Endogenous opioid function mediates the association between laboratory-evoked pain sensitivity and morphine analgesic responses

Psychophysics of Pain: A Methodological Introduction

For over 100 years, psychophysics ..÷ the scientific study between physical stimuli and sensation ... has been successfully employed in numerous scientific and healthcare disciplines, as an objective measure of sensory phenomena. This manuscript provides an overview of fundamental psychophysical concepts, emphasizing pain and research application..÷defining common terms, methods, and procedures.Psychophysics can provide systematic and objective measures of sensory perception that can be used by nursing scientists to explore complex, subjective phenomena..÷such as pain perception. While there needs to be improved standardization of terms and techniques, psychophysical approaches are diverse and may be tailored to address or augment current research paradigms. The interdisciplinary nature of psychophysics..÷like nursing..÷provides a unique lens for understanding how our perceptions are influenced by measurable sensations. While the quest to understand human perception is far from complete, nursing science has an opportunity to contribute to pain research by using the techniques and methods available through psychophysical procedures.

Prospective evaluation of cerebrospinal fluid levels of β-Endorphin as a predictor of opioid use after scheduled cesarean delivery

Background

Prior laboratory work indicates that lower endogenous opioid function is associated with greater analgesic and subjective responses to opioid analgesics. We evaluated whether lower preoperative cerebrospinal uid (CSF) levels of the analgesic endogenous opioid β-Endorphin (BE) were associated with increased opioid use after cesarean delivery (CD).

Methods

We enrolled 136 pregnant women without opioid use or chronic pain who were undergoing CD under regional anesthesia. Preoperatively, participants completed validated pain measures and biospecimens were collected to assess BE levels in plasma and CSF. Postoperatively, pain measures at 48 hours and 2 weeks postpartum were assessed. We evaluated the association between CSF BE levels and total opioid use (in morphine milligram equivalents; MMEs) using linear regression controlling for confounding factors (primary analysis). In secondary analyses, we examined: 1) associations between plasma BE levels and total opioid use, and 2) associations between CSF and plasma BE levels and secondary outcomes (inpatient versus outpatient opioid use, pain intensity).

Results

Participants completed surveys with 100% response rate. The majority were non-Hispanic white (65%), college educated (58%), had private insurance (71%), and had a prior cesarean delivery (69%). Psychiatric diagnoses (depression or anxiety) were common, both currently (22%) and in the past (26%).The median total opioid use across the inpatient and 2-week postpartum follow-up period was 89.1 milligram morphine equivalents (IQR 25-138). Preoperative cerebrospinal uid β-Endorphin levels were not associated with total opioid use (beta = -0.05, SE 0.45, p = 0.64). Similar findings were noted for plasma β-Endorphin levels. cerebrospinal uid β-Endorphin levels were only weakly correlated with plasma β-Endorphin levels (r = 0.30, p < .01). Preoperative cerebrospinal uid and plasma β-Endorphin levels were both positively associated with postpartum pain measures (cerebrospinal uid: at 48 hours, beta = 0.19, SE 0.16, p < 0.05; Plasma: at 48-hours, beta = 0.02, SE 0.03, p = 0.02, and at 2-weeks, beta = 0.27, SE 0.03, p < 0.01).

Conclusions

Lower preoperative cerebrospinal uid levels of β-Endorphin are not associated with increased opioid analgesic use after scheduled cesarean delivery. It is possible that unassessed variability in baseline opioid receptor sensitivity may have confounded ability to test associations between β-Endorphin levels and opioid use outcomes.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Toward Composite Pain Biomarkers of Neuropathic Pain—Focus on Peripheral Neuropathic Pain

Chronic pain affects ~10–20% of the U.S. population with an estimated annual cost of $600 billion, the most significant economic cost of any disease to-date. Neuropathic pain is a type of chronic pain that is particularly difficult to manage and leads to significant disability and poor quality of life. Pain biomarkers offer the possibility to develop objective pain-related indicators that may help diagnose, treat, and improve the understanding of neuropathic pain pathophysiology. We review neuropathic pain mechanisms related to opiates, inflammation, and endocannabinoids with the objective of identifying composite biomarkers of neuropathic pain. In the literature, pain biomarkers typically are divided into physiological non-imaging pain biomarkers and brain imaging pain biomarkers. We review both types of biomarker types with the goal of identifying composite pain biomarkers that may improve recognition and treatment of neuropathic pain.

A Theoretical Endogenous Opioid Neurobiological Framework for Co-occurring Pain, Trauma, and Non-suicidal Self-injury

PURPOSE OF REVIEW

Individuals with chronic pain are significantly more likely to have experienced overwhelming trauma early and often in key developmental years. There is increasing acknowledgment that childhood trauma disrupts how individuals process and cope with both physical and emotional pain. Emerging studies acknowledge elevated rates of non-suicidal self-injury (NSSI) in chronic pain populations. This review provides a theoretical framework to understand the relationship between NSSI behavior and pain experience in persons with chronic pain and childhood trauma histories. We discuss how NSSI may act to regulate neurobiological (e.g., endogenous opioid systems) and psychological (e.g., heightened negative affect and emotion dysregulation) systems affected by childhood trauma, leading to temporary pain relief and a cycle of negative reinforcement perpetuating NSSI. As these concepts are greatly understudied in pain populations, this review focuses on key areas relevant to chronic pain that may provide a testable, conceptual framework to support hypothesis generation, future empirical investigation, and intervention efforts.

RECENT FINDINGS

See Fig. 1. See Fig. 1.

Anxiety enhances pain in a model of osteoarthritis and is associated with altered endogenous opioid function and reduced opioid analgesia

Introduction

Negative affect, including anxiety and depression, is prevalent in chronic pain states such as osteoarthritis (OA) and associated with greater use of opioid analgesics, potentially contributing to present and future opioid crises.

Objectives

We tested the hypothesis that the interaction between anxiety, chronic pain, and opioid use results from altered endogenous opioid function.

