Stress-induced analgesia in humans: endogenous opioids and naloxone-reversible depression of pain reflexes

Mechanisms of the Native American pain inequity: predicting chronic pain onset prospectively at 5 years in the Oklahoma Study of Native American Pain Risk

ABSTRACT

A pain inequity exists for Native Americans (NAs), but the mechanisms are poorly understood. The Oklahoma Study of Native American Pain Risk (OK-SNAP) addressed this issue and recruited healthy, pain-free NAs and non-Hispanic Whites (NHWs) to attend 2 laboratory visits and assessed mechanisms consistent with the biopsychosocial model of pain: demographics, physical variables, psychosocial factors, and nociceptive/pain phenotypes. Then participants were surveyed every 6 months to assess for chronic pain onset. Results at the 2-year follow-up found that NAs were ∼3x more likely than NHWs to develop chronic pain. Moreover, psychosocial factors (discrimination, stress, pain-related anxiety), cardiometabolic load (higher body mass index and blood pressure, lower heart rate variability), and impaired inhibition of spinal nociception partly mediated the pain inequity. The present study examined mechanisms of chronic pain at the 5-year follow-up for OK-SNAP. Results found that the NA pain inequity worsened-NAs were 4x more likely to develop chronic pain (OR = 4.025; CI = 1.966, 8.239), even after controlling for baseline age, sex assigned at birth, income, and education. Moreover, serial mediation models replicated paths from the 2-year follow-up that linked psychosocial variables, cardiometabolic load, and impaired inhibition of spinal nociception to chronic pain onset. Further, 2 new significant paths were observed. One linked discrimination, stress, sleep problems, and facilitated pain perception to increased pain risk. The other linked discrimination with higher spinal nociceptive threshold and pain risk. These results provide further evidence for a NA pain inequity and identify multiple psychosocial, cardiometabolic, and pronociceptive targets for primary interventions.

Modeling Neuropathic Corneal Pain: Pulled Nerve Approach With Elevated Krt16 Gene Expression

Purpose

Neuropathic corneal pain (NCP) is a debilitating condition affecting millions of people worldwide. Despite their critical importance, currently available animal models for NCP research are limited by complex surgeries with high-risk strategy. To advance fundamental understanding of NCP, we developed a novel rodent model that explores both structural and functional mechanisms of the disease, offering a comprehensive approach.

Methods

By uplifting (2-3 mm transversely) the long ciliary nerve (LCN) with gentle force (0.09 ± 0.02 newton [N]) and pressure (0.18 ± 0.05 MPa), our pulled nerve model mimics human NCP conditions and was investigated alongside normal control, sham control, and full transection groups. Specifically, we quantified the NCP status by establishing a relationship between pain perception and chemical sensitivity, using Stevens' Power Law concept.

Results

Following surgery, the temporal patterns of heightened pain perception showed consistent trends across different stimulus methods, suggesting that von Frey and chemical tests could effectively evaluate pain progression. The discernable differences in Alpha values (exponent) of the pain-perception curves across the normal control, pulled nerve, and full transection groups (0.175 ± 0.035, 0.235 ± 0.015, and 0.275 ± 0.005, respectively) demonstrate the model's sensitivity to changes in NCP status. Histological analysis revealed LCN elongation, thickening, and corneal alterations in pulled nerve models, with reduced satellite glial cells (SGCs) in trigeminal ganglion compared to the normal control models. Krt16 gene expression was significantly upregulated following pulled nerve surgery.

Conclusions

Our model not only delineates the pathological landscape of NCP but also promises to accelerate the development of targeted therapies.

Cannabinoid and Serotonergic Systems: Unraveling the Pathogenetic Mechanisms of Stress-Induced Analgesia

The perception of "stress" triggers many physiological and behavioral responses, collectively called the stress response. Such a complex process allows for coping with stress and also triggers severe pathology. Because of the multidirectional effect of stress on the body, multiple systems participate in its pathogenesis, with the endogenous cannabinoid and the serotoninergic ones among them. These two systems also take part in the pain perception decrease, known as stress-induced analgesia (SIA), which can then be taken as an indirect indicator of the stress response. The aim of our study was to study the changes in cold SIA (c-SIA) resulting from the exogenous activation of cannabinoid receptor type 1 (CB1) and 5-hydroxytryptamine (5-HT, serotonin) receptor type 1A (5-HT1A). Various combinations of agonists and/or antagonists of CB1 and 5-HT1A, before or after 1 h of cold exposure, were applied, since we presumed that the exogenous activation of the receptors before the cold exposure would influence the pathogenesis of the stress response, while their activation after the stressful trigger would influence the later development. Our results show that the serotonergic system "maintained" c-SIA in the pre-stress treatment, while the cannabinoids' modulative effect was more prominent in the post-stress treatment. Here, we show the interactions of the two systems in the stress response. The interpretation and understanding of the mechanisms of interaction between CB1 and 5-HT1A may provide information for the prevention and control of adverse stress effects, as well as suggest interesting directions for the development of targeted interventions for the control of specific body responses.

