Nociceptive flexion reflex and pain rating responses during endogenous opiate blockade with naltrexone in healthy young adults

Naltrexone accelerated oral traumatic ulcer healing and downregulated TLR-4/NF-kB pathway in Wistar rats

OBJECTIVE

To assess the effect of naltrexone on oral mucosal healing using a traumatic ulcer model DESIGN: Wistar rats (n = 112) received distilled water (control) or naltrexone (0.5, 10, or 50 mg/kg/day). Ulcers were induced on the buccal mucosa using a round skin biopsy punch (diameter 6 mm). Euthanasia was performed on days 1, 3, 7, and 14. Healing was assessed by ulcer area, histological scores, histomorphometric analysis (number of polymorphonuclears, mononuclears, and fibroblasts), and collagen percentage. Immunohistochemistry for TLR-2, TLR-4, NF-kB, and CD31 was evaluated. Nociceptive threshold was measured daily.

RESULTS

The 50 mg/kg group showed reduced ulcer area on days 1 (p < 0.001), 3 (p < 0.05), and 14 (p < 0.01). In this group, there was, on day 14, an increase in the percentage of reepithelization (p = 0.043) and collagen (p < 0.05), an increase in connective tissue maturation (p = 0.016), and on day 7 an increase in fibroblasts (p < 0.001). The 10 mg/kg dose reduced the ulcer area on day 1 (p < 0.001). The 50 mg/kg group showed lower expression of TLR-4 (p < 0.001) on day 1, NF-kB on days 1 (p < 0.05) and 14 (p < 0.05), and CD31 on day 14 (p < 0.05). The 0.5 and 10 mg/kg doses reduced TLR-4 expression on day 1 (p < 0.05; p < 0.01, respectively). Nociceptive threshold increased in the 50 mg/kg group (p < 0.01).

CONCLUSION

Naltrexone enhanced traumatic oral ulcer healing by reducing TLR-4/NF-kB signaling and promoting fibroblast proliferation and collagen deposition. Additionally, naltrexone reduced pain in rats.

Neurobiological mechanisms of early life adversity, blunted stress reactivity and risk for addiction

Blunted stress reactivity resulting from early exposure to stress during childhood and adolescence may increase vulnerability to addiction. Early life adversity (ELA) affects brain structure and function and results in blunted stress axis reactivity. In this review, we focus on the underlying neurobiological mechanisms associated with a blunted response to stress, ELA, and risk for addictive disorders. ELA and blunted reactivity are accompanied by unstable mood regulation, impulsive behaviors, and reduced cognitive function. Neuroimaging studies reveal cortical and subcortical changes in persons exposed to ELA and those who have a genetic disposition for addiction. We propose a model in which blunted stress reactivity may be a marker of risk for addiction through an altered motivational and behavioral reactivity to stress that contribute to disinhibited behavioral reactivity and impulsivity leading in turn to increased vulnerability for substance use. Evidence supporting this hypothesis in the context of substance use initiation, maintenance, and risk for relapse is presented. The effects of ELA on persons at risk for addiction may lead to early experimentation with drugs of abuse. Early adoption of drug intake may alter neuroregulation in such vulnerable persons leading to a permanent dysregulation of motivational responses consistent with dependence. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

The Endocannabinoid System Contributes to Electroacupuncture Analgesia

The extensive involvement of the endocannabinoid system (ECS) in vital physiological and cognitive processes of the human body has inspired many investigations into the role of the ECS and drugs, and therapies that target this system and its receptors. Activation of cannabinoid receptors 1 and 2 (CB₁ and CB₂) by cannabinoid treatments, including synthetic cannabinoids, alleviates behavioral responses to inflammatory and neuropathic pain. An increasing body of scientific evidence details how electroacupuncture (EA) treatments achieve effective analgesia and reduce inflammation by modulating cannabinoid signaling, without the adverse effects resulting from synthetic cannabinoid administration. CB₁ receptors in the ventrolateral area of the periaqueductal gray are critically important for the mechanisms of the EA antinociceptive effect, while peripheral CB₂ receptors are related to the anti-inflammatory effects of EA. This review explores the evidence detailing the endocannabinoid mechanisms involved in EA antinociception.

