyzed using behavioral studies. The amount of cytokine expression in the dorsal root ganglion was also analyzed. Repeated morphine treatment induced hyperalgesia and marked induction of phosphorylated ERK1/2 in the neurons and satellite glial cells on day 3. An opioid receptor antagonist, toll like receptor-4 inhibitor, MAP/ERK kinase (MEK) inhibitor and gap junction inhibitor inhibited morphine-induced hyperalgesia and ERK1/2 phosphorylation. Morphine treatment induced alteration of cytokine expression, which was inhibited by the opioid receptor antagonist, toll like receptor-4 inhibitor, MEK inhibitor and gap junction inhibitor. Dexamethasone inhibited morphine-induced hyperalgesia and ERK1/2 phosphorylation after morphine treatment. The peripherally restricted opioid receptor antagonist, methylnaltrexone, inhibited hyperalgesia and ERK1/2 phosphorylation. Morphine activates ERK1/2 in neurons and satellite glial cells in the dorsal root ganglion via the opioid receptor and toll like receptor-4. ERK1/2 phosphorylation is gap junction-dependent and is associated with the alteration of cytokine expression. Inhibition of neuroinflammation by activation of neurons and glia might be a promising target to prevent opioid-induced hyperalgesia.

Pharmacological Interventions for Opioid-Induced Hyperalgesia: A Scoping Review of Preclinical Trials

BACKGROUND

Opioid analgesics are the most effective pharmacological agents for moderate and severe pain. However, opioid use has several limitations such as opioid-induced hyperalgesia (OIH), which refers to the increased pain sensitivity that occurs once analgesia wears off after opioid administration. Several pharmacological interventions have been suggested for OIH, but the current literature does not provide guidelines on which interventions are the most effective and whether they differ depending on the opioid that induces hyperalgesia. This scoping review aimed to identify and describe all the preclinical trials investigating pharmacological interventions for OIH caused by remifentanil, fentanyl, or morphine as the first step towards evaluating whether the most effective OIH interventions are different for different opioids.

METHODS

Electronic database searches were carried out in Embase, PubMed, and Web of Science. Detailed data extraction was conducted on the eligible trials.

RESULTS

72 trials were eligible for the review. Of these, 27 trials investigated remifentanil, 14 trials investigated fentanyl, and 31 trials investigated morphine. A total of 82 interventions were identified. The most studied interventions were ketamine (eight trials) and gabapentin (four trials). The majority of the interventions were studied in only one trial. The most common mechanism suggested for the interventions was inhibition of N-methyl-D-aspartate (NMDA) receptors.

CONCLUSION

This scoping review identified plenty of preclinical trials investigating pharmacological interventions for OIH. Using the current literature, it is not possible to directly compare the effectiveness of the interventions. Hence, to identify the most effective interventions for each opioid, the interventions must be indirectly compared in a meta-analysis.

Attenuation of Morphine Dependence by Ganoderma lucidum Extract in Mice

Background: Opioids are the principal drugs of choice for managing acute severe pain; however, physical dependence is still reported as one of the main limiting factors in the clinical application of these drugs. In the present study, the effect of Ganoderma lucidum (G. lucidum) was assessed on morphine dependence in mice. Methods: A 19-day administration schedule was applied to induce morphine dependence in male adult NMRI mice. The mice were given intraperitoneal (i.p.) morphine sulfate once daily in an increasing dose of 10, 20, and 40 mg/kg. Then, G. lucidum hydroalcoholic extract (12.5, 25, and 50 mg/kg, i.p.) was given to the mice from days 10 to 18. Another group of mice received single doses of the extract (50, 100, and 200 mg/kg, i.p.) only on the 19th day. Naloxone (3 mg/kg, i.p.) was used to precipitate withdrawal syndrome. Normal saline and diazepam (0.25 mg/kg) were used as the negative and positive controls, respectively. Results: The administration of single doses of G. lucidum extract (100 and 200 mg/kg, i.p.) significantly decreased the number of jumps, leanings, and diarrhea in mice subjected to morphine dependence. The repeated administration of G. lucidum extract (25 and 50 mg/kg for nine days) significantly attenuated the number of jumps, leanings, and diarrhea in morphine-dependent mice. Conclusions: Overall, G. lucidum extract attenuates induced morphine dependence and inhibits withdrawal syndrome symptoms in mice.

Metformin prevents morphine-induced apoptosis in rats with diabetic neuropathy: a possible mechanism for attenuating morphine tolerance

Morphine is a drug of choice for the treatment of severe and chronic pain, but tolerance to the antinociceptive effect limits its use. The development of tolerance to morphine has recently been associated with neuronal apoptosis. In this study, our aim was to investigate the effects of metformin on morphine-induced neuronal apoptosis and antinociceptive tolerance in diabetic rats. Three days of cumulative dosing were administered to establish morphine tolerance in rats. The antinociceptive effects of metformin (50 mg/kg) and test dose of morphine (5 mg/kg) were considered at 30-min intervals by thermal antinociceptive tests. To induce diabetic neuropathy, streptozotocin (STZ, 65 mg/kg) was injected intraperitoneally. ELISA kits were used to measure caspase-3, bax, and bcl-2 levels from dorsal root ganglion (DRG) tissue. Semi-quantitative scoring system was used to evaluate apoptotic cells with the the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) method. The findings suggest that co-administration of metformin with morphine to diabetic rats showed a significant increase in antinociceptive effect compared to morphine alone. The antinociceptive tests indicated that metformin significantly attenuated morphine antinociceptive tolerance in diabetic rats. In addition, metformin decreased the levels of apoptotic proteins caspase 3 and Bax in DRG neurons, while significantly increased the levels of antiapoptotic Bcl-2. Semi-quantitative scoring showed that metformin provided a significant reduction in apoptotic cell counts in diabetic rats. These data revealed that metformin demonstrated antiapoptotic activity in diabetic rat DRG neurons and attenuated morphine tolerance. The antiapoptotic activity of metformin probably plays a significant role in reducing morphine tolerance.

Sex differences in morphine sensitivity are associated with differential glial expression in the brainstem of rats with neuropathic pain

Chronic pain is more prevalent and reported to be more severe in women. Opioid analgesics are less effective in women and result in stronger nauseant effects. The neurobiological mechanisms underlying these sex differences have yet to be clearly defined, though recent research has suggested neuronal–glial interactions are likely involved. We have previously shown that similar to people, morphine is less effective at reducing pain behaviors in female rats. In this study, we used the immunohistochemical detection of glial fibrillary acidic protein (GFAP) expression to investigate sex differences in astrocyte density and morphology in six medullary regions known to be modulated by pain and/or opioids. Morphine administration had small sex‐dependent effects on overall GFAP expression, but not on astrocyte morphology, in the rostral ventromedial medulla, the subnucleus reticularis dorsalis, and the area postrema. Significant sex differences in the density and morphology of GFAP immunopositive astrocytes were detected in all six regions. In general, GFAP‐positive cells in females showed smaller volumes and reduced complexity than those observed in males. Furthermore, females showed lower overall GFAP expression in all regions except for the area postrema, the critical medullary region responsible for opioid‐induced nausea and emesis. These data support the possibility that differences in astrocyte activity might underlie the sex differences seen in the processing of opioids in the context of chronic neuropathic pain.

