Analgesic Efficacy Of Morphine Research Articles

The efficacy of intranasal leptin for opioid-induced respiratory depression depends on sex and obesity state.

Introduction: Opioid-induced respiratory depression (OIRD) is the primary cause of death associated with opioids and individuals with obesity are particularly susceptible due to comorbid obstructive sleep apnea (OSA). Repeated exposure to opioids, as in the case of pain management, results in diminished therapeutic effect and/or the need for higher doses to maintain the same effect. With limited means to address the negative impact of repeated exposure it is critical to develop drugs that prevent deaths induced by opioids without reducing beneficial analgesia. Methods: We hypothesized that OIRD as a result of chronic opioid use can be attenuated by administration of IN leptin while also maintaining analgesia in both lean mice and mice with diet-induced obesity (DIO) of both sexes. To test this hypothesis, an opioid tolerance protocol was developed and a model of OIRD in mice chronically receiving morphine and tolerant to morphine analgesia was established. Subsequently, breathing was recorded by barometric plethysmography in four experimental groups: obese male, obese female, lean male, and lean female following acute administration of IN leptin. Respiratory data were complemented with measures of arterial blood gas. Operant behavioral assays were used to determine the impact of IN leptin on the analgesic efficacy of morphine. Results: Acute administration of IN leptin significantly attenuated OIRD in DIO male mice decreasing the apnea index by 58.9% and apnea time by 60.1%. In lean mice leptin was ineffective. Blood gas measures confirmed the effectiveness of IN leptin for preventing respiratory acidosis in DIO male mice. However, IN leptin was not effective in lean mice of both sexes and appeared to exacerbate acid-base disturbances in DIO female mice. Additionally, morphine caused a complete loss of temperature aversion which was not reduced by intranasal leptin indicating IN leptin does not decrease morphine analgesia. Discussion: IN leptin effectively treated OIRD in morphine-tolerant DIO male mice without impacting analgesia. In contrast, IN leptin had no effect in lean mice of either sex or DIO female mice. The arterial blood gas data were consistent with ventilatory findings showing that IN leptin reversed morphine-induced respiratory acidosis only in DIO male mice but not in other mouse groups. Finally, a hypercapnic sensitivity study revealed that IN leptin rescued minute ventilation under hypercapnic conditions only in DIO male mice, which suggests that differential responses to IN leptin are attributable to different leptin sensitivities depending on sex and the obesity status.

Intranasal Leptin Prevents Opioid-Induced Respiratory Depression in Obese, Opioid Tolerant Mice

Opioid-induced respiratory depression (OIRD) is the primary cause of death associated with opioids and individuals with obesity are particularly susceptible due to comorbid obstructive sleep apnea (OSA). Repeated exposure to opioids, as in the case of pain management, results in diminished therapeutic effect and/or the need for higher doses to maintain the same effect. With limited means to address the negative impact of repeated exposure it is critical to develop drugs that prevent deaths induced by opioids without reducing beneficial analgesia or causing withdrawal symptoms. We have previously shown that intranasal (IN) leptin can reverse apneas, hypoventilation, and upper airway obstruction while enhancing analgesia following acute morphine administration. Here we hypothesize that OIRD as a result of chronic opioid use can be attenuated by administration of IN leptin without acute withdrawal while also maintaining analgesia in both lean mice and mice with diet-induced obesity (DIO). To test this hypothesis, an opioid tolerance protocol was developed and a model of OIRD in mice chronically receiving morphine and tolerant to morphine analgesia was established. Subsequently, sleep and breathing were recorded by barometric plethysmography following acute or chronic administration of IN leptin. Operant behavioral assays and Pavlovian conditioning procedures were used to determine the impact of IN leptin on the analgesic efficacy of morphine and opioid withdrawal symptoms. Acute administration of IN leptin significantly attenuated OIRD in DIO male mice decreasing the apnea index by 58.9% and apnea time by 60.1%. In lean mice leptin was ineffective. Chronic IN leptin induced > 50% reduction in the OIRD in morphine tolerant DIO male mice suggesting the protective effect of leptin remained after the two-week treatment. Morphine caused a complete loss of temperature aversion which was not reduced by intranasal leptin indicating IN leptin does not decrease morphine analgesia. IN leptin did not modify naloxone conditional place aversion or naloxone withdrawal symptoms, suggesting that IN treatment does not induce opioid withdrawal. We conclude that IN leptin is ineffective in lean mice but prevents OIRD in obesity by increasing hypercapnic sensitivity when leptin resistance at the BBB is present without reducing analgesia or inducing withdrawal. NIH R61HL156240; NIH R01HL128970; NIH R41DA056239 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Coordinated activity of a central pathway drives associative opioid analgesic tolerance.