Sex differences in stress-induced hyperalgesia and its mechanisms

Chronic stress induces a variety of physiological and/or psychological abnormalities, including hyperalgesia. Researchers have discovered sex differences in the prevalence of stress-induced hyperalgesia (SIH) in recent years. Sex differences may be one of the reasons for the heterogeneity of susceptibility to stress-related diseases. In this review, the potential mechanisms of sex differences in SIH are discussed, such as hypothalamus-pituitary-adrenal axis responses, regulation of sex hormones, and immune system responses.

Pain, Fear, Anxiety, and Stress: Relations to the Endogenous Opioid System

Pain, fear, stress, and anxiety are separate yet interrelated phenomena. Each of these concepts has an extensive individual body of research, with some more recent work focusing on points of conceptual overlap. The role of the endogenous opioid system in each of these phenomena is only beginning to be examined and understood. Research examining the ways in which endogenous opioids (e.g., beta-endorphin; βE) may mediate the relations among pain, fear, stress, and anxiety is even more nascent. This chapter explores the extant evidence for endogenous opioid activity as an underpinning mechanism of these related constructs, with an emphasis on research examining βE.

How Do Expectations Modulate Pain? A Motivational Perspective

Expectations can profoundly modulate pain experience, during which the periaqueductal gray (PAG) plays a pivotal role. In this article, we focus on motivationally evoked neural activations in cortical and brainstem regions both before and during stimulus administration, as has been demonstrated by experimental studies on pain-modulatory effects of expectations, in the hope of unraveling how the PAG is involved in descending and ascending nociceptive processes. This motivational perspective on expectancy effects on the perception of noxious stimuli sheds new light on psychological and neuronal substrates of pain and its modulation, thus having important research and clinical implications.

Stress-induced hyperalgesia instead of analgesia in patients with chronic musculoskeletal pain

Many individuals with chronic musculoskeletal pain (CMP) show impairments in their pain-modulatory capacity. Although stress plays an important role in chronic pain, it is not known if stress-induced analgesia (SIA) is affected in patients with CMP. We investigated SIA in 22 patients with CMP and 18 pain-free participants. Pain thresholds, pain tolerance and suprathreshold pain ratings were examined before and after a cognitive stressor that typically induces pain reduction (SIA). Whereas the controls displayed a significant increase in pain threshold in response to the stressor, the patients with CMP showed no analgesia. In addition, increased pain intensity ratings after the stressor indicated hyperalgesia (SIH) in the patients with CMP compared to controls. An exploratory analysis showed no significant association of SIA or SIH with spatial pain extent. We did not observe significant changes in pain tolerance or pain unpleasantness ratings after the stressor in patients with CMP or controls. Our data suggest that altered stress-induced pain modulation is an important mechanism involved in CMP. Future studies need to clarify the psychobiological mechanisms of these stress-induced alterations in pain processing and determine the role of contributing factors such as early childhood trauma, catastrophizing, comorbidity with mental disorders and genetic predisposition.

Shifting the Balance: How Top-Down and Bottom-Up Input Modulate Pain via the Rostral Ventromedial Medulla

The sensory experience of pain depends not only on the transmission of noxious information (nociception), but on the state of the body in a biological, psychological, and social milieu. A brainstem pain-modulating system with its output node in the rostral ventromedial medulla (RVM) can regulate the threshold and gain for nociceptive transmission. This review considers the current understanding of how RVM pain-modulating neurons, namely ON-cells and OFF-cells, are engaged by “top-down” cognitive and emotional factors, as well as by “bottom-up” sensory inputs, to enhance or suppress pain.

Stress-induced analgesia: an evaluation of effects on temporal summation of pain and the role of endogenous opioid mechanisms

Acute stress reduced the initial pain rating in a temporal summation protocol via nonopioid mechanisms but did not affect temporal summation slope, an indicator of central sensitization.

Introduction:

Objectives:

Methods:

Results:

Conclusions:

Stress and pain: modality-specific opioid mediation of stress-induced analgesia

Preclinical research has demonstrated that exposure to acute stress is associated with attenuated pain perception, so called stress-induced analgesia (SIA). Mechanisms of SIA in humans have not been reliably demonstrated. This study examined the role of the endogenous opioid system in the impact of combined interpersonal and mental stressors on evoked pain responses in 84 participants (34 women). Using a within-subject, double-blinded, counterbalanced design, participants were administered either oral placebo or the opioid antagonist naltrexone (50 mg) across two testing sessions. In each session, they experienced two evoked pain stimuli (cold pressor test [CPT], heat pain) after an extended rest period (rest condition) and after exposure to an acute stressor (a combination of public speaking and mental arithmetic challenge; stress condition). Results showed that both stress and opioid blockade produced significant changes in hormonal and cardiovascular measures, consistent with successful induction of acute stress. Stress was associated with attenuated pain perception (SIA) as indicated by significantly increased CPT tolerance. These effects were particularly pronounced in individuals experiencing the stress condition first. More importantly, SIA effects on CPT tolerance were abolished by opioid blockade. There were no significant SIA effects on heat pain responses. This study demonstrates that the endogenous opioid system may mediate effects of acute stress on pain perception, although this effect seems to be qualified by the type of evoked pain stimuli experienced.