Yokukansan Inhibits the Development of Morphine Tolerance by Regulating Presynaptic Proteins in DRG Neurons

Opioids, such as morphine, are used in clinical settings for the management of acute and chronic pain. However, long-term use of morphine leads to antinociceptive tolerance and hypersensitivity. The cellular and molecular mechanisms of morphine tolerance seem to be quite complex, with suggestions including internalization of the μ-opioid receptor (MOR), neuroinflammation with activation of microglia and astrocytes, and changes in synaptic function in the central nervous system. Yokukansan (YKS), a traditional Kampo medicine consisting of seven herbs, has been used to treat emotional instability, neurosis, and insomnia. Interestingly, recent studies have begun to reveal the inhibitory effect of YKS on the development of morphine tolerance. In the present study, we determined the effect of YKS on morphine tolerance formation and its mechanisms in a rat model, focusing on the synapses between primary sensory neurons and spinal dorsal horn secondary neurons. We found that morphine tolerance formation was significantly inhibited by YKS (0.3 or 1.0 g/kg/day) preadministration for 7 days. Repeated administration of morphine (10 mg/kg/day) increased the expression of presynaptic proteins, including synaptotagmin I, in the spinal cord, which was suppressed by YKS. Furthermore, these changes in presynaptic protein expression were more pronounced at isolectin B4 (IB4)-positive excitatory synapses around the lamina II of the dorsal horn. These results suggest that YKS suppresses the development of morphine tolerance by inhibiting the enhancement of presynaptic function of dorsal root ganglia neurons projecting to spinal dorsal horn neurons caused by continuous morphine administration.

Toll-like receptors change morphine-induced antinociception, tolerance and dependence: Studies using male and female TLR and signalling gene KO mice

Toll-like receptors (TLR) have been proposed as a site of action that alters opioid pharmacodynamics. However, a comprehensive assessment of acute opioid antinociception, tolerance and withdrawal behaviours in genetic null mutant strains with altered innate immune signalling has not been performed. Nor has the impact of genetic deletion of TLR2/4 on high-affinity opioid receptor binding. Here we show that diminished TLR signalling potentiates acute morphine antinociception equally in male and female mice. However, only male TIR8 null mutant mice showed reduced morphine analgesia. Analgesic tolerance was prevented in TLR2 and TLR4 null mutants, but not MyD88 animals. Withdrawal behaviours were only protected in TLR2^-/- mice. In silico docking simulations revealed opioid ligands bound preferentially to the LPS binding pocket of MD-2 rather than TLR4. There was no binding of [³H](-)-naloxone or [³H]diprenorphine to TLR4 in the concentrations explored. These data confirm that opioids have high efficacy activity at innate immune pattern recognition binding sites but do not bind to TLR4 and identify critical pathway and sex-specific effects of the complex innate immune signalling contributions to opioid pharmacodynamics. These data further support the behavioural importance of the TLR-opioid interaction but fail to demonstrate direct evidence for high-affinity binding of the TLR4 signalling complex to ligands.

New pharmacological perspectives and therapeutic options for opioids: Differences matter

Opioids remain the major drug class for the treatment of acute, chronic and cancer pain, but have major harmful effects such as dependence and opioid-induced ventilatory impairment. Although no new typical opioids have come onto the market in the past almost 50 years, a plethora of new innovative formulations has been developed to meet the clinical need. This review is intended to shed light on new understanding of the molecular pharmacology of opioids, which has arisen largely due to the genomic revolution, and what new drugs may become available in the coming years. Atypical opioids have and are being developed which not only target the mu opioid receptor but other targets in the pain pathway. Biased mu agonists have been developed but remain 'unbiased' clinically. The contribution of drugs targeting non-mu opioid receptors either alone or as heterodimers shows potential promise but remains understudied. That gene splice variants of the mu opioid receptor produce multiple receptor isoforms in different brain regions, and may change with pain chronicity and phenotype, presents new challenges but also opportunities for precision pain medicine. Finally, that opioids also have pro-inflammatory effects not aligned with mu opioid receptor binding affinity implicates a fresh understanding of their role in chronic pain, whether cancer or non-cancer. Hopefully, a new understanding of opioid analgesic drug action may lead to new drug development and better precision medicine in acute and chronic pain relief with less patient harm.

Pharmacological blockade of neurokinin1 receptor restricts morphine-induced tolerance and hyperalgesia in the rat

OBJECTIVES

The most notable adverse side effects of chronic morphine administration include tolerance and hyperalgesia. This study investigated the involvement of dorsal root ganglion (DRG) protein kinase Cɛ (PKCɛ) expression during chronic morphine administration and also considered the relationship between DRG PKCɛ expression and the substance P- neurokinin1 receptor (SP- NK1R) activity.

METHODS

Thirty-six animals were divided into six groups (n=6) in this study. In the morphine and sham groups, rats received 10 µg intrathecal (i.t.) morphine or saline for eight consecutive days, respectively. Behavioral tests were performed on days 1 and 8 before and after the first injections and then 48 h after the last injection (day 10). In the treatment groups, rats received NK1R antagonist (L-732,138, 25 µg) daily, either alone or 10 min before a morphine injection, Sham groups received DMSO alone or 10 min before a morphine injection. Animals were sacrificed on days 8 and 10, and DRG PKCɛ and SP expression were analyzed by western blot and immunohistochemistry techniques, respectively.

RESULTS

Behavioral tests indicated that tolerance developed following eight days of chronic morphine injection. Hyperalgesia was induced 48 h after the last morphine injection. Expression of SP and PKCɛ in DRG significantly increased in rats that developed morphine tolerance on day 8 and hyperalgesia on day 10, respectively. NK1R antagonist (L-732,138) not only blocked the development of hyperalgesia and the increase of PKCɛ expression but also alleviated morphine tolerance.

CONCLUSIONS

Our results provide evidence that DRG PKCɛ and SP-NK1R most likely participated in the generation of morphine tolerance and hyperalgesia. Pharmacological inhibition of SP-NK1R activity in the spinal cord suggests a role for NK1R and in restricting some side effects of chronic morphine. All experiments were performed by the National Institute of Health (NIH) Guidelines for the Care and Use of Laboratory Animals (NIH Publication No. 80-23, revised1996) and were approved by the Animal Ethics Committee of Shahid Beheshti University of Medical Sciences, Tehran, Iran (IR.SBMU.MSP.REC.1396.130).

