Pain-related behavior is associated with increased joint innervation, ipsilateral dorsal horn gliosis, and dorsal root ganglia activating transcription factor 3 expression in a rat ankle joint model of osteoarthritis

Neuroinflammation in osteoarthritis: From pain to mood disorders

Osteoarthritis (OA) is the most common form of musculoskeletal disease, and its prevalence is increasing due to the aging of the population. Chronic pain is the most burdensome symptom of OA that significantly lowers patients' quality of life, also due to its frequent association with emotional comorbidities, such as anxiety and depression. In recent years, both chronic pain and mood alterations have been linked to the development of neuroinflammation in the peripheral nervous system, spinal cord and supraspinal brain areas. Thus, mechanisms at the basis of the development of the neuroinflammatory process may indicate promising targets for novel treatment for pain and affective comorbidities that accompany OA. In order to assess the key role of neuroinflammation in the maintenance of chronic pain and its potential involvement in development of psychiatric components, the monoiodoacetate (MIA) model of OA in rodents has been used and validated. In the present commentary article, we aim to summarize up-to-date results achieved in this experimental model of OA, focusing on glia activation and cytokine production in the sciatic nerve, dorsal root ganglia (DRGs), spinal cord and brain areas. The association of a neuroinflammatory state with the development of pain and anxiety- and depression-like behaviors are discussed. Results suggest that cells and molecules involved in neuroinflammation may represent novel targets for innovative pharmacological treatments of OA pain and mood comorbidities.

Therapeutic exercise interventions in rat models of arthritis

Arthritis is the leading cause of musculoskeletal pain and disability worldwide. Nearly 50% of individuals over the age of 65 have arthritis, which contributes to limited function, articular pain, physical inactivity, and diminished quality of life. Therapeutic exercise is often recommended in clinical settings for patients experiencing arthritic pain, however, there is little practical guidance regarding the use of therapeutic exercise to alleviate arthritic musculoskeletal pain. Rodent models of arthritis allow researchers to control experimental variables, which cannot be done with human participants, providing an opportunity to test therapeutic approaches in preclinical models. This literature review provides a summary of published findings in therapeutic exercise interventions in rat models of arthritis as well as gaps in the existing literature. We reveal that preclinical research in this field has yet to adequately investigate the impact of experimental variables in therapeutic exercise including their modality, intensity, duration, and frequency on joint pathophysiology and pain outcomes.

Experimental models to study osteoarthritis pain and develop therapeutics

Pain is the predominant symptom of osteoarthritis (OA) that drives patients to seek medical care. Currently, there are no pharmacological treatments that can reverse or halt the progression of OA. Safe and efficacious medications for long-term management of OA pain are also unavailable. Understanding the mechanisms behind OA pain generation at onset and over time is critical for developing effective treatments. In this narrative review, we first summarize our current knowledge on the innervation of the knee joint, and then discuss the molecular mechanism(s) currently thought to underlie OA pain. In particular, we focus on the contribution of each joint component to the generation of pain. Next, the current experimental models for studying OA pain are summarized, and the methods to assess pain in rodents are presented. The potential application of emerging microphysiological systems in OA pain research is especially highlighted. Lastly, we discuss the current challenge in standardizing models and the selection of appropriate systems to address specific questions.

Osteoarthritis Pain in Old Mice Aggravates Neuroinflammation and Frailty: The Positive Effect of Morphine Treatment

Knee osteoarthritis is a common cause of pain and disability in old subjects. Pain may predispose to the development of frailty. Studies on mechanisms underlying pain in osteoarthritis models during aging are lacking. In this work, we used the monosodium iodoacetate model of osteoarthritis in adult (11-week-old) and old (20-month-old) C57BL/6J mice to compare hypersensitivity, locomotion, neuroinflammation, and the effects of morphine treatment. After osteoarthritis induction in adult and old mice, weight-bearing asymmetry, mechanical allodynia, and thermal hyperalgesia similarly developed, while locomotion and frailty were more affected in old than in adult animals. When behavioral deficits were present, the animals were treated for 7 days with morphine. This opioid counteracts the behavioral alterations and the frailty index worsening both in adult and old mice. To address the mechanisms that underlie pain, we evaluated neuroinflammatory markers and proinflammatory cytokine expression in the sciatic nerve, DRGs, and spinal cord. Overexpression of cytokines and glia markers were present in osteoarthritis adult and old mice, but the activation was qualitatively and quantitatively more evident in aged mice. Morphine was able to counteract neuroinflammation in both age groups. We demonstrate that old mice are more vulnerable to pain's detrimental effects, but prompt treatment is successful at mitigating these effects.

Analgesic dorsal root ganglion field stimulation blocks both afferent and efferent spontaneous activity in sensory neurons of rats with monosodium iodoacetate-induced osteoarthritis

OBJECTIVES

Chronic joint pain is common in patients with osteoarthritis (OA). Non-steroidal anti-inflammatory drugs and opioids are used to relieve OA pain, but they are often inadequately effective. Dorsal root ganglion field stimulation (GFS) is a clinically used neuromodulation approach, although it is not commonly employed for patients with OA pain. GFS showed analgesic effectiveness in our previous study using the monosodium iodoacetate (MIA) - induced OA rat pain model. This study was to evaluate the mechanism of GFS analgesia in this model.

METHODS

After osteoarthritis was induced by intra-articular injection of MIA, pain behavioral tests were performed. Effects of GFS on the spontaneous activity (SA) were tested with in vivo single-unit recordings from teased fiber saphenous nerve, sural nerve, and dorsal root.

RESULTS

Two weeks after intra-articular MIA injection, rats developed pain-like behaviors. In vivo single unit recordings from bundles teased from the saphenous nerve and third lumbar (L3) dorsal root of MIA-OA rats showed a higher incidence of SA than those from saline-injected control rats. GFS at the L3 level blocked L3 dorsal root SA. MIA-OA reduced the punctate mechanical force threshold for inducing AP firing in bundles teased from the L4 dorsal root, which reversed to normal with GFS. After MIA-OA, there was increased retrograde SA (dorsal root reflex), which can be blocked by GFS.

CONCLUSIONS

These results indicate that GFS produces analgesia in MIA-OA rats at least in part by producing blockade of afferent inputs, possibly also by blocking efferent activity from the dorsal horn.

Macrophages and glial cells: Innate immune drivers of inflammatory arthritic pain perception from peripheral joints to the central nervous system

Millions of people suffer from arthritis worldwide, consistently struggling with daily activities due to debilitating pain evoked by this disease. Perhaps the most intensively investigated type of inflammatory arthritis is rheumatoid arthritis (RA), where, despite considerable advances in research and clinical management, gaps regarding the neuroimmune interactions that guide inflammation and chronic pain in this disease remain to be clarified. The pain and inflammation associated with arthritis are not isolated to the joints, and inflammatory mechanisms induced by different immune and glial cells in other tissues may affect the development of chronic pain that results from the disease. This review aims to provide an overview of the state-of-the-art research on the roles that innate immune, and glial cells play in the onset and maintenance of arthritis-associated pain, reviewing nociceptive pathways from the joint through the dorsal root ganglion, spinal circuits, and different structures in the brain. We will focus on the cellular mechanisms related to neuroinflammation and pain, and treatments targeting these mechanisms from the periphery and the CNS. A comprehensive understanding of the role these cells play in peripheral inflammation and initiation of pain and the central pathways in the spinal cord and brain will facilitate identifying new targets and pathways to aide in developing therapeutic strategies to treat joint pain associated with RA.