Methods

A genetic model of negative affect, the Wistar-Kyoto (WKY) rat, was combined with intra-articular injection of monosodium iodoacetate (MIA; 1 mg) to mimic clinical presentation. Effects of systemic morphine (0.5-3.5 mg·kg^-1) on pain behaviour and spinal nociceptive neuronal activity were compared in WKY and normo-anxiety Wistar rats 3 weeks after MIA injection. Endogenous opioid function was probed by the blockade of opioid receptors (0.1-1 mg·kg^-1 systemic naloxone), quantification of plasma β-endorphin, and expression and phosphorylation of spinal mu-opioid receptor (MOR).

Results

Monosodium iodoacetate-treated WKY rats had enhanced OA-like pain, blunted morphine-induced analgesia, and greater mechanical hypersensitivity following systemic naloxone, compared with Wistar rats, and elevated plasma β-endorphin levels compared with saline-treated WKY controls. Increased MOR phosphorylation at the master site (serine residue 375) in the spinal cord dorsal horn of WKY rats with OA-like pain (P = 0.0312) indicated greater MOR desensitization.

Conclusions

Reduced clinical analgesic efficacy of morphine was recapitulated in a model of high anxiety and OA-like pain, in which endogenous opioid tone was altered, and MOR function attenuated, in the absence of previous exogenous opioid ligand exposure. These findings shed new light on the mechanisms underlying the increased opioid analgesic use in high anxiety patients with chronic pain.

Does aerobic exercise training alter responses to opioid analgesics in individuals with chronic low back pain? A randomized controlled trial

ABSTRACT

We tested whether aerobic exercise training altered morphine analgesic responses or reduced morphine dosages necessary for adequate analgesia. Patients with chronic back pain were randomized to an 18-session aerobic exercise intervention (n = 38) or usual activity control (n = 45). Before and after the intervention, participants underwent 3 laboratory sessions (double-blinded, crossover) to assess effects of saline placebo, i.v. morphine (0.09 mg/kg), and i.v. naloxone (12 mg) on low back pain and evoked heat pain responses. Differences in evoked and back pain measures between the placebo and morphine conditions indexed morphine analgesia, with pre-post intervention changes the primary outcome. Endogenous opioid analgesia was indexed by differences in evoked and low back pain measures between the naloxone and placebo conditions. A Sex X Intervention interaction on the analgesic effects of morphine on visual analogue scale back pain intensity was observed (P = 0.046), with a similar trend for evoked pain threshold (P = 0.093). Male exercisers showed reduced morphine analgesia pre-post intervention, whereas male controls showed increased analgesia (with no differences in females). Of clinical significance were findings that relative to the control group, aerobic exercise produced analgesia more similar to that observed after receiving ≈7 mg morphine preintervention (P < 0.045). Greater pre-post intervention increases in endogenous opioid function (from any source) were significantly associated with larger pre-post intervention decreases in morphine analgesia (P < 0.046). The overall pattern of findings suggests that regular aerobic exercise has limited direct effects on morphine responsiveness, reducing morphine analgesia in males only.

Are endogenous opioid mechanisms involved in the effects of aerobic exercise training on chronic low back pain? A randomized controlled trial

Aerobic exercise is believed to be an effective chronic low back pain (CLBP) intervention, although its mechanisms remain largely untested. This study evaluated whether endogenous opioid (EO) mechanisms contributed to the analgesic effects of an aerobic exercise intervention for CLBP. Individuals with CLBP were randomized to a 6-week, 18-session aerobic exercise intervention (n = 38) or usual activity control (n = 44). Before and after the intervention, participants underwent separate laboratory sessions to assess responses to evoked heat pain after receiving saline placebo or intravenous naloxone (opioid antagonist) in a double-blinded, crossover fashion. Chronic pain intensity and interference were assessed before and after the intervention. Endogenous opioid analgesia was indexed by naloxone-placebo condition differences in evoked pain responses (blockade effects). Relative to controls, exercise participants reported significantly greater pre-post intervention decreases in chronic pain intensity and interference (Ps < 0.04) and larger reductions in placebo condition evoked pain responsiveness (McGill Pain Questionnaire-Short Form [MPQ]-Total). At the group level, EO analgesia (MPQ-Total blockade effects) increased significantly pre-post intervention only among female exercisers (P = 0.03). Dose-response effects were suggested by a significant positive association in the exercise group between exercise intensity (based on meeting heart rate targets) and EO increases (MPQ-Present Pain Intensity; P = 0.04). Enhanced EO analgesia (MPQ-Total) was associated with a significantly greater improvement in average chronic pain intensity (P = 0.009). Aerobic exercise training in the absence of other interventions appears effective for CLBP management. Aerobic exercise-related enhancements in endogenous pain inhibition, in part EO-related, likely contribute to these benefits.

Greater Conditioned Pain Modulation Is Associated With Enhanced Morphine Analgesia in Healthy Individuals and Patients With Chronic Low Back Pain

OBJECTIVES

Conditioned pain modulation (CPM) protocols index magnitude of descending pain inhibition. This study evaluated whether the degree of CPM, controlling for CPM expectancy confounds, was associated with analgesic and subjective responses to morphine and whether chronic pain status or sex moderated these effects.

MATERIALS AND METHODS

Participants included 92 individuals with chronic low back pain and 99 healthy controls, none using daily opioid analgesics. In a cross-over design, participants attended 2 identical laboratory sessions during which they received either intravenous morphine (0.08 mg/kg) or saline placebo before undergoing evoked pain assessment. In each session, participants engaged in ischemic forearm and heat pain tasks, and a CPM protocol combining ischemic pain (conditioning stimulus) and heat pain (test stimulus). Placebo-controlled morphine outcomes were derived as differences in pain and subjective effects across drug conditions.

RESULTS

In hierarchical regressions controlling for CPM expectancies, greater placebo-condition CPM was associated with less subjective morphine unpleasantness (P=0.001) and greater morphine analgesia (P's<0.05) on both the ischemic pain task (Visual Analog Scale Pain Intensity and Unpleasantness) and heat pain task (Visual Analog Scale Pain Intensity, McGill Pain Questionnaire-Sensory, and Present Pain Intensity subscales). There was no moderation by sex or chronic low back pain status, except for the ischemic Present Pain Intensity outcome for which a significant 2-way interaction (P<0.05) was noted, with men showing a stronger positive relationship between CPM and morphine analgesia than women.

DISCUSSION

Results suggest that CPM might predict analgesic and subjective responses to opioid administration. Further evaluation of CPM as an element of precision pain medicine algorithms may be warranted.