Opioid analgesic tolerance, a root cause of opioid overdose and misuse, can develop through an associative learning. Despite intensive research, the locus and central pathway subserving the associative opioid analgesic tolerance (AOAT) remains unclear. Using a combination of chemo/optogenetic manipulation with calcium imaging and slice physiology, here we identify neuronal ensembles in a hierarchically organized pathway essential for AOAT. The association of morphine-induced analgesia with an environmental condition drives glutamatergic signaling from ventral hippocampus (vHPC) to dorsomedial prefrontal cortex (dmPFC) cholecystokininergic (CCKergic) neurons. Excitation of CCKergic neurons, which project and release CCK to basolateral amygdala (BLA) glutamatergic neurons, relays AOAT signal through inhibition of BLA μ-opioid receptor function, thereby leading to further loss of morphine analgesic efficacy. This work provides evidence for a circuit across different brain regions distinct for opioid analgesic tolerance. The components of this pathway are potential targets to treat opioid overdose and abuse.

The YTHDF1–TRAF6 pathway regulates the neuroinflammatory response and contributes to morphine tolerance and hyperalgesia in the periaqueductal gray

Long-term use of opioids such as morphine has negative side effects, such as morphine analgesic tolerance and morphine-induced hyperalgesia (MIH). These side effects limit the clinical use and analgesic efficacy of morphine. Elucidation of the mechanisms and identification of feasible and effective methods or treatment targets to solve this clinical phenomenon are important. Here, we discovered that YTHDF1 and TNF receptor-associated factor 6 (TRAF6) are crucial for morphine analgesic tolerance and MIH. The m6A reader YTHDF1 positively regulated the translation of TRAF6 mRNA, and chronic morphine treatments enhanced the m6A modification of TRAF6 mRNA. TRAF6 protein expression was drastically reduced by YTHDF1 knockdown, although TRAF6 mRNA levels were unaffected. By reducing inflammatory markers such as IL-1β, IL-6, TNF-α and NF-κB, targeted reduction of YTHDF1 or suppression of TRAF6 activity in ventrolateral periaqueductal gray (vlPAG) slows the development of morphine analgesic tolerance and MIH. Our findings provide new insights into the mechanism of morphine analgesic tolerance and MIH indicating that YTHDF1 regulates inflammatory factors such as IL-1β, IL-6, TNF-α and NF-κB by enhancing TRAF6 protein expression.

Chemokine CXCL10 regulates pain behaviors via PI3K-AKT signaling pathway in mice

The analgesic efficacy of morphine can be affected by a variety of factors. Our previous studies demonstrated that chemokine (CXC motif) ligand 10 (CXCL10) could induce algesia directly and attenuate the analgesic effect produced by a single dose of morphine. However, the underlying mechanism remains unclear. In the present study, we aimed to further investigate the mechanism of CXCL10-mediated inhibition on morphine analgesic effect. According to our findings, recombinant CXCL10 protein (rmCXCL10) could increase the phosphorylation of serine-threonine kinase AKT reduced by morphine in spinal cord. Blocking AKT activation by phosphoinositide 3-kinase (PI3K) inhibitor could effectively attenuate CXCL10-induced algesia, and reverse the decrease of paw withdrawal thresholds caused by the co-administration of morphine and rmCXCL10. Furthermore, rmCXCL10 could enhance the spinal expression of pro-inflammatory cytokines, including TNF-α, IL-6, and IL-1β, which could be blocked by PI3K inhibitor. In summary, these findings suggest that PI3K-AKT signaling pathway mediates the effect of CXCL10 on the regulation of morphine analgesia and the release of cytokines in spinal cord. Our study provides a new insight into the mechanism of chemokine-relative pain regulation.

Perioperative Intravenous Patient-Controlled Analgesic Efficacy of Morphine with Combined Nefopam and Parecoxib versus Parecoxib in Gynecologic Surgery: A Randomized, Double-Blind Study.

