Pain modulatory network is influenced by sex and age in a healthy state and during osteoarthritis progression in rats

Osteoclasts and osteoarthritis: Novel intervention targets and therapeutic potentials during aging

Osteoarthritis (OA), a chronic degenerative joint disease, is highly prevalent among the aging population, and often leads to joint pain, disability, and a diminished quality of life. Although considerable research has been conducted, the precise molecular mechanisms propelling OA pathogenesis continue to be elusive, thereby impeding the development of effective therapeutics. Notably, recent studies have revealed subchondral bone lesions precede cartilage degeneration in the early stage of OA. This development is marked by escalated osteoclast-mediated bone resorption, subsequent imbalances in bone metabolism, accelerated bone turnover, and a decrease in bone volume, thereby contributing significantly to the pathological changes. While the role of aging hallmarks in OA has been extensively elucidated from the perspective of chondrocytes, their connection with osteoclasts is not yet fully understood. There is compelling evidence to suggest that age-related abnormalities such as epigenetic alterations, proteostasis network disruption, cellular senescence, and mitochondrial dysfunction, can stimulate osteoclast activity. This review intends to systematically discuss how aging hallmarks contribute to OA pathogenesis, placing particular emphasis on the age-induced shifts in osteoclast activity. It also aims to stimulate future studies probing into the pathological mechanisms and therapeutic approaches targeting osteoclasts in OA during aging.

Molecular pathogenesis of OA pain: Past, present, and future

OBJECTIVE

To provide a historical perspective and narrative review on research into the molecular pathogenesis of osteoarthritis pain.

DESIGN

PubMed databases were searched for combinations of "osteoarthritis", "pain" and "animal models" for papers that represented key phases in the history of osteoarthritis pain discovery research including epidemiology, pathology, imaging, preclinical modeling and clinical trials.

RESULTS

The possible anatomical sources of osteoarthritis pain were identified over 50 years ago, but relatively slow progress has been made in understanding the apparent disconnect between structural changes captured by radiography and symptom severity. Translationally relevant animal models of osteoarthritis have aided in our understanding of the structural and molecular drivers of osteoarthritis pain, including molecules such as nerve growth factor and C-C motif chemokine ligand 2. Events leading to persistent osteoarthritis pain appear to involve a two-step process involving changes in joint innervation, including neo-innervation of the articular cartilage, as well as sensitization at the level of the joint, dorsal root ganglion and central nervous system.

CONCLUSIONS

There remains a great need for the development of treatments to reduce osteoarthritis pain in patients. Harnessing all that we have learned over the past several decades is helping us to appreciate the important interaction between structural disease and pain, and this is likely to facilitate development of new disease modifying therapies in the future.

Sex differences in visceral sensitivity and brain activity in a rat model of comorbid pain: a longitudinal study

ABSTRACT

Temporomandibular disorder (TMD) and irritable bowel syndrome (IBS) are 2 chronic overlapping pain conditions (COPCs) that present with significant comorbidity. Both conditions are more prevalent in women and are exacerbated by stress. While peripheral mechanisms might contribute to pain hypersensitivity for each individual condition, mechanisms underlying the comorbidity are poorly understood, complicating pain management when multiple conditions are involved. In this study, longitudinal behavioral and functional MRI-based brain changes have been identified in an animal model of TMD-like pain (masseter muscle inflammation followed by stress) that induces de novo IBS-like comorbid visceral pain hypersensitivity in rats. In particular, data indicate that increased activity in the insula and regions of the reward and limbic systems are associated with more pronounced and longer-lasting visceral pain behaviors in female rats, while the faster pain resolution in male rats may be due to increased activity in descending pain inhibitory pathways. These findings suggest the critical role of brain mechanisms in chronic pain conditions and that sex may be a risk factor of developing COPCs.

Modulation of Brain Network Topological Properties in Knee Osteoarthritis by Electroacupuncture in Rats

Introduction

Osteoarthritis is a chronic, ongoing disease that affects patients, and pain is considered a key factor affecting patients, but the brain changes during the development of osteoarthritis pain are currently unclear. In this study, we used electroacupuncture (EA) to intervene the rat model of knee osteoarthritis and analyzed the changes in topological properties of brain networks using graph theory.