Recent advances in the opioid mu receptor based pharmacotherapy for rheumatoid arthritis

INTRODUCTION

Opioids are used for severe forms of acute and cancer pain. Over the last years, their potential use in patients with noncancer pain such as those with rheumatoid arthritis (RA) has been postulated. A recent population-based comparative study showed that chronic opioid use was 12% vs. 4% among RA and non-RA patients, respectively. Another study showed an increase from 7.4% to 16.9% (2002 to 2015). In general, there has been an increasing tendency to use opioids in recent years.

AREAS COVERED

The authors have performed an extensive literature search using PubMed for articles including noncancer pain and the use of the mu opioid receptor (MOR) agonists in patients with RA.

EXPERT OPINION

Data is not sufficient to support opioid use for the treatment of chronic pain in patients with RA. Data is scarce and inconclusive. Rheumatologists should think and ponder the question: Why is this patient in pain? Differential diagnosis should include a disease flare, degenerative changes of the musculoskeletal system, and fibromyalgia. And while there are new strategies for opioid administration currently being researched, unfortunately, they are far from being applied to human subjects in the everyday clinical setting, and are still being evaluated at an experimental level. CNS: Central nervous system; DORs: delta opioid receptor agonists; GI: Gastrointestinal; GPCRs: G protein-coupled receptors; IL: Interleukin; JAK: Janus kinase; KORs: kappa opioid receptor agonists; MCPs: Metacarpophalangeal joints; MORs: Mu opioid receptor agonists; MTPs: Metatarsophalangeal joints; NSAIDs: Non-steroidal anti-inflammatory drugsOA: Osteoarthritis; ORs: Opioid receptors; PD: Pharmacodynamic; PIPs: Proximal interphalangeal joints; PK: Pharmacokinetic; PNS: Peripheral nervous system; RA: Rheumatoid arthritis; RGS: Regulator of G protein signaling; SSRIs: Selective serotonin reuptake inhibitors; TNF: Tumor necrosis factor.

Common Biological Modulators of Acute Pain: An Overview Within the AAAPT Project (ACTTION-APS-AAPM Acute Pain Taxonomy)

OBJECTIVE

The ACTTION-APS-AAPM Acute Pain Taxonomy (AAAPT) project relies on the identification of modulators to improve characterization and classification of acute pain conditions. In the frame of the AAAPT effort, this paper presents an overview of common biological modulators of acute pain.

METHODS

Nonsystematic overview.

RESULTS

Females may experience more acute pain than males, but the clinical significance may be modest. Increasing age is associated with decreasing analgesic requirement and decreasing pain intensity after surgery and with higher risk of acute low back pain. Racial and ethnic minorities have worse pain, function, and perceived well-being. Patients with preexisting chronic pain and opioid use are at higher risk of severe acute pain and high opioid consumption. The OPRM1 gene A118G polymorphism is associated with pain severity and opioid consumption, with modest quantitative impact. Most studies have found positive associations between pain sensitivity and intensity of acute clinical pain. However, the strength of the association is unclear. Surgical techniques, approaches, and complications influence postoperative pain.

CONCLUSIONS

Sex, age, race, ethnicity, preexisting chronic pain and opioid use, surgical approaches, genetic factors, and pain sensitivity are biological modulators of acute pain. Large studies with multisite replication will quantify accurately the association between modulators and acute pain and establish the value of modulators for characterization and classification of acute pain conditions, as well as their ability to identify patients at risk of uncontrolled pain. The development and validation of quick, bed-side pain sensitivity tests would allow their implementation as clinical screening tools. Acute nonsurgical pain requires more investigation.

Geopersonality of Preventable Death in the United States: Anger-Prone States and Opioid Deaths

BACKGROUND

Opioid overdoses have reached epidemic levels in the United States and have clustered in Northeastern and "Rust Belt" states. Five Factor Model (FFM) personality traits also vary at the state level, with anger-prone traits clustered in the Northeast region. This study tested the hypothesis that state-level anger proneness would be associated with a greater increase in rates of opioid overdose death.

METHODS

This was a secondary analysis of state-level data on FFM traits, opioid overdose deaths, and other classes of preventable death. Robust mixed models tested whether change in rates of opioid overdose death from 2008 to 2016 was moderated by state-level anger proneness.

RESULTS

State-level anger proneness was significantly associated with greater increases in rates of opioid overdose deaths (B = 1.01, standard error = 0.19, P < .001, 95% confidence interval: 0.63-1.39). The slope of increase in opioid overdose death rates was 380% greater in anger-prone states and held after adjustment for potential confounders such as state-level prevalence of major depressive disorder, number of mental health facilities, and historical patterns of manufacturing decline. A similar pattern was observed between state-level anger proneness and benzodiazepine overdose deaths but was not significant for the latter after adjustment for potential confounders.

CONCLUSION

These findings suggest that states characterized as more anger prone have experienced greater increases in opioid overdose deaths.

Is Resolution of Chronic Pain Associated With Changes in Blood Pressure-related Hypoalgesia?

Background

In healthy individuals, elevated resting blood pressure (BP) is associated with reduced pain responsiveness and lower temporal summation. Prior work indicates that this BP-related hypoalgesia is reduced in individuals with chronic pain.

Purpose

This study evaluated whether resolution of chronic pain was associated with greater BP-related hypoalgesia compared to nonresolution.

Methods

From a prospective sample of adolescents and young adults diagnosed with chronic functional abdominal pain an average of 9 years earlier, 99 individuals in whom the condition had resolved and 51 individuals with ongoing abdominal pain were studied. Resting systolic BP was assessed, followed by evaluation of thermal pain threshold and tolerance, and assessment of temporal summation to thermal pain stimuli.

Results

Higher resting systolic BP was significantly associated with higher pain threshold and tolerance, and lower temporal summation only in the group with resolved functional abdominal pain (p < .05). Hierarchical regressions revealed that interactions between BP and resolution of chronic pain were significant only for pain tolerance (p < .05). Analyses by sex indicated that interactions between BP and resolution status were significant for the temporal summation outcome in males but not in females.