The influence of physical activity on the nociceptive flexion reflex in healthy people

BACKGROUND

The nociceptive flexion reflex (NFR) is a spinal reflex induced by painful stimuli resulting in an appropriate withdrawal response. The NFR is considered to be an objective physiological correlate of spinal nociception. Previous research has already demonstrated that physical activity (PA) can influence pain assessments. To date, no studies have directly examined the relationship between PA and spinal nociception. Hence, this study aimed to investigate whether the NFR threshold can be predicted by report-based and monitor-based measures of PA in healthy adults.

METHODS

PA and the NFR threshold of 58 healthy adults were assessed. PA was evaluated by self-report using the International Physical Activity Questionnaire and by monitor-based accelerometry data. The NFR threshold was elicited through transcutaneous electrical stimulation of the sural nerve and quantified by the biceps femoris muscle electromyogram. Hierarchical linear regression analyses were performed to determine the relationship between PA and the NFR, while controlling for confounders.

RESULTS

Monitor-based measured step count and activities of moderate- to vigorous-intensity predicted the NFR threshold accounting for 23.0% (p = .047) to 37.1% (p = .002) of the variance. Larger amounts of step counts and higher participation in moderate- to vigorous-intensity activities predicted higher NFR thresholds. Monitor-based activities of sedentary intensity predicted the NFR threshold accounting for 35.8% (p = .014) to 35.9% (p = .014) of the variance. Spending more time per day on activities of sedentary intensity predicted lower NFR thresholds.

CONCLUSIONS

The study provides preliminary evidence indicating that a physically active lifestyle may reduce spinal nociception in healthy adults, while a sedentary lifestyle enhances spinal nociception.

SIGNIFICANCE

The present study provides preliminary evidence that the influencing effects of physical activity on pain are the result of a strong descending control and do not purely rely on supraspinal mechanisms. These study results highlight the importance of considering physical activity levels when evaluating nociceptive processing, given the prognostic value of physical activity in spinal nociception. Furthermore, this study encourages future research to examine the effects of moderate- to vigorous-intensity exercise programmes on spinal nociception in chronic pain populations.

Stress and Addiction: When a Robust Stress Response Indicates Resiliency

OBJECTIVE

Stress reactivity research has traditionally focused on the idea that exaggerated responses to stress may have adverse effects on health. Accumulating evidence suggests that attenuated responses to stress and delayed recovery may also be problematic.

METHODS

This review focuses on the role of the stress response of the hypothalamic-pituitary-adrenocortical axis, the endogenous opioid system, and the cardiovascular system in hypertension, pain perception, and addictive behaviors. Results from multiple methods of assessment and stress paradigms conducted in our laboratory over the past two decades are integrated with research from other investigators and with existing theories.

RESULTS

Research indicates that exaggerated biological and physiological responses to stress and attenuated pain perception are associated with hypertension and risk for cardiovascular diseases. This research complements work linking reduced stress responses with enhanced pain sensitivity and discomfort. Multiple studies have also demonstrated that an attenuated stress response is linked to exacerbation of withdrawal symptoms and relapse in nicotine addiction. Evidence indicates important moderators (i.e., sex, personality traits, and early life adversity) and hypothalamic-pituitary-adrenocortical- and endogenous opioid system-related mechanisms in the altered response to stress. I integrate these findings in a conceptual model emphasizing that robust stress responses in the context of addiction and relapse should be considered as a marker of resiliency.

CONCLUSIONS

A blunted stress response may indicate long-term physiological dysregulation that could usher harmful consequences for cardiovascular disease, pain perception, and addictive disorders. The impact of dysregulation is influenced by multiple individual and situational factors that should be considered in evaluating the clinical significance of stress response dysregulation.

