Underlying mechanisms of pronociceptive consequences of prolonged morphine exposure

Dualism, allosteric modulation, and biased signaling of opioid receptors: Future therapeutic potential

Opioids are still the drugs of choice for the treatment of acute post-surgical pain and chronic cancer pain. Overprescribing of these drugs has given rise to an "opioid crisis" in some countries. In this context, attention has been drawn to the therapeutic potential of various ligands that act as allosteric modulators of orthosteric binding sites and modulate the drug's activity, affinity, potency, and even efficacy.

Morphine acts in vitro to directly prime nociceptors

Hyperalgesic priming is a preclinical model of the transition from acute to chronic pain characterized by a leftward shift in the dose-response curve for and marked prolongation of prostaglandin E2 (PGE2)-induced mechanical hyperalgesia, in vivo. In vitro, priming in nociceptors is characterized by a leftward shift in the concentration dependence for PGE2-induced nociceptor sensitization. In the present in vitro study we tested the hypothesis that a mu-opioid receptor (MOR) agonist opioid analgesic, morphine, can produce priming by its direct action on nociceptors. We report that treatment of nociceptors with morphine, in vitro, produces a leftward shift in the concentration dependence for PGE2-induced nociceptor sensitization. Our findings support the suggestion that opioids act directly on nociceptors to induce priming.

Short-chain fatty acid, butyrate prevents morphine-and paclitaxel-induced nociceptive hypersensitivity

Nociceptive hypersensitivity is a significant side effect with the chronic administration of opioids as well as chemotherapeutics. Both opioid-induced hypersensitivity (OIH) and chemotherapy-induced hypersensitivity (CIH) are characterized by an increased sensitivity to painful stimuli which can significantly reduce the quality of life for individuals on either drug(s). Here we demonstrate the nociceptive hypersensitivity associated with repeated administration of morphine (opioid) and paclitaxel (chemotherapeutic) treatment can be reversed by oral supplementation with the short-chain fatty acid (SCFA) sodium butyrate (NaBut). In two separate mouse behavioral models for nociceptive hypersensitivity, we found that thermal hyperalgesia (for OIH) and cold allodynia (for CIH) were prevented by treatment with oral butyrate (p.o, b.i.d). Electrophysiological recordings of small diameter dorsal root ganglia (DRG) neurons from morphine and paclitaxel treated mice showed an increase in neuronal hyperexcitability in both drug models which was likewise prevented by oral butyrate treatment. Using colonic conditioned media obtained from excised colon segments we found that gut mediators of morphine treated mice can induce hyperexcitability in naïve DRG neurons, but such enhanced excitability is not present when animals are co-treated with NaBut suggesting gut derived mediators modulate neuronal hyperexcitability. In-vitro NaBut treatment did not prevent morphine-induced excitability, suggesting an indirect role of butyrate in modulating neuronal hypersensitivity. These data taken together suggest that gut derived mediators affect opioid and chemotherapeutic-induced neuronal hypersensitivity that is prevented by the SCFA butyrate.

Engineering human spinal microphysiological systems to model opioid-induced tolerance

pioids are commonly used for treating chronic pain. However, with continued use, they may induce tolerance and/or hyperalgesia, which limits therapeutic efficacy. The human mechanisms of opioid-induced tolerance and hyperalgesia are significantly understudied, in part, because current models cannot fully recapitulate human pathology. Here, we engineered novel human spinal microphysiological systems (MPSs) integrated with plug-and-play neural activity sensing for modeling human nociception and opioid-induced tolerance. Each spinal MPS consists of a flattened human spinal cord organoid derived from human stem cells and a 3D printed organoid holder device for plug-and-play neural activity measurement. We found that the flattened organoid design of MPSs not only reduces hypoxia and necrosis in the organoids, but also promotes their neuron maturation, neural activity, and functional development. We further demonstrated that prolonged opioid exposure resulted in neurochemical correlates of opioid tolerance and hyperalgesia, as measured by altered neural activity, and downregulation of μ-opioid receptor expression of human spinal MPSs. The MPSs are scalable, cost-effective, easy-to-use, and compatible with commonly-used well-plates, thus allowing plug-and-play measurements of neural activity. We believe the MPSs hold a promising translational potential for studying human pain etiology, screening new treatments, and validating novel therapeutics for human pain medicine.

The Downregulation of Opioid Receptors and Neuropathic Pain

Neuropathic pain (NP) refers to pain caused by primary or secondary damage or dysfunction of the peripheral or central nervous system, which seriously affects the physical and mental health of 7-10% of the general population. The etiology and pathogenesis of NP are complex; as such, NP has been a hot topic in clinical medicine and basic research for a long time, with researchers aiming to find a cure by studying it. Opioids are the most commonly used painkillers in clinical practice but are regarded as third-line drugs for NP in various guidelines due to the low efficacy caused by the imbalance of opioid receptor internalization and their possible side effects. Therefore, this literature review aims to evaluate the role of the downregulation of opioid receptors in the development of NP from the perspective of dorsal root ganglion, spinal cord, and supraspinal regions. We also discuss the reasons for the poor efficacy of opioids, given the commonness of opioid tolerance caused by NP and/or repeated opioid treatments, an angle that has received little attention to date; in-depth understanding might provide a new method for the treatment of NP.

Fentanyl Structure as a Scaffold for Opioid/Non-Opioid Multitarget Analgesics

One of the strategies in the search for safe and effective analgesic drugs is the design of multitarget analgesics. Such compounds are intended to have high affinity and activity at more than one molecular target involved in pain modulation. In the present contribution we summarize the attempts in which fentanyl or its substructures were used as a μ-opioid receptor pharmacophoric fragment and a scaffold to which fragments related to non-opioid receptors were attached. The non-opioid ‘second’ targets included proteins as diverse as imidazoline I₂ binding sites, CB₁ cannabinoid receptor, NK₁ tachykinin receptor, D₂ dopamine receptor, cyclooxygenases, fatty acid amide hydrolase and monoacylglycerol lipase and σ₁ receptor. Reviewing the individual attempts, we outline the chemistry, the obtained pharmacological properties and structure-activity relationships. Finally, we discuss the possible directions for future work.

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).

Toll-Like Receptor 4 in Pain: Bridging Molecules-to-Cells-to-Systems

Pain impacts the lives of billions of people around the world - both directly and indirectly. It is complex and transcends beyond an unpleasant sensory experience to encompass emotional experiences. To date, there are no successful treatments for sufferers of chronic pain. Although opioids do not provide any benefit to chronic pain sufferers, they are still prescribed, often resulting in more complications such as hyperalgesia and dependence. In order to develop effective and safe medications to manage, and perhaps even treat pain, it is important to evaluate novel contributors to pain pathologies. As such, in this chapter we review the role of Toll-like receptor 4, a receptor of the innate immune system, that continues to gain substantial attention in the field of pain research. Positioned in the nexus of the neuro and immune systems, TLR4 may provide one of the missing pieces in understanding the complexities of pain. Here we consider how TLR4 enables a mechanistical understanding of pain as a multidimensional biopsychosocial state from molecules to cells to systems and back again.

