Modulation of pain perceptions following treadmill running with different intensities in females

We aimed to compare the effects of three intensities of treadmill running on exercise-induced hypoalgesia (EIH) in healthy individuals. We anticipated that the primary and secondary changes in pain perception and modulation may differ between running intensities. Sixty-six women were randomly assigned to one of three treadmill running intensities for 35 min: 40% reserved heart rate (HRR), 55% HRR, or 70% HRR. The effects of EIH were assessed using pressure pain thresholds (PPT) and tolerance thresholds (PPTol). We measured conditional pain modulation (CPM). Compared with baseline, PPT and PPTol significantly increased in all groups during running and at the 5-10-min follow-up. The PPT and PPTol changes in the moderate- and low-intensity groups were significantly higher than those in the high-intensity group during running and 24 h after running, while the CPM responses of the high-intensity group were significantly reduced at the 24-h follow-up. Moderate- and low-intensity running may elicit significant primary and secondary (persisting over 24 h) EIH effects and increase CPM responses in females. However, high-intensity running induced only limited analgesic effects and reduced CPM responses, which may be attributed to the activation of endogenous pain modulation.

Evidence for dopaminergic involvement in endogenous modulation of pain relief

Relief of ongoing pain is a potent motivator of behavior, directing actions to escape from or reduce potentially harmful stimuli. Whereas endogenous modulation of pain events is well characterized, relatively little is known about the modulation of pain relief and its corresponding neurochemical basis. Here, we studied pain modulation during a probabilistic relief-seeking task (a 'wheel of fortune' gambling task), in which people actively or passively received reduction of a tonic thermal pain stimulus. We found that relief perception was enhanced by active decisions and unpredictability, and greater in high novelty-seeking trait individuals, consistent with a model in which relief is tuned by its informational content. We then probed the roles of dopaminergic and opioidergic signaling, both of which are implicated in relief processing, by embedding the task in a double-blinded cross-over design with administration of the dopamine precursor levodopa and the opioid receptor antagonist naltrexone. We found that levodopa enhanced each of these information-specific aspects of relief modulation but no significant effects of the opioidergic manipulation. These results show that dopaminergic signaling has a key role in modulating the perception of pain relief to optimize motivation and behavior.

A web app-based music intervention reduces experimental thermal pain: A randomized trial on preferred versus least-liked music style

Digital technologies are increasingly being used to strengthen national health systems. Music is used as a management technique for pain. The objective of this study is to demonstrate the effects of a web app-based music intervention on pain. The participants were healthy adults and underwent three conditions: Conditioned Pain Modulation (CPM), Most-Liked Music (MLM) and Least-Liked Music (LLM). The music used is MUSIC CARE©, a web app-based personalized musical intervention ("U" Sequence based on a musical composition algorithm). Thermal pain was measured before starting the 20-min music intervention and after three time points for each music condition: 2.20, 11.30, and 20 min. Mean pain perceptions were significantly reduced under both LLM and MLM conditions. Pain decrease was more important under MLM condition than LLM condition at 2.20 min with a mean difference between both conditions of 9.7 (±3.9) (p = 0.0195) and at 11.30 min [9.2 (±3.3), p = 0.0099]. LLM is correlated with CPM but not MLM, suggesting different mechanisms between LLM and MLM. Musical intervention, a simple method of application, fits perfectly into a multidisciplinary global approach and helps to treat the pain and anxiety disorders of participants. Clinical trial registration: [https://clinicaltrials.gov/ct2/show/NCT04862832], ClinicalTrials.gov [NCT04862832].

Temporal Summation and Aftersensations of Second Pain in Women with Myofascial Temporomandibular Disorder Differ by Presence of Temporomandibular Joint Pain

Purpose

Mechanisms underlying myofascial temporomandibular disorder (mTMD) are poorly understood. One theory is dysfunction in the central mediation of pain, specifically in enhanced facilitatory pain modulation. Because mechanisms leading to central sensitization may differ for joint and muscle pain, this study of mTMD addressed phenotypic heterogeneity by temporomandibular (TM) joint pain in the examination of quantitative sensory testing (QST).

Patients and Methods

The stimulus dependent increase in second pain (temporal summation (TS)) and associated aftersensations (AS) were examined across groups of women with mTMD with TM joint pain and without, and a demographically matched control group.

