Attenuated Morphine Tolerance Research Articles

Blockade of neuronal dopamine D2 receptor attenuates morphine tolerance in mice spinal cord.

Tolerance induced by morphine remains a major unresolved problem and significantly limits its clinical use. Recent evidences have indicated that dopamine D2 receptor (D2DR) is likely to be involved in morphine-induced antinociceptive tolerance. However, its exact effect and molecular mechanism remain unknown. In this study we examined the effect of D2DR on morphine antinociceptive tolerance in mice spinal cord. Chronic morphine treatment significantly increased levels of D2DR in mice spinal dorsal horn. And the immunoreactivity of D2DR was newly expressed in neurons rather than astrocytes or microglia both in vivo and in vitro. Blockade of D2DR with its antagonist (sulpiride and L-741,626, i.t.) attenuated morphine antinociceptive tolerance without affecting basal pain perception. Sulpiride (i.t.) also down-regulated the expression of phosphorylation of NR1, PKC, MAPKs and suppressed the activation of astrocytes and microglia induced by chronic morphine administration. Particularly, D2DR was found to interact with μ opioid receptor (MOR) in neurons, and chronic morphine treatment enhanced the MOR/D2DR interactions. Sulpiride (i.t.) could disrupt the MOR/D2DR interactions and attenuate morphine tolerance, indicating that neuronal D2DR in the spinal cord may be involved in morphine tolerance possibly by interacting with MOR. These results may present new opportunities for the treatment and management of morphine-induced antinociceptive tolerance which often observed in clinic.

Opening of the Adenosine Triphosphate-sensitive Potassium Channel Attenuates Morphine Tolerance by Inhibiting JNK and Astrocyte Activation in the Spinal Cord.

In the present study, we investigated the role of adenosine triphosphate (ATP)-sensitive potassium (KATP) channels in chronic morphine tolerance. Male mice were injected intrathecally with morphine or saline, respectively (each in 10 μL). Different doses of the KATP opener cromakalim (0.3, 1, or 3 μg/10 μL/mouse) were administered 15 minutes before the morphine (10 μg/10 μL/mouse) challenge daily for 7 consecutive days. Half an hour after morphine injection, the tail-flick latency was measured to evaluate the antinociceptive effect of morphine. On the seventh day, mice were euthanized with sodium pentobarbital (100 mg/kg) at 1 hour after morphine injection, and their spinal cords were removed for the assays of Western blot, immunofluorescence, and quantitative real-time polymerase chain reaction. Opening of the KATP channel attenuates chronic morphine tolerance, suppresses astrocyte activation inhibits the increase in interleukin-1β at the transcriptional and the translational levels, and reduces the upregulation of phosphorylated c-Jun N-terminal kinase mitogen-activated protein kinase in the spinal cord after chronic morphine treatment. Moreover, transcriptional levels of spinal cord astrocyte KATP channel subunits, named the inwardly rectifying potassium (Kir) 6.1 and sulfonylurea receptor 1, are decreased in morphine-tolerant mice. Cromakalim suppresses morphine-induced astrocyte activation significantly by suppressing the c-Jun N-terminal kinase pathway, resulting in a reduced release of interleukin-1β and the attenuation of morphine chronic antinociceptive tolerance.

Low-Dose Cannabinoid Type 2 Receptor Agonist Attenuates Tolerance to Repeated Morphine Administration via Regulating μ-Opioid Receptor Expression in Walker 256 Tumor-Bearing Rats