Background Nefopam is a non-NSAIDs and opioid sparing centrally acting drug which is effective for a multimodal postoperative analgesia. The present study aimed to evaluate the analgesic efficacy of nefopam combined with parecoxib for gynecologic surgery. Methods This randomized double-blinded control trial recruited participants (n = 72) who underwent gynecologic surgeries and divided them into either a nefopam or control group. The study group received parecoxib 40 mg plus nefopam 20 mg, while the control group received parecoxib 40 mg plus normal saline solution intravenously during open abdominal gynecological surgery. Both groups then received either nefopam or normal saline every 6 hours postoperatively for 24 hours. Intravenous patient-controlled analgesia with morphine was given for breakthrough pain within 24 h. The participants were evaluated for morphine consumption within 24 hours and postoperative pain using a verbal numerical rating scale (VNRS) at a postanesthetic care unit, at 6-, 12-, and 24-hour postoperative periods. Adverse effects were recorded. Results Morphine consumption within 24 hours and adverse effects were not significantly different between both groups. Mean difference and 95% confident interval of morphine consumption between both groups was 1.00 (−4.56, 4.76), P=0.97. The VNRS on movement at 6 hours after surgery of the nefopam group was significantly different from that of the control group [mean (SD), 4.14 (2.11) vs. 5.14 (1.80), P=0.04]. The VNRS of the nefopam group at 12 hours after operation during resting and on movement was significantly different from that of the control group ([mean (SD), 1.47 (1.80) vs. 2.54 (2.15), P=0.03], [mean (SD), 3.22 (1.84) vs 4.17 (1.74), P=0.03]), respectively. Conclusions The combined administration of nefopam and parecoxib during gynecologic surgery slightly reduced the VNRS at 6 and 12 hours postoperatively more than treatment with parecoxib.

Effect of crocin on morphine tolerance and serum BDNF levels in a rat model of neuropathic pain.

Chronic use of morphine treatment for neuropathic pain leads to morphine-induced analgesic tolerance. Crocin contained in Crocus sativus L., exerts anti-inflammatory and analgesic effects. This study examined the effects of crocin on morphine tolerance and serum BDNF levels on neuropathic pain induced by chronic constriction injury (CCI) in rats. CCI model of neuropathic pain was done in male Wistar rats (200-250g). Rats were treated with crocin (15 or 30mg/kg, intraperitoneally) alone or simultaneously with morphine (10mg/kg, subcutaneously) during or after induction of CCI. Pain behavioral responses including mechanical allodynia and thermal hyperalgesia were measured from days of 15-27 after CCI. Then, rats were evaluated for serum BDNF levels on days 14 and/or 27. We found that morphine tolerance developed after the induction of neuropathic pain. The injection of crocin (15 and 30mg/kg) was able to enhance analgesic effect of morphine by reduction of mechanical allodynia on days 15-27 post-surgery in CCI rats. While preemptive administration of crocin at a lower dose (15mg/kg) maintained the analgesic effect of morphine. Morphine injection and/or co-administration with crocin (15, 30mg/kg) decreased serum BDNF levels in CCI rats. These findings indicate that crocin may have a therapeutic effect to maintain morphine analgesic efficacy and also to prevent the development of morphine tolerance in neuropathic pain, but probably not through BDNF.

The effects of radio-frequency radiation (RFR) exposure on the analgesic efficacy of morphine in healthy rats and rats with inflammation