MTEP, a Selective mGluR5 Antagonist, Had a Neuroprotective Effect but Did Not Prevent the Development of Spontaneous Recurrent Seizures and Behavioral Comorbidities in the Rat Lithium-Pilocarpine Model of Epilepsy

Metabotropic glutamate receptors (mGluRs) are expressed predominantly on neurons and glial cells and are involved in the modulation of a wide range of signal transduction cascades. Therefore, different subtypes of mGluRs are considered a promising target for the treatment of various brain diseases. Previous studies have demonstrated the seizure-induced upregulation of mGluR5; however, its functional significance is still unclear. In the present study, we aimed to clarify the effect of treatment with the selective mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) on epileptogenesis and behavioral impairments in rats using the lithium–pilocarpine model. We found that the administration of MTEP during the latent phase of the model did not improve survival, prevent the development of epilepsy, or attenuate its manifestations in rats. However, MTEP treatment completely prevented neuronal loss and partially attenuated astrogliosis in the hippocampus. An increase in excitatory amino acid transporter 2 expression, which has been detected in treated rats, may prevent excitotoxicity and be a potential mechanism of neuroprotection. We also found that MTEP administration did not prevent the behavioral comorbidities such as depressive-like behavior, motor hyperactivity, reduction of exploratory behavior, and cognitive impairments typical in the lithium–pilocarpine model. Thus, despite the distinct neuroprotective effect, the MTEP treatment was ineffective in preventing epilepsy.

Cannabinoid Receptor Type 2 Agonist Reduces Morphine Tolerance via Mitogen Activated Protein Kinase Phosphatase Induction and Mitogen Activated Protein Kinase Dephosphorylation

Morphine is an opioid drug often used in treating moderate to severe pain. However, morphine tolerance in patients limits its used in clinical settings. Our previous study showed that a cannabinoid type 2 (CB2) receptor agonist attenuated morphine tolerance. However, the exact mechanism by which CB2 agonists reduce morphine tolerance remains unclear. In this study, we investigated the effect of mitogen activated protein kinase (MAPK) and mitogen activated protein kinase phosphatases 1 and 3 (MKP-1 and MKP-3) on the regulation of morphine tolerance by CB2 receptor agonist. Chronic morphine treatments for 7 days reduced the protein expression of MKP-1 and MKP-3 in the spinal cord and increased the phosphorylation of p38, ERK1/2 and the level of proinflammatory mediator, such as IL-1β, IL-6 and TNF-α. Coadministration of CB2 receptor agonist AM1241 alleviated the inhibition of MKP-1 and MKP-3 by chronic morphine administration and reduced the expression of phosphorylated MAPK and proinflammatory factors. The effect of the CB2 receptor agonist on morphine-induced downregulation of MKP-1 and MKP-3 was reversed by the MKP-1 and MKP-3 antagonist triptolide. Our findings suggested that CB2 receptor agonist may induce the expression of MKP-1 and MKP-3 to promote MAPK dephosphorylation and reduce the production of downstream cytokine, thereby reducing morphine tolerance. This finding suggested that MKPs may serve as a new target for alleviating morphine tolerance.

Ibudilast attenuates cocaine self-administration and prime- and cue-induced reinstatement of cocaine seeking in rats

Ibudilast is a non-selective phosphodiesterase (PDE) inhibitor and glial cell modulator which has shown great promise for the treatment of drug and alcohol use disorders in recent clinical studies. However, it is unknown whether and how ibudilast affects cocaine seeking behavior. Here we show that systemic administration of ibudilast dose-dependently reduced cocaine self-administration under fixed- and progressive-ratio reinforcement schedules in rats and shifted cocaine dose-response curves downward. In addition, ibudilast decreased cocaine prime- and cue-induced reinstatement of cocaine seeking. These results indicate that ibudilast was effective in reducing the reinforcing effects of cocaine and relapse to cocaine seeking. Chronic cocaine exposure induces cAMP-related neuroadaptations in the reward circuitry of the brain. To investigate potential mechanisms for ibudilast-induced attenuation of cocaine self-administration, we recorded from ventral tegmental area (VTA) dopamine neurons in ex vivo midbrain slices prepared from rats that had undergone saline and cocaine self-administration. We found cocaine self-administration led to a decrease in inhibitory postsynaptic currents (IPSCs), an increase in the AMPAR/NMDAR ratio, and an increase in the excitation to inhibition (E/I) ratio. Ibudilast pretreatments enhanced GABAergic inhibition and did not further change cocaine-induced potentiation of excitation, leading to normalization of the E/I ratio. Restoration of the balance between excitation and inhibition in VTA dopamine neurons may contribute to the attenuation of cocaine self-administration by ibudilast.

Interaction of Opioids with TLR4-Mechanisms and Ramifications

Simple Summary

Recent evidence indicates that opioids can be active at a receptor that is abundantly expressed on innate immune cells as well as cancer cells: the receptor is termed toll-like receptor 4 (TLR4). TLR4 is increasingly recognised as playing key roles in tumour biology and anticancer defences. However, the issue of whether TLR4 mediates some of the effects of opioids on tumour growth and metastasis is entirely unknown. We review existing evidence, mechanisms, and functional consequences of the action of opioids at TLR4. This opens new avenues of research on the role of opioids in cancer.

Abstract

The innate immune receptor toll-like receptor 4 (TLR4) is known as a sensor for the gram-negative bacterial cell wall component lipopolysaccharide (LPS). TLR4 activation leads to a strong pro-inflammatory response in macrophages; however, it is also recognised to play a key role in cancer. Recent studies of the opioid receptor (OR)-independent actions of opioids have identified that TLR4 can respond to opioids. Opioids are reported to weakly activate TLR4, but to significantly inhibit LPS-induced TLR4 activation. The action of opioids at TLR4 is suggested to be non-stereoselective, this is because OR-inactive (+)-isomers of opioids have been shown to activate or to inhibit TLR4 signalling, although there is some controversy in the literature. While some opioids can bind to the lipopolysaccharide (LPS)-binding cleft of the Myeloid Differentiation factor 2 (MD-2) co-receptor, pharmacological characterisation of the inhibition of opioids on LPS activation of TLR4 indicates a noncompetitive mechanism. In addition to a direct interaction at the receptor, opioids affect NF-κB activation downstream of both TLR4 and opioid receptors and modulate TLR4 expression, leading to a range of in vivo outcomes. Here, we review the literature reporting the activity of opioids at TLR4, its proposed mechanism(s), and the complex functional consequences of this interaction.

Ventral tegmental area serotonin 5-HT1A receptors and corticolimbic cFos/BDNF/GFAP signaling pathways mediate dextromethorphan/morphine anti-allodynia

AIMS

The present study examined the role of ventral tegmental area (VTA) serotonergic 5HT_1A receptors in dextromethorphan/morphine-induced anti-allodynia and the possible changes of corticolimbic cFos, brain-derived neurotrophic factor (BDNF), and glial fibrillary acidic protein (GFAP) following the treatments.

MATERIALS AND METHODS

The VTA cannulation and the chronic constriction of the sciatic nerve were performed in male Wistar rats. Flexion withdrawal thresholds to mechanical stimulation in the hind-limb were determined using von Frey hairs. The expressions of cFos, BDNF, and GFAP were evaluated using the Western blotting technique.

