Experimental evidence for weaker endogenous inhibition of trigeminal pain than extra-trigeminal pain in healthy individuals

Sex differences in descending control of nociception (DCN) responses after chronic orofacial pain induction in rats and the contribution of kappa opioid receptors

Descending control of nociception (DCN), a measure of efficiency of descending pain inhibition, can be assessed in animals by the combined application of test and conditioning noxious stimuli. Evidence from pre-clinical and clinical studies indicates that this mechanism of pain control may differ between sexes and might be impaired in many chronic pain states. However, little is known about sex differences in DCN efficiency in models of acute and chronic orofacial pain. Herein, we first evaluated DCN responses in male and female rats by the applying formalin into the upper lip or capsaicin into the forepaw as the conditioning stimulus, followed by mechanical stimulation (Randall-Selitto) of the hind paw as the test stimulus. The same protocol (i.e., capsaicin in the forepaw followed by mechanical stimulation of the hind paw) was evaluated in male and female rats on day 3 after intraoral incision and on day 15 and 30 after chronic constriction injury of the infraorbital nerve (CCI-ION). Additionally, we assessed the effect of the kappa opioid receptor (KOR) antagonist Norbinaltorphimine (nor-BNI) on DCN responses of female nerve-injured rats. This study shows that naïve female rats exhibit less efficient DCN compared to males. Postoperative pain did not alter DCN responses in female and male rats, but CCI-ION induced loss of DCN responses in females but not in males. Systemic pretreatment with nor-BNI prevented the loss of DCN induced by CCI-ION in female rats. The results reveal sex differences in DCN responses and female-specific impairment of DCN following chronic orofacial pain. Moreover, the findings suggest that, at least for females, blocking KOR could be a promising therapeutic approach to prevent maladaptive changes in chronic orofacial pain.

Clinical phenotypes and prognostic factors in persons with hip osteoarthritis undergoing total hip arthroplasty: protocol for a longitudinal prospective cohort study (HIPPROCLIPS)

BACKGROUND

Large heterogeneity exists in the clinical manifestation of hip osteoarthritis (OA). It is therefore not surprising that pain and disability in individuals with hip OA and after total hip arthroplasty (THA) cannot be explained by biomedical variables alone. Indeed, also maladaptive pain-related cognitions and emotions can contribute to pain and disability, and can lead to poor treatment outcomes. Traumatic experiences, mental disorders, self-efficacy and social support can influence stress appraisal and strategies to cope with pain, but their influence on pain and disability has not yet been established in individuals with hip OA undergoing THA. This study aims (1) to determine the influence of traumatic experiences and mental disorders on pain processing before and shortly after THA (2) to identify preoperative clinical phenotypes in individuals with hip OA eligible for THA, (3) to identify pre- and early postoperative prognostic factors for outcomes in pain and disability after THA, and (4) to identify postoperative clinical phenotypes in individuals after THA.

METHODS

This prospective longitudinal cohort study will investigate 200 individuals undergoing THA for hip OA. Phenotyping variables and candidate prognostic factors include pain-related fear-avoidance behaviour, perceived injustice, mental disorders, traumatic experiences, self-efficacy, and social support. Peripheral and central pain mechanisms will be assessed with thermal quantitative sensory testing. The primary outcome measure is the hip disability and osteoarthritis outcome score. Other outcome measures include performance-based measures, hip muscle strength, the patient-specific functional scale, pain intensity, global perceived effect, and outcome satisfaction. All these measurements will be performed before surgery, as well as 6 weeks, 3 months, and 12 months after surgery. Pain-related cognitions and emotions will additionally be assessed in the early postoperative phase, on the first, third, fifth, and seventh day after THA. Main statistical methods that will be used to answer the respective research questions include: LASSO regression, decision tree learning, gradient boosting algorithms, and recurrent neural networks.

DISCUSSION

The identification of clinical phenotypes and prognostic factors for outcomes in pain and disability will be a first step towards pre- and postoperative precision medicine for individuals with hip OA undergoing THA.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT05265858. Registered on 04/03/2022.

Enhanced pain facilitation rather than impaired pain inhibition in burning mouth syndrome female patients

Background

Deficient endogenous pain modulation has been implicated in the development and exacerbation of chronic orofacial pain. To date, relatively little is known regarding the function of the endogenous pain modulation in patients with burning mouth syndrome (BMS). This case–control study investigated endogenous pain modulation in women with BMS.