The aim of this study, conducted at the Military Institute of Hygiene and Epidemiology in Warsaw in 2017, was to evaluate the effects of a single (15 min) and repeated (5 times for 15 min) radio-frequency radiation (RFR) exposure of 1800 MHz frequency on the analgesic efficacy of morphine in healthy rats and rats with complete Freund's adjuvant (CFA) induced inflammation. Rats were injected intraperitoneally with morphine (MF) in the dose of 8 mg/kg or drug vehicle 15 min before RFR exposure. The authors used the plantar analgesia meter and the radiant heat paw-withdrawal test to assess the pain threshold. A single RFR exposure slightly influenced paw withdrawal latency (PWL) in healthy rats in the single exposure baseline group, and influenced PWL, 30 and 60 min after morphine or vehicle injection, in the repeated exposure group. There were differences between the sham-exposed groups (vehicle), 30, 60 and 90 min after injection, both in the single and repeated RFR-exposure groups. The antinociceptive effect of morphine in healthy rats was slightly decreased by RFR exposure at 60 and 90 min, both in the single and repeated exposure groups. The PWL was slightly decreased, both in the single and repeated exposure groups with inflammation (CFA and CFA/MF), at 30, 60 and 90 min, and PWL was increased in the sham-exposed groups (CFA and CFA/MF), both in the single and repeated exposure groups, at 30, 60 and 90 min. The antinociceptive effect of morphine in healthy rats was significantly increased by RFR exposure at 30 min after drug injection in the single exposure group, and increased at 30 and 60 min in the repeated exposure group. The authors observed a minor influence of RFR exposure on the antinociceptive effects of morphine in healthy rats after repeated exposures and a statistically significant influence of repeated exposure on morphine mediated antinociceptive effects in the inflammation group. Int J Occup Med Environ Health. 2019;32(4):465-74.

Chronic morphine exposure potentiates p-glycoprotein trafficking from nuclear reservoirs in cortical rat brain microvessels.

The rates of opioid prescription and use have continued to increase over the last few decades resulting in a greater number of opioid tolerant patients. Treatment of acute pain from surgery and injury is a clinical challenge for these patients. Several pain management strategies including prescribing increased opioids are used clinically with limited success; all currently available strategies have significant limitations. Many opioids are a substrate for p-glycoprotein (p-gp), an efflux transporter at the blood-brain barrier (BBB). Increased p-gp is associated with a decreased central nervous system uptake and analgesic efficacy of morphine. Our laboratory previously found that acute peripheral inflammatory pain (PIP) induces p-gp trafficking from the nucleus to the luminal surface of endothelial cells making up the BBB concomitant with increased p-gp activity and decreased morphine analgesic efficacy. In the current study, we tested whether PIP-induced p-gp trafficking could contribute to decreased opioid efficacy in morphine tolerant rats. A 6-day continuous dosing of morphine from osmotic minipumps was used to establish morphine tolerance in female rats. PIP induced p-gp trafficking away from nuclear stores showed a 2-fold increase in morphine tolerant rats. This observation suggests that p-gp trafficking contributes to the decreased morphine analgesic effects in morphine tolerant rats experiencing an acute pain stimulus. Attenuating p-gp trafficking during an acute pain stimulus could improve pain management by increasing the amount of opioid that could reach CNS analgesic targets and decrease the need for the dose escalation that is a serious challenge in pain management.

Role of Neuroinflammation in Opioid Tolerance: Translational Evidence from Human-to-Rodent Studies.

Opioid analgesics remain the most effective and widely used analgesics for the management of moderate to severe pain, including cancer pain and chronic non-cancer pain. However, the efficacy of long-term opioid analgesics is attenuated by tolerance and/or hyperalgesia after long-term use, preventing adequate pain relief under stable opioid dosages for chronic pain patients. Classical neuron-centered concepts about tolerance, such as internalization of opioid receptors, upregulation of N-methyl-D-aspartate receptor function, or downregulation of glutamate transporter activity, can only partially explain the phenomenon of tolerance. Recent evidence revealing glial activation and upregulation of inflammatory mediators in the rodent central nervous system has confirmed the pivotal role of neuroinflammation in neuropathic pain or opioid tolerance, or both. However, human evidence is still sparse.Based on our clinical practice, we conducted translational research by investigating the cerebrospinal fluid (CSF) cytokine and chemokine profiles of opioid-tolerant patients after research ethic committee approval. CSF samples from opioid-tolerant patients and opioid-naive subjects were compared. We found CXCL1, CXCL12, and leukemia inhibitory factor (LIF) were significantly upregulated among the opioid-tolerant patients and positively correlated with the opioid dosage.We translated these findings back to lab animal experiment; after induction of tolerance by morphine infusion, the spinal cord expression of CXCL1, CXCL12, and LIF were all upregulated. Although CXCL1 and CXCL12 infusion alone did not affect baseline tail-flick latency, morphine analgesic efficacy dropped significantly after intrathecal infusion of CXCL1 and CXCL12. After establishing tolerance by intrathecal continuous infusion of morphine, tolerance development was accelerated by co-administration of CXCL1 and CXCL12. In parallel, the effect was attenuated by co-administration of CXCL1- or CXCL12-neutralizing antibody or concordant receptor antagonists.On the contrary, although chronic morphine administration still induced LIF upregulation in rat spinal cords, intrathecal injection of LIF potentiated the analgesic action of morphine and delayed the development of morphine tolerance. Upregulation of endogenously released LIF by long-term use of opioids might counterbalance the tolerance induction effects of other pro-inflammatory cytokines.CXCL1, CXCL12, and LIF are upregulated in both opioid-tolerant patients and rodents. The onset and extent of opioid tolerance were affected by modulating the intrathecal CXCL1/CXCR2, CXCL12/CXCR4, and LIF signaling and could be novel drug targets for the treatment of opioid tolerance.

