Dose-dependent expression of neuronal injury markers during experimental osteoarthritis induced by monoiodoacetate in the rat

The Implications of Brain-Derived Neurotrophic Factor in the Biological Activities of Platelet-Rich Plasma

Platelet-rich plasma (PRP) is a biological blood-derived therapeutic obtained from whole blood that contains higher levels of platelets. PRP has been primarily used to mitigate joint degeneration and chronic pain in osteoarthritis (OA). This clinical applicability is based mechanistically on the release of several proteins by platelets that can restore joint homeostasis. Platelets are the primary source of brain-derived neurotrophic factor (BDNF) outside the central nervous system. Interestingly, BDNF and PRP share key biological activities with clinical applicability for OA management, such as anti-inflammatory, anti-apoptotic, and antioxidant. However, the role of BDNF in PRP therapeutic activities is still unknown. Thus, this work aimed to investigate the implications of BDNF in therapeutic outcomes provided by PRP therapy in vitro and in-vivo, using the MIA-OA animal model in male Wistar rats. Initially, the PRP was characterized, obtaining a leukocyte-poor-platelet-rich plasma (LP-PRP). Our assays indicated that platelets activated by Calcium release BDNF, and suppression of M1 macrophage polarization induced by LP-PRP depends on BDNF full-length receptor, Tropomyosin Kinase-B (TrkB). OA animals were given LP-PRP intra-articular and showed functional recovery in gait, joint pain, inflammation, and tissue damage caused by MIA. Immunohistochemistry for activating transcriptional factor-3 (ATF-3) on L4/L5 dorsal root ganglia showed the LP-PRP decreased the nerve injury induced by MIA. All these LP-PRP therapeutic activities were reversed in the presence of TrkB receptor antagonist. Our results suggest that the therapeutic effects of LP-PRP in alleviating OA symptoms in rats depend on BDNF/TrkB activity.

Platelet-rich plasma alleviates neuropathic pain in osteoarthritis by downregulating microglial activation

BACKGROUND

The development of neuropathic pain (NP) is one of the reasons why the pain is difficult to treat, and microglial activation plays an important role in NP. Recently, platelet-rich plasma (PRP) has emerged as a novel therapeutic method for knee osteoarthritis (KOA). However, it's unclarified whether PRP has analgesic effects on NP induced by KOA and the underlying mechanisms unknown.

PURPOSE

To observe the analgesic effects of PRP on NP induced by KOA and explore the potential mechanisms of PRP in alleviating NP.

METHODS

KOA was induced in male rats with intra-articular injections of monosodium iodoacetate (MIA) on day 0. The rats received PRP or NS (normal saline) treatment at days 15, 17, and 19 after modeling. The Von Frey and Hargreaves tests were applied to assess the pain-related behaviors at different time points. After euthanizing the rats with deep anesthesia at days 28 and 42, the corresponding tissues were taken for subsequent experiments. The expression of activating transcription factor 3 (ATF3) in dorsal root ganglia (DRG) and ionized-calcium-binding adapter molecule-1(Iba-1) in the spinal dorsal horn (SDH) was detected by immunohistochemical staining. In addition, the knee histological assessment was performed by hematoxylin-eosin (HE) staining.

RESULTS

The results indicated that injection of MIA induced mechanical allodynia and thermal hyperalgesia, which could be reversed by PRP treatment. PRP downregulated the expression of ATF3 within the DRG and Iba-1 within the SDH. Furthermore, an inhibitory effect on cartilage degeneration was observed in the MIA + PRP group only on day 28.

CONCLUSION

These results indicate that PRP intra-articular injection therapy may be a potential therapeutic agent for relieving NP induced by KOA. This effect could be attributed to downregulation of microglial activation and reduction in nerve injury.

Skeletal interoception in osteoarthritis

The interoception maintains proper physiological conditions and metabolic homeostasis by releasing regulatory signals after perceving changes in the internal state of the organism. Among its various forms, skeletal interoception specifically regulates the metabolic homeostasis of bones. Osteoarthritis (OA) is a complex joint disorder involving cartilage, subchondral bone, and synovium. The subchondral bone undergoes continuous remodeling to adapt to dynamic joint loads. Recent findings highlight that skeletal interoception mediated by aberrant mechanical loads contributes to pathological remodeling of the subchondral bone, resulting in subchondral bone sclerosis in OA. The skeletal interoception is also a potential mechanism for chronic synovial inflammation in OA. In this review, we offer a general overview of interoception, specifically skeletal interoception, subchondral bone microenviroment and the aberrant subchondral remedeling. We also discuss the role of skeletal interoception in abnormal subchondral bone remodeling and synovial inflammation in OA, as well as the potential prospects and challenges in exploring novel OA therapies that target skeletal interoception.