KEY FINDINGS

BDNF (in the hippocampus), and GFAP (in the targeted sites) levels were increased following neuropathic pain. Morphine administration induced an anti-allodynic effect with a decrease in the amygdala BDNF level. Dextromethorphan/morphine-induced anti-allodynia was accompanied by the decrease of hippocampus/amygdala/PFC GFAP and amygdala cFos expressions. The PFC BDNF expression level was increased in dextromethorphan/morphine-treated rats. Intra-VTA microinjection of (S)-WAY100135 (1 µg/rat), a selective 5-HT_1A receptor antagonist, inhibited the anti-allodynic effect of dextromethorphan/morphine. This treatment increased the cFos level in the hippocampus and the amygdala while decreased the PFC level of cFos. The hippocampal BDNF expression was significantly increased, while the amygdala and the PFC expressions of BDNF were decreased under treatment. (S)-WAY100135 plus dextromethorphan/morphine increased the hippocampal/amygdala and PFC levels of GFAP.

SIGNIFICANCE

These findings indicate that dextromethorphan could potentiate the analgesic effect of morphine via the implication of the VTA serotonin 5-HT_1A receptors. It seems that the changes in the corticolimbic cFos/BDNF/GFAP signaling pathway may be involved in the observed anti-allodynic effect.

Activation of the spinal EGFR signaling pathway in a rat model of cancer-induced bone pain with morphine tolerance

Cancer-induced bone pain (CIBP) is considered to be one of the most difficult pain conditions to treat. Morphine, an analgesic drug, is widely used in clinical practice, and long-term use of morphine can lead to drug tolerance. Recent reports have suggested that inhibitors of epidermal growth factor receptor (EGFR) may have analgesic effects in cancer patients suffering from pain. Therefore, we sought to determine whether EGFR signaling was involved in morphine tolerance (MT) in a rat model of cancer-induced bone pain. In this study, Walker 256 mammary gland carcinoma cells were inoculated into the tibias of rats to provoke cancer-induced bone pain. Then, morphine was intrathecally administered twice daily for seven consecutive days to induce drug tolerance. We observed sustained increased in the protein levels of EGFR, p-EGFR, ERK1/2, and p-ERK1/2 during the development of morphine tolerance in rats with cancer-induced bone pain by western blotting. The EGFR level was significantly increased in the MT and CIBP + MT groups, and EGFR was colocalized with markers of microglia and neurons in the spinal cords of rats. Inhibition of EGFR by a small molecule inhibitor markedly attenuated the degree of morphine tolerance and decreased the number of microglia, and the protein levels of EGFR, p-EGFR, ERK1/2, and p-ERK1/2 were also reduced. In summary, our results suggest that the activation of the EGFR signaling pathway in spinal microglia plays an important modulatory role in the development of morphine tolerance and that inhibition of EGFR may provide a new therapeutic option for cancer-induced bone pain.

Contributions of neuroimmune and gut-brain signaling to vulnerability of developing substance use disorders

Epidemiology and clinical research indicate that only a subset of people who are exposed to drugs of abuse will go on to develop a substance use disorder. Numerous factors impact individual susceptibility to developing a substance use disorder, including intrinsic biological factors, environmental factors, and interpersonal/social factors. Given the extensive morbidity and mortality that is wrought as a consequence of substance use disorders, a substantial body of research has focused on understanding the risk factors that mediate the shift from initial drug use to pathological drug use. Understanding these risk factors provides a clear path for the development of risk mitigation strategies to help reduce the burden of substance use disorders in the population. Here we will review the rapidly growing body of literature that examines the importance of interactions between the peripheral immune system, the gut microbiome, and the central nervous system (CNS) in mediating the transition to pathological drug use. While these systems had long been viewed as distinct, there is growing evidence that there is bidirectional communication between both the immune system and the gut microbiome that drive changes in neural and behavioral plasticity relevant to substance use disorders. Further, both of these systems are highly sensitive to environmental perturbations and are implicated in numerous neuropsychiatric conditions. While the field of study examining these interactions in substance use disorders is in its relative infancy, clarifying the relationship between gut-immune-brain signaling and substance use disorders has potential to improve our understanding of individual propensity to developing addiction and yield important insight into potential treatment options.

Microglial TLR4-induced TAK1 phosphorylation and NLRP3 activation mediates neuroinflammation and contributes to chronic morphine-induced antinociceptive tolerance

BACKGROUND AND PURPOSE

The aim of this work was to investigate the role and signal transduction of toll-like receptor 4 (TLR4), TGF-β-activated kinase 1 (TAK1) and nod-like receptor protein 3 (NLRP3) in microglial in the development of morphine-induced antinociceptive tolerance.

METHODS

TLR4 and NLRP3 knockout mice and 5Z-7-oxozeaeno (a selective inhibitor against TAK1 activity) were used to observe their effect on the development of morphine tolerance. Intrathecal injections of morphine (0.75 mg/kg once daily for 7 days) were used to establish anti-nociceptive tolerance, which was measured by the tail-flick test. Spinal TLR4, TAK1, and NLRP3 expression levels and phosphorylation of TAK1 were evaluated by Western blotting and immunofluorescence.

RESULTS

Repeated treatment with morphine increased total expression of spinal TLR4, TAK1, and NLRP3 and phosphorylation of TAK1 in wild-type mice. TLR4 knockout attenuated morphine-induced tolerance and inhibited the chronic morphine-induced increase in NLRP3 and phosphorylation of TAK1. Compared with controls, mice that received 5Z-7-oxozeaenol showed decreased development of morphine tolerance and inhibition on repeated morphine-induced increase of NLRP3 but not TLR4. NLRP3 knockout mice showed resistance to morphine-induced analgesic tolerance with no effect on chronic morphine-induced expression of TLR4 and TAK1. TLR4, TAK1, and NLRP3 were collectively co-localized together and with the microglia marker Iba1.

CONCLUSIONS

Microglial TLR4 regulates TAK1 expression and phosphorylation and results in NLRP3 activation contributes to the development of morphine tolerance through regulating neuroinflammation. Targeting TLR4-TAK1-NLRP3 signaling to regulate neuro-inflammation will be alternative therapeutics and strategies for chronic morphine-induced antinociceptive tolerance.

Mesoporous Polydopamine Nanoparticles Attenuate Morphine Tolerance in Neuropathic Pain Rats by Inhibition of Oxidative Stress and Restoration of the Endogenous Antioxidant System

Oxidative stress resulting from reactive oxygen species (ROS) is known to play a key role in numerous neurological disorders, including neuropathic pain. Morphine is one of the commonly used opioids for pain management. However, long-term administration of morphine results in morphine antinociceptive tolerance (MAT) through elevation of ROS and suppression of natural antioxidant defense mechanisms. Recently, mesoporous polydopamine (MPDA) nanoparticles (NPS) have been known to possess strong antioxidant properties. We speculated that morphine delivery through an antioxidant nanocarrier might be a reasonable strategy to alleviate MAT. MPDAs showed a high drug loading efficiency of ∼50%, which was much higher than conventional NPS. Spectral and in vitro studies suggest a superior ROS scavenging ability of NPS. Results from a rat neuropathic pain model demonstrate that MPDA-loaded morphine (MPDA@Mor) is efficient in minimizing MAT with prolonged analgesic effect and suppression of pro-inflammatory cytokines. Additionally, serum levels of liver enzymes and levels of endogenous antioxidants were measured in the liver. Treatment with free morphine resulted in elevated levels of liver enzymes and significantly lowered the activities of endogenous antioxidant enzymes in comparison with the control and MPDA@Mor-treated group. Histopathological examination of the liver revealed that MPDA@Mor can significantly reduce the hepatotoxic effects of morphine. Taken together, our current work will provide an important insight into the development of safe and effective nano-antioxidant platforms for neuropathic pain management.