Effect of age factor on analgesic efficacy of morphine during recovery from remifentanil-based anesthesia in pediatric patients undergoing minor surgery

Objective To evaluate the effect of age factor on the analgesic efficacy of morphine during recovery from remifentanil-based anesthesia in pediatric patients undergoing minor surgery. Methods Fifty pediatric patients of both sexes, aged 3-10 yr, with body mass index ≤30 kg/m2, of American Society of Anesthesiologists physical status Ⅰor Ⅱ, scheduled for elective tonsillectomy, were divided into preschool group (3-5 yr, n=30) and school-age group (6-10 yr, n=20) according to age.Anesthesia was induced by inhaling 8% sevoflurane and IV morphine 0.1 mg/kg.Pediatric patients were mechanically ventilated after tracheal intubation, and end-tidal pressure of carbon dioxide was maintained between 35-45 mmHg.Anesthesia was maintained by inhalation of 2%-3% sevoflurane and IV infusion of remifentanil 0.2 μg·kg-1·min-1.Pain was evaluated using Faces Pain Scale (FPS) and Face Legs Activity Cry Consolability (FLACC) scale during the recovery period in the postanesthesia care unit.When FPS or FLACC scores ≥4, morphine 0.05 mg/kg was intravenously injected.When pain was still unrelieved after morphine was given for 2 times (time interval 5 min), fentanyl 1 μg/kg was intravenously injected until FPS and FLACC scores <4.The requirement for analgesics and consumption of analgesics (fentanyl consumption was converted into morphine consumption) were recorded.The development of nausea and vomiting, pruritus and respiratory depression was also recorded. Results Compared with preschool group, the requirement for analgesics and consumption of analgesics were significantly decreased during recovery from anesthesia (P 0.05). No pruritus or respiratory depression was found in two groups. Conclusion The analgesic efficacy of morphine is affected by age factors during recovery from remifentanil-based anesthesia in pediatric patients undergoing minor surgery, and morphine produces better analgesic efficacy in school-age pediatric patients than in preschool pediatric patients. Key words: Age factors; Morphine; Piperidines; Child; Analgesia

A blinded randomised placebo-controlled trial investigating the efficacy of morphine analgesia for procedural pain in infants: Trial protocol

Infant pain has both immediate and long-term negative consequences, yet in clinical practice it is often undertreated. To date, few pain-relieving drugs have been tested in infants. Morphine is a potent analgesic that provides effective pain relief in adults, but there is inconclusive evidence for its effectiveness in infants. The purpose of this study is to establish whether oral morphine provides effective analgesia for procedural pain in infants. A blinded, placebo-controlled, parallel-group randomized, phase II, clinical trial will be undertaken to determine whether morphine sulphate administered orally prior to clinically-required retinopathy of prematurity (ROP) screening and heel lancing provides effective analgesia. 156 infants between 34 and 42 weeks’ gestational age who require a clinical heel lance and ROP screening on the same test occasion will be included in the trial. Infants will be randomised to receive either a single dose of morphine sulphate (100 μg/kg) or placebo. Each infant will be monitored for 48 hours and safety data will be collected during the 24 hours following drug administration. The primary outcome will be the Premature Infant Pain Profile–revised (PIPP-R) score 30 seconds after ROP screening. The co-primary outcome will be the magnitude of nociceptive-specific brain activity evoked by a clinically-required heel lance. Infant clinical stability will be assessed by comparing the number of episodes of bradycardia, tachycardia, desaturation and apnoea, and changes in respiratory support requirements in the 24-hour periods before and after the clinical intervention. In addition, drug safety will be assessed by considering the occurrence of apnoeic and hypotensive episodes requiring intervention in the 24-hour period following drug administration. This study has been published as an Accepted Protocol Summary by The Lancet.