Morphine is widely used in patients with moderate and severe cancer pain, whereas the development of drug tolerance remains a major problem associated with opioid use. Previous studies have shown that cannabinoid type 2 (CB2) receptor agonists induce morphine analgesia, attenuate morphine tolerance in normal and neuropathic pain animals, induce transcription of the μ-opioid receptor (MOR) gene in Jurkat T cells, and increase morphine analgesia in cancer pain animals. However, no studies of the effects of CB2 receptor agonists on morphine tolerance in cancer pain have been performed. Therefore, we investigated the effect of repeated intrathecal (IT) injection of the low-dose CB2 receptor agonist AM1241 on the development of morphine tolerance in walker 256 tumor-bearing rats. We also tested the influence of the CB2 receptor agonist AM1241 on MOR protein and messenger ribonucleic acid (mRNA) expression in the rat spinal cord and dorsal root ganglia (DRG). Walker 256 cells were implanted into the plantar region of each rat's right hindpaw. Tumor-bearing rats received IT injection of the CB2 receptor agonist AM1241 or antagonist AM630 with or without morphine subcutaneously twice daily for 8 days. Rats receiving drug vehicle only served as the control group. Mechanical paw withdrawal threshold and thermal paw withdrawal latency were assessed by a von Frey test and hot plate test 30 minutes after drug administration every day. MOR protein and mRNA expression in the spinal cord and DRG were detected after the last day (day 8) of drug administration via Western blot and real-time reverse transcription polymerase chain reaction. The data were analyzed via analysis of variance followed by Student t test with Bonferroni correction for multiple comparisons. Repeated morphine treatments reduced the mechanical withdrawal threshold and thermal latency. Coadministration of a nonanalgetic dose of the CB2 receptor agonist AM1241 with morphine significantly inhibited the development of morphine tolerance and increased the MOR protein expression in the spinal cord and DRG and mRNA expression in the spinal cord in tumor-bearing rats. Our findings indicate that IT injection of a nonanalgetic dose of a CB2 receptor agonist increased the analgesia effect and alleviated tolerance to morphine in tumor-bearing rats, potentially by regulating MOR expression in the spinal cord and DRG. This receptor may be a new target for prevention of the development of opioid tolerance in cancer pain.

The effect of Gly–Gln [ß-endorphin30–31] on morphine-evoked serotonin and GABA efflux in the nucleus accumbens of conscious rats

Glycyl-L-glutamine (Gly–Gln; β-endorphin30–31) is an endogenous dipeptide synthesized through the post-translational processing of β-endorphin1–31. Central Gly–Gln administration inhibits the rewarding properties of morphine and attenuates morphine tolerance, dependence and withdrawal although it does not interfere with morphine analgesia. In an earlier study, we found that Gly–Gln inhibits morphine-induced dopamine efflux in the nucleus accumbens (NAc), consistent with its ability to inhibit morphine reward. To further investigate the mechanism responsible for its central effects we tested whether i.c.v. Gly–Gln administration influences the rise in extracellular serotonin and GABA concentrations evoked by morphine in the NAc. Conscious rats were treated with Gly–Gln (100nmol/5μl) or saline i.c.v. followed, 2min later, by morphine (2.5mg/kg) or saline i.p. and extracellular serotonin and GABA concentrations were analyzed by microdialysis and HPLC. Morphine administration increased extracellular serotonin and GABA concentrations significantly within 20min, as shown previously. Unexpectedly, Gly–Gln also increased extracellular serotonin concentrations significantly in control animals. Combined treatment with Gly–Gln+morphine also elevated extracellular serotonin concentrations although the magnitude of the response did not differ significantly from the effect of Gly–Gln or morphine, given alone suggesting that Gly–Gln suppressed morphine induced serotonin efflux. Gly–Gln abolished the morphine-induced rise in extracellular GABA concentrations but had no effect on extracellular GABA when given alone to otherwise untreated animals. These data show that Gly–Gln stimulates NAc serotonin efflux and, together with earlier studies, support the hypothesis that Gly–Gln inhibits the rewarding effects of morphine by modulating morphine induced dopamine, GABA and serotonin efflux in the NAc.

Attenuation of morphine tolerance and dependence by thymoquinone in mice.