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

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

Immune cell-mediated opioid analgesia

Pathological pain is regulated by a balance between pro-algesic and analgesic mechanisms. Interactions between opioid peptide-producing immune cells and peripheral sensory neurons expressing opioid receptors represent a powerful intrinsic pain control in animal models and in humans. Therefore, treatments based on general suppression of immune responses have been mostly unsuccessful. It is highly desirable to develop strategies that specifically promote neuro-immune communication mediated by opioids. Promising examples include vaccination-based recruitment of opioid-containing leukocytes to painful tissue and the local reprogramming of pro-algesic immune cells into analgesic cells producing and secreting high amounts of opioid peptides. Such approaches have the potential to inhibit pain at its origin and be devoid of central and systemic side effects of classical analgesics. In support of these concepts, in this article, we describe the functioning of peripheral opioid receptors, migration of opioid-producing immune cells to inflamed tissue, opioid peptide release, and the consequent pain relief. Conclusively, we provide clinical evidence and discuss therapeutic opportunities and challenges associated with immune cell-mediated peripheral opioid analgesia.

Differential role of specific cardiovascular neuropeptides in pain regulation: Relevance to cardiovascular diseases

In many instances, the perception of pain is disproportionate to the strength of the algesic stimulus. Excessive or inadequate pain sensation is frequently observed in cardiovascular diseases, especially in coronary ischemia. The mechanisms responsible for individual differences in the perception of cardiovascular pain are not well recognized. Cardiovascular disorders may provoke pain in multiple ways engaging molecules released locally in the heart due to tissue ischemia, inflammation or cellular stress, and through neurogenic and endocrine mechanisms brought into action by hemodynamic disturbances. Cardiovascular neuropeptides, namely angiotensin II (Ang II), angiotensin-(1-7) [Ang-(1-7)], vasopressin, oxytocin, and orexins belong to this group. Although participation of these peptides in the regulation of circulation and pain has been firmly established, their mutual interaction in the regulation of pain in cardiovascular diseases has not been profoundly analyzed. In the present review we discuss the regulation of the release, and mechanisms of the central and systemic actions of these peptides on the cardiovascular system in the context of their central and peripheral nociceptive (Ang II) and antinociceptive [Ang-(1-7), vasopressin, oxytocin, orexins] properties. We also consider the possibility that they may play a significant role in the modulation of pain in cardiovascular diseases. The rationale for focusing attention on these very compounds was based on the following premises (1) cardiovascular disturbances influence the release of these peptides (2) they regulate vascular tone and cardiac function and can influence the intensity of ischemia - the factor initiating pain signals in the cardiovascular system, (3) they differentially modulate nociception through peripheral and central mechanisms, and their effect strongly depends on specific receptors and site of action. Accordingly, an altered release of these peptides and/or pharmacological blockade of their receptors may have a significant but different impact on individual sensation of pain and comfort of an individual patient.

Effects of stress on the corticolimbic system: implications for chronic pain

Stress has multifaceted effects on pain. On the one hand, it is a powerful inhibitor of nociception and inflammation; on the other hand, it contributes to enhanced pathological states including the establishment and continuation of chronic pain. These seemingly paradoxical effects can be better understood by investigating how stress-induced plasticity in particular brain circuitry contributes to the chronic pain state. This review presents the rationale and evidence for the interactions between stress and pain, emphasizing underlying mechanisms and putting forth the hypothesis that stress partly mediates the deleterious effects of pain on the corticolimbic system. First, a general description of the corticolimbic circuitry predisposing and amplifying chronic pain will be discussed, followed by an overview of the neurotoxic effects of stress hormones on this circuitry. Recent studies show that the resulting perturbations to these brain circuits have significant consequences both for chronic pain and for general regulation of the stress response, primarily through feedback mechanisms controlling the hypothalamic-pituitary-adrenal axis. This overlap in effected circuitry provides a key point of comparison between stress and pain, and the similarities between the plasticity induced by chronic pain and chronic stress will be examined here. Chronic pain patients have been shown to exhibit maladaptive stress responses in general and in response to pain; the cause of this response and its consequence on pain severity will then be reviewed. Finally, factors that have been shown to lead to resilience or vulnerability for chronic pain and maladaptive stress responses will be summarized.