Comparison of the Tolerability Profile of Controlled-Release Oral Morphine and Oxycodone for Cancer Pain Treatment. An Open-Label Randomized Controlled Trial

ContextOxycodone and morphine are recommended as first-choice opioids for moderate/severe cancer pain, but evidence about their relative tolerability has significant methodological limitations. ObjectivesThis study was mainly aimed at comparing the risk of developing adverse events (AEs) with controlled-release oral morphine vs. oxycodone; secondary aims were comparing their analgesic efficacy and testing heterogeneity in tolerability across different age and renal function subgroups. MethodsAn open-label multicenter RCT (EudraCT number: 2006-003151-21) was carried out in patients with moderate/severe cancer pain. At baseline, 7 and 14 days, patients scored on 0–10 rating scales (0–10 numerical rating scale) the intensity of pain and of a list of common opioid side effects. The primary end point was the percentage of patients reporting an AE (a worsening ≥ 2 points on any of the listed side effects); tolerability by subgroups and average follow-up pain intensity were compared through regression models. ResultsOne hundred eighty-seven patients were enrolled (47% of originally planned). Intention to treat (ITT) analysis (N = 185, morphine 94, oxycodone 91) did not show any difference in the risk of developing AEs (risk difference −0.6%, 95% CI −11.0% to 9.9%) nor in analgesia (0–10 numerical rating scale pain intensity difference −0.28, 95% CI −0.83 to 0.27). No evidence of heterogeneity of tolerability across age and renal function patient subgroups emerged. ConclusionThis trial failed to show any difference in tolerability and analgesic efficacy of morphine and oxycodone as first-line treatment for moderate/severe cancer pain but results interpretation is difficult due to lack of power, potential bias from open-label design, and concerns about assay sensitivity. These data, however, can significantly contribute to future meta-analyses comparing WHO Step-III opioids and are relevant in designing future randomized studies.

Evaluation of the effects of specific opioid receptor agonists in a rodent model of spinal cord injury.

ObjectiveThe current study aimed to evaluate the contribution(s) of specific opioid receptor systems to the analgesic and detrimental effects of morphine, observed after spinal cord injury in prior studies.Study DesignWe used specific opioid receptor agonists to assess the effects of µ- (DAMGO), δ- (DPDPE), and κ- (GR89696) opioid receptor activation on locomotor (BBB, tapered beam, ladder tests) and sensory (girdle, tactile, and tail-flick tests) recovery in a rodent contusion model (T12). We also tested the contribution of non-classic opioid binding using [+]- morphine.MethodsFirst, a dose-response curve for analgesic efficacy was generated for each opioid agonist. Baseline locomotor and sensory reactivity was assessed 24 h after injury. Subjects were then treated with an intrathecal dose of a specific agonist and re-tested after 30 min. To evaluate effects on recovery, subjects were treated with a single dose of an agonist and both locomotor and sensory function were monitored for 21 d.ResultsAll agonists for the classic opioid receptors, but not the [+]- morphine enantiomer, produced antinociception at a concentration equivalent to a dose of morphine previously shown to produce strong analgesic effects (0.32 μmol). DAMGO and [+]- morphine did not affect long-term recovery. GR89696, however, significantly undermined recovery of locomotor function at all doses tested.ConclusionsBased on these data, we hypothesize that the analgesic efficacy of morphine is primarily mediated by binding to the classic μ-opioid receptor. Conversely, the adverse effects of morphine may be linked to activation of the κ-opioid receptor. Ultimately, elucidating the molecular mechanisms underlying the effects of morphine is imperative in order to develop safe and effective pharmacological interventions in a clinical setting.SettingUSA

Epigenetic regulation of spinal cord gene expression contributes to enhanced postoperative pain and analgesic tolerance subsequent to continuous opioid exposure.