Results

TS was slightly more evident in mTMD without joint pain vs with (p = 0.035), but AS were most robustly persistent in the group with joint pain vs without (p < 0.002).

Conclusion

While both subgroups demonstrated evidence of central sensitization relative to controls on one of two measures, differences in QST results, if replicated, may point to possible differences in the mechanisms that yield central sensitization. Alternatively, it may represent methodological artifacts that need to be addressed. Therefore, greater consideration should be given to symptom-based phenotypes in studies examining TS and AS.

The translocator protein gene is associated with endogenous pain modulation and the balance between glutamate and γ-aminobutyric acid in fibromyalgia and healthy subjects: a multimodal neuroimaging study

ABSTRACT

A cerebral upregulation of the translocator protein (TSPO), a biomarker of glial activation, has been reported in fibromyalgia subjects (FMS). The TSPO binding affinity is genetically regulated by the Ala147Thr polymorphism in the TSPO gene (rs6971) and allows for a subject classification into high affinity binders (HABs) and mixed/low affinity binders (MLABs). The aim of the present multimodal neuroimaging study was to examine the associations of the TSPO polymorphism with: (1) conditioned pain modulation, (2) expectancy-modulated pain processing assessed during functional magnetic resonance imaging, and (3) the concentration and balance of glutamate and γ-aminobutyric acid in the rostral anterior cingulate cortex and thalamus using proton magnetic resonance spectroscopy in FMS (n = 83) and healthy controls (n = 43). The influence of TSPO on endogenous pain modulation presented in the form of TSPO HABs, as opposed to MLABs, displaying less efficient descending pain inhibition and expectancy-induced reduction of pain. Translocator protein HABs in both groups (FM and healthy controls) were found to have higher thalamic glutamate concentrations and exhibit a pattern of positive correlations between glutamate and γ-aminobutyric acid in the rostral anterior cingulate cortex, not seen in MLABs. Altogether, our findings point to TSPO-related mechanisms being HAB-dependent, brain region-specific, and non-FM-specific, although in FMS the disadvantage of an aberrant pain regulation combined with an HAB genetic set-up might hamper pain modulation more strongly. Our results provide evidence for an important role of TSPO in pain regulation and brain metabolism, thereby supporting the ongoing drug development targeting TSPO-associated mechanisms for pain relief.

Pain and executive function: no association between remote exercise-induced hypoalgesia and cognitive inhibition in pain-free participants

OBJECTIVES

Cognitive inhibition, which denotes the ability to suppress predominant or automatic responses, has been associated with lower pain sensitivity and larger conditioned pain modulation in humans. Studies exploring the association between cognitive inhibition and other pain inhibitory phenomena, like exercise-induced hypoalgesia (EIH), are scarce. The primary aim was to explore the association between cognitive inhibition and EIH at exercising (local) and non-exercising (remote) muscles after isometric exercise. The secondary aim was to explore the association between cognitive inhibition and pressure pain sensitivity.

METHODS

Sixty-six pain-free participants (28.3 ± 8.9 years old, 34 women) completed two cognitive inhibition tasks (stop-signal task and Stroop Colour-Word task), a 3-min isometric wall squat exercise, and a quiet rest control condition with pre- and post-assessments of manual pressure pain thresholds at a local (thigh) and a remote site (shoulder). In addition, cuff pressure pain thresholds, pain tolerance and temporal summation of pain were assessed at baseline.

RESULTS

No association was found between remote EIH and cognitive inhibition (Stroop interference score: r=0.12, [-0.15; 0.37], p=0.405, BF01=6.70; stop-signal reaction time: r=-0.08, [-0.32; 0.17], p=0.524, BF01=8.32). Unexpectedly, individuals with worse performance on the Stroop task, as indicated by a higher Stroop interference score, showed higher local EIH (r=0.33; [0.10; 0.53], p=0.007, BF01=0.29). No associations were observed between pain sensitivity and any of the cognitive inhibition performance parameters.

CONCLUSIONS

The present findings do not support previous evidence on positive associations between exercise-induced hypoalgesia and cognitive inhibition, as well as baseline pain sensitivity and cognitive inhibition.