Dependence and tolerance are major restricting factors in the clinical use of opioid analgesics. In the present study, the effects of thymoquinone, the major constituent of Nigella sativa seeds, on morphine dependence and tolerance were investigated in mice. Male adult NMRI mice were made tolerant and dependent by repeated injections of morphine (50, 50, and 75 mg/kg, i.p. on 9 a.m., 1 p.m., and 5 p.m., respectively) during a 3-day administration schedule. The hot-plate test was used to assess tolerance to the analgesic effects of morphine. Naloxone (2 mg/kg, i.p.) was injected to precipitate withdrawal syndrome in order to assess the morphine dependence. To evaluate the effects of thymoquinone on tolerance and dependence to morphine, different single or repeated doses of thymoquinone were administered in mice. Rotarod was used to assess the motor coordination. Administration of single or repeated doses of thymoquinone (20 and 40 mg/kg, i.p.) significantly decreased the number of jumps in morphine dependent animals. Repeated administration of thymoquinone (20 and 40 mg/kg, for 3 days) and also single injection of thymoquinone (40 mg/kg, on the fourth day) attenuated tolerance to the analgesic effect of morphine. None of the thymoquinone doses (10, 20, and 40 mg/kg) produced any antinociceptive effects on their own. Motor coordination of animals was impaired by the high dose of thymoquinone (40 mg/kg). Based on these results, it can be concluded that thymoquinone prevents the development of tolerance and dependence to morphine.

Downregulation of miR-219 enhances brain-derived neurotrophic factor production in mouse dorsal root ganglia to mediate morphine analgesic tolerance by upregulating CaMKIIγ.

BackgroundIncreasing evidence suggests that microRNAs are functionally involved in the initiation and maintenance of pain hypersensitivity, including chronic morphine analgesic tolerance, through the posttranscriptional regulation of pain-related genes. We have previously demonstrated that miR-219 regulates inflammatory pain in the spinal cord by targeting calcium/calmodulin-dependent protein kinase II gamma (CaMKIIγ). However, whether miR-219 regulates CaMKIIγ expression in the dorsal root ganglia to mediate morphine tolerance remains unclear.ResultsMiR-219 expression was downregulated and CaMKIIγ expression was upregulated in mouse dorsal root ganglia following chronic morphine treatment. The changes in miR-219 and CaMKIIγ expression closely correlated with the development of morphine tolerance, which was measured using the reduction of percentage of maximum potential efficiency to thermal stimuli. Morphine tolerance was markedly delayed by upregulating miR-219 expression using miR-219 mimics or downregulating CaMKIIγ expression using CaMKIIγ small interfering RNA. The protein and mRNA expression of brain-derived neurotrophic factor were also induced in dorsal root ganglia by prolonged morphine exposure in a time-dependent manner, which were transcriptionally regulated by miR-219 and CaMKIIγ. Scavenging brain-derived neurotrophic factor via tyrosine receptor kinase B-Fc partially attenuated morphine tolerance. Moreover, functional inhibition of miR-219 via miR-219-sponge in naive mice elicited thermal hyperalgesia and spinal neuronal sensitization, which were both suppressed by CaMKIIγ small interfering RNA or tyrosine receptor kinase B-Fc.ConclusionsThese results demonstrate that miR-219 contributes to the development of chronic tolerance to morphine analgesia in mouse dorsal root ganglia by targeting CaMKIIγ and enhancing CaMKIIγ-dependent brain-derived neurotrophic factor expression.

Altered nociception and morphine tolerance in neuropeptide FF receptor type 2 over-expressing mice.