Emotional Modulation of Pain and Spinal Nociception in Sexual Assault Survivors

OBJECTIVE

Sexual assault (SA) is associated with an increased risk for chronic pain and affective distress. Given that emotional processes modulate pain (e.g., negative emotions enhance pain, positive emotions inhibit pain), increased pain risk in SA survivors could stem from a disruption of emotional modulation processes.

METHODS

A well-validated affective picture-viewing paradigm was used to study emotional modulation of pain in 33 healthy, pain-free SA survivors and a control group of 33 healthy, pain-free individuals with no reported history of SA (matched on age, sex, race, and number of non-SA traumas). Unpleasant (mutilation), neutral, and pleasant (erotic) pictures were presented, while painful electrocutaneous stimulations were delivered at the ankle. Pain intensity ratings and nociceptive flexion reflex (NFR) magnitudes (a physiologic measure of spinal nociception) were recorded in response to electric stimuli. Multilevel models were used to analyze the data with group (SA versus non-SA) and content (mutilation, neutral, erotic) as independent variables.

RESULTS

Both groups demonstrated similar emotional modulation of pain (FGroupbyContent(2,646.52) = 0.44, p = .65), but a main effect of group (FGroup(1,65.42) = 4.24, p = .043) indicated the SA group experienced more overall pain from electric stimuli (hyperalgesia). A significant group by content interaction for NFR (p = .035) indicated that emotional modulation of NFR was present for the non-SA group (FContentSimpleEffect(2,684.55) = 12.43, p < .001), but not the SA group (FContentSimpleEffect(2,683.38) = 1.71, p = .18).

CONCLUSIONS

These findings suggest that SA survivors have difficulty emotionally engaging brain-to-spinal cord mechanisms to modulate spinal nociception. A disruption of descending inhibition plus hyperalgesia could contribute to comorbidity between sexual trauma and chronic pain.

Mechanisms of placebo analgesia: A dual-process model informed by insights from cross-species comparisons

Placebo treatments are pharmacologically inert, but are known to alleviate symptoms across a variety of clinical conditions. Associative learning and cognitive expectations both play important roles in placebo responses, however we are just beginning to understand how interactions between these processes lead to powerful effects. Here, we review the psychological principles underlying placebo effects and our current understanding of their brain bases, focusing on studies demonstrating both the importance of cognitive expectations and those that demonstrate expectancy-independent associative learning. To account for both forms of placebo analgesia, we propose a dual-process model in which flexible, contextually driven cognitive schemas and attributions guide associative learning processes that produce stable, long-term placebo effects. According to this model, the placebo-induction paradigms with the most powerful effects are those that combine reinforcement (e.g., the experience of reduced pain after placebo treatment) with suggestions and context cues that disambiguate learning by attributing perceived benefit to the placebo. Using this model as a conceptual scaffold, we review and compare neurobiological systems identified in both human studies of placebo analgesia and behavioral pain modulation in rodents. We identify substantial overlap between the circuits involved in human placebo analgesia and those that mediate multiple forms of context-based modulation of pain behavior in rodents, including forebrain-brainstem pathways and opioid and cannabinoid systems in particular. This overlap suggests that placebo effects are part of a set of adaptive mechanisms for shaping nociceptive signaling based on its information value and anticipated optimal response in a given behavioral context.

Psychosocial Stress-Induced Analgesia: An Examination of Effects on Heat Pain Threshold and Tolerance and of Neuroendocrine Mediation

Stress-induced analgesia (SIA) is an adaptive response of reduced nociception following demanding acute internal and external stressors. Although a psychobiological understanding of this phenomenon is of importance for stress-related psychiatric and pain conditions, comparably little is known about the psychobiological mechanisms of SIA in humans. The aim of this study was to investigate the effects of acute psychosocial stress on heat pain perception and its possible neuroendocrine mediation by salivary cortisol levels and α-amylase activity in healthy men. Employing an intra-individual assessment of heat pain parameters, acute psychosocial stress did not influence heat pain threshold but significantly, albeit slightly, increased heat pain tolerance. Using linear mixed-model analysis, this effect of psychosocial stress on heat pain tolerance was not mediated by increases of salivary cortisol and state anxiety levels or by the activity of α-amylase. These results show that while psychosocial stress is selectively analgesic for heat pain tolerance, this observed effect is not mediated by stress-induced increases of salivary cortisol and α-amylase activity, as proxies of both the hypothalamus-pituitary-adrenal axis and the autonomic nervous system activation.