Somatosensory dysfunction in patients with posttraumatic headache: A systematic review

OBJECTIVES

Aim of the review is to summarize the knowledge about the sensory function and pain modulatory systems in posttraumatic headache and discuss its possible role in patients with posttraumatic headache.

BACKGROUND

Posttraumatic headache is the most common complication after traumatic brain injury, and significantly impacts patients' quality of life. Even though it has a high prevalence, its origin and pathophysiology are poorly understood. Thereby, the existing treatment options are insufficient. Identifying its mechanisms can be an important step forward to develop target-based personalized treatment.

METHODS

We searched the PubMed database for studies examining pain modulation and/or quantitative sensory testing in individuals with headache after brain injury.

RESULTS

The studies showed heterogenous alterations in sensory profiles (especially in heat and pressure pain perception) compared to healthy controls and headache-free traumatic brain injury-patients. Furthermore, pain inhibition capacity was found to be diminished in subjects with posttraumatic headache.

CONCLUSIONS

Due to the small number of heterogenous studies a distinct sensory pattern for patients with posttraumatic headache could not be identified. Further research is needed to clarify the underlying mechanisms and biomarkers for prediction of development and persistence of posttraumatic headache.

Developments in Understanding Diffuse Noxious Inhibitory Controls: Pharmacological Evidence from Pre-Clinical Research

Bulbospinal pathways regulate nociceptive processing, and inhibitory modulation of nociception can be achieved via the activity of diffuse noxious inhibitory controls (DNIC), a unique descending pathway activated upon application of a conditioning stimulus (CS). Numerous studies have investigated the effects of varied pharmacological systems on the expression status of a) DNIC (as measured in anaesthetised animals) and b) the descending control of nociception (DCN), a surrogate measure of DNIC-like effects in conscious animals. However, the complexity of the underlying circuitry that governs initiation of a top-down inhibitory response in reaction to a CS, coupled with the methodological limitations associated with using pharmacological tools for its study, has often obscured the exact role(s) of a given drug. In this literature review, we discuss the pharmacological manipulation interrogation strategies that have hitherto been used to examine the functionality of DNIC and DCN. Discreet administration of a substance in the spinal cord or brain is considered in the context of action on one of four hypothetical systems that underlie the functionality of DNIC/DCN, where interpreting the outcome is often complicated by overlapping qualities. Systemic pharmacological modulation of DNIC/DCN is also discussed despite the fact that the precise location of drug action(s) cannot be pinpointed. Chiefly, modulation of the noradrenergic, serotonergic and opioidergic transmission systems impacts DNIC/DCN in a manner that relates to drug class, route of administration and health/disease state implicated. The advent of increasingly sophisticated interrogation tools will expedite our full understanding of the circuitries that modulate naturally occurring pain-inhibiting pathways.

Transcutaneous electrical nerve stimulation (TENS): towards the development of a clinic-friendly method for the evaluation of excitatory and inhibitory pain mechanisms

Background: Temporal summation and conditioned pain modulation (CPM) can be measured using a thermode and cold pressor test (CPTest). Unfortunately, these complex and expensive tools are ill-suited for routine clinical assessments.

Aims: We aimed to compare the temporal summation and CPM obtained with the thermode + CPTest paradigm to those obtained with a novel paradigm using transcutaneous electrical nerve stimulation (TENS).

Methods: We assessed temporal summation and CPM in 29 healthy participants, using two paradigms (random order): TENS, and thermode + CPTest. In the TENS paradigm, both the conditioning stimulus (CS) and the test stimulus (TS) were delivered using TENS; in the thermode + CPTest paradigm, the CS consisted of a CPTest and the TS was delivered using a thermode. We compared the average temporal summation and CPM evoked by the two paradigms.

Results: Average temporal summation was similar for both modalities (P = 0.90), and the number of participants showing temporal summation was similar in both paradigms (19 with thermode vs. 18 with TENS; P = 1.00). Average CPM response was larger following the thermode + CPTest than following the TENS (P = 0.005), and more participants showed CPM with the thermode + CPTest paradigm compared to the TENS paradigm (24 vs. 14; P = 0.01).

Conclusions: Both paradigms were roughly equivalent in the ability to evoke temporal summation (although response to one modality did not predict response to the other), but the TENS paradigm appeared to be less apt to induce a CPM response than the thermode + CPTest paradigm.