Conclusions

Results suggest that BP-related hypoalgesic mechanisms may be more effective in individuals in whom chronic pain has resolved compared to those with ongoing chronic pain. Findings hint at sex differences in the extent to which resolution of chronic pain is associated with BP-related hypoalgesia. Whether greater BP-related hypoalgesia is a consequence of, or alternatively a contributor to, resolution of chronic pain warrants further investigation.

Effect of morphine added to multimodal cocktail on infiltration analgesia in total knee arthroplasty: A meta-analysis of randomized controlled trials

BACKGROUND

The local injection of multimodal cocktail is currently commonly used in the treatment of postoperative pain after total knee arthroplasty (TKA). It is still inconclusive whether the morphine added to the intraoperative injection mixture could make some difference. This meta-analysis aimed to evaluate the efficacy and safety of additional morphine injection on postoperative analgesia in TKA, and provide some useful information on morphine usage in clinical practice.

METHODS

The randomized controlled trials (RCTs) in databases including PubMed, Web of Science, Embase, Cochrane Library, Chinese biomedical literature database (CBM), and Chinese National Knowledge Infrastructure (CNKI) databases were systematically searched. Of 623 records identified, 8 RCTs involving 1093 knees were eligible for data extraction and meta-analysis according to criteria included.

RESULTS

Meta-analysis showed that the use of local morphine injection was not associated with significant pain relief within 48 hours postoperatively at rest and on motion (P > .05, all). The use of morphine reduced postoperative total systemic opioids consumption (P < .05). This study found no significant differences in other outcomes including knee flexion range of motion (ROM) (P > .05), extension ROM (P > .05), The Western Ontario and McMaster Universities Arthritis Index (WOMAC) scores (P > .05), Post-operative nausea and vomiting occurrence (P > .05) regardless of the presence of morphine or not in the injections.

CONCLUSION

Additional morphine added to multimodal cocktail did not decrease the postoperative pain scores significantly based on our outcomes, but it reduced the systemic postoperative opioids consumption in total knee arthroplasty.

AAAPT Diagnostic Criteria for Acute Pain Following Breast Surgery

Acute pain after breast surgery decreases the quality of life of cancer survivors. Previous studies using a variety of definitions and methods report prevalence rates between 10% and 80%, which suggests the need for a comprehensive framework that can be used to guide assessment of acute pain and pain-related outcomes after breast surgery. A multidisciplinary task force with clinical and research expertise performed a focused review and synthesis and applied the 5 dimensional framework of the AAAPT (Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks [ACTTION], American Academy of Pain Medicine [AAPM], American Pain Society [APS] Pain Taxonomy) to acute pain after breast surgery. Application of the AAAPT taxonomy yielded the following: 1) Core Criteria: Location, timing, severity, and impact of breast surgery pain were defined; 2) Common Features: Character and expected trajectories were established in relevant surgical subgroups, and common pain assessment tools for acute breast surgery pain identified; 3) Modulating Factors: Biological, psychological, and social factors that modulate interindividual variability were delineated; 4) Impact/Functional Consequences: Domains of impact were outlined and defined; 5) Neurobiologic Mechanisms: Putative mechanisms were specified ranging from nerve injury, inflammation, peripheral and central sensitization, to affective and social processing of pain. PERSPECTIVE: The AAAPT provides a framework to define and guide improved assessment of acute pain after breast surgery, which will enhance generalizability of results across studies and facilitate meta-analyses and studies of interindividual variation, and underlying mechanism. It will allow researchers and clinicians to better compare between treatments, across institutions, and with other types of acute pain.

An Open Trial of Morning Bright Light Treatment Among US Military Veterans with Chronic Low Back Pain: A Pilot Study

OBJECTIVE

To examine the feasibility, acceptability, and effects of a home-based morning bright light treatment on pain, mood, sleep, and circadian timing in US veterans with chronic low back pain.

DESIGN

An open treatment trial with a seven-day baseline, followed by 13 days of a one-hour morning bright light treatment self-administered at home. Pain, pain sensitivity, mood, sleep, and circadian timing were assessed before, during, and after treatment.

SETTING

Participants slept at home, with weekly study visits and home saliva collections.

PARTICIPANTS

Thirty-seven US veterans with medically verified chronic low back pain.

METHODS

Pain, mood, and sleep quality were assessed with questionnaires. Pain sensitivity was assessed using two laboratory tasks: a heat stimulus and an ischemia stimulus that gave measures of threshold and tolerance. Sleep was objectively assessed with wrist actigraphy. Circadian timing was assessed with the dim light melatonin onset.

RESULTS

Morning bright light treatment led to reduced pain intensity, pain behavior, thermal pain threshold sensitivity, post-traumatic stress disorder symptoms, and improved sleep quality (P < 0.05). Phase advances in circadian timing were associated with reductions in pain interference (r = 0.55, P < 0.05).

CONCLUSIONS

Morning bright light treatment is a feasible and acceptable treatment for US veterans with chronic low back pain. Those who undergo morning bright light treatment may show improvements in pain, pain sensitivity, and sleep. Advances in circadian timing may be one mechanism by which morning bright light reduces pain. Morning bright light treatment should be further explored as an innovative treatment for chronic pain conditions.

The Endogenous Opioid System: Role and Dysfunction Caused by Opioid Therapy

The endogenous opioid system is comprised of a wide array of receptors and ligands that are present throughout the central and peripheral nervous system, the gastrointestinal tract, and the immune system. This explains the multitude of physiological functions it is responsible for including analgesia, mood regulation, and modulation of the stress response. It also plays a pivotal role in modulating the brain's reward center with behavioral and social implications on mood disorders and addiction. Exogenous opioid therapy hijacks the endogenous system and alters its functions contributing to an imbalance that is responsible for the pathogenesis of several disease states.