Lifetime Modulation of the Pain System via Neuroimmune and Neuroendocrine Interactions

Chronic pain is a debilitating condition that still is challenging both clinicians and researchers. Despite intense research, it is still not clear why some individuals develop chronic pain while others do not or how to heal this disease. In this review, we argue for a multisystem approach to understand chronic pain. Pain is not only to be viewed simply as a result of aberrant neuronal activity but also as a result of adverse early-life experiences that impact an individual's endocrine, immune, and nervous systems and changes which in turn program the pain system. First, we give an overview of the ontogeny of the central nervous system, endocrine, and immune systems and their windows of vulnerability. Thereafter, we summarize human and animal findings from our laboratories and others that point to an important role of the endocrine and immune systems in modulating pain sensitivity. Taking "early-life history" into account, together with the past and current immunological and endocrine status of chronic pain patients, is a necessary step to understand chronic pain pathophysiology and assist clinicians in tailoring the best therapeutic approach.

Multidisciplinary assessment and treatment of self-injurious behavior in autism spectrum disorder and intellectual disability: integration of psychological and biological theory and approach

The objective of this review is to consider the psychological (largely behavioral) and biological [neurochemical, medical (including genetic), and pharmacological] theories and approaches that contribute to current thinking about the etiology and treatment of self-injurious behavior (SIB) in individuals with autism spectrum disorder and/or intellectual disability. Algorithms for the assessment and treatment of SIB in this context, respectively, from a multidisciplinary, integrative perspective are proposed and challenges and opportunities that exist in clinical and research settings are discussed.

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.

Neurobiology of stress-induced hyperalgesia

The intensity and severity of perceived pain does not correlate consistently with the degree of peripheral or central nervous system tissue damage or with the intensity of primary afferent or spinal nociceptive neurone activity. In this respect, the modulation of pain by emotion and context is now widely recognized. In particular, stress, fear and anxiety exert potent, but complex, modulatory influences on pain. Stress can either suppress pain (stress-induced analgesia) or exacerbate it (stress-induced hyperalgesia; SIH) depending on the nature, duration and intensity of the stressor. Herein, we review the methods and models used to study the phenomenon of SIH in rodents and humans and then present a detailed discussion of our current understanding of neural substrates and neurobiological mechanisms. The review provides perspectives and challenges for the current and future treatment of pain and the co-morbidity of pain with stress-related psychiatric disorders including anxiety and depression.

Classical conditioning and pain: conditioned analgesia and hyperalgesia

This article reviews situations in which stimuli produce an increase or a decrease in nociceptive responses through basic associative processes and provides an associative account of such changes. Specifically, the literature suggests that cues associated with stress can produce conditioned analgesia or conditioned hyperalgesia, depending on the properties of the conditioned stimulus (e.g., contextual cues and audiovisual cues vs. gustatory and olfactory cues, respectively) and the proprieties of the unconditioned stimulus (e.g., appetitive, aversive, or analgesic, respectively). When such cues are associated with reducers of exogenous pain (e.g., opiates), they typically increase sensitivity to pain. Overall, the evidence concerning conditioned stress-induced analgesia, conditioned hyperalagesia, conditioned tolerance to morphine, and conditioned reduction of morphine analgesia suggests that selective associations between stimuli underlie changes in pain sensitivity.

Changes in pain perception and hormones pre- and post-kumdo competition

The psychological stress of competition is a powerful stimulus affecting numerous hormones, which in turn change how pain is perceived. This study investigated whether a kumdo (kendo) team competition may be related to changes in hormones and pain. Seventeen healthy male kumdo practitioners participated in this experiment. Pain experiments were conducted by applying noxious stimuli with a thermal stimulator 10 min before a kumdo competition and 30 min post-competition. Serum testosterone, cortisol, beta-endorphin levels, pain thresholds, pain ratings at 48 °C and during blood sampling (sampling pain), anxiety, blood pressure, and heart rate were measured pre- and post-competition. Anxiety, pain threshold, testosterone/cortisol ratio, and blood pressure were significantly higher pre-competition compared to post-competition, while cortisol and pain ratings were significantly lower pre-competition than post-competition. There were significant correlations between the number of previous competitions and testosterone levels both pre-competition and post-competition. In pre-competition measurements, sampling pain increased with an increase in systolic blood pressure, heart rate, and beta-endorphins, and a decrease in age. In post-competition measurements, sampling pain increased with an increase in diastolic blood pressure and a decrease in testosterone levels. These results indicate that severe psychological pre-competition stress was associated with reduced pain ratings, perhaps in order to improve athletic performance. This also suggests that competitors may be at risk of potential injury due to changes in pain perception, and careful consideration should be taken to avoid potential injury before and during competition.

Coexistence of ipsilateral pain-inhibitory and facilitatory processes after high-frequency electrical stimulation

BACKGROUND

High-frequency electrical stimulation (HFS) of the human forearm evokes analgesia to blunt pressure in the ipsilateral forehead, consistent with descending ipsilateral inhibitory pain modulation. The aim of the current study was to further delineate pain modulation processes evoked by HFS by examining sensory changes in the arm and forehead; investigating the effects of HFS on nociceptive blink reflexes elicited by supraorbital electrical stimulation; and assessing effects of counter-irritation (electrically evoked pain at the HFS-conditioned site in the forearm) on nociceptive blink reflexes before and after HFS.