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 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.

Assessing pain objectively: the use of physiological markers

Pain diagnosis and management would benefit from the development of objective markers of nociception and pain. Current research addressing this issue has focused on five main strategies, each with its own advantages and disadvantages. These encompass: (i) monitoring changes in the autonomic nervous system; (ii) biopotentials; (iii) neuroimaging; (iv) biological (bio-) markers; and (v) composite algorithms. Although each strategy has shown areas of promise, there are currently no validated objective markers of nociception or pain that can be recommended for clinical use. This article introduces the most important developments in the field and highlights shortcomings, with the aim of allowing the reader to make informed decisions about what trends to watch in the future.

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.

Reduction of conditioned pain modulation in humans by naltrexone: an exploratory study of the effects of pain catastrophizing

The current study tested the hypothesis that conditioned pain modulation is mediated by the release of endogenous opioids with a placebo-controlled (sugar pill) study of naltrexone (50 mg) in 33 healthy volunteers over two counter-balanced sessions. Pain modulation consisted of rating of heat pain (palm) during concurrent cold water immersion (foot). Compared to baseline heat pain ratings, concurrent foot immersion lowered pain intensity ratings, which suggests an inhibitory effect, was reduced with naltrexone, suggesting at least partial dependence of inhibition on endogenous opioids. An exploratory analysis revealed that individual differences in catastrophizing moderated the effects of naltrexone; endogenous opioid blockade abolished modulation in subjects lower in catastrophizing while modulation was unaffected by naltrexone among high catastrophizers. The results suggest a role of endogenous opioids in endogenous analgesia, but hint that multiple systems might contribute to conditioned pain modulation, and that these systems might be differentially activated as a function of individual differences in responses to pain.

What do plasma beta-endorphin levels reveal about endogenous opioid analgesic function?

Plasma levels of beta-endorphin (BE), an endogenous opioid analgesic, are often reported as they relate to acute and chronic pain outcomes. However, little is known about what resting plasma BE levels might reveal about functioning of the endogenous opioid antinociceptive system. This study directly examined associations between resting plasma BE and subsequent endogenous opioid analgesic responses to acute pain in 39 healthy controls and 37 individuals with chronic low back pain (LBP). Resting baseline levels of plasma BE were assessed. Next, participants received opioid blockade (8 mg naloxone i.v.) or placebo in a double-blind, randomized, crossover design. Participants then underwent two acute pain stimuli: finger pressure (FP) pain and ischaemic (ISC) forearm pain. Blockade effects (naloxone minus placebo pain ratings) were derived to index endogenous opioid analgesic function. In placebo condition analyses for both pain stimuli, higher resting BE levels were associated with subsequently greater reported pain intensity (p's < 0.05), with this effect occurring primarily in healthy controls (BE × Participant Type interactions, p's < 0.05). In blockade effect analyses across both pain tasks, higher resting plasma BE predicted less subsequent endogenous opioid analgesia (smaller blockade effects; p's < 0.05). For the ISC task, these links were significantly more prominent in LBP participants (BE × Participant Type Interactions, p's < 0.05). Results suggest that elevated resting plasma BE may be a potential biomarker for reduced endogenous opioid analgesic capacity, particularly among individuals with chronic pain. Potential clinical implications are discussed.

Enhancing Acupuncture by Low Dose Naltrexone

To find appropriate and effective treatment options for chronic pain syndromes is a challenging task. Multimodal treatment approach has been gaining acceptance for chronic pain. However, combining treatments, such as acupuncture, with rational pharmacology is still in its infancy. Acupuncture influences the opioid and cannabinoid system through releasing endogenous receptor ligands. Low dose naltrexone also acts on both these systems, and upregulates the opioid and cannabinoid receptors. The authors hypothesise that low dose naltrexone could enhance the pain-relieving effect of acupuncture.