The association between endogenous opioid function and morphine responsiveness: a moderating role for endocannabinoids

We sought to replicate previous findings that low endogenous opioid (EO) function predicts greater morphine analgesia and extended these findings by examining whether circulating endocannabinoids and related lipids moderate EO-related predictive effects. Individuals with chronic low-back pain (n = 46) provided blood samples for endocannabinoid analyses, then underwent separate identical laboratory sessions under 3 drug conditions: saline placebo, intravenous (i.v.) naloxone (opioid antagonist; 12-mg total), and i.v. morphine (0.09-mg/kg total). During each session, participants rated low-back pain intensity, evoked heat pain intensity, and nonpain subjective effects 4 times in sequence after incremental drug dosing. Mean morphine effects (morphine-placebo difference) and opioid blockade effects (naloxone-placebo difference; to index EO function) for each primary outcome (low-back pain intensity, evoked heat pain intensity, and nonpain subjective effects) were derived by averaging across the 4 incremental doses. The association between EO function and morphine-induced back pain relief was significantly moderated by endocannabinoids [2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamine (AEA)]. Lower EO function predicted greater morphine analgesia only for those with relatively lower endocannabinoids. Endocannabinoids also significantly moderated EO effects on morphine-related changes in visual analog scale-evoked pain intensity (2-AG), drug liking (AEA and 2-AG), and desire to take again (AEA and 2-AG). In the absence of significant interactions, lower EO function predicted significantly greater morphine analgesia (as in past work) and euphoria. Results indicate that EO effects on analgesic and subjective responses to opioid medications are greatest when endocannabinoid levels are low. These findings may help guide development of mechanism-based predictors for personalized pain medicine algorithms.

Designing and conducting proof-of-concept chronic pain analgesic clinical trials

Introduction:

The evolution of pain treatment is dependent on successful development and testing of interventions. Proof-of-concept (POC) studies bridge the gap between identification of a novel target and evaluation of the candidate intervention's efficacy within a pain model or the intended clinical pain population.

Methods:

This narrative review describes and evaluates clinical trial phases, specific POC pain trials, and approaches to patient profiling.

Results:

We describe common POC trial designs and their value and challenges, a mechanism-based approach, and statistical issues for consideration.

Conclusion:

Proof-of-concept trials provide initial evidence for target use in a specific population, the most appropriate dosing strategy, and duration of treatment. A significant goal in designing an informative and efficient POC study is to ensure that the study is safe and sufficiently sensitive to detect a preliminary efficacy signal (ie, a potentially valuable therapy). Proof-of-concept studies help avoid resources wasted on targets/molecules that are not likely to succeed. As such, the design of a successful POC trial requires careful consideration of the research objective, patient population, the particular intervention, and outcome(s) of interest. These trials provide the basis for future, larger-scale studies confirming efficacy, tolerability, side effects, and other associated risks.

Self-reported cumulative medical opioid exposure and subjective responses on first use of opioids predict analgesic and subjective responses to placebo-controlled opioid administration

BACKGROUND AND OBJECTIVES

To expand the evidence base needed to enable personalized pain medicine, we evaluated whether self-reported cumulative exposure to medical opioids and subjective responses on first opioid use predicted responses to placebo-controlled opioid administration.

METHODS

In study 1, a survey assessing cumulative medical opioid exposure and subjective responses on first opioid use was created (History of Opioid Medical Exposure (HOME)) and psychometric features documented in a general sample of 307 working adults. In study 2, 49 patients with chronic low back pain completed the HOME and subsequently rated back pain intensity and subjective opioid effects four times after receiving saline placebo or intravenous morphine (four incremental doses) in two separate double-blinded laboratory sessions. Placebo-controlled morphine effects were derived for all outcomes.

RESULTS

Two HOME subscales were supported: cumulative opioid exposure and euphoric response, both demonstrating high test-retest reliability (Intraclass Correlation Coefficients > 0.93) and adequate internal consistency (Revelle's Omega Total = 0.73-0.77). In study 2, higher cumulative opioid exposure scores were associated with significantly greater morphine-related reductions in back pain intensity (p=0.02), but not with subjective drug effects. Higher euphoric response subscale scores were associated with significantly lower overall perceived morphine effect (p=0.003), less sedation (p=0.04), greater euphoria (p=0.03) and greater desire to take morphine again (p=0.02).

DISCUSSION

Self-reports of past exposure and responses to medical opioid analgesics may have utility for predicting subsequent analgesic responses and subjective effects. Further research is needed to establish the potential clinical and research utility of the HOME.

TRIAL REGISTRATION NUMBER

NCT02469077.

Expectancy Effects on Conditioned Pain Modulation Are Not Influenced by Naloxone or Morphine

BACKGROUND

Recent studies suggest that participant expectations influence pain ratings during conditioned pain modulation testing. The present study extends this work by examining expectancy effects among individuals with and without chronic back pain after administration of placebo, naloxone, or morphine.

PURPOSE

This study aims to identify the influence of individual differences in expectancy on changes in heat pain ratings obtained before, during, and after a forearm ischemic pain stimulus.

METHODS

Participants with chronic low back pain (n = 88) and healthy controls (n = 100) rated heat pain experience (i.e., "test stimulus") before, during, and after exposure to ischemic pain (i.e., "conditioning stimulus"). Prior to testing, participants indicated whether they anticipated that their heat pain would increase, decrease, or remain unchanged during ischemic pain.

RESULTS

Analysis of the effects of expectancy (pain increase, decrease, or no change), drug (placebo, naloxone, or morphine), and group (back pain, healthy) on changes in heat pain revealed a significant main effect of expectancy (p = 0.001), but no other significant main effects or interactions. Follow-up analyses revealed that individuals who expected lower pain during ischemia reported significantly larger decreases in heat pain as compared with those who expected either no change (p = 0.004) or increased pain (p = 0.001).

CONCLUSIONS

The present findings confirm that expectancy is an important contributor to conditioned pain modulation effects, and therefore significant caution is needed when interpreting findings that do not account for this individual difference. Opioid mechanisms do not appear to be involved in these expectancy effects.

Do Resting Plasma β-Endorphin Levels Predict Responses to Opioid Analgesics?

OBJECTIVES

Clinically feasible predictors of opioid analgesic responses for use in precision pain medicine protocols are needed. This study evaluated whether resting plasma β-endorphin (BE) levels predicted responses to an opioid analgesic, and whether chronic pain status or sex moderated these effects.

METHODS

Participants included 73 individuals with chronic low back pain (CLBP) and 88 pain-free controls, all using no daily opioid analgesics. Participants attended 2 identical laboratory sessions during which they received either intravenous morphine (0.08 mg/kg) or saline placebo, with blood samples obtained before drug administration to assay resting plasma BE levels. Once peak drug activity was achieved in each session, participants engaged in an ischemic forearm pain task (ISC) and a heat pain task. Morphine analgesic effects were derived reflecting the difference in pain outcomes between placebo and morphine conditions.