Microstructural alterations in medial forebrain bundle are associated with interindividual pain sensitivity

The perception of pain to noxious stimuli, also known as pain sensitivity, varies among individuals. The comprised brain structures and their white matter pathways are complex and elusive. Here, we aimed to investigate whether variation of microstructure of the medial forebrain bundle (MFB), a tract connecting the basal forebrain with the brain stem, is associated with interindividual pain sensitivity. We assessed interindividual pain sensitivity as a rating of pain intensity to heat stimuli (45, 47, and 48.9°C) in 38 healthy men (age: 27.05 ± 5.7 years). We also reconstructed the MFB using multitensor tractography from diffusion magnetic resonance imaging (dMRI) and calculated free‐water corrected dMRI measures of fractional anisotropy (FA_t), radial diffusivity (RD_t), and axial diffusivity (AD_t). Lower ratings of interindividual pain intensity correlated with higher FA_t and lower RD_t of the MFB. As changes in FA_t and RD_t may reflect abnormalities in myelination, the results might be interpreted as that a lower pain rating is associated with higher degree of myelination of the MFB and could represent an inhibitory pathway of pain. Our results suggest that alteration of microstructure in the MFB contributes to the interindividual variation of pain perception.

Pain Control by Co-adaptive Learning in a Brain-Machine Interface

Innovation in the field of brain-machine interfacing offers a new approach to managing human pain. In principle, it should be possible to use brain activity to directly control a therapeutic intervention in an interactive, closed-loop manner. But this raises the question as to whether the brain activity changes as a function of this interaction. Here, we used real-time decoded functional MRI responses from the insula cortex as input into a closed-loop control system aimed at reducing pain and looked for co-adaptive neural and behavioral changes. As subjects engaged in active cognitive strategies orientated toward the control system, such as trying to enhance their brain activity, pain encoding in the insula was paradoxically degraded. From a mechanistic perspective, we found that cognitive engagement was accompanied by activation of the endogenous pain modulation system, manifested by the attentional modulation of pain ratings and enhanced pain responses in pregenual anterior cingulate cortex and periaqueductal gray. Further behavioral evidence of endogenous modulation was confirmed in a second experiment using an EEG-based closed-loop system. Overall, the results show that implementing brain-machine control systems for pain induces a parallel set of co-adaptive changes in the brain, and this can interfere with the brain signals and behavior under control. More generally, this illustrates a fundamental challenge of brain decoding applications-that the brain inherently adapts to being decoded, especially as a result of cognitive processes related to learning and cooperation. Understanding the nature of these co-adaptive processes informs strategies to mitigate or exploit them.

Aberrant pain modulation in trigeminal neuralgia patients

Objectives The present study attempts to understand the role of supraspinal nociceptive pain modulation in typical trigeminal neuralgia (TN) patients by using a conditioned pain modulation paradigm and estimation of plasma levels of two important neuromodulators; Calcitonin Gene-Related Peptide and β-endorphin. Methods Twenty TN patients and 20 healthy, age and gender-matched subjects participated in the study. The participants' hot pain thresholds (HPT) were measured over their affected side on the face. Testing sites were matched for healthy controls. For the conditioned pain modulation their contralateral foot was immersed in noxious cold (5 °C) water bath (conditioning stimuli) for 30 s and HPT (testing stimuli) was determined before, during and till 5 min after the immersion. Plasma Calcitonin Gene-Related Peptide and β-endorphin levels were estimated to understand their role in disease pathogenesis and pain modulation. Results Change in HPT during foot immersion was significantly higher in healthy controls compared to TN patients (p<0.0001). The changes recorded in HPT in patients, were significant only in 2nd and 3rd minute post immersion. While in healthy controls, the effect lasted till the 4th minute. The concentration of beta-endorphin was significantly lower in TN patients (p=0.003) when compared to healthy controls. Conclusions The results suggest that there is an impairment in supraspinal pain modulation also known as Diffuse Noxious Inhibitory Controls in typical TN and that the reduced levels of β-endorphin may contribute to the chronic pain state experienced by patients.

