Age-related differences in conditioned pain modulation of sensitizing and desensitizing trends during response dependent stimulation

Sexual Dimorphism in the Mechanism of Pain Central Sensitization

It has long been recognized that men and women have different degrees of susceptibility to chronic pain. Greater recognition of the sexual dimorphism in chronic pain has resulted in increasing numbers of both clinical and preclinical studies that have identified factors and mechanisms underlying sex differences in pain sensitization. Here, we review sexually dimorphic pain phenotypes in various research animal models and factors involved in the sex difference in pain phenotypes. We further discuss putative mechanisms for the sexual dimorphism in pain sensitization, which involves sex hormones, spinal cord microglia, and peripheral immune cells. Elucidating the sexually dimorphic mechanism of pain sensitization may provide important clinical implications and aid the development of sex-specific therapeutic strategies to treat chronic pain.

PAQR8 and PAQR9 expression is altered in the ventral tegmental area of aged rats infected with varicella zoster virus

Infection with varicella zoster virus (VZV) results in chicken pox and reactivation of VZV results in herpes zoster (HZ) or what is often referred to as shingles. Patients with HZ experience decreased motivation and increased emotional distress consistent with functions of the ventral tegmental area (VTA) of the brain. In addition, activity within the ventral tegmental area is altered in patients with HZ. HZ primarily affects individuals that are older and the VTA changes with age. To begin to determine if the VTA has a role in HZ symptoms, a screen of 10,000 genes within the VTA in young and old male rats was completed after injecting the whisker pad with VZV. The two genes that had maximal change were membrane progesterone receptors PAQR8 (mPRβ) and PAQR9 (mPRε). Neurons and non-neuronal cells expressed both PAQR8 and PAQR9. PAQR8 and PAQR9 protein expression was significantly reduced after VZV injection of young males. In old rats PAQR9 protein expression was significantly increased after VZV injection and PAQR9 protein expression was reduced in aged male rats versus young rats. Consistent with previous results, pain significantly increased after VZV injection of the whisker pad and aged animals showed significantly more pain than young animals. Our data suggests that PAQR8 and PAQR9 expression is altered by VZV injection and that these changes are affected by age.

Pain and aging: A unique challenge in neuroinflammation and behavior

Chronic pain is one of the most common, costly, and potentially debilitating health issues facing older adults, with attributable costs exceeding $600 billion annually. The prevalence of pain in humans increases with advancing age. Yet, the contributions of sex differences, age-related chronic inflammation, and changes in neuroplasticity to the overall experience of pain are less clear, given that opposing processes in aging interact. This review article examines and summarizes pre-clinical research and clinical data on chronic pain among older adults to identify knowledge gaps and provide the base for future research and clinical practice. We provide evidence to suggest that neurodegenerative conditions engender a loss of neural plasticity involved in pain response, whereas low-grade inflammation in aging increases CNS sensitization but decreases PNS sensitivity. Insights from preclinical studies are needed to answer mechanistic questions. However, the selection of appropriate aging models presents a challenge that has resulted in conflicting data regarding pain processing and behavioral outcomes that are difficult to translate to humans.

Central Sensıtızatıon ın Axıal Spondyloarthrıtıs: An Exploratıve Study wıth Quantıtatıve Sensory Testıng and Clınıcal Scales

OBJECTIVE

To evaluate the central sensitization (CS) and the related parameters in patients with axial spondyloarthritis (axSpA).

METHODS

Quantitative sensory testing (QST) which consists of pressure pain threshold (PPT), temporal summation (TS), and conditioned pain modulation (CPM) were applied to the participants. Disease activity, functional status, sleep quality, pain, depression, and fatigue were assessed. Patients were divided as the ones with and without CS according to the central sensitization inventory (CSI) and the results were compared.

RESULTS

One hundred patients and fifty controls were recruited. Sixty axSpA patients had CS. When QST results were compared between the patient and control groups, all PPT scores were found lower (p<0.05) in patients. Regarding the comparison of the patients with and without CS, sacroiliac, and trapezius PPT scores were found lower in the patients with CS (p<0.05). On the other hand, there was no significant difference in the mean TS scores (p>0.05) between patients and controls, and in patients with and without CS. All investigated comorbidities were found to be significantly more frequent (p<0.001) in the patients with CS. In regression analysis female gender, morning stiffness duration, CPM, depression, and fatigue were detected as related parameters with CSI scores.

CONCLUSION

CS and related comorbidities were found to be increased in axSpA patients. This increase should be taken into consideration in the management of these patients.