RESULTS

In hierarchical regressions, significant Type (CLBP vs. control)×BE interactions (Ps<0.05) were noted for morphine effects on ISC tolerance, ISC intratask pain ratings, and thermal VAS unpleasantness ratings. These interactions derived primarily from associations between higher BE levels and smaller morphine effects restricted to the CLBP subgroup. All other BE-related effects, including sex interactions, for predicting morphine analgesia failed to reach statistical significance.

DISCUSSION

BE was a predictor of morphine analgesia for only 3 out of 9 outcomes examined, with these effects moderated by chronic pain status but not sex. On the whole, results do not suggest that resting plasma BE levels are likely to be a clinically useful predictor of opioid analgesic responses.

Psychosocial factors predict opioid analgesia through endogenous opioid function

Use of opioid analgesics for management of chronic nonmalignant pain has become common, yet there are presently no well-validated predictors of optimal opioid analgesic efficacy. We examined whether psychosocial factors (eg, depressive symptoms) predicted changes in spontaneous low back pain after administration of opioid analgesics, and whether endogenous opioid (EO) function mediated these relationships. Participants with chronic low back pain but who were not chronic opioid users (N = 89) underwent assessment of low back pain intensity pre- and post-drug in 3 (counterbalanced) conditions: (1) placebo, (2) intravenous naloxone, and (3) intravenous morphine. Comparison of placebo condition changes in back pain intensity to those under naloxone and morphine provided indexes of EO function and opioid analgesic responses, respectively. Results showed that (1) most psychosocial variables were related significantly and positively to morphine analgesic responses for low back pain, (2) depressive symptoms, trait anxiety, pain catastrophizing, and pain disability were related negatively to EO function, and (3) EO function was related negatively to morphine analgesic responses for low back pain. Bootstrapped mediation analyses showed that links between morphine analgesic responses and depressive symptoms, trait anxiety, pain catastrophizing, and perceived disability were partially mediated by EO function. Results suggest that psychosocial factors predict elevated analgesic responses to opioid-based medications, and may serve as markers to identify individuals who benefit most from opioid therapy. Results also suggest that people with greater depressive symptoms, trait anxiety, pain catastrophizing, and perceived disability may have deficits in EO function, which may predict enhanced response to opioid analgesics.

Ethnic Differences in the Effects of Naloxone on Sustained Evoked Pain: A Preliminary Study

Ethnic differences in pain response have been well documented, with non-Hispanic Black (NHB) participants reporting enhanced clinical pain and greater laboratory-evoked pain sensitivity to a variety of quantitative sensory testing (QST) methods compared to non-Hispanic Whites (NHW). One potential mechanism that may contribute to these disparities is differential functioning of endogenous pain-regulatory systems. To evaluate endogenous opioid (EO) mechanisms in pain responses, we examined group differences in response to tonic capsaicin pain following double-blinded crossover administration of saline and the opioid antagonist, naloxone. Ten percent topical capsaicin cream and a thermode were applied to the dorsum of the non-dominant hand, maintaining a constant temperature of 40°C for 90 min. Naloxone (0.1 mg/kg) or saline placebo was administered at the 25 min mark and post-drug pain intensity ratings were obtained every 5 min thereafter. As an index of EO function, blockade effects were derived for each participant, reflecting the difference between mean post-drug pain intensity ratings under the saline versus naloxone conditions, with higher positive scores reflecting greater EO inhibition of pain. Thirty-nine healthy, young individuals (19 non-Hispanic Black [NHB], 20 non-Hispanic White [NHW]) participated. Group difference in EO function were identified, with NHB participants displaying lower EO function scores (mean=-10.8, SD=10.1) as compared to NHW participants (mean=-0.89, SD=11.5; p=0.038). NHB participants experienced significant paradoxical analgesia with naloxone. Thirty five percent of the NHW participants showed a positive blockade effect indicating EO analgesia (i.e., an increase in pain with naloxone), while only 10% of the NHB participants exhibited evidence of EO analgesia. These findings suggest differential functioning of the endogenous opioid pain regulatory system between NHB and NHW participants. Future research is warranted to examine whether these differences contribute to the disparities observed in clinical pain between groups.

Endogenous Opioid Function and Responses to Morphine: The Moderating Effects of Anger Expressiveness

Long-term use of opioid analgesics may be ineffective or associated with significant negative side effects for some people. At present, there is no sound method of identifying optimal opioid candidates. Individuals with chronic low back pain (n = 89) and healthy control individuals (n = 102) underwent ischemic pain induction with placebo, opioid blockade (naloxone), and morphine in counterbalanced order. They completed the Spielberger Anger-Out subscale. Endogenous opioid function × Anger-out × Pain status (chronic pain, healthy control) interactions were tested for morphine responses to ischemic threshold, tolerance, and pain intensity (McGill Sensory and Affective subscales) and side effects. For individuals with chronic pain and healthy control participants, those with low endogenous opioid function and low anger-out scores exhibited the largest morphine analgesic responses, whereas those with high anger-out and low endogenous opioid function showed relatively weaker morphine analgesic responses. Further, individuals with chronic pain with low endogenous opioid function and low anger-out scores also reported the fewest negative effects to morphine, whereas those with low endogenous opioid function and high anger-out reported the most. Findings point toward individuals with chronic pain who may strike a favorable balance of good analgesia with few side effects, as well as those who have an unfavorable balance of poor analgesia and many side effects.

PERSPECTIVE

We sought to identify optimal candidates for opioid pain management. Low back pain patients who express anger and also have deficient endogenous opioid function may be poor candidates for opioid therapy. In contrast, low back patients who tend not to express anger and who also have deficient endogenous opioid function may make optimal candidates for opioid therapy.

Evaluating Persistent Postoperative Pain in One Tertiary Hospital: Incidence, Quality of Life, Associated Factors, and Treatment

Background

Persistent postoperative pain (PPP) is defined as persistent pain after surgery of greater than three months’ duration.

Objectives

Identify the incidence of PPP in our hospital and its associated factors; evaluate quality of life (QoL) and treatment of patients.