Negative Affectivity, Depression, and Resting Heart Rate Variability (HRV) as Possible Moderators of Endogenous Pain Modulation in Functional Somatic Syndromes

Background: Several studies have shown that patients with functional somatic syndromes (FSS) have, on average, deficient endogenous pain modulation (EPM), as well as elevated levels of negative affectivity (NA) and high comorbidity with depression and reduced resting heart rate variability (HRV) compared to healthy controls (HC). The goals of this study were (1) to replicate these findings and (2) to investigate the moderating role of NA, depression, and resting HRV in EPM efficiency within a patient group with fibromyalgia and/or chronic fatigue syndrome (CFS). Resting HRV was quantified as the root mean square of successive differences between inter-beat intervals (RMSSD) in rest, a vagally mediated time domain measure of HRV.

Methods: Seventy-eight patients with fibromyalgia and/or CFS and 33 HC completed a counter-irritation paradigm as a measure of EPM efficiency. Participants rated the painfulness of electrocutaneous stimuli (of individually calibrated intensity) on the ankle before (baseline phase), during (counter-irritation phase) and after (recovery phase) the application of a cold pain stimulus on the forearm. A larger reduction in pain in the counter-irritation phase compared to the baseline phase reflects a more efficient EPM.

Results: In contrast to our expectations, there was no difference between pain ratings in the baseline compared to counter-irritation phase for both patients and HC. Therefore, reliable conclusions on the moderating effect of NA, depression, and RMSSD could not be made. Surprisingly, patients reported more pain in the recovery compared to the counter-irritation and baseline phase, while HC did not. This latter effect was more pronounced in patients with comorbid depression, patients who rated the painfulness of the counter-irritation stimulus as high and patients who rated the painfulness of the electrocutaneous stimuli as low. We did not manage to successfully replicate the counter-irritation effect in HC or FSS patients. Therefore, no valid conclusions on the association between RMSSD, depression, NA and EPM efficiency can be drawn from this study. Possible reasons for the lack of the counter-irritation effect are discussed.

Preserved Capacity for Placebo Analgesia in the Elderly

The prevalence of chronic pain rises with increasing age. It has been suggested that the mechanisms responsible for the development of chronic pain overlap with mechanisms involved in aging, potentially implicating age-related changes in descending modulatory pathways. This observation raises the question whether other forms of endogenous pain modulation, in particular placebo analgesia, become compromised with age. Because of the known contribution of placebo effects to analgesic treatment outcomes this question is of important clinical relevance. In this study, we compared the response to thermal painful stimuli and the capacity for endogenous pain modulation between younger and older adults using a well established placebo analgesia paradigm involving expectancy and conditioning components. We recruited 30 younger (age 23-40 years, mean = 27.04, standard error of the mean ± .61) and 24 older adults (60-80 years, mean = 69.3, standard error of the mean ± .89). We observed increased heat pain thresholds and higher pain intensity ratings (in response to physically identical heat stimulation) in the older compared with the younger group. However, the placebo analgesic response was comparable between both age groups of healthy participants. The preserved capacity for placebo analgesia in our sample of older participants highlights the potential to use nonpharmacological analgesic treatment strategies in this age group and to exploit placebo mechanisms as an add-on to existing analgesic (pharmacological) treatment strategies.

PERSPECTIVE

In contrast to the commonly shared view that endogenous pain modulation declines with age we found a comparable capacity for placebo analgesia in a group of healthy older and younger adults.

Visceral pain perception in patients with irritable bowel syndrome and healthy volunteers is affected by the MRI scanner environment

BACKGROUND

The MRI scanner environment induces marked psychological effects, but specific effects on pain perception and processing are unknown and relevant to all brain imaging studies.

OBJECTIVES AND METHODS

We performed visceral and somatic quantitative sensory and pain testing and studied endogenous pain modulation by heterotopic stimulation outside and inside the functional MRI scanner in 11 healthy controls and 13 patients with irritable bowel syndrome.