Age Differences in Multimodal Quantitative Sensory Testing and Associations With Brain Volume

BACKGROUND AND OBJECTIVES

Somatosensory function is critical for successful aging. Prior studies have shown declines in somatosensory function with age; however, this may be affected by testing site, modality, and biobehavioral factors. While somatosensory function declines are associated with peripheral nervous system degradation, little is known regarding correlates with the central nervous system and brain structure in particular. The objectives of this study were to examine age-related declines in somatosensory function using innocuous and noxious stimuli, across 2 anatomical testing sites, with considerations for affect and cognitive function, and associations between somatosensory function and brain structure in older adults.

RESEARCH DESIGN AND METHODS

A cross-sectional analysis included 84 "younger" (n = 22, age range: 19-24 years) and "older" (n = 62, age range: 60-94 years) healthy adults who participated in the Neuromodulatory Examination of Pain and Mobility Across the Lifespan study. Participants were assessed on measures of somatosensory function (quantitative sensory testing), at 2 sites (metatarsal and thenar) using standardized procedures, and completed cognitive and psychological function measures and structural magnetic resonance imaging.

RESULTS

Significant age × test site interaction effects were observed for warmth detection (p = .018,η p 2 = 0.10) and heat pain thresholds (p = .014,η p 2 = 0.12). Main age effects were observed for mechanical, vibratory, cold, and warmth detection thresholds (ps < .05), with older adults displaying a loss of sensory function. Significant associations between somatosensory function and brain gray matter structure emerged in the right occipital region, the right temporal region, and the left pericallosum.

DISCUSSION AND IMPLICATIONS

Our findings indicate healthy older adults display alterations in sensory responses to innocuous and noxious stimuli compared to younger adults and, furthermore, these alterations are uniquely affected by anatomical site. These findings suggest a nonuniform decline in somatosensation in older adults, which may represent peripheral and central nervous system alterations part of aging processes.

Advancing our understanding of neuropathic pain in diabetes mellitus using conditioned pain modulation: further considerations for age and testing site

ABSTRACT

Neuropathy is common among individuals with diabetes mellitus, and is associated with decreased quality of life, greater comorbidity, and substantial economic burden. However, the mechanisms underlying painful diabetic polyneuropathy has yet to be fully elucidated. While it is recognized that diabetic polyneuropathy places patients at a greater risk for developing neuropathic pain, it is still not clear why some individuals develop pain and others do not. Similar to other chronic pain conditions, painful diabetic neuropathy is likely driven by alterations in both the peripheral and central nervous system. Experimental conditioned pain modulation paradigms have contributed substantially to our current understanding of chronic pain across various disease states. In a new study, researchers have extended this work by examining the efficiency of conditioned pain modulation in patients with painful and non-painful diabetic polyneuropathy. Surprisingly, the results indicate individuals with painful neuropathy experience greater endogenous pain inhibition, which may seem counterintuitive at first blush. Here, we discuss potential explanations and directions for future research, including consideration for age effects, testing site, and disease type, with the goal of further advancing this important line of research.

Test–Retest Instability of Temporal Summation and Conditioned Pain Modulation Measures in Older Adults

Objective

The temporal stability (test–retest reliability) of temporal summation of pain (TS) and conditioned pain modulation (CPM) has yet to be established in healthy older adults. The purpose of this study was to compare the temporal stability of TS and CPM in healthy older and younger adults and to investigate factors that might influence TS and CPM stability.

Methods

In a test–retest study, 40 healthy older adults and 30 healthy younger adults completed two sessions of quantitative sensory testing within a two-week period that included TS of heat pain, TS of mechanical pain, and CPM with pressure pain thresholds and suprathreshold heat pain as test stimuli and a cold water immersion as a conditioning stimulus. Participants also completed self-report measures of situational catastrophizing, anxiety, clinical pain, and physical activity. Absolute and relative stability were examined for each variable. Bivariate correlations examined the associations of age, clinical, behavioral, and psychological variables with the intra-individual stability of TS and CPM.

Results

The results revealed moderate to excellent stability for the TS measures and poor to moderate stability for CPM. The results also revealed significant age differences for two of the TS measures and CPM, with younger adults having greater stability compared with older adults. Additionally, the magnitude and stability of psychological factors were correlated with stability of TS.

Conclusions

These findings suggest that TS and CPM may be more reliable in younger compared with older adults. Furthermore, psychological states may be an important factor influencing the stability of TS in healthy adults.