Methods

Sixteen SD rat models of right-knee osteoarthritis with anterior cruciate ligament transection (ACLT) were randomly divided into electroacupuncture intervention group and control group. The electroacupuncture group was intervened on Zusanli (ST36) and Futu (ST32) for 20 min each time, five times a week for 3 weeks, while the control group was applied sham stimulation. Both groups were measured for pain threshold. The small-world properties and node properties of the brain network between the two groups after the intervention were statistically analyzed by graph theory methods.

Results

The differences are mainly in the changes in node attributes between the two groups, such as degree centrality, betweenness centrality, and so on in different brain regions (P<0.05). Both groups showed no small-world characteristics in the brain networks of the two groups. The mechanical thresholds and thermal pain thresholds were significantly higher in the EA group than in the control group (P<0.05).

Conclusion

The study demonstrated that electroacupuncture intervention enhanced the activity of nodes related to pain circuit and relieved pain in osteoarthritis, which provides a complementary basis for explaining the effect of electroacupuncture intervention on pain through graphical analysis of changes in brain network topological properties and helps to develop an imaging model for pain affected by electroacupuncture.

Rodent models of knee osteoarthritis for pain research

Osteoarthritis (OA) is a chronic degenerative joint disease and a leading cause of disability worldwide. Pain is the main symptom, yet no current treatment can halt disease progression or effectively provide symptomatic relief. Numerous animal models have been described for studying OA and some for the associated OA pain. This review aims to update on current models used for studying OA pain, focusing on mice and rats. These models include surgical, chemical, mechanical, and spontaneous OA models. The impact of sex and age will also be addressed in the context of OA modelling. Although no single animal model has been shown ideal for studying OA pain, increased efforts to phenotype OA will likely impact the choice of models for pre-clinical and basic research studies.

Chronic Pain in the Elderly: Mechanisms and Perspectives

Chronic pain affects a large part of the population causing functional disability, being often associated with coexisting psychological disorders, such as depression and anxiety, besides cognitive deficits, and sleep disturbance. The world elderly population has been growing over the last decades and the negative consequences of chronic pain for these individuals represent a current clinical challenge. The main painful complaints in the elderly are related to neurodegenerative and musculoskeletal conditions, peripheral vascular diseases, arthritis, and osteoarthritis, contributing toward poorly life quality, social isolation, impaired physical activity, and dependence to carry out daily activities. Organ dysfunction and other existing diseases can significantly affect the perception and responses to chronic pain in this group. It has been proposed that elderly people have an altered pain experience, with changes in pain processing mechanisms, which might be associated with the degeneration of circuits that modulate the descending inhibitory pathways of pain. Aging has also been linked to an increase in the pain threshold, a decline of painful sensations, and a decrease in pain tolerance. Still, elderly patients with chronic pain show an increased risk for dementia and cognitive impairment. The present review article is aimed to provide the state-of-art of pre-clinical and clinical research about chronic pain in elderly, emphasizing the altered mechanisms, comorbidities, challenges, and potential therapeutic alternatives.

Voluntary exercise blocks ongoing pain and diminishes bone remodeling while sparing protective mechanical pain in a rat model of advanced osteoarthritis pain

ABSTRACT

Exercise is the most common treatment recommended by healthcare providers for the treatment of musculoskeletal pain. We examined whether voluntary running wheel exercise improves pain and bone remodeling in rats with monosodium iodoacetate-induced unilateral knee joint pain. During acquisition of wheel running before osteoarthritis (OA) treatment, rats separated into 2 groups characterized by either high or low levels of voluntary wheel running as indicated by distance and peak speed. After the induction of knee joint OA, all rats showed diminished voluntary wheel running throughout the study. Voluntary wheel running failed to alter evoked nociceptive responses evaluated as weight asymmetry or hind paw tactile thresholds at any timepoint of the study. By contrast, relief of ongoing pain was demonstrated by conditioned place preference produced by lidocaine injection into the monosodium iodoacetate-treated knee in high but not low-running rats. Both high and low voluntary runners showed diminished trabecular bone loss compared with sedentary controls. These observations indicate that both high-intensity and low-intensity exercise is beneficial in protecting against bone remodeling in advanced OA. The data suggest that similar to clinical observation, bone remodeling does not correlate with pain. In addition, these results suggest that higher intensity exercise may relieve persistent ongoing OA pain while maintaining movement-evoked nociception. The relief of ongoing pain can potentially offer significant improvement in quality of life, whereas preservation of responses to movement-evoked pain may be especially important in protecting the joint from damage because of overuse.