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.

TissueGene-C induces long-term analgesic effects through regulation of pain mediators and neuronal sensitization in a rat monoiodoacetate-induced model of osteoarthritis pain

OBJECTIVE

TissueGene-C (TG-C), a combination of human allogeneic chondrocytes and irradiated GP2-293 cells engineered to overexpress transforming growth factor-β1 (TGF-β1), has been developed as a novel cell-based gene therapy and a candidate for disease modifying osteoarthritis drug (DMOAD). We aim to investigate analgesic mechanism of TG-C in a pre-clinical animal model with monoiodoacetate (MIA)-induced pain.

DESIGN

We used a rat MIA model of osteoarthritis (OA) pain. We examined that TG-C can regulate pain by inhibiting the upregulation of various pain mediators in both knee joint tissue and dorsal root ganglia (DRG) (n = 112) and alleviating pain behavior (n = 41) and neuronal hyperexcitability in DRG (n = 60), afferent nerve fiber (n = 24), and spinal cord (n = 35).

RESULTS

TG-C significantly alleviated pain-related behavior by restoring altered dynamic weight bearing and reduced mechanical threshold of the affected hindlimb. TG-C significantly suppressed the expression of nerve growth factor (NGF) and calcitonin gene-related peptide (CGRP) in inflamed joint tissue. TG-C significantly suppressed the upregulation of tropomyosin receptor kinase A (TrkA) and nerve injury/regeneration protein (GAP43) and activation of Iba1-positive microglial cells in DRG. TG-C significantly recovered neuronal hyperexcitability by restoring RMP and firing threshold and frequency of DRG neurons, attenuating firing rates of mechanosensitive C- or Aδ-nerve fiber innervating knee joint, and lowering increased miniature and evoked excitatory postsynaptic currents (mEPSCs and eEPSCs) in the spinal cord.

CONCLUSION

Our results demonstrated that TG-C exerted potent analgesic effects in a rat MIA model of OA pain by inhibiting the upregulation of pain mediators and modulating neuronal sensitization.

Translational evaluation of gait behavior in rodent models of arthritic disorders with the CatWalk device - a narrative review

Arthritic disorders have become one of the main contributors to the global burden of disease. Today, they are one of the leading causes of chronic pain and disability worldwide. Current therapies are incapable of treating pain sufficiently and preventing disease progression. The lack of understanding basic mechanisms underlying the initiation, maintenance and progression of arthritic disorders and related symptoms represent the major obstacle in the search for adequate treatments. For a long time, histological evaluation of joint pathology was the predominant outcome parameter in preclinical arthritis models. Nevertheless, quantification of pain and functional limitations analogs to arthritis related symptoms in humans is essential to enable bench to bedside translation and to evaluate the effectiveness of new treatment strategies. As the experience of pain and functional deficits are often associated with altered gait behavior, in the last decades, automated gait analysis has become a well-established tool for the quantitative evaluation of the sequalae of arthritic disorders in animal models. The purpose of this review is to provide a detailed overview on the current literature on the use of the CatWalk gait analysis system in rodent models of arthritic disorders, e.g., Osteoarthritis, Monoarthritis and Rheumatoid Arthritis. Special focus is put on the assessment and monitoring of pain-related behavior during the course of the disease. The capability of evaluating the effect of distinct treatment strategies and the future potential for the application of the CatWalk in rodent models of arthritic disorders is also addressed in this review. Finally, we discuss important consideration and provide recommendations on the use of the CatWalk in preclinical models of arthritic diseases.