The neuropeptide FF system is thought to act as an anti-opioid modulator and plays a role in nociception, morphine antinociception and dependence. Two receptor subtypes, NPFFR1 and NPFFR2, have been identified, but their respective roles in these processes remain uncertain. In the present study, the role of NPFFR2 was investigated using transgenic mice over-expressing NPFFR2 in addition to a NPFFR2 agonist AC-263093. NPFFR2 Tg mice exhibited increased sensitivity to both mechanical and thermal noxious stimuli compared to the WT mice, while the antinociceptive effects of morphine at three different doses (6.25, 12.5 and 25mg/kg, s.c.) were similar in both strains. The development of tolerance to morphine antinociception after chronic morphine treatment (12.5mg/kg, s.c.; twice daily×5days) was attenuated in NPFFR2 Tg mice when compared to WT mice. Similarly, WT mice receiving AC-263093 pretreatment (2.5mg/kg, i.p.) showed attenuated morphine tolerance compared to vehicle controls. Most naloxone-precipitated morphine withdrawal symptoms were not attenuated in NPFFR2 Tg mice, with the exception of wet dog shake that was significantly reduced. Both NPFFR2 Tg and WT mice displayed similar degree of morphine rewarding. Our results suggest that neuropeptide FF R2 is mainly involved in the modulation of nociception and tolerance to morphine antinociception.

Predicting Neuroinflammation in Morphine Tolerance for Tolerance Therapy from Immunostaining Images of Rat Spinal Cord.

Long-term morphine treatment leads to tolerance which attenuates analgesic effect and hampers clinical utilization. Recent studies have sought to reveal the mechanism of opioid receptors and neuroinflammation by observing morphological changes of cells in the rat spinal cord. This work proposes a high-content screening (HCS) based computational method, HCS-Morph, for predicting neuroinflammation in morphine tolerance to facilitate the development of tolerance therapy using immunostaining images for astrocytes, microglia, and neurons in the spinal cord. HCS-Morph first extracts numerous HCS-based features of cellular phenotypes. Next, an inheritable bi-objective genetic algorithm is used to identify a minimal set of features by maximizing the prediction accuracy of neuroinflammation. Finally, a mathematic model using a support vector machine with the identified features is established to predict drug-treated images to assess the effects of tolerance therapy. The dataset consists of 15 saline controls (1 μl/h), 15 morphine-tolerant rats (15 μg/h), and 10 rats receiving a co-infusion of morphine (15 μg/h) and gabapentin (15 μg/h, Sigma). The three individual models of astrocytes, microglia, and neurons for predicting neuroinflammation yielded respective Jackknife test accuracies of 96.67%, 90.00%, and 86.67% on the 30 rats, and respective independent test accuracies of 100%, 90%, and 60% on the 10 co-infused rats. The experimental results suggest that neuroinflammation activity expresses more predominantly in astrocytes and microglia than in neuron cells. The set of features for predicting neuroinflammation from images of astrocytes comprises mean cell intensity, total cell area, and second-order geometric moment (relating to cell distribution), relevant to cell communication, cell extension, and cell migration, respectively. The present investigation provides the first evidence for the role of gabapentin in the attenuation of morphine tolerance from phenotypic changes of astrocytes and microglia. Based on neuroinflammation prediction, the proposed computer-aided image diagnosis system can greatly facilitate the development of tolerance therapy with anti-inflammatory drugs.

Role of neurokinin-1 receptor antagonist in attenuating morphine tolerance in rats

Role of neurokinin-1 receptor antagonist in attenuating morphine tolerance in rats

Early single Aspirin-triggered Lipoxin blocked morphine anti-nociception tolerance through inhibiting NALP1 inflammasome: Involvement of PI3k/Akt signaling pathway.