Methods

Conditioned pain modulation (CPM) was assessed upon temporal summation (TSP) of thermal pain. Forty female subjects, 20 BMS patients and 20 age-matched control subjects, were included in a 2 session-protocol. Mechanical and thermal pain thresholds were measured on the forearm and hand. TSP was obtained using repetitive laser-evoked thermal stimuli applied on the non-dominant hand, at an intensity yielding to moderate pain. During TSP, CPM was produced by immersing the contralateral foot in a water bath at painful cold (8 °C) temperature. In control conditions, the foot was immersed in a water bath at not painful (30 °C) temperature.

Results

BMS was not associated with any impairment in thermal as well as mechanical extracephalic pain thresholds. TSP and CPM efficacy were similar in BMS patients and control subjects. However, BMS patients exhibited enhanced extracephalic heat hyperalgesia.

Conclusion

This study reveals that there is no impairment of endogenous pain inhibition mechanisms in BMS patients, but rather an increase in pain facilitation.

To Calibrate or not to Calibrate? A Methodological Dilemma in Experimental Pain Research

To calibrate or not to calibrate? This question is raised by almost everyone designing an experimental pain study with supra-threshold stimulation. The dilemma is whether to individualize stimulus intensity to the pain threshold / supra-threshold pain level of each participant or whether to provide the noxious stimulus at a fixed intensity so that everyone receives the identical input. Each approach has unique pros and cons which need to be considered to i) accurately design an experiment, ii) enhance statistical inference in the given data and, iii) reduce bias and the influence of confounding factors in the individual study e.g., body composition, differences in energy absorption and previous experience. Individualization requires calibration, a procedure already irritating the nociceptive system but allowing to match the pain level across individuals. It leads to a higher variability of the stimulus intensity, thereby influencing the encoding of "noxiousness" by the central nervous system. Results might be less influenced by statistical phenomena such as ceiling/floor effects and the approach does not seem to rise ethical concerns. On the other hand, applying a fixed (standardized) intensity reduces the problem of intensity encoding leading to a large between-subjects variability in pain responses. Fixed stimulation intensities do not require pre-exposure. It can be proposed that one method is not preferable over another, however the choice depends on the study aim and the desired level of external validity. This paper discusses considerations for choosing the optimal approach for experimental pain studies and provides recommendations for different study designs. PERSPECTIVE: To calibrate pain or not? This dilemma is related to almost every experimental pain research. The decision is a trade-off between statistical power and greater control of stimulus encoding. The article decomposes both approaches and presents the pros and cons of either approach supported by data and simulation experiment.

Offset analgesia is increased intra-orally

BACKGROUND

Offset analgesia (OA) is commonly used to quantify endogenous pain inhibition. However, the potential role of afferent inputs and the subsequent peripheral factors from different body areas on the underlying mechanisms are still unclear.

OBJECTIVES

The aim of this cross-sectional study was to compare the magnitude of OA in four different body areas representing a) glabrous and non-glabrous skin, b) trigeminal and extra-trigeminal areas, and c) intra- and extra-oral tissue.

METHODS

OA was assessed at the oral mucosa of the lower lip, at the skin of the cheek, the forearm and the palm of the hand in 32 healthy and pain-free participants. OA testing included two trials: (1) a constant trial (30 seconds of constant heat stimulation at an individualized temperature of Pain₅₀ (pain intensity of 50 out of 100)), and (2) an offset trial (10 seconds of individualized Pain₅₀ , followed by 5 seconds at Pain₅₀ +1°C and 15 seconds at Pain₅₀ ). Participants continuously rated their pain during each trial with a computerized visual analog scale.

RESULTS

A significant OA response was recorded at the oral mucosa (p<0.001, d=1.24), the cheek (p<0.001, d=0.84) and the forearm (p<0.001, d=1.04), but not at the palm (p=0.19, d=0.24). Significant differences were shown for OA recorded at the cheek versus the mucosa (p=0.02), and between palm and mucosa (p=0.007), but not between the remaining areas (p>0.05).

CONCLUSION

This study suggests that intra-oral endogenous pain inhibition assessed with OA is enhanced and supports the role of peripheral mechanisms contributing to the OA response.

Preclinical models of deep craniofacial nociception and temporomandibular disorder pain

Chronic pain in temporomandibular disorder (TMD) is a common health problem. Cumulating evidence indicates that the etiology of TMD pain is complex with multifactorial experience that could hamper the developments of treatments. Preclinical research is a resource to understand the mechanism for TMD pain, whereas limitations are present as a disease-specific model. It is difficult to incorporate multiple risk factors associated with the etiology that could increase pain responses into a single animal. This article introduces several rodent models which are often employed in the preclinical studies and discusses their validities for TMD pain after the elucidations of the neural mechanisms based on the clinical reports. First, rodent models were classified into two groups with or without inflammation in the deep craniofacial tissues. Next, the characteristics of each model and the procedures to identify deep craniofacial pain were discussed. Emphasis was directed on the findings of the effects of chronic psychological stress, a major risk factor for chronic pain, on the deep craniofacial nociception. Preclinical models have provided clinically relevant information, which could contribute to better understand the basis for TMD pain, while efforts are still required to bridge the gap between animal and human studies.