Effects of cyanocobalamin and its combination with morphine on neuropathic rats and the relationship between these effects and thrombospondin-4 expression

Background

Thrombospondin-4 (TSP4) upregulates in the spinal cord following peripheral nerve injury and contributes to the development of neuropathic pain (NP). We investigated the effects of cyanocobalamin alone or in combination with morphine on pain and the relationship between these effects and spinal TSP4 expression in neuropathic rats.

Methods

NP was induced by chronic constriction injury (CCI) of the sciatic nerve. Cyanocobalamin (5 and 10 mg/kg/day) was administered 15 days before CCI and then for 4 and 14 postoperative days. Morphine (2.5 and 5 mg/kg/day) was administered only post-CCI. Combination treatment included cyanocobalamin and morphine, 10 and 5 mg/kg/day, respectively. All drugs were administered intraperitoneally. Nociceptive thresholds were detected by esthesiometer, analgesia meter, and plantar test, and TSP4 expression was assessed by western blotting and fluorescence immunohistochemistry.

Results

CCI decreased nociceptive thresholds in all tests and induced TSP4 expression on the 4th postoperative day. The decrease in nociceptive thresholds persisted except for the plantar test, and the increased TSP4 expression reversed on the 14th postoperative day. Cyanocobalamin and low-dose morphine alone did not produce any antinociceptive effects. High-dose morphine improved the decreased nociceptive thresholds in the esthesiometer when administered alone but combined with cyanocobalamin in all tests. Cyanocobalamin and morphine significantly induced TSP4 expression when administered alone in both doses for 4 or 14 days. However, this increase was less when the two drugs are combined.

Conclusions

The combination of cyanocobalamin and morphine is more effective in antinociception and partially decreased the induced TSP4 expression compared to the use of either drug alone.

Sex Differences in Behavioral and Brainstem Transcriptomic Neuroadaptations following Neonatal Opioid Exposure in Outbred Mice

The opioid epidemic led to an increase in the number of neonatal opioid withdrawal syndrome (NOWS) cases in infants born to opioid-dependent mothers. Hallmark features of NOWS include weight loss, severe irritability, respiratory problems, and sleep fragmentation. Mouse models provide an opportunity to identify brain mechanisms that contribute to NOWS. Neonatal outbred Swiss Webster Cartworth Farms White (CFW) mice were administered morphine (15 mg/kg, s.c.) twice daily from postnatal day 1 (P1) to P14, an approximation of the third trimester of human gestation. Female and male mice underwent behavioral testing on P7 and P14 to determine the impact of opioid exposure on anxiety and pain sensitivity. Ultrasonic vocalizations (USVs) and daily body weights were also recorded. Brainstems containing pons and medulla were collected during morphine withdrawal on P14 for RNA sequencing. Morphine induced weight loss from P2 to P14, which persisted during adolescence (P21) and adulthood (P50). USVs markedly increased at P7 in females, emerging earlier than males. On P7 and P14, both morphine-exposed female and male mice displayed hyperalgesia on the hot plate and tail-flick assays, with females showing greater hyperalgesia than males. Morphine-exposed mice exhibited increased anxiety-like behavior in the open-field arena on P21. Transcriptome analysis of the brainstem, an area implicated in opioid withdrawal and NOWS, identified pathways enriched for noradrenergic signaling in females and males. We also found sex-specific pathways related to mitochondrial function and neurodevelopment in females and circadian entrainment in males. Sex-specific transcriptomic neuroadaptations implicate unique neurobiological mechanisms underlying NOWS-like behaviors.

Opioids: Understanding How Acute Actions Impact Chronic Consequences

In the last decade, critical-care nurses have seen a surge in acute opioid overdose admissions to intensive care units; there have also been significant increases in intensive care unit admissions due to opioid-related illness such as dependence, tolerance, and hyperalgesia. Despite these issues, opioids continue to be the criterion standard of pain management, and the search for opioid alternatives has not produced a clear replacement. A contributor to this problem has been the prevailing opinion that once bound to a receptor, all opioids engaged in the same types of intracellular signaling, which resulted in the same types of responses, only differing in the magnitude of those responses. Contemporary research with G-protein-coupled receptor models (eg, opioids) has demonstrated that this oversimplification is incorrect or incomplete. Understanding the complexity of opioid pharmacodynamics and pharmacokinetics helps us to grasp the intricacies of opioid-related adverse effects. Although there are many potential adverse effects related to opioids, this review focuses on the major adverse effects commonly seen in critical care, namely, respiratory depression, tolerance, hyperalgesia, and central sensitization. In addition, a case study has been incorporated to aid in understanding of strategies nurses can incorporate into their practices: that help mitigate the development of these effects.

Addressing opioid tolerance and opioid-induced hypersensitivity: Recent developments and future therapeutic strategies

Opioids are a commonly prescribed and efficacious medication for the treatment of chronic pain but major side effects such as addiction, respiratory depression, analgesic tolerance, and paradoxical pain hypersensitivity make them inadequate and unsafe for patients requiring long-term pain management. This review summarizes recent advances in our understanding of the outcomes of chronic opioid administration to lay the foundation for the development of novel pharmacological strategies that attenuate opioid tolerance and hypersensitivity; the two main physiological mechanisms underlying the inadequacies of current therapeutic strategies. We also explore mechanistic similarities between the development of neuropathic pain states, opioid tolerance, and hypersensitivity which may explain opioids' lack of efficacy in certain patients. The findings challenge the current direction of analgesic research in developing non-opioid alternatives and we suggest that improving opioids, rather than replacing them, will be a fruitful avenue for future research.

κ-Opioid receptor antagonism reverses heroin withdrawal-induced hyperalgesia in male and female rats

Although opioids are potent analgesics, a consequence of chronic opioid use is hyperalgesia during withdrawal, which may contribute to opioid misuse. Dynorphin, the endogenous ligand of κ-opioid receptors (KORs), is upregulated in opioid-dependent rats and in animal models of chronic pain. However, the role of KORs in opioid withdrawal-induced hyperalgesia remains to be determined. We hypothesized that KOR antagonism would reverse opioid withdrawal-induced hyperalgesia in opioid-dependent rats. Male and female Wistar rats received daily injections of heroin (2-6 mg/kg, SC) and were tested for mechanical sensitivity in the electronic von Frey test 4-6 h into withdrawal. Female rats required significantly more heroin than male rats to reach comparable levels of both heroin-induced analgesia and hyperalgesia (6 mg/kg vs. 2 mg/kg). Once hyperalgesia was established, we tested the effects of the KOR antagonists nor-binaltorphimine (norBNI; 30 mg/kg, SC) and 5'-guanidinonaltrindole (5'GNTI; 30 mg/kg, SC). When the animals continued to receive their daily heroin treatment (or saline treatment in the repeated saline group) five times per week throughout the experiment, both KOR antagonists reversed heroin withdrawal-induced hyperalgesia. The anti-hyperalgesia effect of norBNI was more prolonged in males than in females (14 days vs. 7 days), whereas 5'GNTI had more prolonged effects in females than in males (14 days vs. 4 days). The behavioral effects of 5'GNTI coincided with higher 5'GNTI levels in the brain than in plasma when measured at 24 h, whereas 5'GNTI did not reverse hyperalgesia at 30 min posttreatment when 5'GNTI levels were higher in plasma than in the brain. Finally, we tested the effects of 5'GNTI on naloxone-induced and spontaneous signs of opioid withdrawal and found no effect in either male or female rats. These findings indicate a functional role for KORs in heroin withdrawal-induced hyperalgesia that is observed in rats of both sexes.