Patients and Methods

We conducted an observational prospective study in adults proposed to various types of surgery using the brief pain inventory short form preoperatively (T0), one day after surgery, and three months later (T3). If the patient had pain at T3 and other causes of pain were excluded, they were considered to have PPP, and the McGill Pain Questionnaire Short Form was applied. QoL was measured with the EuroQol 5-dimension questionnaire (EQ-5D).

Results

One hundred seventy-five patients completed the study. The incidence of PPP was 28%, and the affected patients presented lower QoL. The majority referred to a moderate to severe level of interference in their general activity. Cholecystectomies were less associated with PPP, and total knee/hip replacements were more associated with it. Preoperative pain, preoperative benzodiazepines or antidepressants, and more severe acute postoperative pain were associated with the development of PPP. Half of the patients with PPP were under treatment, and they refer a mean symptomatic relief of 69%.

Conclusions

This study, apart from attempting to better characterize the problem of PPP, emphasizes the lack of its treatment.

An Evaluation of Central Sensitization in Patients With Sickle Cell Disease

Central sensitization (CS), nociceptive hyperexcitability known to amplify and maintain clinical pain, has been identified as a leading culprit responsible for maintaining pain in several chronic pain conditions. Recent evidence suggests that it may explain differences in the symptom experience of individuals with sickle cell disease (SCD). Quantitative sensory testing (QST) can be used to examine CS and identify individuals who may have a heightened CS profile. The present study categorized patients with SCD on the basis of QST responses into a high or low CS phenotype and compared these groups according to measures of clinical pain, vaso-occlusive crises, psychosocial factors, and sleep continuity. Eighty-three adult patients with SCD completed QST, questionnaires, and daily sleep and pain diaries over a 3-month period, weekly phone calls for 3 months, and monthly phone calls for 12 months. Patients were divided into CS groups (ie, no/low CS [n = 17] vs high CS [n = 21]), on the basis of thermal and mechanical temporal summation and aftersensations, which were norm-referenced to 47 healthy control subjects. High CS subjects reported more clinical pain, vaso-occlusive crises, catastrophizing, and negative mood, and poorer sleep continuity (Ps < .05) over the 18-month follow-up period. Future analyses should investigate whether psychosocial disturbances and sleep mediate the relationship between CS and pain outcomes.

PERSPECTIVE

In general, SCD patients with greater CS had more clinical pain, more crises, worse sleep, and more psychosocial disturbances compared with the low CS group.

Obstructive sleep apnea, pain, and opioids: is the riddle solved?

PURPOSE OF REVIEW

Perioperative opioid-based pain management of patients suffering from obstructive sleep apnea (OSA) may present challenges because of concerns over severe ventilatory compromise. The interaction between intermittent hypoxia, sleep fragmentation, pain, and opioid responses in OSA, is complex and warrants a special focus of perioperative outcomes research.

RECENT FINDINGS

Life-threatening opioid-related respiratory events are rare. Epidemiologic evidence suggests that OSA together with other serious renal and heart disease, is among those conditions predisposing patients for opioid-induced ventilatory impairment (OIVI) in the postoperative period. Both intermittent hypoxia and sleep fragmentation, two distinct components of OSA, enhance pain. Intermittent hypoxia may also potentiate opioid analgesic effects. Activation of major inflammatory pathways may be responsible for the effects of sleep disruption and intermittent hypoxia on pain and opioid analgesia. Recent experimental evidence supports that these, seemingly contrasting, phenotypes of pain-increasing and opioid-enhancing effects of intermittent hypoxia, are not mutually exclusive. Although the effect of intermittent hypoxia on OIVI has not been elucidated, opioids worsen postoperative sleep-disordered breathing in OSA patients. A subset of these patients, characterized by decreased chemoreflex responsiveness and high arousal thresholds, might be at higher risk for OIVI.

SUMMARY

OSA may complicate opioid-based perioperative management of pain by altering both pain processing and sensitivity to opioid effect.

The Contribution of Differential Opioid Responsiveness to Identification of Opioid Risk in Chronic Pain Patients

The Screener and Opioid Assessment for Patients with Pain-Revised (SOAPP-R) predicts increased risk of opioid misuse in chronic pain patients. We evaluated whether higher SOAPP-R scores are associated with greater opioid reinforcing properties, potentially contributing to their predictive utility. Across 2 counterbalanced laboratory sessions, 55 chronic low back pain sufferers completed the SOAPP-R at baseline and measures of back pain intensity, evoked pain responsiveness (thermal, ischemic), and subjective opioid effects after receiving intravenous morphine (.08 mg/kg) or saline placebo. Morphine effect measures were derived for all outcomes, reflecting the difference between morphine and placebo condition values. Higher SOAPP-R scores were significantly associated with greater desire to take morphine again, less feeling down and feeling bad, and greater reductions in sensory low back pain intensity following morphine administration. This latter effect was due primarily to SOAPP-R content assessing medication-specific attitudes and behavior. Individuals exceeding the clinical cutoff (18 or higher) on the SOAPP-R exhibited significantly greater morphine liking, desire to take morphine again, and feeling sedated; less feeling bad; and greater reductions in sensory low back pain following morphine. The SOAPP-R may predict elevated opioid risk in part by tapping into individual differences in opioid reinforcing effects.

PERSPECTIVE

Based on placebo-controlled morphine responses, associations were observed between higher scores on a common opioid risk screener (SOAPP-R) and greater desire to take morphine again, fewer negative subjective morphine effects, and greater analgesia. Opioids may provide the best analgesia in those patients at greatest risk of opioid misuse.