Acupuncture Enhances Dorsal Raphe Functional Connectivity in Knee Osteoarthritis With Chronic Pain

INTRODUCTION

Knee osteoarthritis is a common disease in the elderly. Patients suffer from long-term chronic pain and reduced life quality. Acupuncture has been proven to be an effective treatment for KOA. However, the neural mechanism of acupuncture is unclear, so far. Periaqueductal gray (PAG) and raphe nuclei (RPN) are essential structures associated with chronic pain in human brains. This study aims to investigate functional connectivity (FC) changes of PAG and RPN in KOA to interpret the neural mechanism of acupuncture.

METHODS

In 15 patients with KOA and 15 healthy controls (HC), we acquired Visual Analog Scale (VAS) scores and resting-state fMRI images of each participant before and after acupuncture stimulation on EX-LE5 acupoint. Then, PAG and RPN were selected as seeds to perform FC analysis based on resting-state fMRI images. Finally, we compared FC patterns of PAG and RPN between patients with KOA and HC, then between pre-acupuncture and post-acupuncture. Correlations between FC values and VAS scores were calculated as well.

RESULTS

For PAG, FC of patients with KOA was lower in the right lingual gyrus at post-acupuncture compared with HC (p <0.001, uncorrected). For dorsal RPN, FC of patients with KOA was significantly higher in right putamen at post-acupuncture compared with HC (p <0.001, corrected with FDR), and FC changes were significant between pre-acupuncture and post-acupuncture in patients with KOA. Post-acupuncture FC values between dorsal RPN and right putamen were correlated with VAS scores. For medial RPN, FC of patients with KOA was lower in the right cerebellum at post-acupuncture compared with HC (p <0.001, uncorrected), but no significant FC changes were found between pre-acupuncture and post-acupuncture in patients with KOA. FC values between medial RPN and right cerebellum were not correlated with VAS scores at pre-acupuncture and post-acupuncture.

DISCUSSION

Our study demonstrated that acupuncture enhanced FC between dorsal RPN and the right putamen in patients with KOA, which was associated with chronic pain intensity. This result suggests that acupuncture stimulation can enhance FC between dorsal raphe and striatum, illustrating a neural mechanism that acupuncture can drive the patients' brain, with KOA, to perceive pain.

Electroacupuncture Attenuates Cancer-Induced Bone Pain via NF-κB/CXCL12 Signaling in Midbrain Periaqueductal Gray

Electroacupuncture (EA) is effective in various chronic pains. NF-κB and CXCL12 modulate the formation of chronic pain. Herein, we hypothesized that EA alleviates cancer-induced bone pain (CIBP) through NF-κB/CXCL12 axis in midbrain periaqueductal gray (PAG), which participates in "top-down" pain modulatory circuits. In order to filter the optimum EA frequency for CIBP treatment, 2, 100, or 2/100 Hz EA was set up. In addition, ipsilateral, contralateral, and bilateral EA groups were established to affirm the optimal EA scheme. Bilateral 2/100 Hz EA was considered as the optimal therapeutic scheme and was applied in a subsequent experiment. Western blotting along with immunofluorescence illustrated that CIBP induces a rapid and substantial increase in CXCL12 protein level and NF-κB phosphorylation in vlPAG from day 6 to day 12. Anti-CXCL12 neutralizing antibody and pAAV-U6-shRNA(CXCL12)-CMV-EGFP-WPRE in vlPAG remarkably improved the mechanical pain threshold of the hind paw in CIBP model relative to the control. EA inhibited the upregulation of pNF-κB and CXCL12 in vlPAG of CIBP. The recombinant CXCL12 and pAAV-CMV-CXCL12-EF1a-EGFP-3Xflag-WPRE reversed the abirritation of EA in the CIBP rat model. NF-κB phosphorylation mediated-CXCL12 expression contributed to CIBP allodynia, whereas EA suppressed NF-κB phosphorylation in CIBP. According to the above evidence, we conclude that bilateral 2/100 Hz EA is an optimal therapeutic scheme for CIBP. The abirritation mechanism of EA might reduce the expression of CXCL12 by inhibiting the activation of NF-κB, which might lead to the restraint of descending facilitation of CIBP.