Multifunctional Opioid-Derived Hybrids in Neuropathic Pain: Preclinical Evidence, Ideas and Challenges

When the first- and second-line therapeutics used to treat neuropathic pain (NP) fail to induce efficient analgesia—which is estimated to relate to more than half of the patients—opioid drugs are prescribed. Still, the pathological changes following the nerve tissue injury, i.a. pronociceptive neuropeptide systems activation, oppose the analgesic effects of opiates, enforcing the use of relatively high therapeutic doses in order to obtain satisfying pain relief. In parallel, the repeated use of opioid agonists is associated with burdensome adverse effects due to compensatory mechanisms that arise thereafter. Rational design of hybrid drugs, in which opioid ligands are combined with other pharmacophores that block the antiopioid action of pronociceptive systems, delivers the opportunity to ameliorate the NP-oriented opioid treatment via addressing neuropathological mechanisms shared both by NP and repeated exposition to opioids. Therewith, the new dually acting drugs, tailored for the specificity of NP, can gain in efficacy under nerve injury conditions and have an improved safety profile as compared to selective opioid agonists. The current review presents the latest ideas on opioid-comprising hybrid drugs designed to treat painful neuropathy, with focus on their biological action, as well as limitations and challenges related to this therapeutic approach.

Opioid-Induced Hyperalgesic Priming in Single Nociceptors

Clinical µ-opioid receptor (MOR) agonists produce hyperalgesic priming, a form of maladaptive nociceptor neuroplasticity, resulting in pain chronification. We have established an in vitro model of opioid-induced hyperalgesic priming (OIHP), in male rats, to identify nociceptor populations involved and its maintenance mechanisms. OIHP was induced in vivo by systemic administration of fentanyl and confirmed by prolongation of prostaglandin E₂ (PGE₂) hyperalgesia. Intrathecal cordycepin, which reverses Type I priming, or the combination of Src and mitogen-activated protein kinase (MAPK) inhibitors, which reverses Type II priming, both partially attenuated OIHP. Parallel in vitro experiments were performed on small-diameter (<30 µm) dorsal root ganglion (DRG) neurons, cultured from fentanyl-primed rats, and rats with OIHP treated with agents that reverse Type I or Type II priming. Enhancement of the sensitizing effect of a low concentration of PGE₂ (10 nm), another characteristic feature of priming, measured as reduction in action potential (AP) rheobase, was found in weakly isolectin B4 (IB4)-positive and IB4-negative (IB4-) neurons. In strongly IB4-positive (IB4+) neurons, only the response to a higher concentration of PGE₂ (100 nm) was enhanced. The sensitizing effect of 10 nm PGE₂ was attenuated in weakly IB4+ and IB4- neurons cultured from rats whose OIHP was reversed in vivo Thus, in vivo administration of fentanyl induces neuroplasticity in weakly IB4+ and IB4- nociceptors that persists in vitro and has properties of Type I and Type II priming. The mechanism underlying the enhanced sensitizing effect of 100 nm PGE₂ in strongly IB4+ nociceptors, not attenuated by inhibitors of Type I and Type II priming, remains to be elucidated.SIGNIFICANCE STATEMENT Commonly used clinical opioid analgesics, such as fentanyl and morphine, can produce hyperalgesia and chronification of pain. To uncover the nociceptor population mediating opioid-induced hyperalgesic priming (OIHP), a model of pain chronification, and elucidate its underlying mechanism, at the cellular level, we established an in vitro model of OIHP. In dorsal root ganglion (DRG) neurons cultured from rats primed with fentanyl, robust nociceptor population-specific changes in sensitization by prostaglandin E₂ (PGE₂) were observed, when compared with nociceptors from opioid naive rats. In DRG neurons cultured from rats with OIHP, enhanced PGE₂-induced sensitization was observed in vitro, with differences identified in non-peptidergic [strongly isolectin B4 (IB4)-positive] and peptidergic [weakly IB4-positive (IB4+) and IB4-negative (IB4-)] nociceptors.

Neonatal morphine exposure and maternal deprivation alter nociceptive response and central biomarkers' levels throughout the life of rats

In the present study, we investigated the effect of repeated neonatal morphine exposure and/or maternal deprivation(MD) on the nociceptive response and central biomarkers' BDNF, IL-1β, and IL-4 levels at postnatal days 16(PND16), 30(PND30), and 60(PND60). At birth, the litters were standardized to contain 8 pups/dam (n = 58). From PND1 to PND10, the pups of the deprived groups were separated daily from their mothers for 3 h and divided into 5 groups: control(C), saline(S), morphine(M), deprived-saline(DS), and deprived-morphine(DM). The pups received subcutaneous injections of saline/morphine (5 μg) in the mid-scapular area between PND8 and PND14. Nociceptive responses were assessed by hot plate(HP) and tail-flick(TFL) tests and biomarker levels by ELISA. Thermal hyperalgesia(HP) was found in all assessments for the M, DS, and DM groups, and a decrease in nociceptive threshold(TFL) was found in the DS group at PND16; M and DM groups at PND30; and M, DS, and DM groups at PND60. There were interactions between treatment/deprivation/timepoint in all central biomarkers' levels. The current study indicates that neonatal exposure to morphine and MD, which occurs in the pediatric ICU, can alter the nociceptive and neuroinflammatory responses.

Dopamine receptor D3 agonist (Pramipexole) reduces morphine-induced cardiac fibrosis

Morphine is routinely used for pain management in heart failure patients. However, extended morphine exposure associates with major adverse cardiovascular events. Reports link the dopamine receptor D2-family with morphine-induced nociception modulation. This study first assessed whether morphine induces cardiac remodeling in healthy mice, then whether DRD3 agonist (DRD3ag, D2-family member) adjunct therapy prevents morphine-induced cardiac remodeling. Mice received morphine (2 mg/kg/day i. p.) for 7 days (D7) and were either euthanized at D7 or kept 7 more days without morphine (i.e. withdrawal period, D8-D14): G1, morphine; G2, morphine/DRD3ag; G3, morphine + withdrawal; G4, morphine/DRD3ag + withdrawal; G5, morphine + withdrawal/DRD3ag. A separate cohort of animals were used as naïve tissues. We evaluated functional and molecular parameters of cardiac remodeling. Although we did not observe significant differences in systolic function, morphine induced both interstitial fibrosis and cardiomyocyte hypertrophy. Interestingly, DRD3ag abolished these effects. Compared to naïve tissues, collagen 1 increased after withdrawal in G3 and G4 and collagen 3 increased in G1-G4 but at higher levels in G1 and G2. Only G5 did not show collagen differences compared to naïve, suggesting DRD3ag treatment during withdrawal may be beneficial and prevent morphine-induced fibrosis. Smad2/3 phosphorylation increased during withdrawal, indicating a likely upstream pathway for the observed morphine-induced fibrosis. Overall, our data suggest that DRD3ag adjunct therapy decreases morphine-induced adverse cardiac remodeling.