Clinical usage of opioids in pain relief is dampened by analgesic tolerance after chronic exposure, which is related to opioid-associated neuroinflammation. In the current study, which is based on a chronic morphine tolerance rat model and sustained morphine treatment on primary neuron culture, it was observed that Akt phosphorylation, cleaved-Caspase-1-dependent NALP1 inflammasome activation and IL-1β maturation in spinal cord neurons were significantly enhanced by morphine. Moreover, treatment with LY294002, a specific inhibitor of PI3k/Akt signaling, significantly reduced Caspase-1 cleavage, NALP1 inflammasome activation and attenuated morphine tolerance. Tail-flick tests demonstrated that pharmacological inhibition on Caspase-1 activation or antagonizing IL-1β dramatically blocked the development of morphine tolerance. The administration of an exogenous analogue of lipoxin, Aspirin-triggered Lipoxin (ATL), caused a decline in Caspase-1 cleavage, inflammasome activation and mature IL-1β production and thus attenuated the development of morphine tolerance by inhibiting upstream Akt phosphorylation. Additionally, treatment with DAMGO, a selective μ-opioid receptor peptide, significantly induced Akt phosphorylation, Caspase-1 cleavage and anti-nociception tolerance, all of which were attenuated by ATL treatment. Taken together, the present study revealed the involvement of spinal NALP1 inflammasome activation in the development of morphine tolerance and the role of the μ-receptor/PI3k-Akt signaling/NALP1 inflammasome cascade in this process. By inhibiting this signaling cascade, ATL blocked the development of morphine tolerance.

Ketamine coadministration attenuates morphine tolerance and leads to increased brain concentrations of both drugs in the rat.

Background and PurposeThe effects of ketamine in attenuating morphine tolerance have been suggested to result from a pharmacodynamic interaction. We studied whether ketamine might increase brain morphine concentrations in acute coadministration, in morphine tolerance and morphine withdrawal.Experimental ApproachMorphine minipumps (6 mg·day–1) induced tolerance during 5 days in Sprague–Dawley rats, after which s.c. ketamine (10 mg·kg–1) was administered. Tail flick, hot plate and rotarod tests were used for behavioural testing. Serum levels and whole tissue brain and liver concentrations of morphine, morphine-3-glucuronide, ketamine and norketamine were measured using HPLC-tandem mass spectrometry.Key ResultsIn morphine-naïve rats, ketamine caused no antinociception whereas in morphine-tolerant rats there was significant antinociception (57% maximum possible effect in the tail flick test 90 min after administration) lasting up to 150 min. In the brain of morphine-tolerant ketamine-treated rats, the morphine, ketamine and norketamine concentrations were 2.1-, 1.4- and 3.4-fold, respectively, compared with the rats treated with morphine or ketamine only. In the liver of morphine-tolerant ketamine-treated rats, ketamine concentration was sixfold compared with morphine-naïve rats. After a 2 day morphine withdrawal period, smaller but parallel concentration changes were observed. In acute coadministration, ketamine increased the brain morphine concentration by 20%, but no increase in ketamine concentrations or increased antinociception was observed.Conclusions and ImplicationsThe ability of ketamine to induce antinociception in rats made tolerant to morphine may also be due to increased brain concentrations of morphine, ketamine and norketamine. The relevance of these findings needs to be assessed in humans.

Con-T[M8Q] potently attenuates the expression and development of morphine tolerance in mice

As a variant of peptide conantokin-T (con-T), con-T[M8Q] is derived from the venom of Conus tulipa. Our previous study has demonstrated that con-T[M8Q] selectively targets N-methyl-d-aspartate receptor (NMDAR) NR2B subunit. In the present study, we determined the effects of con-T[M8Q] on the expression and development of morphine tolerance using hot plate test and acetic acid writhing test. Our results demonstrated that con-T[M8Q] could efficiently attenuate the expression and development of morphine analgesic tolerance in mice at low doses (5–20nmol/kg), and it exhibited more potent effects compared with ifenprodil, a typical small-molecule antagonist of NMDAR. In addition, low doses of con-T[M8Q] (5–20nmol/kg) did not cause drug resistance and apparent analgesic activity compared with morphine. Taken together, con-T[M8Q] could be a promising new candidate in attenuating morphine tolerance.