Anakinra, an interleukin-1 receptor antagonist, increases the morphine analgesic effect and decreases morphine tolerance development by modulating oxidative stress and endoplasmic reticulum stress in rats

Background/aim

Recent studies have shown that inflammation plays a role in morphine analgesia and tolerance development. Anakinra is a competitive inhibitor of IL-1 receptors and an antiinflammatory protein regulating IL-1β’s biological activity by avoiding signal transduction. In this study, we aimed to examine the effects of anakinra on morphine analgesia and tolerance.

Materials and methods

In this study, 36 Wistar Albino (230–250 g) male rats were used. Animals were divided into 6 groups: saline (S), 100 mg/kg anakinra (A), 5mg/kg morphine (M), M+A, morphine tolerance (MT), and MT+A. The resulting analgesic effect was measured with hot plate and tail-flick analgesia tests. After the analgesia tests, the dorsal root ganglions (DRG) tissues were removed. Oxidative stress parameters [total antioxidant status (TAS), total oxidant status (TOS)], endoplasmic reticulum (ER) stress, and apoptosis proteins [E74-like factor 2 (elF-2α), activating transcription factor 4 (ATF-4), C/EBP homologous protein (CHOP), caspase-3, and bcl-2-associated X protein (bax)] were measured in DRG tissues.

Results

Anakinra showed an antinociceptive effect when given alone (P < 0.001). In addition, anakinra increased the analgesic effect of morphine (P < 0.05 to P < 0.001), and also decreased the tolerance to morphine at a significant level (P < 0.05 to P < 0.001). Moreover, it decreased oxidative stress and ER-stress when given as a single-dose morphine and tolerance induction (P < 0.01 to P < 0.001). Furthermore, anakinra decreased apoptosis proteins after tolerance development (P < 0.001).

Conclusion

Anakinra has antinociceptive properties, and it increases the analgesic effect of morphine and also prevents tolerance development. These effects probably occur by the modulation of oxidative stress and ER-stress pathways.

Aberrations in peripheral inflammatory cytokine levels in substance use disorders: a meta-analysis of 74 studies

AIMS

To characterize the peripheral inflammatory cytokine profile in people with substance use disorders (SUDs).

DESIGN

Systematic review and meta-analysis.

SETTING

Clinical studies that evaluated peripheral blood inflammatory cytokine levels in patients with SUDs and healthy controls PARTICIPANTS: SUD patients and healthy controls.

MEASUREMENTS

PubMed and Web of Science were systematically searched for relevant studies. Two investigators independently selected studies and extracted data. A total of 77 articles were included in the meta-analysis, containing 5649 patients with SUDs and 4643 healthy controls. Data were pooled using a random-effects model by the Comprehensive Meta-Analysis version 2 software.

FINDINGS

Concentrations of interleukin (IL)-6) in 32 studies, tumor necrosis factor (TNF)-α in 28 studies, IL-10 in 20 studies, IL-8 in 17 studies, C-reactive protein in 14 studies, IL-4 in 10 studies, IL-12 in seven studies, monocyte chemoattractant protein (MCP)-1 in 6 studies, TNF-receptor 2 (TNF-R2) in four studies and granulocyte-macrophage colony-stimulating factor (GM-CSF) in three studies were significantly higher in patients with SUDs compared with healthy controls, while concentrations of leptin in 14 studies were significantly lower in patients with SUDs compared with healthy controls. The findings were inconclusive for the associations between interferon-γ, IL-1β, IL-2, IL-1 receptor antagonist (IL-1RA), transforming growth factor (TGF)-β1, G-CSF, C-C motif chemokine 11, TGF-α and SUDs.

CONCLUSIONS

People with substance use disorders (SUDs) appear to have higher peripheral concentrations of IL-4, IL-6, IL-8, IL-10, IL-12, TNF-α, C-reactive protein, MCP-1, TNF-R2 and GM-CSF and lower peripheral concentrations of leptin than people without SUDs. This strengthens the view that SUD is accompanied by an inflammatory response.

Anakinra Reduces Epileptogenesis, Provides Neuroprotection, and Attenuates Behavioral Impairments in Rats in the Lithium-Pilocarpine Model of Epilepsy

Temporal lobe epilepsy is a widespread chronic disorder that manifests as spontaneous seizures and is often characterized by refractoriness to drug treatment. Temporal lobe epilepsy can be caused by a primary brain injury; therefore, the prevention of epileptogenesis after a primary event is considered one of the best treatment options. However, a preventive treatment for epilepsy still does not exist. Neuroinflammation is directly involved in epileptogenesis and neurodegeneration, leading to the epileptic condition and cognitive decline. In the present study, we aimed to clarify the effect of treatment with a recombinant form of the Interleukin-1 receptor antagonist (anakinra) on epileptogenesis and behavioral impairments in rats using the lithium–pilocarpine model. We found that anakinra administration during the latent phase of the model significantly suppressed the duration and frequency of spontaneous recurrent seizures in the chronic phase. Moreover, anakinra administration prevented some behavioral impairments, including motor hyperactivity and disturbances in social interactions, during both the latent and chronic periods. Histological analysis revealed that anakinra administration decreased neuronal loss in the CA1 and CA3 areas of the hippocampus but did not prevent astro- and microgliosis. The treatment increased the expression level of the solute carrier family 1 member 2 gene (Slc1a2, encoding excitatory amino acid transporter 2 (EAAT2)) in the hippocampus, potentially leading to a neuroprotective effect. However, the increased gene expression of proinflammatory cytokine genes (Interleukin-1β (Il1b) and tumor necrosis factor α (Tnfa)) and astroglial marker genes (glial fibrillary acidic protein (Gfap) and inositol 1,4,5-trisphosphate receptor type 2 (Itpr2)) in experimental rats was not affected by anakinra treatment. Thus, our data demonstrate that the administration of anakinra during epileptogenesis has some beneficial disease-modifying effects.