The role of CGRP receptor antagonist (CGRP8-37) and Endomorphin-1 combination therapy on neuropathic pain alleviation and expression of Sigma-1 receptors and antioxidants in rats

BACKGROUND

Spinal cord injury is one of the most common causes of neuropathic pain which is not responsive to common treatments. Owing to the adverse effects of drugs, it seems that the use of Calcitonin Gene-Related Protein (CGRP) receptor antagonist or Morphine and their combination could be an appropriate strategy for pain alleviation.

METHOD

To achieve the objective, fifty six male Wistar rats were divided into seven groups. CGRP8-37 and Endomorphin-1 alone, and in combinated administration, as bolus and continues dose. Both mechanical and cold allodynia, and mechanical hyperalgesia were evaluated before and also15 and 60 min after injection to indicate the efficacy of the therapies in the acute and chronic circumstances on pain induced by spinal cord compression injury. Sigma-1 receptor experssion, oxidant and antioxidant activity after the seven days of the drug adminestration were evaluated.

RESULT

The results showed that Endomorphin-1and CGRP8-37 injections were able to reduce neuropathic pain after spinal cord compression injury. Compared to Endomorphin-1, or CGRP8-37 monotherapy, combination therapy did not show more attenuating effects on the pain threshold. Compared to the continous administration of Endomorphin-1 alone, and CGRP8-37 alone, the continous combination therapy did not reduce the pain further. Molecular studies disclosed the increased expression of the Sigma1 receptor, in the spinal cord after administration of Endomorphin-1, and CGRP8-37 alone, as well as combination therapy. Although, an increase in GPx and SOD activity, and decrease in MDA activity was observed in the combination therapy.

CONCLUSION

Our results demonstrate that either Endomorphin-1 or CGRP receptor antagonist is able to decrease the neuropathic pain after SCI but combination therapy by a CGRP receptor antagonist and Endomorphin-1 did not make any further reduction in pain sensation.

The Complex Balance between Analgesic Efficacy, Change of Dose and Safety Profile Over Time, in Cancer Patients Treated with Opioids: Providing the Clinicians with an Evaluation Tool

Background: Scanty data exist on the integration between the analgesic effect of opioids, dose changes, and adverse events in cancer patients. Methods: To provide further information on this issue, we analysed data on 498 advanced-stage cancer patients treated with strong opioids. At baseline and three visits (at days 7, 14, and 21), pain intensity, oral morphine-equivalent daily dose, and the prevalence of major adverse events were measured. The proportion of responders (pain intensity decrease ≥30% from baseline) and non-responders, as well as of patients with low or high dose escalation, was calculated. Results: Pain intensity strongly decreased from baseline (pain intensity difference −4.0 at day 7 and −4.2 at day 21) in responders, while it was quite stable in non-responders (pain intensity difference −0.8 at day 7 and −0.9 at day 21). In low dose escalation patients (82.4% at final visit), daily dose changed from 52.3 to 65.3 mg; in high dose escalation patients (17.6%), it varied from 94.1 to 146.7 mg. Among responders, high dose escalation patients experienced significantly more frequent adverse events compared to low or high dose escalation patients, while no differences were observed in non-responders. Conclusions: The response to opioids results from the combination of three clinical aspects, which are strongly interrelated. These results provide some thoughts to help clinical evaluations and therapeutic decisions regarding opioid use.

Cytokines in Pain: Harnessing Endogenous Anti-Inflammatory Signaling for Improved Pain Management

Current pain therapeutics offer inadequate relief to patients with chronic pain. A growing literature supports that pro-inflammatory cytokine signaling between immune, glial, and neural cells is integral to the development of pathological pain. Modulation of these communications may hold the key to improved pain management. In this review we first offer an overview of the relationships between pro-inflammatory cytokine and chemokine signaling and pathological pain, with a focus on the actions of cytokines and chemokines in communication between glia (astrocytes and microglia), immune cells (macrophages and T cells), and neurons. These interactions will be discussed in relation to both peripheral and central nervous system locations. Several novel non-neuronal drug targets for controlling pain are emerging as highly promising, including non-viral IL-10 gene therapy, which offer the potential for substantial pain relief through localized modulation of targeted cytokine pathways. Preclinical investigation of the mechanisms underlying the success of IL-10 gene therapy revealed the unexpected discovery of the powerful anti-nociceptive anti-inflammatory properties of D-mannose, an adjuvant in the non-viral gene therapeutic formulation. This review will include gene therapeutic approaches showing the most promise in controlling pro-inflammatory signaling via increased expression of anti-inflammatory cytokines like interleukin-10 (IL-10) or IL-4, or by directly limiting the bioavailability of specific pro-inflammatory cytokines, as with tumor necrosis factor (TNF) by the TNF soluble receptor (TNFSR). Approaches that increase endogenous anti-inflammatory signaling may offer additional opportunities for pain therapeutic development in patients not candidates for gene therapy. Promising novel avenues discussed here include the disruption of lymphocyte function-associated antigen (LFA-1) activity, antagonism at the cannabinoid 2 receptor (CB2R), and toll-like receptor 4 (TLR4) antagonism. Given the partial efficacy of current drugs, new strategies to manipulate neuroimmune and cytokine interactions hold considerable promise.

Sensitized brain response to acute pain in patients using prescription opiates for chronic pain: A pilot study

BACKGROUND

Chronic opiate use leads to a sensitized behavioral response to acute pain, which in turn, leads to escalating doses of opiates. This study was designed to test the hypothesis that chronic opiate usage is also associated with a sensitized neurobiological response to acute pain in individuals that have used prescription opiates for 6 or more months.

METHODS

Fourteen patients with non-alcoholic chronic pancreatitis that have been taking prescription opiates for 6 or more months and 14 gender matched, non-opiate using controls were enrolled. Functional neuroimaging data was acquired while participants received blocks of thermal stimulation to their wrist (individually-tailored to their pain threshold).

RESULTS

Self-reported pain was significantly greater in opiate using patients (3.4 ± 3.4) than controls (0.2 ± 0.8: Brief Pain Inventory p < 0.005), however no significant difference between groups was observed in the individually-tailored pain thresholds. Opiate using patients evidenced a significantly greater response to pain than controls in two established nodes of the "Pain Matrix": somatosensory cortex (p_FWE≤0.001) and anterior cingulate cortex (p ≤ 0.01). This response was positively correlated with prescribed morphine equivalent dosages (average: 133.5 ± 94.8 mg/day).

CONCLUSION

The findings suggest that in chronic pancreatitis patients, a dose of opiates that normalizes their behavioral response to acute pain is associated with an amplified neural response to acute pain. Further longitudinal studies are needed to determine if this neural sensitization hastens a behavioral tolerance to opiates or the development of an opioid use disorder.