RESULTS

Rectal pain intensity (VAS 0-100) during identical distension pressures increased from 39 (95% confidence interval: 35-42) outside the scanner to 53 (43-63) inside the scanner in irritable bowel syndrome, and from 42 (31-52) to 49 (39-58), respectively, in controls (ANOVA for scanner effect: p = 0.006, group effect: p = 0.92). The difference in rectal pain outside versus inside correlated significantly with stress (r = -0.76, p = 0.006), anxiety (r = -0.68, p = 0.02) and depression scores (r = -0.67, p = 0.02) in controls, but not in irritable bowel syndrome patients, who a priori had significantly higher stress and anxiety scores. ANOVA analysis showed trends for effect of the scanner environment and subject group on endogenous pain modulation (p = 0.09 and p = 0.1, respectively), but not on somatic pain (p > 0.3).

CONCLUSION

The scanner environment significantly increased visceral, but not somatic, pain perception in irritable bowel syndrome patients and healthy controls in a protocol specifically aimed at investigating visceral pain. Psychological factors, including anxiety and stress, are the likely underlying causes, whereas classic endogenous pain modulation pathways activated by heterotopic stimulation play a lesser role. These results are highly relevant to a wide range of imaging applications and need to be taken into account in future pain research. Further controlled studies are indicated to clarify these findings.

Dysfunctional endogenous pain modulation in patients with functional dyspepsia

BACKGROUND

Endogenous pain modulation (EPM) is central to the processing of sensory information. Visceral and somatic EPM are abnormal in irritable bowel syndrome, but have not been studied in functional dyspepsia (FD).

METHODS

Visceral EPM was assessed in 34 FD patients and 42 healthy controls. Gastric pain was induced with oral capsaicin and EPM was studied by adding heterotopic thermal foot stimulation or distraction by STROOP test. Somatic EPM was assessed using foot heat stimulation with heterotopic hand electrical stimulation.

KEY RESULTS

Endogenous pain modulation by distraction reduced mean gastric pain by 11.9 on the 0-100 visual analog scale (95% CI: 3.8-20.1) in controls (p = 0.006) and by 2.0 (-6.18 to 10.44) in FD (p = 0.6), with greater EPM in controls than in FD (difference -13.3 [-26.1 to -0.5]; p = 0.04). Endogenous pain modulation by heterotopic foot stimulation reduced gastric pain by 6.5 (-0.7 to 13.6) in controls (p = 0.07) and by 7.1 (-2.29 to 16.47) in FD (p = 0.1), with no significant difference in EPM between controls and FD (-2.0 [-14.5 to 10.5]; p = 0.75). In patients with prominent FD pain, greater pain correlated with decreased visceral EPM by distraction (r = 0.51, p = 0.04). Somatic EPM by heterotopic stimulation significantly decreased foot pain in controls (p = 0.004), but not in FD (p = 0.80).

CONCLUSIONS & INFERENCES

In FD, visceral pain modulation by distraction was dysfunctional compared to controls. Somatic pain modulation was also decreased in FD. These data and the correlation of abnormal pain modulation by distraction with clinical pain in pain-predominant FD suggest a potential pathophysiological significance of abnormal pain modulation in FD.

Endogenous inhibition of somatic pain is impaired in girls with irritable bowel syndrome compared with healthy girls

Endogenous pain inhibition is often deficient in adults with chronic pain conditions including irritable bowel syndrome (IBS). It is unclear whether deficiencies in pain inhibition are present in young children with IBS. The present study compared endogenous pain inhibition, somatic pain threshold, and psychosocial distress in young girls with IBS versus controls. Girls with IBS did not show significant endogenous pain inhibition of heat pain threshold during a cold-pressor task in contrast to controls, who had significant pain inhibition. Girls with IBS did not differ from peers on measures of somatic pain but had more symptoms of depression, somatization, and anxiety than controls. When psychological variables were included as covariates, the difference in pain inhibition was no longer significant, although poor achieved power limits interpretation of these results. Higher-order cognitive processes including psychological variables may be contributing to observed pain inhibition. In girls with IBS, pain inhibition was positively related to the number of days without a bowel movement. To our knowledge, this is the first study to demonstrate deficiencies of endogenous pain inhibition in young children with IBS. Findings have implications for better understanding of onset and maintenance of IBS and other chronic pain conditions.

PERSPECTIVE

This study found that young girls with IBS have deficient endogenous pain inhibition compared to healthy girls, which is consistent with the literature on adults. This information can facilitate clinicians in identification of risk factors for onset/maintenance of IBS and other chronic pain conditions.