Reversal of morphine tolerance by a compound with NPFF receptor subtype-selective actions

Neuropeptide FF (NPFF) modulates opiate actions. It has pro-nociceptive effects, primarily through the NPFF receptor 1 subtype, and anti-nociceptive effects, primarily through the NPFFR2 subtype. AC-263093 is a small l, organic, systemically active molecule that was previously shown to functionally activate NPFFR2, but not NPFFR1. It was hypothesized that AC-263093 would attenuate morphine tolerance. Rats were tested for radiant heat tail-flick latency before and after 5mg/kg morphine sulfate s.c. They were then rendered morphine-tolerant by continuous subcutaneous infusion of 17.52mg/kg/day morphine sulfate. On the seventh day of infusion, they were retested for analgesia 10 and 20min after 5mg/kg morphine sulfate s.c. Tolerance was indicated by reduction of morphine analgesia from the pre-infusion test. Fifty minutes prior to morphine challenge, rats received either 10mg/kg i.p. AC-263093 or injection vehicle alone. AC-2623093-treated rats had far smaller tolerance scores than control rats. This drug effect was significant, p=0.015. The same dose of AC-263093 had almost no analgesic effect in non-tolerant, saline-infused rats. In vitro experiments revealed that AC-263093 had equal affinity for NPFFR1 and NPFFR2, and functionally inactivated NPFFR1, in addition to its previously shown ability to activate NPFFR2. Thus, altering the balance between activation of NPFF receptor subtypes may provide one approach to reversing opiate tolerance.

Curcumin, demethoxycurcumin and bisdemethoxycurcumin differentially inhibit morphine's rewarding effect in rats.

Recent animal studies reported that curcumin, the active constituent of Curcuma longa, has several central actions and may attenuate morphine tolerance. In the present study, we utilized the intracranial self-stimulation (ICSS) paradigm to examine the effects of the commercially available curcuminoid mixture and each one of its components, individually, on brain stimulation reward and on the reward-facilitating effect of morphine. Male Sprague-Dawley rats were implanted with an electrode into the medial forebrain bundle and trained to respond for electrical stimulation using a rate-frequency paradigm. In the first study, rats were injected with graded doses either of the curcuminoid mixture, or curcumin I, or II, or III. In the second study, we examined whether a low dose of the curcuminoid mixture or each individual curcumin analogue composing it could counteract the reward-facilitating effect of morphine. At low doses, both the curcuminoid mixture and curcumin I did not affect brain stimulation reward, whereas, higher doses increased ICSS thresholds. Curcumin II and curcumin III did not affect brain stimulation reward at any doses. Subthreshold doses of the curcuminoid mixture and curcumin I inhibited the reward-facilitating effect of morphine. Both the curcuminoid mixture and curcumin I lack hedonic properties and moderate the reward-facilitating effect of morphine. Our data suggest that curcumin interferes with brain reward mechanisms responsible for the expression of the acute reinforcing properties of opioids and provide evidence that curcumin may be a promising adjuvant for attenuating morphine's rewarding effects in patients who are under long-term opioid therapy.

Gabapentin Enhances the Morphine Anti-Nociceptive Effect in Neuropathic Pain via the Interleukin-10-Heme Oxygenase-1 Signalling Pathway in Rats

In the present study, we investigated the anti-inflammatory mechanisms by which gabapentin enhances morphine anti-nociceptive effect in neuropathic pain in rats and the interaction between the anti-nociceptive effects of gabapentin on morphine and the interleukin (IL)-10-heme-oxygenase (HO)-1 signal pathway in a rat model of neuropathic pain. The neuropathic pain model was induced via a left L5/6 spinal nerve ligation (SNL) in rats. The anti-nociceptive effect of gabapentin and IL-10 on morphine was examined over a 7-day period, and the effects of the anti-IL-10 and HO-1 inhibitor zinc protoporphyrin (ZnPP) on gabapentin/morphine co-injection were assessed. Drug administration was given over 7 days, and on day 8, both anti-inflammatory cytokine IL-10, a stress-induced protein HO-1 and pro-inflammatory cytokines IL-1β, IL-6 and TNF-α were measured. Gabapentin attenuated morphine tolerance over 7 days of co-administration, and reduced the expression of pro-inflammatory cytokines but increased IL-10 and HO-1 expression. The effect of gabapentin on morphine was partially blocked using the anti-IL-10 antibody or the HO-1 inhibitor zinc protoporphyrin. Our findings indicated that the anti-nociceptive effects of gabapentin on morphine might be caused by activation of the IL-10-HO-1 signalling pathway, which resulted in the inhibition of the expression of pro-inflammatory cytokines in neuropathic pain in the rat spinal cord.