Electroacupuncture alleviates morphine‑induced hyperalgesia by regulating spinal CB1 receptors and ERK1/2 activity

Electroacupuncture (EA), a traditional Chinese therapeutic technique, is considered an effective method for treating certain painful neuropathies induced by various neuropathological damage. The current study investigated the effect of EA on intrathecal (IT) morphine-induced hyperalgesia (MIH) and examined the hypothesis that activation of cannabinoid receptor 1 (CB1) could enhance the antinociceptive effect of EA on MIH via regulation of the extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway. Using a rat model of IT MIH, mechanical and thermal hyperalgesia were evaluated by an electronic von Frey filament and hotplate at baseline (1 day before IT administration) and at days 1, 3, 5 and 7 after IT administration. Rats received IT normal saline, IT morphine or IT morphine + EA at ST36-GB34. The protein levels of ERK1/2, phosphorylated (p)-ERK1/2 and CB1 in the spinal cord were assayed by western blotting. Furthermore, the effect of IT injection of the CB1 agonist WIN 55,212-2 and the CB1 antagonist SR141716 on the antinociceptive effect of EA in rats with MIH was investigated. Nociceptive behavior and ERK1/2, phosphorylated (p)-ERK1/2 and CB1 protein levels were evaluated as mentioned above. The results revealed that chronic IT injections of morphine induced a significant decrease in mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) accompanied with remarkable upregulation of p-ERK1/2 in the spinal cord, which could be attenuated by EA at the ST36-GB34 acupoints. In the rat model of MIH, IT injection of WIN 55,212-2 combined with EA induced a significant increase in MWT and TWL accompanied with a significant decrease in p-ERK1/2 and a significant increase in CB1 protein level compared with EA alone, while SR141716 induced the opposite results. The present study suggests that EA alleviates hyperalgesia induced by IT injection of morphine partially through the inhibition of ERK1/2 activation. Activation of the CB1 receptor enhances the antinociceptive effect of EA in rats with MIH partly through the regulation of the spinal CB1-ERK1/2 signaling pathway.

The Benzimidazole Derivatives, B1 (N-[(1H-Benzimidazol-2-yl)Methyl]-4-Methoxyaniline) and B8 (N-{4-[(1H-Benzimidazol-2-yl)Methoxy]Phenyl}Acetamide) Attenuate Morphine-Induced Paradoxical Pain in Mice

Despite being routinely used for pain management, opioid use is limited due to adverse effects such as development of tolerance and paradoxical pain, including thermal hyperalgesia and mechanical allodynia. Evidence indicates that continued morphine administration causes increased expression of proinflammatory mediators such as tumor necrosis factor-alpha (TNF-α). The objectives of the present study were to determine the effects of B1 (N-[(1H-benzimidazol-2-yl)methyl]-4-methoxyaniline) and B8 (N-{4-[(1H-benzimidazol-2-yl)methoxy]phenyl}acetamide), benzimidazole derivatives, on thermal nociception and mechanical allodynia during repeated morphine (intraperitoneal; 5 mg/kg twice daily for 6 days)-induced paradoxical pain and TNF-α expression in the spinal cord in mice. Our data indicate that administration of benzimidazole derivatives attenuated morphine-induced thermal hyperalgesia and mechanical allodynia. Benzimidazole derivatives also reduced TNF-α expression in mice. Taken together, these results suggest that benzimidazole derivatives might be useful for the treatment of neuroinflammatory consequences of continued morphine administration and could be potential drug candidates for the management of opioid-induced paradoxical pain.

Systemic Morphine Produces Dose-dependent Nociceptor-mediated Biphasic Changes in Nociceptive Threshold and Neuroplasticity

We investigated the dose dependence of the role of nociceptors in opioid-induced side-effects, hyperalgesia and pain chronification, in the rat. Systemic morphine produced a dose-dependent biphasic change in mechanical nociceptive threshold. At lower doses (0.003-0.03 mg/kg, s.c.) morphine induced mechanical hyperalgesia, while higher doses (1-10 mg/kg, s.c.) induced analgesia. Intrathecal (i.t.) oligodeoxynucleotide (ODN) antisense to mu-opioid receptor (MOR) mRNA, attenuated both hyperalgesia and analgesia. 5 days after systemic morphine (0.03-10 mg/kg s.c.), mechanical hyperalgesia produced by intradermal (i.d.) prostaglandin E₂ (PGE₂) was prolonged, indicating hyperalgesic priming at the peripheral terminal of the nociceptor. The hyperalgesia induced by i.t. PGE₂ (400 ng/10 µl), in groups that received 0.03 (that induced hyperalgesia) or 3 mg/kg (that induced analgesia) morphine, was also prolonged, indicating priming at the central terminal of the nociceptor. The prolongation of the hyperalgesia induced by i.d. or i.t. PGE₂, in rats previously treated with either a hyperalgesic (0.03 mg/kg, s.c.) or analgesic (3 mg/kg, s.c.) dose, was reversed by i.d. or i.t. injection of the protein translation inhibitor cordycepin (1 µg), indicative of Type I priming at both terminals. Although pretreatment with MOR antisense had no effect on priming induced by 0.03 mg/kg morphine, it completely prevented priming by 3 mg/kg morphine, in both terminals. Thus, the induction of hyperalgesia, but not priming, by low-dose morphine, is MOR-dependent. In contrast, induction of both hyperalgesia and priming by high-dose morphine is MOR-dependent. The receptor at which low-dose morphine acts to produce priming remains to be established.

Why mu-opioid agonists have less analgesic efficacy in neuropathic pain?

Injury to peripheral nerves often leads to abnormal pain states (hyperalgesia, allodynia and spontaneous pain), which can remain long after the injury heals. Although opioid agonists remain the gold standard for the treatment of moderate to severe pain, they show reduced efficacy against neuropathic pain. In addition to analgesia, opioid use is also associated with hyperalgesia and analgesia tolerance, whose underlying mechanisms share some commonalities with nerve injury-induced hypersensitivity. Here, we reviewed up-to-day research exploring the contribution of mu-opioid receptor (MOR) on the pathophysiology of neuropathic pain and on analgesic opioid actions under these conditions. We focused on the specific contributions of MOR populations at peripheral, spinal and supraspinal level. Moreover, evidences of neuroplastic changes that may underlie the low efficacy of MOR agonists under neuropathic pain conditions are reviewed and discussed. Sensitization processes leading to pain hypersensitivity, molecular changes in signalling pathways triggered by MOR and glial activation are some of these mechanisms elicited by both nerve injury and opioid exposure. Nerve injury-induced pain hypersensitivity might be masking the initial analgesic effects of opioid agonists, and alternatively, sustained opioid treatment to individuals already suffering from neuropathic pain could aggravate their pathophysiological state. Finally, some combined therapies that can increase opioid analgesic effectiveness in neuropathic pain treatment are highlighted. SIGNIFICANCE: This review provides evidence of the low benefit of opioid monotherapy in neuropathic pain and analyses the reasons of this reduced effectiveness. Opioid agonists along with drugs targeted to block the sensitization processes induced by MOR stimulation might result in a better management of neuropathic pain.

Evidence of opioid-induced hyperalgesia in clinical populations after chronic opioid exposure: a systematic review and meta-analysis

BACKGROUND

Opioid-induced hyperalgesia (OIH) is well documented in preclinical studies, but findings of clinical studies are less consistent. The objective was to undertake a systematic review and meta-analysis of studies examining evidence for OIH in humans after opioid exposure.

METHODS

Systematic electronic searches utilised six research databases (Embase, Medline, PubMed, CINAHL Plus, Web of Science, and OpenGrey). Manual 'grey' literature searches were also undertaken. The Population, Interventions, Comparators, Outcomes, and Study design (PICOS) framework was used to develop search strategies, and findings are reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement. Data synthesis and subgroup analyses were undertaken using a random effects model (DerSimonian-Laird method).