Investigation of Correlations Between Pain Modulation Paradigms

OBJECTIVE

Endogenous pain modulation can be quantified through the use of various paradigms. Commonly used paradigms include conditioned pain modulation (CPM), offset analgesia (OA), spatial summation of pain (SSP), and temporal summation of pain (TSP), which reflect spatial and temporal aspects of pro- and antinociceptive processing. Although these paradigms are regularly used and are of high clinical relevance, the underlying physiological mechanisms are not fully understood.

DESIGN

The aim of this study is therefore to assess the association between these paradigms by using comparable protocols and methodological approaches.

SETTING

University campus.

SUBJECTS

Healthy and pain-free volunteers (n = 48) underwent psychophysical assessment of CPM, OA, SSP, and TSP (random order) at the same body area (volar nondominant forearm) with individualized noxious stimuli.

METHODS

CPM included heat stimuli before, during, and after a noxious cold-water bath, whereas for OA, three heat stimuli were applied: baseline trial, offset trial, and constant trial. For the SSP paradigm, two differently sized heat stimulation areas were evaluated, whereas for TSP, the first and last stimulus of 10 consecutive short heat stimuli were assessed. A computerized visual analog scale was used to continuously evaluate pain intensity. The magnitudes of all associations between all paradigm pairs were analyzed with Spearman's correlation, and individual influencing factors were assessed with a multivariate linear regression model.

RESULTS

Weak to moderate correlations among all four paradigms were found (P > 0.05), and no distinct influencing factors were identified.

CONCLUSIONS

A limited association between pain modulation paradigms suggests that CPM, OA, SSP, and TSP assess distinct aspects of endogenous analgesia with different underlying physiological mechanisms.

Effects of acute mental stress on conditioned pain modulation in temporomandibular disorders patients and healthy individuals

Stress is a contributing factor to painful temporomandibular disorders (TMD). Nevertheless, the underpinnings of this relationship are not fully understood.

Nonlinear increase of pain in distance-based and area-based spatial summation

ABSTRACT

When nociceptive stimulation affects a larger body area, pain increases. This effect is called spatial summation of pain (SSp). The aim of this study was to describe SSp as a function of the size or distance of a stimulated area(s) and to test how this function is shaped by the intensity and SSp test paradigm. Thirty-one healthy volunteers participated in a within-subject experiment. Participants were exposed to area-based and distanced-based SSp. For area-based SSp, electrocutaneous noxious stimuli were applied by up to 5 electrodes (5 areas) forming a line-like pattern; for distance-based SSp, the same position and lengths of stimuli were used but only 2 electrodes were stimulated. Each paradigm was repeated using pain of low, moderate, and high intensity. It was found that the pattern of pain intensity followed a logarithmic (power) rather than a linear function. The dynamics of the pain increase were significantly different across pain intensities, with more summation occurring when pain was perceived as low. Results indicated that area-based SSp is more painful than distance-based SSp when low and moderate but not when high pain intensity is induced. Presented findings have important implications for all studies in which the spatial dimension of pain is measured. When the area or separation between nociceptive stimulation increases, pain does not increase linearly and the pattern of the pain increase is a result of the interaction between intensity and the number of nociceptive sites. A power function should be considered when predicting the size of a nociceptive source.

The effect of acute-experimental pain models on offset analgesia

BACKGROUND

Offset analgesia (OA) is characterized by a disproportionately large decrease in pain perception after a slight decrease in noxious stimulation. In patients with ongoing pain, this response is reduced. The effect is pronounced in painful body areas. The influence of acute pain has not been sufficiently investigated. The aim of this study was to investigate the influence of two experimental acute pain models, measured within the area of acute pain and on the non-affected opposite side, thereby considering the possible somatotopic nature of OA.

METHODS

Healthy, pain-free volunteers (n = 75) were randomly assigned to one of three groups (cold water, exercise and control group). The 'cold water group' immersed one hand into cold water for 3 min (Cold Pressor Task), while the 'exercise group' performed an isometric grip exercise for 3 min. There was no manipulation in the control group. Each experimental pain stimulus was performed at both (dominant, non-dominant) forearms. The individualized OA paradigm consisted of offset and constant temperature trials. Offset analgesia was measured immediately before, during and after the experimental pain stimuli.