The Decline of Endogenous Pain Modulation With Aging: A Meta-Analysis of Temporal Summation and Conditioned Pain Modulation

The purpose of this article was to examine age-related changes in conditioned pain modulation (CPM) and temporal summation (TS) of pain using meta-analytic techniques. Five electronic databases were searched for studies, which compared measures of CPM and TS among healthy, chronic pain-free younger, middle-aged, and older adults. Eleven studies were included in the final review for TS and 11 studies were included in the review of CPM. The results suggested a moderate magnitude of difference in TS among younger and middle-aged/older adults, with the older cohorts exhibiting enhanced TS of pain. Considerable variability existed in the magnitude of the effect sizes, which was likely due to the different experimental methodologies used across studies (ie, interstimulus interval, stimulus type, and body location). In regards to CPM, the data revealed a large magnitude of difference between younger and older adults, with younger adults exhibiting more efficient pain inhibition. Differences in CPM between middle-aged and older adults were minimal. The magnitude of pain inhibition during CPM in older adults may depend on the use of concurrent versus nonconcurrent protocols. In summary, the data provided strong quantitative evidence of a general age-related decline in endogenous pain modulatory function as measured by TS and CPM. PERSPECTIVE: This review compared CPM and TS of pain among younger, middle-aged, and older adults. These findings enhance our understanding of the decline in endogenous pain modulatory function associated with normal aging.

Mechanisms of Below-Level Pain Following Spinal Cord Injury (SCI)

Mechanisms of below-level pain are discoverable as neural adaptations rostral to spinal injury. Accordingly, the strategy of investigations summarized here has been to characterize behavioral and neural responses to below-level stimulation over time following selective lesions of spinal gray and/or white matter. Assessments of human pain and the pain sensitivity of humans and laboratory animals following spinal injury have revealed common disruptions of pain processing. Interruption of the spinothalamic pathway partially deafferents nocireceptive cerebral neurons, rendering them spontaneously active and hypersensitive to remaining inputs. The spontaneous activity among these neurons is disorganized and unlikely to generate pain. However, activation of these neurons by their remaining inputs can result in pain. Also, injury to spinal gray matter results in a cascade of secondary events, including excitotoxicity, with rostral propagation of excitatory influences that contribute to chronic pain. Establishment and maintenance of below-level pain results from combined influences of injured and spared axons in the spinal white matter and injured neurons in spinal gray matter on processing of nociception by hyperexcitable cerebral neurons that are partially deafferented. A model of spinal stenosis suggests that ischemic injury to the core spinal region can generate below-level pain. Additional questions are raised about demyelination, epileptic discharge, autonomic activation, prolonged activity of C nocireceptive neurons, and thalamocortical plasticity in the generation of below-level pain. PERSPECTIVE: An understanding of mechanisms can direct therapeutic approaches to prevent development of below-level pain or arrest it following spinal cord injury. Among the possibilities covered here are surgical and other means of attenuating gray matter excitotoxicity and ascending propagation of excitatory influences from spinal lesions to thalamocortical systems involved in pain encoding and arousal.

The silent epidemic of chronic pain in older adults

Chronic pain is highly prevalent among older adults where it is associated with significant suffering, disability, social isolation, and greater costs and burden to health care systems. Pharmaceutical treatment of chronic pain in older adults is usually only partially effective and is often limited by side effects including urinary retention, constipation, sedation, cognitive impairment, and increased risk of falls. Since older adults are underrepresented in clinical trials testing treatments for chronic pain, the potential impacts of polypharmacy and frailty on reported outcomes and side effect profiles are largely unknown. Thus, for current treatments, providers and patients must balance anticipated benefits of pain reduction with the known and unknown risks of treatment. Chronic pain is also a risk factor for premature death as well as accelerated cognitive decline, suggesting potential shared mechanisms between persistent pain (or its treatment) and dementia. Cognitive decline and dementia may also impact pain perception and the ability to report pain, complicating treatment decisions. Associations between persistent pain and the risks of premature death and accelerated cognitive decline make estimates for chronic pain in these populations particularly challenging. Future research is needed to improve estimates for chronic pain in older adults, to elucidate underlying mechanisms of pain with aging, and to develop and advance safer, more effective treatment options for chronic pain in older adults.

High Temporal Summation of Pain Predicts Immediate Analgesic Effect of Acupuncture in Chronic Pain Patients-A Prospective Cohort Study

Objectives: This prospective cohort study explored whether two distinguished sensory parameters predicted acupuncture effects in chronic pain patients; namely high temporal summation of pain (TS) indicating spinal synaptic facilitation as well as a low vibration detection threshold (VDT) indicating a loss of Aβ-fiber function.

Methods: Pinprick induced TS and VDT were assessed by standardized, validated methods at the most painful body site and a pain free control site in 100 chronic pain patients receiving six acupuncture sessions as part of an interdisciplinary multimodal pain treatment (IMPT). Immediate change in pain intensity after the first acupuncture session (first treatment on the first day of IMPT) was assessed by the verbal rating scale (VRS, 0–100). After 4 weeks of treatment, patients indicated in a questionnaire whether acupuncture had relieved pain immediately and whether it had contributed to overall pain reduction and well-being after IMPT.