RESULTS

A total of 6167 articles were identified. After abstract and full-text reviews, 26 articles (involving 2706 participants) were included in the review. There was evidence of OIH, assessed by pain tolerance, in response to noxious thermal (hot and cold) stimuli, but not electrical stimuli. There was no evidence of OIH when assessing pain detection thresholds. OIH was more evident in patients with opioid use disorder than in patients with pain, and in patient groups treated with N-methyl-d-aspartate receptor antagonists (primarily evidenced in methadone-maintained populations).

CONCLUSIONS

OIH was evident in patients after chronic opioid exposure, but findings were dependent upon pain modality and assessment measures. Further studies should consider evaluating both pain threshold and pain tolerance across a range of modalities to ensure assessment validity. Significant subgroup findings suggest that potential confounders of pain judgements, such as illicit substance use, affective characteristics, or coping styles, should be rigorously controlled in future studies.

Drug dependence in patients with chronic pain: A retrospective study

Drug dependence, which can exist concurrently with chronic pain, is seen as one of the major causes of rapidly increasing medical expenses. However, drug dependence in patients with chronic pain has not been evaluated. The aim of this study was to identify the risk factors for drug dependence in patients with chronic noncancer pain.This retrospective study included 151 patients with chronic noncancer pain (43 males, 108 females; mean age, 72 years). Low back pain (LBP) occurred in 96 patients, whereas 22 had shoulder pain, 8 had hip pain, and 77 had knee pain. Patients were divided into drug dependence and nondrug dependence groups based on the Severity of Dependence Scale (SDS) scores. Patients with SDS scores ≥5 and <5 were classified into drug dependence and nondrug dependence groups, respectively. All patients completed self-report questionnaires. Factors that predict drug dependence were identified by performing univariate and multivariate analyses.Sixty (40%) of the 151 patients met the SDS criteria for drug dependence. Significant differences were found between patients with and without drug dependence for the LBP, hip pain, number of medications, and for the Numerical Rating Scale, Pain Disability Assessment Scale (PDAS), Hospital Anxiety and Depression Scale, and Pain Catastrophizing Scale (PCS) scores. Multiple regression analysis identified LBP, hip pain, PCS, and PDAS scores as factors related to drug dependence in patients with chronic noncancer pain.Drug dependence tends to differ in patients based on the location of their chronic pain. Pain catastrophizing and disability indicated a greater tendency for drug dependence. Thus, PCS and PDAS scores are useful screening tools for predicting drug dependence in patients with chronic pain.

Effects of Ketamine on Postoperative Pain After Remifentanil-Based Anesthesia for Major and Minor Surgery in Adults: A Systematic Review and Meta-Analysis

Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, has been postulated as an adjuvant analgesic for preventing remifentanil-induced hyperalgesia after surgery. This systematic review and meta-analysis aims to assess the effectiveness of ketamine [racemic mixture and S-(+)-ketamine] in reducing morphine consumption and pain intensity scores after remifentanil-based general anesthesia. We performed a literature search of the PubMed, Web of Science, Scopus, Cochrane, and EMBASE databases in June 2017 and selected randomized controlled trials using predefined inclusion and exclusion criteria. To minimize confounding and heterogeneity, studies of NMDA receptor antagonists other than ketamine were excluded and the selected studies were grouped into those assessing minor or major surgery. Methodological quality was evaluated with the PEDro and JADA scales. The data were extracted and meta-analyses were performed where possible. Twelve RCTs involving 156 adults who underwent minor surgery and 413 adults who underwent major surgery were included in the meta-analysis. When used as an adjuvant to morphine, ketamine reduced postoperative morphine consumption in the first 24 h and postoperative pain intensity in the first 2 h in the minor and major surgery groups. It was also associated with significantly reduced pain intensity in the first 24 h in the minor surgery group. Time to the first rescue analgesia was longer in patients who received ketamine and underwent major surgery. No significant differences in the incidence of ketamine-related adverse effects were observed among patients in the intervention group and controls. This systematic review and meta-analysis show that low-dose (≤0.5 mg/kg for iv bolus or ≤5 μg/kg/min for iv perfusion) of ketamine reduces postoperative morphine consumption and pain intensity without increasing the incidence of adverse effects.

Endogenous and Exogenous Opioids in Pain

Opioids are the most commonly used and effective analgesic treatments for severe pain, but they have recently come under scrutiny owing to epidemic levels of abuse and overdose. These compounds act on the endogenous opioid system, which comprises four G protein-coupled receptors (mu, delta, kappa, and nociceptin) and four major peptide families (β-endorphin, enkephalins, dynorphins, and nociceptin/orphanin FQ). In this review, we first describe the functional organization and pharmacology of the endogenous opioid system. We then summarize current knowledge on the signaling mechanisms by which opioids regulate neuronal function and neurotransmission. Finally, we discuss the loci of opioid analgesic action along peripheral and central pain pathways, emphasizing the pain-relieving properties of opioids against the affective dimension of the pain experience.

Opioid misuse in gastroenterology and non-opioid management of abdominal pain

Opioids were one of the earliest classes of medications used for pain across a variety of conditions, but morbidity and mortality have been increasingly associated with their chronic use. Despite these negative consequences, chronic opioid use is increasing worldwide, with the USA and Canada having the highest rates. Chronic opioid use for noncancer pain can have particularly negative effects in the gastrointestinal and central nervous systems, including opioid-induced constipation, narcotic bowel syndrome, worsening psychopathology and addiction. This Review summarizes the evidence of opioid misuse in gastroenterology, including the lack of evidence of a benefit from these drugs, as well as the risk of harm and negative consequences of opioid use relative to the brain-gut axis. Guidelines for opioid management and alternative pharmacological and nonpharmacological strategies for pain management in patients with gastrointestinal disorders are also discussed. As chronic pain is complex and involves emotional and social factors, a multimodal approach targeting both pain intensity and quality of life is best.

Patterns of regional variation of opioid prescribing in primary care in England: a retrospective observational study

BACKGROUND

Opioids are a widely prescribed class of drug with potentially harmful short-term and long-term side effects. There are concerns about the amounts of these drugs being prescribed in England given that they are increasingly considered ineffective in the context of long-term non-cancer pain, which is one of the major reasons for their prescription.

AIM

To assess the amount and type of opioids prescribed in primary care in England, and patterns of regional variation in prescribing.

DESIGN AND SETTING

Retrospective observational study using publicly available government data from various sources pertaining to opioids prescribed in primary practice in England and Indices of Social Deprivation.

METHOD

Official government data were analysed for opioid prescriptions from August 2010 to February 2014. The total amount of opioid prescribed was calculated and standardised to allow for geographical comparisons.

RESULTS

The total amount of opioid prescribed, in equivalent milligrams of morphine, increased (r = 0.48) over the study period. More opioids were prescribed in the north than in the south of England (r = 0.66, P<0.0001), and more opioids were prescribed in areas of greater social deprivation (r = 0.56, P<0.0001).

CONCLUSION

Long-term opioid prescribing is increasing despite poor efficacy for non-cancer pain, potential harm, and incompatibility with best practice. Questions of equality of care arise from higher prescription rates in the north of England and in areas of greater social deprivation. A national registry of patients with high opioid use would improve patient safety for this high-risk demographic, as well as provide more focused epidemiological data regarding patterns of prescribing.