RESULTS

A significant difference in OA was shown during experimental pain when compared to the control condition (exercise vs. control: p < 0.001, cold vs. control: p = 0.001), with no difference between the experimental conditions (p > 0.05). Immediately following the pain stimulation, results were marginally non-significant (p = 0.05).

CONCLUSIONS

Experimental painful stimulation reduced OA. This result should be interpreted with caution due to potential influences of conditioned pain modulation or exercise-induced hypoalgesia as well as possible floor effects.

SIGNIFICANCE

Temporal contrast of pain perception is inhibited in acute pain states. This study showed that reduced offset analgesia is observed when pain is experimentally induced using noxious cold and exercise stimuli.

Individual variability and sex differences in conditioned pain modulation and the impact of resilience, and conditioning stimulus pain unpleasantness and salience

Distinct pain experiences are shaped both by personal attributes and characteristics of noxious stimuli. An Individual's capacity for endogenous pain inhibition (reflected by conditioned pain modulation [CPM]), their resilience, and the pain unpleasantness and salience of painful stimuli can impact their pain perception. Here, we aimed to determine how individual variability in CPM relates to sex and resilience as personal attributes, and pain unpleasantness and salience of the CPM conditioning stimulus (CS). We evaluated CPM in 106 healthy participants (51 female and 55 male) based on the change in test stimulus pain applied concurrently with a painful CS, both delivered by painful heat. The CS reduced test stimulus pain in only half of the participants (CPM subgroup), but did not do so for the other half (no-CPM subgroup), many who exhibited pain facilitation. A regression model explained CPM effects after accounting for sex, resilience, CS pain unpleasantness and salience. In the CPM subgroup regression model, the CPM effect was positively related to CS pain unpleasantness, while the CPM effect was not related to any variable in the no-CPM subgroup model. Correlation analyses revealed that the CPM effect was anticorrelated with resilience in males with no-CPM. The CPM effect was correlated with CS pain unpleasantness in males with CPM and in females with no-CPM. The CPM effect and CS salience were correlated in the whole group more strongly than in the subgroups. These data reveal that the complexity of contributors to CPM variability include both personal attributes and attributes of the CS.

Influence of self-reported physical activity and sleep quality on conditioned pain modulation in the orofacial region

OBJECTIVES

To evaluate the influence of self-reported physical activity and sleep quality on conditioned pain modulation (CPM) in the orofacial region.

MATERIALS AND METHODS

Ninety healthy participants aged 18-50 years old were evenly distributed according to the level of physical activity into low, moderate, and high level. The classification of physical activity was based on modified criteria of the International Physical Activity Questionnaire (IPAQ), considering intensity, duration, and frequency of physical activity. The Pittsburgh Sleep Quality Index (PSQI) assessed sleep quality and the individuals were then classified as good or poor sleepers. CPM was assessed using the pressure pain threshold (PPT) of the anterior temporalis as test stimulus and hand immersion in hot water as conditioning stimulus. ANOVA was applied to the data and Tukey's posttest was applied when the main effects or interactions were significant (p < 0.050).

RESULTS

There was no significant main effect of neither physical activity nor sleep quality on pain modulation. However, individuals who reported high level of physical activity and good quality of sleep had a greater pain modulation (- 0.60 ± 0.34) when compared with those who reported moderate (- 0.10 ± 0.25) and low level of physical activity (- 0.10 ± 0.52) and good sleep quality (p < 0.028).

CONCLUSIONS

Pain modulation seems to be more efficient in individuals who report a good sleep quality and a high level of physical activity.

CLINICAL RELEVANCE

Conditioned pain modulation is highly variable in healthy people. Therefore, a multifactorial approach should be taken into consideration in the evaluation of the efficacy of endogenous analgesia.

Subliminal (latent) processing of pain and its evolution to conscious awareness

By unconscious or covert processing of pain we refer to nascent interactions that affect the eventual deliverance of pain awareness. Thus, internal processes (viz., repeated nociceptive events, inflammatory kindling, reorganization of brain networks, genetic) or external processes (viz., environment, socioeconomic levels, modulation of epigenetic status) contribute to enhancing or inhibiting the presentation of pain awareness. Here we put forward the notion that for many patients, ongoing sub-conscious changes in brain function are significant players in the eventual manifestation of chronic pain. In this review, we provide clinical examples of nascent or what we term pre-pain processes and the neurobiological mechanisms of how these changes may contribute to pain, but also potential opportunities to define the process for early therapeutic interventions.