Endogenous opioid antagonism in physiological experimental pain models: a systematic review

Opioid antagonists are pharmacological tools applied as an indirect measure to detect activation of the endogenous opioid system (EOS) in experimental pain models. The objective of this systematic review was to examine the effect of mu-opioid-receptor (MOR) antagonists in placebo-controlled, double-blind studies using ʻinhibitoryʼ or ʻsensitizingʼ, physiological test paradigms in healthy human subjects. The databases PubMed and Embase were searched according to predefined criteria. Out of a total of 2,142 records, 63 studies (1,477 subjects [male/female ratio = 1.5]) were considered relevant. Twenty-five studies utilized ʻinhibitoryʼ test paradigms (ITP) and 38 studies utilized ʻsensitizingʼ test paradigms (STP). The ITP-studies were characterized as conditioning modulation models (22 studies) and repetitive transcranial magnetic stimulation models (rTMS; 3 studies), and, the STP-studies as secondary hyperalgesia models (6 studies), ʻpainʼ models (25 studies), summation models (2 studies), nociceptive reflex models (3 studies) and miscellaneous models (2 studies). A consistent reversal of analgesia by a MOR-antagonist was demonstrated in 10 of the 25 ITP-studies, including stress-induced analgesia and rTMS. In the remaining 14 conditioning modulation studies either absence of effects or ambiguous effects by MOR-antagonists, were observed. In the STP-studies, no effect of the opioid-blockade could be demonstrated in 5 out of 6 secondary hyperalgesia studies. The direction of MOR-antagonist dependent effects upon pain ratings, threshold assessments and somatosensory evoked potentials (SSEP), did not appear consistent in 28 out of 32 ʻpainʼ model studies. In conclusion, only in 2 experimental human pain models, i.e., stress-induced analgesia and rTMS, administration of MOR-antagonist demonstrated a consistent effect, presumably mediated by an EOS-dependent mechanisms of analgesia and hyperalgesia.

Fibromyalgia Survey Criteria Are Associated with Increased Postoperative Opioid Consumption in Women Undergoing Hysterectomy

Background:

The current study was designed to test the hypothesis that the fibromyalgia survey criteria would be directly associated with increased opioid consumption after hysterectomy even when accounting for other factors previously described as being predictive for acute postoperative pain.

Methods:

Two hundred eight adult patients undergoing hysterectomy between October 2011 and December 2013 were phenotyped preoperatively with the use of validated self-reported questionnaires including the 2011 fibromyalgia survey criteria, measures of pain severity and descriptors, psychological measures, preoperative opioid use, and health information. The primary outcome was the total postoperative opioid consumption converted to oral morphine equivalents.

Results:

Higher fibromyalgia survey scores were significantly associated with worse preoperative pain characteristics, including higher pain severity, more neuropathic pain, greater psychological distress, and more preoperative opioid use. In a multivariate linear regression model, the fibromyalgia survey score was independently associated with increased postoperative opioid consumption, with an increase of 7-mg oral morphine equivalents for every 1-point increase on the 31-point measure (Estimate, 7.0; Standard Error, 1.7; P &lt; 0.0001). In addition to the fibromyalgia survey score, multivariate analysis showed that more severe medical comorbidity, catastrophizing, laparotomy surgical approach, and preoperative opioid use were also predictive of increased postoperative opioid consumption.

Conclusions:

As was previously demonstrated in a total knee and hip arthroplasty cohort, this study demonstrated that increased fibromyalgia survey scores were predictive of postoperative opioid consumption in the posthysterectomy surgical population during their hospital stay. By demonstrating the generalizability in a second surgical cohort, these data suggest that patients with fibromyalgia-like characteristics may require a tailored perioperative analgesic regimen.

Relationship between endogenous opioid function and opioid analgesic adverse effects

BACKGROUND AND OBJECTIVES

Our recent work indicates that endogenous opioid activity influences analgesic responses to opioid medications. This secondary analysis evaluated whether endogenous opioid activity is associated with degree of opioid analgesic adverse effects, and whether chronic pain status and sex affect these adverse effects.

METHODS

Using a double-blind, randomized, placebo-controlled, crossover design, 51 subjects with chronic low back pain and 38 healthy controls participated in 3 separate sessions, undergoing 2 laboratory-evoked pain tasks (ischemic and thermal) after receiving placebo, naloxone, or morphine. Endogenous opioid system function was indexed by the difference in pain responses between the placebo and naloxone conditions. These measures were examined for associations with morphine-related adverse effects.

RESULTS

Chronic pain subjects reported significantly greater itching and unpleasant bodily sensations with morphine than controls (P < 0.05). Across groups, only 6 of 112 possible associations between adverse effects and blockade effects were significant. For the ischemic task, higher endogenous opioid function was associated with greater itching (visual analog scale [VAS]; P < 0.05), numbness (tolerance; P < 0.001), dry mouth (tolerance; P < 0.05), and unpleasant bodily sensations (VAS; P < 0.05). For the thermal task, higher endogenous opioid function was associated with greater numbness (VAS; P < 0.05) and feeling carefree (VAS; P < 0.05). There were no significant main or interaction effects of chronic pain status or sex on these findings.

CONCLUSIONS

No consistent relationships were observed between endogenous opioid function and morphine-related adverse effects. This is in stark contrast to our previous observation of strong relationships between elevated endogenous opioid function and smaller morphine analgesic effects.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Endogenous opioid inhibition of chronic low-back pain influences degree of back pain relief after morphine administration

BACKGROUND AND OBJECTIVES

Factors underlying differential responsiveness to opioid analgesic medications used in chronic pain management are poorly understood. We tested whether individual differences in endogenous opioid inhibition of chronic low-back pain were associated with the magnitude of acute reductions in back pain ratings after morphine administration.

METHODS

In randomized counterbalanced order over three sessions, 50 chronic low-back pain patients received intravenous naloxone (8 mg), morphine (0.08 mg/kg), or placebo. Back pain intensity was rated predrug and again after peak drug activity was achieved using the McGill Pain Questionnaire-Short Form (Sensory and Affective subscales, VAS Intensity measure). Opioid blockade effect measures to index degree of endogenous opioid inhibition of back pain intensity were derived as the difference between predrug to postdrug changes in pain intensity across placebo and naloxone conditions, with similar morphine responsiveness measures derived across placebo and morphine conditions.

RESULTS

Morphine significantly reduced back pain compared with placebo (McGill Pain Questionnaire-Short Form Sensory, VAS; P < 0.01). There were no overall effects of opioid blockade on back pain intensity. However, individual differences in opioid blockade effects were significantly associated with the degree of acute morphine-related reductions in back pain on all measures, even after controlling for effects of age, sex, and chronic pain duration (P < 0.03). Individuals exhibiting greater endogenous opioid inhibition of chronic back pain intensity reported less acute relief of back pain with morphine.

CONCLUSIONS

Morphine appears to provide better acute relief of chronic back pain in individuals with lower natural opioidergic inhibition of chronic pain intensity. Possible implications for personalized medicine are discussed.