Model of methadone-induced hyperalgesia in rats and effect of memantine

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.

Chronic Opioid Therapy and Central Sensitization in Sickle Cell Disease

Chronic opioid therapy (COT) for chronic non-cancer pain is frequently debated, and its effectiveness is unproven in sickle cell disease (SCD). The authors conducted a descriptive study among 83 adult SCD patients and compared the severity of disease and pain symptoms among those who were prescribed COT (n=29) with those who were not using COT. All patients completed baseline laboratory pain assessment and questionnaires between January 2010 and June 2014. Thereafter, participants recorded daily pain, crises, function, and healthcare utilization for 90 days using electronic diaries. Analyses were conducted shortly after the final diary data collection period. Patients on COT did not differ on age, sex, or measures of disease severity. However, patients on COT exhibited greater levels of clinical pain (particularly non-crisis); central sensitization; and depression and increased diary measures of pain severity, function, and healthcare utilization on crisis and non-crisis diary days, as well as a greater proportion of days in crisis. Including depressive symptoms in multivariate models did not change the associations between COT and pain, interference, central sensitization, or utilization. Additionally, participants not on COT displayed the expected positive relationship between central sensitization and clinical pain, whereas those on COT demonstrated no such relationship, despite having both higher central sensitization and higher clinical pain. Overall, the results point out a high symptom burden in SCD patients on COT, including those on high-dose COT, and suggest that nociceptive processing in SCD patients on COT differs from those who are not.

Opioid-induced central immune signaling: implications for opioid analgesia

Despite being the mainstay of pain management, opioids are limited in their clinical utility by adverse effects, such as tolerance and paradoxical hyperalgesia. Research of the past 15 years has extended beyond neurons, to implicate central nervous system immune signaling in these adverse effects. This article will provide an overview of these central immune mechanisms in opioid tolerance and paradoxical hyperalgesia, including those mediated by Toll-like receptor 4, purinergic, ceramide, and chemokine signaling. Challenges for the future, as well as new lines of investigation will be highlighted.

Role of Methadone in Induction and/or Exacerbation of Cluster Headache in Patients Treated for Opioid Addiction

Methadone is a potent opioid agonist widely used in opioid maintenance therapy. In some countries, methadone is available for pain treatment. We report the cases of two patients with history of substance abuse (mainly heroin), who presented with cluster headache possibly related to high-dose methadone. One possible explanation for the severe pain described in these cases is hyperalgesia induced by high doses of methadone.

The effect of repeated intramuscular alfentanil injections on experimental pain and abuse liability indices in healthy males

OBJECTIVE

Opioid-induced hyperalgesia (OIH), increased sensitivity to noxious stimuli after repeated opioid exposures, has been demonstrated in preclinical studies. However, there is no accepted, prospective model of OIH after repeated opioid exposures currently available in humans. This study assessed a potential prospective OIH model.

METHODS

Double-blind intramuscular injections of a short-acting opioid (alfentanil 15 mcg/kg; N=8) were compared to active placebo (diphenhydramine 25 mg; N=3) on cold and pressure pain testing and standard abuse liability measures in eight 10-hour sessions (1 injection/session) over 4 to 5 weeks in healthy, pain-free males. Decreases from session baseline pain threshold (PThr) and tolerance (PTol) were calculated to represent hyperalgesia, and were assessed both within and across sessions.

RESULTS

Mean decreases in cold PTol were seen in the alfentanil group at 180 minutes (-3.8 s, ±26.5) and 480 minutes (-1.63 s, ±31.5) after drug administration. There was a trend for differences between conditions on cold PThr hyperalgesia but not for pressure PThr. Alfentanil participants had greater mean ratings on Liking and High visual analog scales at peak effects (30 min), but these scores did not change across sessions.

DISCUSSION

Repeated alfentanil exposures over 4 to 5 weeks resulted in within session decreases in cold pain tolerance from baseline but these differences were not substantially different from diphenhydramine controls. The results did not support the phenomenon of OIH in this model, although definitive conclusions regarding the existence of OIH in humans likely requires a larger sample size or an alternative model.

Complications of long-term opioid therapy for management of chronic pain: the paradox of opioid-induced hyperalgesia

While opioids remain a valid and effective analgesic strategy for patients suffering from a wide variety of painful conditions, they are not a panacea. Increasingly, physicians must balance patient expectations of adequate pain control with known limitations of opioid pharmaceuticals including adverse effects, tolerance, addiction, withdrawal, and drug diversion. Further complicating the issue over the last decade is a growing body of evidence suggesting chronic opioid use may unexpectedly worsen the perception of pain in some individuals. This syndrome, termed opioid-induced hyperalgesia (OIH), fundamentally changes our understanding of opioid pharmacodynamics and may influence our approach to management of chronic pain. This manuscript describes the concept OIH and provides an overview of basic science and clinical research to date attempting to characterize this syndrome, as well as ascertain its clinical relevance. The potential existence of OIH in humans is framed within the context of our current understanding of opioids and our prescribing patterns so that physicians may begin to incorporate these ideas into their philosophy of pain management as further information develops. Animal studies reliably validate OIH in controlled models. Rigorous research protocols in humans are lacking, and we cannot yet confidently conclude that OIH manifests in clinically significant ways. However, clinicians should consider the possibility of OIH when evaluating outcomes of patients on chronic opioid therapy.