Site-Specific Regulation of P2X7 Receptor Function in Microglia Gates Morphine Analgesic Tolerance

Tolerance to the analgesic effects of opioids is a major problem in chronic pain management. Microglia are implicated in opioid tolerance, but the core mechanisms regulating their response to opioids remain obscure. By selectively ablating microglia in the spinal cord using a saporin-conjugated antibody to Mac1, we demonstrate a causal role for microglia in the development, but not maintenance, of morphine tolerance in male rats. Increased P2X7 receptor (P2X7R) activity is a cardinal feature of microglial activation, and in this study we found that morphine potentiates P2X7R-mediated Ca²⁺ responses in resident spinal microglia acutely isolated from morphine tolerant rats. The increased P2X7R function was blocked in cultured microglia by PP2, a Src family protein tyrosine kinase inhibitor. We identified Src family kinase activation mediated by μ-receptors as a key mechanistic step required for morphine potentiation of P2X7R function. Furthermore, we show by site-directed mutagenesis that tyrosine (Y_382-384) within the P2X7R C-terminus is differentially modulated by repeated morphine treatment and has no bearing on normal P2X7R function. Intrathecal administration of a palmitoylated peptide corresponding to the Y_382-384 site suppressed morphine-induced microglial reactivity and preserved the antinociceptive effects of morphine in male rats. Thus, site-specific regulation of P2X7R function mediated by Y_382-384 is a novel cellular determinant of the microglial response to morphine that critically underlies the development of morphine analgesic tolerance.SIGNIFICANCE STATEMENT Controlling pain is one of the most difficult challenges in medicine and its management is a requirement of a large diversity of illnesses. Although morphine and other opioids offer dramatic and impressive relief of pain, their impact is truncated by loss of efficacy (analgesic tolerance). Understanding why this occurs and how to prevent it are of critical importance in improving pain therapies. We uncovered a novel site (Y_382-384) within the P2X7 receptor that can be targeted to blunt the development of morphine analgesic tolerance, without affecting normal P2X7 receptor function. Our findings provide a critical missing mechanistic piece, site-specific modulation by Y_382-384, that unifies P2X7R function to the activation of spinal microglia and the development of morphine tolerance.

Recent Advances in the Realm of Allosteric Modulators for Opioid Receptors for Future Therapeutics

Opioids, and more specifically μ-opioid receptor (MOR) agonists such as morphine, have long been clinically used as therapeutics for severe pain states but often come with serious side effects such as addiction and tolerance. Many studies have focused on bringing about analgesia from the MOR with attenuated side effects, but its underlying mechanism is not fully understood. Recently, focus has been geared toward the design and elucidation of the orthosteric site with ligands of various biological profiles and mixed subtype opioid activities and selectivities, but targeting the allosteric site is an area of increasing interest. It has been shown that allosteric modulators play key roles in influencing receptor function such as its tolerance to a ligand and affect downstream pathways. There has been a high variance of chemical structures that provide allosteric modulation at a given receptor, but recent studies and reviews tend to focus on the altered cellular mechanisms instead of providing a more rigorous description of the allosteric ligand's structure-function relationship. In this review, we aim to explore recent developments in the structural motifs that potentiate orthosteric binding and their influences on cellular pathways in an effort to present novel approaches to opioid therapeutic design.

CaMKIIα may modulate fentanyl-induced hyperalgesia via a CeLC-PAG-RVM-spinal cord descending facilitative pain pathway in rats

Each of the lateral capsular division of central nucleus of amygdala(CeLC), periaqueductal gray (PAG), rostral ventromedial medulla(RVM) and spinal cord has been proved to contribute to the development of opioid-induced hyperalgesia(OIH). Especially, Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα) in CeLC and spinal cord seems to play a key role in OIH modulation. However, the pain pathway through which CaMKIIα modulates OIH is not clear. The pathway from CeLC to spinal cord for this modulation was explored in the present study. Mechanical and thermal hyperalgesia were tested by von Frey test or Hargreaves test, respectively. CaMKIIα activity (phospho-CaMKIIα, p-CaMKIIα) was evaluated by western blot analysis. CaMKIIα antagonist (KN93) was micro-infused into CeLC, spinal cord or PAG, respectively, to evaluate its effect on behavioral hyperalgesia and p-CaMKIIα expression in CeLC, PAG, RVM and spinal cord. Then the underlying synaptic mechanism was explored by recording miniature excitatory postsynaptic currents (mEPSCs) on PAG slices using whole-cell voltage-clamp methods. Results showed that inhibition of CeLC, PAG or spinal CaMKIIα activity respectively by KN93, reversed both mechanical and thermal hyperalgesia. Microinjection of KN93 into CeLC decreased p-CaMKIIα expression in CeLC, PAG, RVM and spinal cord; while intrathecal KN93 can only block spinal but not CeLC CaMKIIα activity. KN93 injected into PAG just decreased p-CaMKIIα expression in PAG, RVM and spinal cord, but not in the CeLC. Similarly, whole-cell voltage-clamp recording found the frequency and amplitude of mEPSCs in PAG cells were decreased by KN93 added in PAG slice or micro-infused into CeLC in vivo. These results together with previous findings suggest that CaMKIIα may modulate OIH via a CeLC-PAG-RVM-spinal cord descending facilitative pain pathway.

Adenosine-A1 receptor agonist induced hyperalgesic priming type II

We have recently shown that repeated exposure of the peripheral terminal of the primary afferent nociceptor to the mu-opioid receptor (MOR) agonist DAMGO ([D-Ala, N-Me-Phe, Gly-ol]-enkephalin acetate salt) induces a model of transition to chronic pain that we have termed type II hyperalgesic priming. Similar to type I hyperalgesic priming, there is a markedly prolonged response to subsequent administration of proalgesic cytokines, prototypically prostaglandin E2 (PGE2). However, type II hyperalgesic priming differs from type I in being rapidly induced, protein kinase A (PKA), rather than PKCε dependent, not reversed by a protein translation inhibitor, occurring in female as well as in male rats, and isolectin B4-negative neuron dependent. We report that, as with the repeated injection of a MOR agonist, the repeated administration of an agonist at the A1-adenosine receptor, also a Gi-protein coupled receptor, N-cyclopentyladenosine (CPA), also produces priming similar to DAMGO-induced type II hyperalgesic priming. In this study, we demonstrate that priming induced by repeated exposure to this A1-adenosine receptor agonist shares the same mechanisms, as MOR-agonist induced priming. However, the prolongation of PGE2 hyperalgesia induced by repeated administration of CPA depends on G-protein αi subunit activation, differently from DAMGO-induced type II priming, in which it depends on the β/γ subunit. These data implicate a novel form of Gi-protein signaling pathway in the type II hyperalgesic priming induced by repeated administration of an agonist at A1-adenosine receptor to the peripheral terminal of the nociceptor.