Results: Logistic regression analysis revealed an association between high TS at the control site and a reduction in pain intensity of at least 30% (VRS) after the first acupuncture (OR [95%-CI] 4.3 [1.6–11.8]). Questionnaire ratings of immediate pain relief after acupuncture were associated with high TS at the control site (OR [95%-CI] 3.8 [1.4–10.2] any pain relief, OR [95%-CI] 5.5 [1.7–17.1] over 50% pain reduction) and at the pain site (OR [95%-CI] 3.2 [1.2–8.9] any pain relief). Appraisals of the contribution of acupuncture to overall pain reduction and well-being after IMPT were not associated with TS. The VDT was not associated with any outcome.

Conclusion: This explorative study provides first-time evidence that high TS, especially at a pain free control site, but not VDT, might predict immediate analgesic response to acupuncture in chronic pain patients. Thus, highly centrally sensitized chronic pain patients might respond particularly well to acupuncture.

Reduced Modulation of Pain in Older Adults After Isometric and Aerobic Exercise

Laboratory-based studies show that acute aerobic and isometric exercise reduces sensitivity to painful stimuli in young healthy individuals, indicative of a hypoalgesic response. However, little is known regarding the effect of aging on exercise-induced hypoalgesia (EIH). The purpose of this study was to examine age differences in EIH after submaximal isometric exercise and moderate and vigorous aerobic exercise. Healthy older and younger adults completed 1 training session and 4 testing sessions consisting of a submaximal isometric handgrip exercise, vigorous or moderate intensity stationary cycling, or quiet rest (control). The following measures were taken before and after exercise/quiet rest: 1) pressure pain thresholds, 2) suprathreshold pressure pain ratings, 3) pain ratings during 30 seconds of prolonged noxious heat stimulation, and 4) temporal summation of heat pain. The results revealed age differences in EIH after isometric and aerobic exercise, with younger adults experiencing greater EIH compared with older adults. The age differences in EIH varied across pain induction techniques and exercise type. These results provide evidence for abnormal pain modulation after acute exercise in older adults.

PERSPECTIVE

This article enhances our understanding of the influence of a single bout of exercise on pain sensitivity and perception in healthy older compared with younger adults. This knowledge could help clinicians optimize exercise as a method of pain management.

Age-Dependent Changes in the Inflammatory Nociceptive Behavior of Mice

The processing of pain undergoes several changes in aging that affect sensory nociceptive fibers and the endogenous neuronal inhibitory systems. So far, it is not completely clear whether age-induced modifications are associated with an increase or decrease in pain perception. In this study, we assessed the impact of age on inflammatory nociception in mice and the role of the hormonal inhibitory systems in this context. We investigated the nociceptive behavior of 12-month-old versus 6–8-week-old mice in two behavioral models of inflammatory nociception. Levels of TRP channels, and cortisol as well as cortisol targets, were measured by qPCR, ELISA, and Western blot in the differently aged mice. We observed an age-related reduction in nociceptive behavior during inflammation as well as a higher level of cortisol in the spinal cord of aged mice compared to young mice, while TRP channels were not reduced. Among potential cortisol targets, the NF-κB inhibitor protein alpha (IκBα) was increased, which might contribute to inhibition of NF-κB and a decreased expression and activity of the inducible nitric oxide synthase (iNOS). In conclusion, our results reveal a reduced nociceptive response in aged mice, which might be at least partially mediated by an augmented inflammation-induced increase in the hormonal inhibitory system involving cortisol.

Successful aging: Advancing the science of physical independence in older adults

The concept of 'successful aging' has long intrigued the scientific community. Despite this long-standing interest, a consensus definition has proven to be a difficult task, due to the inherent challenge involved in defining such a complex, multi-dimensional phenomenon. The lack of a clear set of defining characteristics for the construct of successful aging has made comparison of findings across studies difficult and has limited advances in aging research. A consensus on markers of successful aging is furthest developed is the domain of physical functioning. For example, walking speed appears to be an excellent surrogate marker of overall health and predicts the maintenance of physical independence, a cornerstone of successful aging. The purpose of the present article is to provide an overview and discussion of specific health conditions, behavioral factors, and biological mechanisms that mark declining mobility and physical function and promising interventions to counter these effects. With life expectancy continuing to increase in the United States and developed countries throughout the world, there is an increasing public health focus on the maintenance of physical independence among all older adults.