Gabapentin attenuates morphine tolerance through interleukin-10

In this study, we examined the anti-inflammatory mechanism by which gabapentin attenuates morphine tolerance in rats. Gabapentin enhanced the antinociceptive effect of morphine and attenuated chronic morphine tolerance when administered intrathecally with morphine in naive rats. We found that a 7-day chronic intrathecal injection of morphine increased the expression of proinflammatory cytokines and decreased interleukin-10 (IL-10) levels in the rat spinal cord. These changes were minimized when gabapentin was combined with morphine. In addition, the effects of gabapentin were reversed by coadministration of the anti-IL-10 antibody. Our findings indicate that enhancement of the antinociceptive effect of morphine by gabapentin may occur through upregulation of the anti-inflammatory cytokine IL-10 and inhibition of proinflammatory cytokines in the rat spinal cord.

Orally Administered Nanocurcumin to Attenuate Morphine Tolerance: Comparison between Negatively Charged PLGA and Partially and Fully PEGylated Nanoparticles

We have formulated hydrophobic curcurmin [1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] into stable nanoparticle suspensions (nanocurcumin) to overcome its relatively low bioavailability, high rate of metabolism, and rapid elimination and clearance from the body. Employing the curcumin nanoformulations as the platform, we discovered that curcumin has the potential to alleviate morphine tolerance. The two types of stable polymeric nanoparticles, poly(lactic-co-glycolic acid) (PLGA) and poly(ethylene glycol)-b-poly(lactic acid) (PEG-b-PLA), and the hybrid of the two were generated using flash nanoprecipitation integrated with spray drying. The optimized formulations have high drug loading (>45%), small particles size with narrow distribution, and controlled surface properties. Mice behavioral studies (tail-flick and hot-plate tests) were conducted to verify the effects of nanocurcumin on attenuating morphine tolerance. Significant analgesia was observed in mice during both tail-flick and hot-plate tests using orally administered nanocurcumin following subcutaneous injections of morphine. However, unformulated curcumin at the same dose showed no effect. Compared with PEGylated nanocurcumin, negatively charged PLGA nanoparticles showed better functionality.

Blockade of Toll-like receptor 4 attenuates morphine tolerance and facilitates the pain relieving properties of morphine.

The ventrolateral periaqueductal gray (vlPAG) is an integral locus for morphine action. Although it is clear that glia contribute to the development of morphine tolerance, to date, the investigation of their role has been limited to spinal and medullary loci. Opioids induce a neuroinflammatory response that opposes acute and long-term analgesia, thereby limiting their efficacy as therapeutic agents. Recent data suggest that the innate immune receptor Toll-like receptor 4 (TLR4), along with its coreceptor myeloid differentiation factor-2 (MD-2), mediates these effects. To date, the brain loci through which TLR4 modulates morphine tolerance have not been identified. We have previously demonstrated that chronic subcutaneous morphine results in tolerance that is accompanied by increases in vlPAG glial cell activity. Using in vivo pharmacological manipulations of vlPAG glia and TLR4 in the adult male rat, we show that intra-vlPAG administration of the general glial cell metabolic inhibitor propentofylline or the astrocyte activity inhibitor fluorocitrate attenuate tolerance to morphine. Characterization of MD-2 expression within the PAG revealed dense MD-2 expression throughout the vlPAG. Further, antagonizing vlPAG TLR4 dose dependently prevented the development of morphine tolerance, and vlPAG microinjections of TLR4 agonists dose dependently produced a "naive" tolerance to subsequent challenge doses of morphine. Finally, using a model of persistent inflammatory pain and pharmacological manipulation of TLR4 we demonstrate that systemic antagonism of TLR4 potentiated acute morphine antihyperalgesia. These results, together, indicate that vlPAG glia regulate morphine tolerance development via TLR4 signaling, and implicate TLR4 as a potential therapeutic target for the treatment of pain.

