Conditioned pain modulation and offset analgesia: Influence of sex, sex hormone levels and menstrual cycle on the magnitude and retest reliability in healthy participants

Body-site effect on CPM efficiency in healthy subjects: Central vs. peripheral stimulation

Structural changes in the peripheral nerve system in neuropathic states alter sensory capacity of the affected area, thus biasing results of conditioned pain modulation (CPM) responses. The aim of this study was to evaluate CPM efficiency of central (i.e. trunk) vs. peripheral (i.e. limb) application of 'test' and 'conditioning' stimuli. Methods: Healthy volunteers (ages 18-73 yrs) underwent two CPM protocols: 'CPM Limb' and 'CPM Trunk'. Each included two types of test stimuli (Ts) (pressure pain threshold: PPT; and contact heat) conditioned either to hand immersion in cold noxious water (CPM limb), or to noxious contact heat applied on lower back (CPM trunk). Results: Both protocols generated efficient pain inhibition for each of the applied Ts; the PPT-based protocol induced more efficient CPM when the conditioned stimulus was applied on the trunk (p = 0.016). Moreover, the PPT-based CPM responses were significantly correlated (ρ = 0.349; p = 0.007). Conclusions: An efficient CPM induced by both central and peripheral stimulation, along with significant correlation between PPT-based responses, advances using the central 'CPM Trunk' protocol in patients with peripheral neuropathy.

How to Modulate Peripheral and Central Nervous System to Treat Acute Postoperative Pain and Prevent Pain Persistence

Chronic postoperative pain (CPSP) is a major issue after surgery, which may impact on patient's quality of life. Traditionally, CPSP is believed to rely on maladaptive hyperalgesia and risk factors have been identified that predispose to CPSP, including acute postoperative pain. Despite new models of prediction are emerging, acute pain is still a modifiable factor that can be challenged with perioperative analgesic strategies. In this review we present the issue of CPSP, focusing on molecular mechanism underlying the development of acute and chronic hyperalgesia. Also, we focus on how perioperative strategies can impact directly or indirectly (by reducing postoperative pain intensity) on the development of CPSP.

Generation of a whole-brain hemodynamic response function and sex-specific differences in cerebral processing of mechano-sensation in mice detected by BOLD fMRI

BOLD fMRI has become a prevalent method to study cerebral sensory processing in rodent disease models, including pain and mechanical hypersensitivity. fMRI data analysis is frequently combined with a general-linear-model (GLM) -based analysis, which uses the convolution of a hemodynamic response function (HRF) with the stimulus paradigm. However, several studies indicated that the HRF differs across species, sexes, brain structures, and experimental factors, including stimulation modalities or anesthesia, and hence might strongly affect the outcome of BOLD analyzes. While considerable work has been done in humans and rats to understand the HRF, much less is known in mice. As a prerequisite to investigate mechano-sensory processing and BOLD fMRI data in male and female mice, we (1) designed a rotating stimulator that allows application of two different mechanical modalities, including innocuous von Frey and noxious pinprick stimuli and (2) determined and statistically compared HRFs across 30 brain structures and experimental conditions, including sex and, stimulus modalities. We found that mechanical stimulation lead to brain-wide BOLD signal changes thereby allowing extraction of HRFs from multiple brain structures. However, we did not find differences in HRFs across all brain structures and experimental conditions. Hence, we computed a whole-brain mouse HRF, which is based on 88 functional scans from 30 mice. A comparison of this mouse-specific HRF with our previously reported rat-derived HRF showed significantly slower kinetics in mice. Finally, we detected pronounced differences in cerebral BOLD activation between male and female mice with mechanical stimulation, thereby exposing divergent processing of noxious and innocuous stimuli in both sexes.

Factors Associated with Low Inter-Session Reliability of Conditioned Pain Modulation in Older People with or Without Chronic Musculoskeletal Pain

Purpose

Conditioned pain modulation (CPM) is a measurement of the descending pain pathways that inhibit or facilitate afferent noxious stimuli. The reliability of CPM in older individuals with or without chronic musculoskeletal pain has not been sufficiently reported. This study aimed to examine the inter-session reliability of CPM in these cohorts and the factors in CPM reliability.

Patients and Methods

Individuals aged 65 or older were recruited in Narita, Japan. The measurements were performed on separate days 2 weeks apart (sessions 1 and 2). Each participant's hand was immersed in cold water, and we measured pressure pain threshold (PPT) before and after the immersion. The ratio before and after PPT measurements was presented as CPM index. The autonomic activities (heart rate variability, heart rate, and blood pressure) were simultaneously measured. An absolute reliability of CPM index was analyzed by the adjusted two-way analysis of variance (ANOVA) and the Bland Altman plot, and relative reliability was analyzed by intraclass correlation coefficient (ICC). Spearman's rho correlation and the adjusted multivariate regression analysis were utilized for examining the CPM reliability factors.

Results

Thirty-two participants were divided into two groups: chronic pain (n=19) and non-chronic pain (n=13) groups. The mean difference between session 1 and 2 in CPM index showed a systematic error in the chronic pain group at 17.3 (confidence interval, CI: 15.0 to 19.7), but none in the non-chronic pain group at 3.7 (CI: -0.02 to 7.4). The adjusted two-way ANOVA for CPM index did not identify any differences. ICC was not significant at p=-0.247 in the non-chronic and 0.167 in chronic pain. Multivariate regression analysis revealed total power and low/high frequencies as significant factors for CPM index.

Conclusion

This study identified low inter-session reliability in older adults with chronic musculoskeletal pain and autonomic nervous system activities as factors in CPM reliability.