The role of gut-immune-brain signaling in substance use disorders

Substance use disorders (SUDs) are debilitating neuropsychiatric conditions that exact enormous costs in terms of loss of life and individual suffering. While much progress has been made defining the neurocircuitry and intracellular signaling cascades that contribute to SUDs, these studies have yielded limited effective treatment options. This has prompted greater exploration of non-traditional targets in addiction. Emerging data suggest inputs from peripheral systems, such as the immune system and the gut microbiome, impact multiple neuropsychiatric diseases, including SUDs. Until recently the gut microbiome, peripheral immune system, and the CNS have been studied independently; however, current work shows the gut microbiome and immune system critically interact to modulate brain function. Additionally, the gut microbiome and immune system intimately regulate one another via extensive bidirectional communication. Accumulating evidence suggests an important role for gut-immune-brain communication in the pathogenesis of substance use disorders. Thus, a better understanding of gut-immune-brain signaling could yield important insight to addiction pathology and potential treatment options.

Sphingosine-1-phosphate receptor subtype 1 activation in the central nervous system contributes to morphine withdrawal in rodents

Opioid therapies for chronic pain are undermined by many adverse side effects that reduce their efficacy and lead to dependence, abuse, reduced quality of life, and even death. We have recently reported that sphingosine-1-phosphate (S1P) 1 receptor (S1PR1) antagonists block the development of morphine-induced hyperalgesia and analgesic tolerance. However, the impact of S1PR1 antagonists on other undesirable side effects of opioids, such as opioid-induced dependence, remains unknown. Here, we demonstrate that naloxone-precipitated morphine withdrawal in mice altered de novo sphingolipid metabolism in the dorsal horn of the spinal cord and increased S1P that accompanied the manifestation of several withdrawal behaviors. Blocking de novo sphingolipid metabolism with intrathecal administration of myriocin, an inhibitor of serine palmitoyltransferase, blocked naloxone-precipitated withdrawal. Noteworthy, we found that competitive (NIBR-15) and functional (FTY720) S1PR1 antagonists attenuated withdrawal behaviors in mice. Mechanistically, at the level of the spinal cord, naloxone-precipitated withdrawal was associated with increased glial activity and formation of the potent inflammatory/neuroexcitatory cytokine interleukin-1β (IL-1β); these events were attenuated by S1PR1 antagonists. These results provide the first molecular insight for the role of the S1P/S1PR1 axis during opioid withdrawal. Our data identify S1PR1 antagonists as potential therapeutics to mitigate opioid-induced dependence and support repurposing the S1PR1 functional antagonist FTY720, which is FDA-approved for multiple sclerosis, as an opioid adjunct.

Fundamental sex differences in morphine withdrawal-induced neuronal plasticity

ABSTRACT

Withdrawal from systemic opioids can induce long-term potentiation (LTP) at spinal C-fibre synapses ("opioid-withdrawal-LTP"). This is considered to be a cellular mechanism underlying opioid withdrawal-induced hyperalgesia, which is a major symptom of the opioid withdrawal syndrome. Opioids can activate glial cells leading to the release of proinflammatory mediators. These may influence synaptic plasticity and could thus contribute to opioid-withdrawal-LTP. Here, we report a sexual dimorphism in the mechanisms of morphine-withdrawal-LTP in adult rats. We recorded C-fibre-evoked field potentials in the spinal cord dorsal horn from deeply anaesthetised male and female rats. In both sexes, we induced a robust LTP through withdrawal from systemic morphine infusion (8 mg·kg-1 bolus, followed by a 1-hour infusion at a rate of 14 mg·kg-1·h-1). This paradigm also induced mechanical hypersensitivity of similar magnitude in both sexes. In male rats, systemic but not spinal application of (-)naloxone blocked the induction of morphine-withdrawal-LTP, suggesting the involvement of descending pronociceptive pathways. Furthermore, we showed that in male rats, the induction of morphine-withdrawal-LTP required the activation of spinal astrocytes and the release of the proinflammatory cytokines tumour necrosis factor and interleukin-1. In striking contrast, in female rats, the induction of morphine-withdrawal-LTP was independent of spinal glial cells. Instead, blocking µ-opioid receptors in the spinal cord was sufficient to prevent a facilitation of synaptic strength. Our study revealed fundamental sex differences in the mechanisms underlying morphine-withdrawal-LTP at C-fibre synapses: supraspinal and gliogenic mechanisms in males and a spinal, glial cell-independent mechanism in females.

Withdrawal from an opioid induces a transferable memory trace in the cerebrospinal fluid

Opioids are the most powerful analgesics available to date. However, they may also induce adverse effects including paradoxical opioid-induced hyperalgesia. A mechanism that might underlie opioid-induced hyperalgesia is the amplification of synaptic strength at spinal C-fibre synapses after withdrawal from systemic opioids such as remifentanil ("opioid-withdrawal long-term potentiation [LTP]"). Here, we show that both the induction as well as the maintenance of opioid-withdrawal LTP were abolished by pharmacological blockade of spinal glial cells. By contrast, the blockade of TLR4 had no effect on the induction of opioid-withdrawal LTP. D-serine, which may be released upon glial cell activation, was necessary for withdrawal LTP. D-serine is the dominant coagonist for neuronal NMDA receptors, which are required for the amplification of synaptic strength on remifentanil withdrawal. Unexpectedly, opioid-withdrawal LTP was transferable through the cerebrospinal fluid between animals. This suggests that glial-cell-derived mediators accumulate in the extracellular space and reach the cerebrospinal fluid at biologically active concentrations, thereby creating a soluble memory trace that is transferable to another animal ("transfer LTP"). When we enzymatically degraded D-serine in the superfusate, LTP could no longer be transferred. Transfer LTP was insensitive to pharmacological blockade of glial cells in the recipient animal, thus representing a rare form of glial cell-independent LTP in the spinal cord.

The Attenuating Effect of Curcumin on Morphine Dependence in Rats: The Involvement of Spinal Microglial Cells and Inflammatory Cytokines

New evidence suggests an important role for spinal glial cells in the development of opioid dependence. Curcumin, a component of the Curcuma Longa, has shown to act as a suppressor of microglial cells. The main goal of this study was to explore the attenuating effects of curcumin on morphine dependence with a focus on spinal microglial cells and inflammatory cytokines. In order to induce morphine dependence in male Wistar rats, morphine was administered intraperitoneally (i.p.) once daily for 9 days in an increasing dose of 10, 20, and 40 mg/kg. Curcumin (2.5, 5, and 10 mg/kg, i.p.) was given from the days 10th to 18th. Naloxone-precipitated abstinence syndrome was used to assess the behavioral symptoms of morphine dependence. Immunofluorescence staining of Iba1 and ELISA test were used to measure spinal microglial activity and inflammatory cytokines levels, respectively.

The results showed that curcumin (2.5, 5, and 10 mg/kg) significantly decreased jumping, leaning, and diarrhea in morphine-dependent rats. In addition, the spinal concentration of TNF-α and IL-6 was reduced by curcumin (2.5, 5, and 10 mg/kg) significantly. Moreover, curcumin showed a potent attenuating effect on the number of Iba1 positive cells in rats which were subjected to morphine dependence. The results of this study demonstrated that curcumin exerts a remarkable reducing effect on morphine dependence in rats. The findings showed that the therapeutic effect of curcumin on morphine dependence is mediated through the suppression of activated microglial cells and reduction of inflammatory cytokines levels in the spinal cord.