METHODS

Before and after HFS conditioning, sensitivity to heat and to blunt and sharp stimuli was assessed at and adjacent to the conditioned site in the forearm and on each side of the forehead. Nociceptive blink reflexes were also assessed before and after HFS with and without counter-irritation of the forearm.

RESULTS

HFS triggered secondary hyperalgesia in the forearm (a sign of central sensitization) and analgesia to blunt pressure in the ipsilateral forehead. Under most conditions, both HFS conditioning and counter-irritation of the forearm suppressed electrically evoked pain in the forehead, and the amplitude of the blink reflex to supraorbital stimuli decreased. Importantly, however, in the absence of forearm counter-irritation, HFS conditioning facilitated ipsilateral blink reflex amplitude to supraorbital stimuli delivered ipsilateral to the HFS-conditioned site.

CONCLUSIONS

These findings suggest that HFS concurrently triggers hemilateral inhibitory and facilitatory influences on nociceptive processing over and above more general effects of counter-irritation. The inhibitory influence may help limit the spread of sensitization in central nociceptive pathways.

Acute stress contributes to individual differences in pain and pain-related brain activity in healthy and chronic pain patients

Individual differences in pain sensitivity and reactivity are well recognized but the underlying mechanisms are likely to be diverse. The phenomenon of stress-induced analgesia is well documented in animal research and individual variability in the stress response in humans may produce corresponding changes in pain. We assessed the magnitude of the acute stress response of 16 chronic back pain (CBP) patients and 18 healthy individuals exposed to noxious thermal stimulations administered in a functional magnetic resonance imaging experiment and tested its possible contribution to individual differences in pain perception. The temperature of the noxious stimulations was determined individually to control for differences in pain sensitivity. The two groups showed similar significant increases in reactive cortisol across the scanning session when compared with their basal levels collected over 7 consecutive days, suggesting normal hypothalamic-pituitary-adrenal axis reactivity to painful stressors in CBP patients. Critically, after controlling for any effect of group and stimulus temperature, individuals with stronger cortisol responses reported less pain unpleasantness and showed reduced blood oxygenation level-dependent activation in nucleus accumbens at the stimulus onset and in the anterior mid-cingulate cortex (aMCC), the primary somatosensory cortex, and the posterior insula. Mediation analyses indicated that pain-related activity in the aMCC mediated the relationship between the reactive cortisol response and the pain unpleasantness. Psychophysiological interaction analysis further revealed that higher stress reactivity was associated with reduced functional connectivity between the aMCC and the brainstem. These findings suggest that acute stress modulates pain in humans and contributes to individual variability in pain affect and pain-related brain activity.

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

Predictors of responsiveness to opioid analgesic medications are not well understood. This study tested whether individual differences in endogenous opioid (EO) function are associated with analgesic responsiveness to morphine. In randomized, counterbalanced order over 3 sessions, 45 chronic low back pain participants and 31 healthy controls received an opioid antagonist (8 mg naloxone), morphine (0.08 mg/kg), or placebo. Participants then engaged in 2 laboratory-evoked pain tasks (ischemic and thermal). Outcomes included pain threshold, pain tolerance, and pain ratings. Indexes of EO function and morphine analgesic responsiveness were derived for each measure as the difference in pain responses between the placebo condition and naloxone or morphine condition, respectively. For all 7 pain measures across the 2 laboratory pain tasks, greater EO function was associated with significantly lower morphine analgesic responsiveness (P<0.001-P=0.02). Morphine reduced pain responses of low EO individuals to levels similar to those of high EO individuals receiving placebo. Higher placebo condition-evoked pain sensitivity was associated with significantly greater morphine analgesic responsiveness for 5 of 7 pain measures (P<0.001-P=0.02). These latter associations were significantly mediated by EO function for 4 of these 5 pain outcomes (all P values<0.05). In the laboratory-evoked pain context, opioid analgesic medications may supplement inadequate EO analgesia, with little incremental benefit in those with preexisting high EO function. Implications for personalized medicine are discussed.