Effect of metabotropic glutamate 5 receptor antagonists on morphine efficacy and tolerance in rats with neuropathic pain

The metabotropic glutamate 5 (mGlu5) receptor is involved in both pain processing and modulation of µ-opioid induced antinociception and antihyperalgesia. Systemic mGlu5 receptor antagonists 2-methyl-6-phenylethynylpyridine (MPEP) or 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine (MTEP) provide antihyperalgesic effects in various pain models, but few studies have shown their interaction with morphine in neuropathic pain models. The aim of this study is to compare the effects of systemic and intrathecal MPEP/MTEP on morphine efficacy and tolerance in rats with chronic neuropathic pain. L5-6 spinal nerve ligation (SNL) was used to establish neuropathic pain model in rats. The Von Frey test and the hot water tail flick test were employed as behavior tests. Low, medium and high doses of MPEP/MTEP were tested for their effect on both acute morphine efficacy and chronic morphine tolerance. SNL provides sustained neuropathic pain on the ipsilateral hind paw of rats. Both systemic and intrathecal MPEP/MTEP had antiallodynia effects and boosted morphine's efficacy in a dose-dependent manner in the Von Frey tests but not in the tail flick tests. In fact, high doses of MTEP and MPEP attenuated morphine's antinociceptive effect in the latter test. After intrathecal chronic co-administration with morphine, low-doses of MTEP/MPEP attenuated morphine tolerance in both tests. Systemically, only MTEP attenuated morphine tolerance, and only in the Von Frey tests, not in the tail flick tests, whereas MPEP had no effect on morphine tolerance in either tests. The therapeutic use of mGlu5 receptor antagonists may have distinct effects in different pain models.

Persistent Peripheral Inflammation Attenuates Morphine-Induced Periaqueductal Gray Glial Cell Activation and Analgesic Tolerance in the Male Rat

Morphine is among the most prevalent analgesics prescribed for chronic pain. However, prolonged morphine treatment results in the development of analgesic tolerance. An abundance of evidence has accumulated indicating that central nervous system glial cell activity facilitates pain transmission and opposes morphine analgesia. While the midbrain ventrolateral periaqueductal gray (vlPAG) is an important neural substrate mediating pain modulation and the development of morphine tolerance, no studies have directly assessed the role of PAG glia. Here we test the hypothesis that morphine-induced increases in vlPAG glial cell activity contribute to the development of morphine tolerance. As morphine is primarily consumed for the alleviation of severe pain, the influence of persistent inflammatory pain was also assessed. Administration of morphine, in the absence of persistent inflammatory pain, resulted in the rapid development of morphine tolerance and was accompanied by a significant increase in vlPAG glial activation. In contrast, persistent inflammatory hyperalgesia, induced by intraplantar administration of complete Freund’s adjuvant (CFA), significantly attenuated the development of morphine tolerance. No significant differences were noted in vlPAG glial cell activation for CFA-treated animals versus controls. These results indicate that vlPAG glia are modulated by a persistent pain state, and implicate vlPAG glial cells as possible regulators of morphine tolerance. PerspectiveThe development of morphine tolerance represents a significant impediment to its use in the management of chronic pain. We report that morphine tolerance is accompanied by increased glial cell activation within the vlPAG, and that the presence of a persistent pain state prevented vlPAG glial activation and attenuated morphine tolerance.