CCR4 Antagonist (C021) Administration Diminishes Hypersensitivity and Enhances the Analgesic Potency of Morphine and Buprenorphine in a Mouse Model of Neuropathic Pain

Neuropathic pain is a chronic condition that remains a major clinical problem owing to high resistance to available therapy. Recent studies have indicated that chemokine signaling pathways are crucial in the development of painful neuropathy; however, the involvement of CC chemokine receptor 4 (CCR4) has not been fully elucidated thus far. Therefore, the aim of our research was to investigate the role of CCR4 in the development of tactile and thermal hypersensitivity, the effectiveness of morphine/buprenorphine, and opioid-induced tolerance in mice exposed to chronic constriction injury (CCI) of the sciatic nerve. The results of our research demonstrated that a single intrathecal or intraperitoneal administration of C021, a CCR4 antagonist, dose dependently diminished neuropathic pain-related behaviors in CCI-exposed mice. After sciatic nerve injury, the spinal expression of CCL17 and CCL22 remained unchanged in contrast to that of CCL2, which was significantly upregulated until day 14 after CCI. Importantly, our results provide evidence that in naive mice, CCL2 may evoke pain-related behaviors through CCR4 because its pronociceptive effects are diminished by C021. In CCI-exposed mice, the pharmacological blockade of CCR4 enhanced the analgesic properties of morphine/buprenorphine and delayed the development of morphine-induced tolerance, which was associated with the silencing of IBA-1 activation in cells and decrease in CCL2 production. The obtained data suggest that the pharmacological blockade of CCR4 may be a new potential therapeutic target for neuropathic pain polytherapy.

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.

Improved efficacy, tolerance, safety, and abuse liability profile of the combination of CR4056 and morphine over morphine alone in rodent models

BACKGROUND AND PURPOSE

Prolonged use of opioids causes analgesic tolerance and adverse effects including constipation and dependence. Compounds targeting imidazoline I2 receptors are known to potentiate opioid analgesia in rodents. We investigated whether combination with the I2 receptor ligand CR4056 could improve efficacy and safety of morphine and explored the mechanisms of the CR4056-opioid interaction.

EXPERIMENTAL APPROACH

We used the complete Freund's adjuvant (CFA) model in rats to study the effects of treatments on hyperalgesia, morphine tolerance and microglia activation as measured by immunofluorescence. Opioid-induced adverse effects were assessed in rodent models of morphine-induced constipation, sedation (open field, sedation rating scale, and rotarod), physical dependence (naloxone-induced withdrawal), and abuse (conditioned place preference-associated reward). Chemiluminescence assays tested CR4056 as allosteric modulator of μ-opioid receptors.

KEY RESULTS

CR4056 (ED50 = 4.88 mg·kg-1 ) and morphine (ED50 = 2.07 mg·kg-1 ) synergized in reducing CFA-induced hyperalgesia (ED50 = 0.52 mg·kg-1 ; 1:1 combination). Consistently, low doses of CR4056 (1 mg·kg-1 ) spared one third of the cumulative morphine dose administered during 4 days and prevented/reversed the development of tolerance to morphine anti-hyperalgesia. These opioid-sparing effects were associated with decreased activation of microglia, independent of CR4056 interactions on μ-opioid receptors. Importantly, the low doses of CR4056 and morphine that synergize in analgesia did not induce constipation, sedation, physical dependence, or place preference.

CONCLUSION AND IMPLICATIONS

We showed selective synergism between CR4056 and morphine as analgesics. Their combination showed an improved safety and abuse liability profile over morphine alone. CR4056 could be developed as an opioid-sparing drug in multimodal analgesia.

Pellino1 Contributes to Morphine Tolerance by Microglia Activation via MAPK Signaling in the Spinal Cord of Mice

Chronic morphine-induced antinociceptive tolerance is a major unresolved issue in clinical practices, which is associated with microglia activation in the spinal cord. E3 ubiquitin ligase Pellino1 (Peli1) is known to be an important microglia-specific regulator. However, it is unclear whether Peli1 is involved in morphine tolerance. Here, we found that Peli1 levels in the spinal cord were significantly elevated in morphine tolerance mouse model. Notably, Peli1 was expressed in a great majority of microglia in the spinal dorsal horn, while downregulation of spinal Peli1 attenuated the development of morphine tolerance and associated hyperalgesia. Our biochemical data revealed that morphine tolerance-induced increase in Peli1 was accompanied by spinal microglia activation, activation of mitogen-activated protein kinase (MAPK) signaling, and production of proinflammatory cytokines. Peli1 additionally was found to promote K63-linked ubiquitination of tumor necrosis factor receptor-associated factor 6 (TRAF6) in the spinal cord after repeated morphine treatment. Furthermore, knocking down Peli1 in cultured BV2 microglial cells significantly attenuated inflammatory reactions in response to morphine challenge. Therefore, we conclude that the upregulation of Peli1 in the spinal cord plays a curial role in the development of morphine tolerance via Peli1-dependent mobilization of spinal microglia, activation of MAPK signaling, and production of proinflammatory cytokines. Modulation of Peli1 may be a potential strategy for the prevention of morphine tolerance.

Mitragynine Attenuates Morphine Withdrawal Effects in Rats-A Comparison With Methadone and Buprenorphine

BACKGROUND

Opiate addiction is a major health problem in many countries. A crucial component of the medical treatment is the management of highly aversive opiate withdrawal signs, which may otherwise lead to resumption of drug taking. In a medication-assisted treatment (MAT), methadone and buprenorphine have been implemented as substitution drugs. Despite MAT effectiveness, there are still limitations and side effects of using methadone and buprenorphine. Thus, other alternative therapies with less side effects, overdosing, and co-morbidities are desired. One of the potential pharmacotherapies may involve kratom's major indole alkaloid, mitragynine, since kratom (Mitragyna speciosa Korth.) preparations have been reported to alleviate opiate withdrawal signs in self-treatment in Malaysian opiate addicts.

METHODS

Based on the morphine withdrawal model, rats were morphine treated with increasing doses from 10 to 50 mg/kg twice daily over a period of 6 days. The treatment was discontinued on day 7 in order to induce a spontaneous morphine abstinence. The withdrawal signs were measured daily after 24 h of the last morphine administration over a period of 28 abstinence days. In rats that developed withdrawal signs, a drug replacement treatment was given using mitragynine, methadone, or buprenorphine and the global withdrawal score was evaluated.

RESULTS

The morphine withdrawal model induced profound withdrawal signs for 16 days. Mitragynine (5-30 mg/kg; i.p.) was able to attenuate acute withdrawal signs in morphine dependent rats. On the other hand, smaller doses of methadone (0.5-2 mg/kg; i.p.) and buprenorphine (0.4-1.6 mg/kg; i.p.) were necessary to mitigate these effects.

CONCLUSIONS

These data suggest that mitragynine may be a potential drug candidate for opiate withdrawal treatment.