Naloxone-reversible modulation of pain circuitry by left prefrontal rTMS

A 20-minute session of 10 Hz repetitive transcranial magnetic stimulation (rTMS) of Brodmann Area (BA) nine of the left dorsolateral prefrontal cortex (DLPFC) can produce analgesic effects on postoperative and laboratory-induced pain. This analgesia is blocked by pretreatment with naloxone, a μ-opioid antagonist. The purpose of this sham-controlled, double-blind, crossover study was to identify the neural circuitry that underlies the analgesic effects of left DLPFC rTMS, and to examine how the function of this circuit, including midbrain and medulla, changes during opioid blockade. Fourteen healthy volunteers were randomized to receive intravenous saline or naloxone immediately before sham and real left DLPFC rTMS on the same experimental visit. One week later, each participant received the novel pretreatment but the same stimulation paradigm. Using short sessions of heat on capsaicin-sensitized skin, hot allodynia was assessed during 3 Tesla functional magnetic resonance imaging (fMRI) scanning at baseline, post-sham rTMS, and post-real rTMS. Data were analyzed using whole-brain voxel-based analysis, as well as time series extractions from anatomically-defined regions of interest representing midbrain and medulla. Consistent with previous findings, real rTMS significantly reduced hot allodynia pain ratings. This analgesia was associated with elevated blood oxygenation-level dependent (BOLD) signal in BAs 9 and 10, and diminished BOLD signal in the anterior cingulate, thalamus, midbrain, and medulla during pain. Naloxone pretreatment largely abolished rTMS-induced analgesia, as well as rTMS-induced attenuation of BOLD signal response to painful stimuli throughout pain processing regions, including midbrain and medulla. These preliminary results suggest that left DLPFC rTMS drives top-down opioidergic analgesia.

Ultra-marathon runners are different: investigations into pain tolerance and personality traits of participants of the TransEurope FootRace 2009

INTRODUCTION

Susceptibility to pain varies among individuals and may predispose to a higher risk for pain disorders. Thus, it is of interest to investigate subjects who exhibit higher resistance to pain. We therefore tested pain tolerance and assessed personality traits of ultra-marathon athletes who are able to run 4487 km (2789 mi) over 64 days without resting days and compare the results to controls.

METHODS

After approval of the local ethics committee and with informed consent, 11 participants of the TransEurope FootRace (TEFR09 participants) and 11 matched (age, sex, and ethnicity) controls without marathon experience in the last 5 years were enrolled. They were tested for cold pain tolerance (cold pressor [CP] test), and the 240 item trait and character inventory (TCI) as well as the general self-efficacy (GSE) test were obtained.

RESULTS

TransEurope FootRace participants had a highly significant greater cold pain tolerance in the CP test than controls (P = 0.0002). While the GSE test showed no differences, the TCI test provided TEFR09 participants to be less cooperative and reward dependent but more spiritually transcendent than the controls. Significant positive correlations were found between the CP test pain score at 180 seconds and several TCI subscales showing that higher pain scores correlate with higher reward dependence, dependence, cooperativeness, empathy, and pure-hearted conscience.

CONCLUSIONS

Personality profiles as well as pain tolerance of our sample of TEFR09 participants differ from normal controls and-as obtained in previous studies-probably also from chronic pain patients. Low pain perception may predispose a person to become a long-distance runner. It remains unclear, however, whether low pain perception is cause or consequence of continuous extreme training.

High frequency electrical stimulation concurrently induces central sensitization and ipsilateral inhibitory pain modulation

BACKGROUND

In healthy humans, analgesia to blunt pressure develops in the ipsilateral forehead during various forms of limb pain. The aim of the current study was to determine whether this analgesic response is induced by ultraviolet B radiation (UVB), which evokes signs of peripheral sensitization, or by high-frequency electrical stimulation (HFS), which triggers signs of central sensitization.

METHODS

Before and after HFS and UVB conditioning, sensitivity to heat and to blunt and sharp stimuli was assessed at and adjacent to the treated site in the forearm. In addition, sensitivity to blunt pressure was measured bilaterally in the forehead. The effect of ipsilateral versus contralateral temple cooling on electrically evoked pain in the forearm was then examined, to determine whether HFS or UVB conditioning altered inhibitory pain modulation.

RESULTS

UVB conditioning triggered signs of peripheral sensitization, whereas HFS conditioning triggered signs of central sensitization. Importantly, ipsilateral forehead analgesia developed after HFS but not UVB conditioning. In addition, decreases in electrically evoked pain at the HFS-treated site were greater during ipsilateral than contralateral temple cooling, whereas decreases at the UVB-treated site were similar during both procedures.

CONCLUSIONS

HFS conditioning induced signs of central sensitization in the forearm and analgesia both in the ipsilateral forehead and the HFS-treated site. This ipsilateral analgesia was not due to peripheral sensitization or other non-specific effects, as it failed to develop after UVB conditioning. Thus, the supra-spinal mechanisms that evoke central sensitization might also trigger a hemilateral inhibitory pain modulation process. This inhibitory process could sharpen the boundaries of central sensitization or limit its spread.