Neurobiological Effects of Morphine after Spinal Cord Injury

Opioids and non-steroidal anti-inflammatory drugs are used commonly to manage pain in the early phase of spinal cord injury (SCI). Despite its analgesic efficacy, however, our studies suggest that intrathecal morphine undermines locomotor recovery and increases lesion size in a rodent model of SCI. Similarly, intravenous (IV) morphine attenuates locomotor recovery. The current study explores whether IV morphine also increases lesion size after a spinal contusion (T12) injury and quantifies the cell types that are affected by early opioid administration. Using an experimenter-administered escalating dose of IV morphine across the first seven days post-injury, we quantified the expression of neuron, astrocyte, and microglial markers at the injury site. SCI decreased NeuN expression relative to shams. In subjects with SCI treated with IV morphine, virtually no NeuN⁺ cells remained across the rostral-caudal extent of the lesion. Further, whereas SCI per se increased the expression of astrocyte and microglial markers (glial fibrillary acidic protein and OX-42, respectively), morphine treatment decreased the expression of these markers. These cellular changes were accompanied by attenuation of locomotor recovery (Basso, Beattie, Bresnahan scores), decreased weight gain, and the development of opioid-induced hyperalgesia (increased tactile reactivity) in morphine-treated subjects. These data suggest that morphine use is contraindicated in the acute phase of a spinal injury. Faced with a lifetime of intractable pain, however, simply removing any effective analgesic for the management of SCI pain is not an ideal option. Instead, these data underscore the critical need for further understanding of the molecular pathways engaged by conventional medications within the pathophysiological context of an injury.

The dark side of opioids in pain management: basic science explains clinical observation

Although there is no doubt about opioids' ability to relieve pain in the short term, it is not always clear why longer-term analgesic efficacy seems to be impaired. Tolerance and hyperalgesia have been suggested as mechanisms for opioid analgesic deterioration. But could there also be an effect of opioids on pain itself?

Introduction:

In the past 2 decades, opioids have been used increasingly for the treatment of persistent pain, and doses have tended to creep up. As basic science elucidates mechanisms of pain and analgesia, the cross talk between central pain and opioid actions becomes clearer.

Objectives:

We aimed to examine the published literature on basic science explaining pronociceptive opioid actions, and apply this knowledge to clinical observation.

Methods:

We reviewed the existing literature on the pronociceptive actions of opioids, both preclinical and clinical studies.

Results:

Basic science provides a rationale for the clinical observation that opioids sometimes increase rather than decrease pain. Central sensitization (hyperalgesia) underlies pain chronification, but can also be produced by high dose and high potency opioids. Many of the same mechanisms account for both central pain and opioid hyperalgesia.

Conclusion:

Newly revealed basic mechanisms suggest possible avenues for drug development and new drug therapies that could alter pain sensitization through endogenous and exogenous opioid mechanisms. Recent changes in practice such as the introduction of titration-to-effect for opioids have resulted in higher doses used in the clinic setting than ever seen previously. New basic science knowledge hints that these newer dosing practices may need to be reexamined. When pain worsens in a patient taking opioids, can we be assured that this is not because of the opioids, and can we alter this negative effect of opioids through different dosing strategies or new drug intervention?

Melatonin as a potential counter-effect of hyperalgesia induced by neonatal morphine exposure

Morphine administration in the neonatal period can induce long-term effects in pain circuitry leading to hyperalgesia induced by the opioid in adult life. This study explored a new pharmacological approach for reversing this effect of morphine. We focused on melatonin owing its well-known antinociceptive and anti-inflammatory effects, and its ability to interact with the opioid system. We used the formalin test to assess the medium and long-term effects of melatonin administration on hyperalgesia induced by morphine in early life. Newborn rats were divided into two groups: the control group, which received saline, and the morphine group, which received morphine (5μg subcutaneously [s.c.]) in the mid-scapular area, once daily for 7days, from P8 (postnatal day 8) until P14. At postnatal days 30 (P30) and 60 (P60), both groups were divided in two subgroups, which received melatonin or melatonin vehicle 30min before the formalin test. The nociceptive responses were assessed by analyzing the total time spent biting, flicking, and licking the formalin-injected hind paw; these responses were recorded during the first 5min (neurogenic/acute phase) and from 15 to 30min (inflammatory/tonic phase). Initially, animals in the morphine/vehicle group showed increased nociceptive behavior in phase II (inflammatory) of the formalin test at P30, and in the neurogenic and inflammatory phases at P60. These increased nociceptive responses were fully reversed by melatonin administration at either age. These findings show that melatonin administration is a potential means for countering hyperalgesia induced by neonatal morphine exposure in young and adult rats.

Inhibition of CaMKIIα in the Central Nucleus of Amygdala Attenuates Fentanyl-Induced Hyperalgesia in Rats

Opioid-induced hyperalgesia (OIH) is a less-studied phenomenon that has been reported in both preclinical and clinical studies. Although the underlying cause is not entirely understood, OIH is a real-life problem that affects millions of patients on a daily basis. Research has implicated the important contribution of Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) to OIH at the level of spinal nociceptors. To expand our understanding of the entire brain circuitry driving OIH, in this study we investigated the role of CaMKIIα in the laterocapcular division of the central amygdala (CeLC), the conjunctive point between the spinal cord and rostro-ventral medulla. OIH was produced by repeated fentanyl administration in the rat. Correlating with the development of mechanical allodynia and thermal hyperalgesia, CaMKIIα activity was significantly elevated in the CeLC in OIH. In addition, the frequency and amplitude of spontaneous miniature excitatory postsynaptic currents (mEPSCs) in CeLC neurons were significantly increased in OIH. 2-[N-(2-hidroxyethyl)-N-(4-methoxy-benzenesulfonyl)]-amino-N-(4-chlorocinnamyl)-N-methylbenzylamine, a CaMKIIα inhibitor, dose dependently reversed sensory hypersensitivity, activation of CeLC CaMKIIα, and mEPSCs in OIH. Taken together, our data for the first time implicate a critical role of CeLC CaMKIIα in OIH.

Activation of dura-sensitive trigeminal neurons and increased c-Fos protein induced by morphine withdrawal in the rostral ventromedial medulla

Aims

Overuse of medications used to treat migraine headache can increase the frequency of headaches. Sudden abstinence from migraine medication can also lead to a period of withdrawal-induced headaches. The aim of this study was to examine the effect of morphine withdrawal localized to the rostral ventromedial medulla (RVM) on the activity of dura-sensitive spinal trigeminal nucleus caudalis (Vc) neurons.

Methods

Rats were implanted with either morphine or placebo pellets for six to seven days before the microinjection of naloxone methiodide or phosphate-buffered saline into the RVM in urethane-anesthetized animals. Dura-sensitive neurons were recorded in the Vc and the production of c-Fos-like immunoreactivity was quantified.

Results

In chronic morphine-treated animals, naloxone methiodide microinjections produced a significant increase both in ongoing and facial heat-evoked activity and an increase in Fos-positive neurons in the Vc and in the nucleus reticularis dorsalis, a brainstem region involved in diffuse noxious inhibitory controls.

Conclusions

These results indicate that activation of pronociceptive neurons in the RVM under conditions of morphine withdrawal can increase the activity of neurons that transmit headache pain. Modulation of the subnucleus reticularis dorsalis by the RVM may explain the attenuation of conditioned pain modulation in patients with chronic headache.

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

Background

Opioids 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.

Results

Four 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.

Conclusions

Spinal 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.