Experimental Colitis Enhances the Rate of Antinociceptive Tolerance to Morphine via Peripheral Opioid Receptors

Opioids are highly effective analgesics, however, their therapeutic use is limited by adverse effects that include respiratory depression, dependence, and tolerance. Inflammation has been implicated as a significant driver for the development of tolerance to opioids. Recent studies show that chronic morphine in mice results in gut microbial dysbiosis and inflammation in the colon. In the present study, we examined whether colonic inflammation results in tolerance to the antinociceptive effects of morphine. Colonic inflammation was induced in mice by intrarectal administration of 2,4,6-trinitro-benzene sulfonic acid. The development of antinociceptive tolerance was determined by warm-water tail-immersion assay in mice implanted with 25-, 50-, or 75-mg morphine pellet. Colonic inflammation significantly enhanced the rate at which tolerance developed in each cohort of chronic morphine-treated mice. At the lowest dose of morphine pellet (25 mg), antinociceptive tolerance only developed in the presence of colonic inflammation, whereas in 50- and 75-mg pelleted mice, tolerance developed faster in the inflamed animals than in the noninflamed mice. The enhanced antinociceptive tolerance was attenuated with daily administration of peripheral opioid receptor antagonist, 6β-N-heterocyclic-substituted naltrexamine derivative [17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(4'pyridyl)acetamido]morphinan (NAP)], irrespective of colonic inflammation. Collectively, these findings show that the rate of tolerance to morphine antinociception is exaggerated in the presence of colonic inflammation, and tolerance is prevented by a peripheral μ-opioid receptor antagonist. These studies suggest a peripheral component to the development of antinociceptive tolerance to opioids. Furthermore, peripherally selective opioid antagonists may be useful adjuncts in opioid-based pain management. SIGNIFICANCE STATEMENT: This study supports the notion that inflammation influences the development of antinociceptive tolerance to chronic morphine exposure. We found that, in the presence of colonic inflammation, the rate of development of tolerance to the antinociceptive effects of morphine increased. We also found that treatment with a peripheral opioid receptor antagonist prevented morphine antinociceptive tolerance. Increasing opioid intake during an inflammatory state would result in decreased analgesia and enhanced analgesic tolerance, which puts patients with inflammatory bowel diseases, inflammatory joint diseases, and sickle cell anemia at risk for heavy opioid use.

Neuroimmune mechanisms of psychostimulant and opioid use disorders

Substance use disorders are global health problems with few effective treatment options. Unfortunately, most potential pharmacological treatments are hindered by abuse potential of their own, limited efficacy, or adverse side effects. As a consequence, there is a pressing need for the development of addiction treatments with limited abuse potential and fewer off target effects. Given the difficulties in developing new pharmacotherapies for substance use disorders, there has been growing interest in medications that act on non-traditional targets. Recent evidence suggests a role for dysregulated immune signaling in the pathophysiology of multiple psychiatric diseases. While there is evidence that immune responses in the periphery and the central nervous system are altered by exposure to drugs of abuse, the contributions of neuroimmune interactions to addictive behaviors are just beginning to be appreciated. In this review, we discuss the data on immunological changes seen in clinical populations with substance use disorders, as well as in translational animal models of addiction. Importantly, we highlight those mechanistic findings showing causal roles for central or peripheral immune mediators in substance use disorder and appropriate animal models. Based on the literature reviewed here, it is clear that brain-immune system interactions in substance use disorders are much more complex and important than previously understood. While much work remains to be done, there are tremendous potential therapeutic implications for immunomodulatory treatments in substance use disorders.

Morphine immunomodulation prolongs inflammatory and postoperative pain while the novel analgesic ZH853 accelerates recovery and protects against latent sensitization

Background

Numerous studies have identified the proinflammatory, pronociceptive effects of morphine which ultimately exacerbate pain. Our novel endomorphin analog ZH853 does not produce proinflammatory effects on its own and gives potent, long-lasting analgesia. This study investigates whether ZH853’s lack of interaction with the neuroimmune system reduces the risk of prolonged pain.

Methods

Adult male Sprague-Dawley rats were subjected to one of two treatment paradigms. Either (1) chronic pain followed by chronic treatment with morphine, ZH853 or vehicle, or (2) chronic drug administered prior to pain induction. Complete Freund’s adjuvant (CFA) was injected or paw incision surgery was performed on the left hind plantar foot pad. Drugs were administered through Alzet osmotic minipumps at a rate of 1 μl/h for 5 days at appropriate doses based on prior experiments. Animals were tested for mechanical allodynia and thermal hyperalgesia using von Frey filaments and the Hargreaves apparatus, respectively. Additionally, several gait parameters were measured using the CatWalk XT. When all animals had recovered from pain, 1 mg/kg of naltrexone was administered to test for development of latent sensitization (LS). A second set of animals was used to investigate dorsal horn inflammation following CFA and drug treatment. ANOVAs were used to assess differences between drug treatment groups.

Results

As expected, morphine increased and prolonged pain in all experiments compared to vehicle treatment. However, ZH853 treatment reduced the overall time spent in pain and the severity of pain scores compared to morphine. ZH853 not only reduced inflammation versus morphine treatment but also, in some instances, acted as an anti-inflammatory drug compared to vehicle treatment. Finally, ZH853 prevented the development of LS while vehicle- and morphine-treated animals showed robust relapse to pain.

Conclusions

ZH853 has a favorable side effect profile versus morphine and provides superior analgesia in a number of pain states. We now know that chronic use of this compound reduces time spent in a chronic pain state, the opposite of common opioids like morphine, and reduces the risk of LS, making ZH853 an excellent candidate for clinical development in humans for inflammatory and postoperative pain.

Inflammatory mediators of opioid tolerance: Implications for dependency and addiction

Each year, over 50 million Americans suffer from persistent pain, including debilitating headaches, joint pain, and severe back pain. Although morphine is amongst the most effective analgesics available for the management of severe pain, prolonged morphine treatment results in decreased analgesic efficacy (i.e., tolerance). Despite significant headway in the field, the mechanisms underlying the development of morphine tolerance are not well understood. The midbrain ventrolateral periaqueductal gray (vlPAG) is a primary neural substrate for the analgesic effects of morphine, as well as for the development of morphine tolerance. A growing body of literature indicates that activated glia (i.e., microglia and astrocytes) facilitate pain transmission and oppose morphine analgesia, making these cells important potential targets in the treatment of chronic pain. Morphine affects glia by binding to the innate immune receptor toll-like receptor 4 (TLR4), leading to the release of proinflammatory cytokines and opposition of morphine analgesia. Despite the established role of the vlPAG as an integral locus for the development of morphine tolerance, most studies have examined the role of glia activation within the spinal cord. Additionally, the role of TLR4 in the development of tolerance has not been elucidated. This review attempts to summarize what is known regarding the role of vlPAG glia and TLR4 in the development of morphine tolerance. These data, together, provide information about the mechanism by which central nervous system glia regulate morphine tolerance, and identify a potential therapeutic target for the enhancement of analgesic efficacy in the clinical treatment of chronic pain.