BackgroundOpioids have become the mainstay for treatment of moderate to severe pain and are commonly used to treat surgical pain. While opioid administration has been shown to cause opioid-induced hyperalgesia and tolerance, interactions between opioid administration and surgery with respect to these problematic adaptations have scarcely been addressed. Accumulating evidence suggests opioids and nociceptive signaling may converge on epigenetic mechanisms in spinal cord to enhance or prolong neuroplastic changes. Epigenetic regulation of Bdnf (brain-derived neurotrophic factor) and Pdyn (prodynorphin) genes may be involved.ResultsFour days of ascending doses of morphine treatment caused opioid-induced hyperalgesia and reduced opioid analgesic efficacy in mice. Both opioid-induced hyperalgesia and the reduced opioid analgesic efficacy were enhanced in mice that received hindpaw incisions. The expression of Bdnf and Pdyn (qPCR) was increased after morphine treatment and incision. Chromatin immunoprecipitation assays demonstrated that the Pdyn and Bdnf promoters were more strongly associated with acetylated H3K9 after morphine plus incision than in the morphine or incision alone groups. Selective tropomyosin-related kinase B (ANA-12) and κ-opioid receptor (nor-binaltorphimine) antagonists were administered intrathecally, both reduced hyperalgesia one or three days after surgery. Administration of ANA-12 or nor-binaltorphimine attenuated the decreased morphine analgesic efficacy on day 1, but only nor-binaltorphimine was effective on day 3 after incision in opioid-exposed group. Coadministration of histone acetyltransferase inhibitor anacardic acid daily with morphine blocked the development of opioid-induced hyperalgesia and attenuated incision-enhanced hyperalgesia in morphine-treated mice. Anacardic acid had similar effects on analgesic tolerance, showing the involvement of histone acetylation in the interactions detected.ConclusionsSpinal epigenetic changes involving Bdnf and Pdyn may contribute to the enhanced postoperative nociceptive sensitization and analgesic tolerance observed after continuous opioid exposure. Treatments blocking the epigenetically mediated up-regulation of these genes or administration of TrkB or κ-opioid receptor antagonists may improve the clinical utility of opioids, particularly after surgery.

Morphine Analgesia Modification in Normotensive and Hypertensive Female Rats after Repeated Fluoxetine Administration.

The purpose of this investigation was to determine through the use of fluoxetine the effect of administering a serotonin reuptake inhibitor over several days on the antinociceptive action of μ-morphine type opioid receptor agonist. Investigations were performed on rats of both sexes, both the WKY normotensive strains as well as on the SHR genetically conditioned hypertensive strains. Results showed that the efficacy of morphine analgesia is higher in the SHR strain compared to normotensive rats (WKY). Surprisingly, repeated administration of fluoxetine reduced morphine analgesia, with the weakening of opioid antinociceptive action comparable to the duration of serotonin reuptake inhibitor administration. It was also concluded that the antinociceptive action of morphine in female rats and the alteration of its efficacy as a result of fluoxetine premedication for several days depend on oestrus cycle phase. The highest sensitivity of female rats to morphine was reported in the dioestrus and oestrus phases; much lower values were reported for the metoestrus phase.

Efficacy of Dopamine D3 Receptor Antagonist SR 21502 in Reducing Opioid Tolerance and Dependence

Dopamine (DA) D3 antagonists modulate the addictive and rewarding properties of both stimulant and opioid drugs by deactivating the drugs' effects on learning and motivation centers in the brain. SR 21502 (SR), a novel DA D3 receptor antagonist, has been shown to be effective in reducing conditioned place preference and cue‐induced relapse in rodent models of cocaine addiction. This study investigated SR's ability to mediate opioid addiction without compromising morphine's analgesic efficacy. We co‐administered morphine and SR, twice daily for 4 days, to assess the ability of SR to reduce morphine‐induced tolerance and dependence. Results showed that SR was able to reduce morphine‐induced tolerance by producing less of a rightward shift in the dose‐response curve. Standard morphine tolerance produced a 12.4 rightward shift where co‐administration with SR produced a 6.9 fold shift. In the assessment of chronic dependence using naloxone to induce withdrawal, we found that SR treated mice exhibited less vertical jumping and had a significantly lower output of urine and feces during a 20 min observation period. It was also observed that the laxation of SR treated mice produced well‐formed fecal boli versus diarrhea that was seen following standard morphine treatment. These findings support our hypothesis that co‐administration of SR throughout morphine treatment would be an effective approach at identifying an equally efficacious lower‐risk treatment option. This research also provides a promising approach at minimizing negative side effects, including tolerance, gastro‐intestinal and addictive tendencies, which may improve patient compliance to these types of therapies.