The effect of intermittent alcohol vapor or pulsatile heroin on somatic and negative affective indices during spontaneous withdrawal in Wistar rats

RATIONALE

Once dependent on alcohol or opioids, negative affect may accompany withdrawal. Dependent individuals are hypothesized to "self-medicate" in order to cope with withdrawal, which promotes escalated alcohol and drug use.

OBJECTIVES

The current study aimed to develop a reliable animal model to assess symptoms that occur during spontaneous alcohol and opioid withdrawal.

METHODS

Dependence was induced using intermittent alcohol exposure or pulsatile heroin delivery and assessed for the presence of withdrawal symptoms during acute withdrawal by measuring somatic signs, behavior in the forced swim test (FST), and air-puff-induced 22-kHz ultrasonic vocalizations (USVs). Additional animals subjected to 8 weeks of alcohol vapor exposure were evaluated for altered somatic signs, operant alcohol self-administration, and 22-kHz USV production, as well as performance in the elevated plus maze (EPM).

RESULTS

During spontaneous withdrawal from pulsatile heroin or intermittent alcohol vapor, animals displayed increased somatic withdrawal signs, FST immobility, and 22-kHz USV production but did not show any behavioral change in the EPM unless the duration of alcohol exposure was extended to 4 weeks. Following 8 weeks of alcohol vapor exposure, animals displayed somatic withdrawal signs, escalated alcohol self-administration, and increased 22-kHz USVs.

CONCLUSIONS

These paradigms provide consistent methods to evaluate the behavioral ramifications, and neurobiological substrates, of alcohol and opioid dependence during spontaneous withdrawal. As immobility in the FST and percent open-arm time in the EPM were dissociable, with 22-kHz USVs paralleling immobility in the FST, assessment of air-puff-induced 22-kHz USVs could provide an ethologically valid alternative to the FST.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Postoperative pain trajectories in chronic pain patients undergoing surgery: the effects of chronic opioid pharmacotherapy on acute pain

For 2 weeks following surgery, 55 patients with preexisting chronic pain (CP) reported daily postoperative pain with movement and at rest. Of these, 30 CP patients used opioid pharmacotherapy for CP management and 25 did not. We modeled pain resolution in each patient using a linear fit so that each patient yielded 2 scores for each pain rating: 1) an intercept, or initial level of pain, immediately after surgery; and 2) a slope, or rate of pain resolution. The patients not using opioid pharmacotherapy had a mean pain with movement intercept of 5.4 and a slope of -.20, while the patients using opioid pharmacotherapy had a significantly higher mean intercept of 7.68 (P = .001) and a slope of -.21, sustaining higher pain levels over days. The opioid pharmacotherapy patients had the same rate of pain resolution as the other CP patients, and both groups resolved their pain more slowly than normal surgery patients. Preexisting CP may predispose a patient undergoing surgery to a slower rate of postoperative pain resolution. Chronic pain patients who use opioids share this predisposition but in addition, they are at risk for markedly higher postoperative pain across the entire pain resolution trajectory.

PERSPECTIVE

This is an observational rather than a randomized controlled study, and as such is less definitive. Nonetheless, these findings are consistent with those of animal studies showing that prolonged exposure to opioids can produce opioid-induced hyperalgesia. Patients with opioid pharmacotherapy for chronic pain who undergo surgery merit special attention for acute pain management.

Exploring the neuroimmunopharmacology of opioids: an integrative review of mechanisms of central immune signaling and their implications for opioid analgesia

Vastly stimulated by the discovery of opioid receptors in the early 1970s, preclinical and clinical research was directed at the study of stereoselective neuronal actions of opioids, especially those played in their crucial analgesic role. However, during the past decade, a new appreciation of the non-neuronal actions of opioids has emerged from preclinical research, with specific appreciation for the nonclassic and nonstereoselective sites of action. Opioid activity at Toll-like receptors, newly recognized innate immune pattern recognition receptors, adds substantially to this unfolding story. It is now apparent from molecular and rodent data that these newly identified signaling events significantly modify the pharmacodynamics of opioids by eliciting proinflammatory reactivity from glia, the immunocompetent cells of the central nervous system. These central immune signaling events, including the release of cytokines and chemokines and the associated disruption of glutamate homeostasis, cause elevated neuronal excitability, which subsequently decreases opioid analgesic efficacy and leads to heightened pain states. This review will examine the current preclinical literature of opioid-induced central immune signaling mediated by classic and nonclassic opioid receptors. A unification of the preclinical pharmacology, neuroscience, and immunology of opioids now provides new insights into common mechanisms of chronic pain, naive tolerance, analgesic tolerance, opioid-induced hyperalgesia, and allodynia. Novel pharmacological targets for future drug development are discussed in the hope that disease-modifying chronic pain treatments arising from the appreciation of opioid-induced central immune signaling may become practical.

Opioid-induced hyperalgesia: clinically relevant or extraneous research phenomenon?

Opioids have become the unequivocal therapy of choice in treating many varieties of chronic pain. With the increased prescription of opioids, some unintended consequences have occurred. After prolonged opioid exposure, opioid-induced hyperalgesia (OIH), the paradoxical effect that opioid therapy may in fact enhance or aggravate preexisting pain, may occur. Over the past several decades, an increasing number of laboratory and clinical reports have suggested lowered pain thresholds and heightened atypical pain unrelated to the original perceived pain sensations as hallmarks of OIH. However, not all evidence supports the clinical importance of OIH, and some question whether the phenomenon exists at all. Here, we present a nonexhaustive, brief review of the recent literature. OIH will be reviewed in terms of preclinical and clinical evidence for and against its existence; recommendations for clinical evaluation and intervention also will be discussed.