Anger suppression predicts pain, emotional, and cardiovascular responses to the cold pressor

BACKGROUND

Manipulated anger suppression has been shown to heighten pain and anger responses to pain.

PURPOSE

We examined whether individual differences in self-reported anger suppression predicted pain, anger, and blood pressure responses to acute pain.

METHODS

Healthy participants (N = 47) underwent an anger-provoking speech task followed by a cold pressor pain task. Participants reported their degree of suppression of thoughts and feelings related to the speech. Pain intensity ratings were obtained throughout the cold pressor. Self-reported anger, anxiety and positive emotion, as well as ratings of sensory, general distress, and anger-specific elements of pain were obtained following the cold pressor. Systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR) were recorded throughout.

RESULTS

Self-reported suppression predicted greater pain intensity ratings, perception of sensory and anger-specific elements of pain, and self-reported anger in response to the cold pressor. Associations between self-reported suppression and pain intensity and ratings of anger-specific elements of pain were statistically mediated by pain-induced changes in self-reported anger, whereas the effect of suppression on sensory pain ratings was not. Self-reported suppression was also correlated inversely with SBP responses to the cold pressor.

CONCLUSIONS

Consistent with an ironic process model and prior studies involving experimental manipulation of suppression, self-reported suppression of anger predicted greater pain intensity and perception of the anger-specific element of pain. Findings also suggest that suppression might attenuate homeostatic pressor responses to acute pain.

Naltrexone fails to increase pain affect in response to inflammatory pain in a novel escape/avoidance paradigm

The non-specific opioid antagonist naltrexone has traditionally been used as treatment for opioid overdose, as well as in research settings as an antagonist to examine opioid and non-opioid mediated analgesia. However, the mechanisms by which this drug operates are not well understood, and its exact effects on sensory and affective pain processes remain uncertain. Various studies have demonstrated that naltrexone behaves in a paradoxical manner, leading to analgesia, no discernable changes, or an increase in pain, depending on the circumstances of the study. This imprecise spectrum of effects leads to difficulty in interpreting results in studies where naltrexone was utilized as an antagonist. Therefore, the purpose of this experiment was to further examine whether naltrexone elicits dose-dependent effects in behavioral tests designed to quantify the sensory and affective components of pain. Naltrexone was not expected to have an effect on carrageenan-induced inflammatory pain in sensory pain measures, but a dose-dependent increase was predicted in behavior related to the affective component of pain. Eighty-eight male Sprague-Dawley rats were used to test these hypotheses by measuring Mechanical Paw Withdrawal Thresholds before and after naltrexone injection and by assessing performance in the Place Escape Avoidance Paradigm test, a novel paradigm to test pain affect, in which naltrexone had not been utilized. The results demonstrated that naltrexone failed to increase place/escape avoidance behavior as anticipated, but rather produced a slight, but non-significant, decrease in escape avoidance behavior. Further research is needed to elucidate the differential effects of naltrexone on various aspects of pain-related behavior.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 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.

Effects of opioid blockade on nociceptive flexion reflex thresholds and nociceptive responding in hypertensive and normotensive individuals

Hypertension and risk for hypertension have been associated with reduced pain sensitivity. It has been hypothesised that endogenous opioids contribute to this hypertensive hypoalgesia. The nociceptive flexion reflex can be used as a tool to investigate modulation of nociceptive transmission at spinal level. The current study employed a double-blind placebo-controlled design to compare the effects of naltrexone, an opioid antagonist, and placebo on nociceptive flexion reflex thresholds and nociceptive responding in unmedicated patients with essential hypertension and normotensive individuals. Neither nociceptive flexion reflex thresholds nor nociceptive responding differed between hypertensives and normotensives during placebo or naltrexone. These data provide no support for the hypothesis that essential hypertension is characterised by higher levels of endogenous opioids in the central nervous system and reveal no association between blood pressure status and nociceptive flexion reflex responses.