P-glycoprotein Modulates Morphine Uptake into the CNS: A Role for the Non-steroidal Anti-inflammatory Drug Diclofenac

Our laboratory has previously demonstrated that peripheral inflammatory pain (PIP), induced by subcutaneous plantar injection of λ-carrageenan, results in increased expression and activity of the ATP-dependent efflux transporter P-glycoprotein (P-gp) that is endogenously expressed at the blood-brain barrier (BBB). The result of increased P-gp functional expression was a significant reduction in CNS uptake of morphine and, subsequently, reduced morphine analgesic efficacy. A major concern in the treatment of acute pain/inflammation is the potential for drug-drug interactions resulting from P-gp induction by therapeutic agents co-administered with opioids. Such effects on P-gp activity can profoundly modulate CNS distribution of opioid analgesics and alter analgesic efficacy. In this study, we examined the ability of diclofenac, a non-steroidal anti-inflammatory drug (NSAID) that is commonly administered in conjunction with the opioids during pain therapy, to alter BBB transport of morphine via P-gp and whether such changes in P-gp morphine transport could alter morphine analgesic efficacy. Administration of diclofenac reduced paw edema and thermal hyperalgesia in rats subjected to PIP, which is consistent with the known mechanism of action of this NSAID. Western blot analysis demonstrated an increase in P-gp expression in rat brain microvessels not only following PIP induction but also after diclofenac treatment alone. Additionally, in situ brain perfusion studies showed that both PIP and diclofenac treatment alone increased P-gp efflux activity resulting in decreased morphine brain uptake. Critically, morphine analgesia was significantly reduced in animals pretreated with diclofenac (3 h), as compared to animals administered diclofenac and morphine concurrently. These novel findings suggest that administration of diclofenac and P-gp substrate opioids during pain pharmacotherapy may result in a clinically significant drug-drug interaction.

Spinal Morphine but not Ziconotide or Gabapentin Analgesia is Affected by Alternative Splicing of Voltage-Gated Calcium Channel Cav2.2 Pre-mRNA

Presynaptic voltage-gated calcium CaV2.2 channels play a privileged role in spinal level sensitization following peripheral nerve injury. Direct and indirect inhibitors of CaV2.2 channel activity in spinal dorsal horn are analgesic in chronic pain states. CaV2.2 channels represent a family of splice isoforms that are expressed in different combinations according to cell-type. A pair of mutually exclusive exons in the CaV2.2 encoding Cacna1b gene, e37a and e37b, differentially influence morphine analgesia. In mice that lack exon e37a, which is enriched in nociceptors, the analgesic efficacy of intrathecal morphine against noxious thermal stimuli is reduced. Here we ask if sequences unique to e37a influence: the development of abnormal thermal and mechanical sensitivity associated with peripheral nerve injury; and the actions of two other classes of analgesics that owe part or all of their efficacy to CaV2.2 channel inhibition. We find that: i) the analgesic efficacy of morphine, but not ziconotide or gabapentin, is reduced in mice lacking e37a, ii) the induction and maintenance of behaviors associated with sensitization that accompany peripheral nerve injury, do not require e37a-specific sequence, iii) intrathecal morphine, but not ziconotide or gabapentin analgesia to thermal stimuli is significantly lower in wild-type mice after peripheral nerve injury, iv) the analgesic efficacy of ziconotide and gabapentin to mechanical stimuli is reduced following nerve injury, and iv) intrathecal morphine analgesia to thermal stimuli in mice lacking e37a is not further reduced by peripheral nerve injury. Our findings show that the analgesic action of morphine, but not ziconotide or gabapentin, to thermal stimuli is linked to which Cacna1b exon, e37a or e37b, is selected during alternative pre-mRNA splicing.

Blockade of PDGFR-β activation eliminates morphine analgesic tolerance

For centuries, opioid drugs have been the mainstay of chronic pain treatment. However, over time analgesic tolerance develops, leaving few treatment options. Here we show that platelet-derived growth factor receptor-β (PDGFR-β)-mediated signaling plays a key role in morphine tolerance. PDGFR-β inhibition selectively eliminates morphine tolerance in rats. PDGFR-β inhibitors are widely used and well tolerated, suggesting that clinical translation of our findings could reduce the suffering endured by individuals with intractable pain.