Endogenous opioids mediate left dorsolateral prefrontal cortex rTMS-induced analgesia

The concurrent rise of undertreated pain and opiate abuse poses a unique challenge to physicians and researchers alike. A focal, noninvasive form of brain stimulation called repetitive transcranial magnetic stimulation (rTMS) has been shown to produce acute and chronic analgesic effects when applied to dorsolateral prefrontal cortex (DLPFC), but the anatomical and pharmacological mechanisms by which prefrontal rTMS induces analgesia remain unclear. Data suggest that DLPFC mediates top-down analgesia via gain modulation of the supraspinal opioidergic circuit. This potential pathway might explain how prefrontal rTMS reduces pain. The purpose of this sham-controlled, double-blind, crossover study was to determine whether left DLPFC rTMS-induced analgesia was sensitive to μ-opioid blockade. Twenty-four healthy volunteers were randomized to receive real or sham TMS after either intravenous saline or naloxone pretreatment. Acute hot and cold pain via quantitative sensory testing and hot allodynia via block testing on capsaicin-treated skin were assessed at baseline and at 0, 20, and 40 minutes after TMS treatment. When compared to sham, real rTMS reduced hot pain and hot allodynia. Naloxone pretreatment significantly reduced the analgesic effects of real rTMS. These results demonstrate that left DLPFC rTMS-induced analgesia requires opioid activity and suggest that rTMS drives endogenous opioidergic pain relief in the human brain. Further studies with chronic dosing regimens of drugs that block or augment the actions of opiates are needed to determine whether TMS can augment opiates in chronic or postoperative pain management.

Autonomic activation and pain in response to low-grade mental stress in fibromyalgia and shoulder/neck pain patients

OBJECTIVE

Psychosocial stress is a risk factor for musculoskeletal pain, but how stress affects musculoskeletal pain is poorly understood. We wanted to examine the relationship between low-grade autonomic activation and stress-related pain in patients with fibromyalgia and localised chronic shoulder/neck pain.

METHODS

Twenty-three female patients with fibromyalgia, 29 female patients with chronic shoulder-neck pain, and 35 healthy women performed a stressful task lasting 60min. With a blinded study design, we recorded continuous blood pressure, heart rate, finger skin blood flow and respiration frequency before (10min), during (60min) and after (30min) the stressful task. The physiological responses were compared with subjective reports of pain.

RESULTS

The increase in diastolic blood pressure and heart rate in response to the stressful task were smaller in fibromyalgia patients compared with the healthy controls. Furthermore, fibromyalgia patients had reduced finger skin blood flow at the end of the stressful task compared to healthy controls. We also found an inverse relationship between the heart rate response and development and recovery of the stress-related pain in fibromyalgia patients.

CONCLUSION

We found abnormal cardiovascular responses to a 60min long stressful task in fibromyalgia patients. Furthermore, we found a negative association between the heart rate response and the pain which developed during the stressful task in the fibromyalgia group, possibly a result of reduced stress-induced analgesia for fibromyalgia patients.

Selection for stress-induced analgesia affects the mouse hippocampal transcriptome

Stress responsiveness, including pain sensitivity and stress-induced analgesia (SIA), depends on genotype and, partially, is mediated by hippocampus. The present study examined differences in constitutive gene expression in hippocampus in lines of mice bred for high (HA) and low (LA) swim SIA. Between the lines, we found 1.5-fold or greater differences in expression of 205 genes in the hippocampus in nonstressed animals. The identity of these genes indicates that selective breeding for swim SIA affected many aspects of hippocampal neurons physiology, including metabolism, structural changes, and cellular signaling. Genes involved in calcium signaling pathway, including Slc8a1, Slc8a2, Prkcc, and Ptk2b, were upregulated in LA mice. In HA mice, robust upregulation of genes coding some transcription factors (Klf5) or receptors for neurotensin (Ntsr2) and GABA (Gabard) suggests the genetic basis for a novel mechanism of the non-opioid type of SIA in HA animals. Additional groups of differentially expressed genes represented functional networks involved in carbohydrate metabolism, gene expression regulation, and molecular transport. Our data indicate that selection for a single and very specific stress response trait, swim SIA, alters hippocampal gene expression. The results suggest that individual stress responsiveness may be associated with characteristics of the constitutive hippocampal transcriptome.

Modulation of peripheral sensory neurons by the immune system: implications for pain therapy

The concept that the immune system can communicate with peripheral sensory neurons to modulate pain is based mostly on documented interactions between opioid ligands and receptors. Such findings may have broad implications for the development of safer pain medication. Innovative strategies take into account that analgesics should be particularly active in pathological states rather than producing a general suppression of the central nervous system, as with conventional morphine- or cannabinoid-like drugs. Inflammation of peripheral tissue leads to increased functionality of opioid receptors on peripheral sensory neurons and to local production of endogenous opioid peptides. In addition, endocannabinoids were detected in leukocytes, but their role in pain modulation has yet to be addressed. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing immune cells to sites of painful injury, and the augmentation of peripheral ligand and receptor synthesis (e.g., by gene therapy). Similar approaches may be pursued for cannabinoids. The ultimate goal is to avoid detrimental side effects of currently available analgesics such as respiratory depression, cognitive impairment, addiction, gastrointestinal bleeding, and thromboembolic complications.