Chronic pain and local pain in usually painless conditions including neuroma may be due to compressive proximal neural lesion

It has been unexplained why chronic pain does not invariably accompany chronic pain-prone disorders. This question-driven, hypothesis-based article suggests that the reason may be varying occurrence of concomitant peripheral compressive proximal neural lesion (cPNL), e.g., radiculopathy and entrapment plexopathies. Transition of acute to chronic pain may involve development or aggravation of cPNL. Nociceptive hypersensitivity induced and/or maintained by cPNL may be responsible for all types of general chronic pain as well as for pain in isolated tissue conditions that are usually painless, e.g., neuroma, scar, and Dupuytren's fibromatosis. Compressive PNL induces focal neuroinflammation, which can maintain dorsal root ganglion neuron (DRGn) hyperexcitability (i.e., peripheral sensitization) and thus fuel central sensitization (i.e., hyperexcitability of central nociceptive pathways) and a vicious cycle of chronic pain. DRGn hyperexcitability and cPNL may reciprocally maintain each other, because cPNL can result from reflexive myospasm-induced myofascial tension, muscle weakness, and consequent muscle imbalance- and/or pain-provoked compensatory overuse. Because of pain and motor fiber damage, cPNL can worsen the causative musculoskeletal dysfunction, which further accounts for the reciprocity between the latter two factors. Sensitization increases nerve vulnerability and thus catalyzes this cycle. Because of these mechanisms and relatively greater number of neurons involved, cPNL is more likely to maintain DRGn hyperexcitability in comparison to distal neural and non-neural lesions. Compressive PNL is associated with restricted neural mobility. Intermittent (dynamic) nature of cPNL may be essential in chronic pain, because healed (i.e., fibrotic) lesions are physiologically silent and, consequently, cannot provide nociceptive input. Not all patients may be equally susceptible to develop cPNL, because occurrence of cPNL may vary as vary patients' predisposition to musculoskeletal impairment. Sensitization is accompanied by pressure pain threshold decrease and consequent mechanical allodynia and hyperalgesia, which can cause unusual local pain via natural pressure exerted by space occupying lesions or by their examination. Worsening of local pain is similarly explainable. Neuroma pain may be due to cPNL-induced axonal mechanical sensitivity and hypersensitivity of the nociceptive nervi nervorum of the nerve trunk and its stump. Intermittence and symptomatic complexity of cPNL may be the cause of frequent misdiagnosis of chronic pain.

Pilot comparison of outcome measures across chemical and surgical experimental models of chronic osteoarthritis in the rat (Rattus norvegicus)

Relatively little work has evaluated both the disease of osteoarthritis (OA) and clinically-relevant pain outcome measures across different OA models in rats. The objective of this study was to compare sensitivity, pain, and histological disease severity across chemical and surgical models of OA in the rat. Stifle OA was induced in Sprague-Dawley rats via intraarticular injection of monoiodoacetate (MIA) or surgical transection of anterior cruciate ligament and/or destabilization of medial meniscus (ACL+DMM or DMM alone). Reflexive (e.g., mechanical and thermal stimuli) measures of sensitivity and non-reflexive assays (e.g., lameness, static hindlimb weight-bearing asymmetry, dynamic gait analysis) of pain were measured over time. Joint degeneration was assessed histologically. Six-weeks post OA-induction, the ACL+DMM animals had significantly greater visually observed lameness than MIA animals; however, both ACL+DMM and MIA animals showed equal pain as measured by limb use during ambulation and standing. The MIA animals showed increased thermal, but not mechanical, sensitivity compared to ACL+DMM animals. Joint degeneration was significantly more severe in the MIA model at 6 weeks. Our pilot data suggest both the ACL+DMM and MIA models are equal in terms of clinically relevant pain behaviors, but the MIA model is associated with more severe histological changes over time potentially making it more suitable for screening disease modifying agents. Future work should further characterize each model in terms of complex pain behaviors and biochemical, molecular, and imaging analysis of the sensory system and joint tissues, which will allow for more informed decisions associated with model selection and investigative outcomes.

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.

Intra-articular injection of monoiodoacetate induces diverse hip osteoarthritis in rats, depending on its dose

BACKGROUND

Monoiodoacetate (MIA)-induced arthritis models are used widely in osteoarthritis (OA) research to develop effective conservative treatments for hip OA, as an alternative to joint replacement surgery. In joint OA models, such as the MIA-induced knee OA model, various doses of MIA are utilized, depending on the purpose of the research. So far, only 2 mg of MIA has been used for MIA-induced hip OA research. We hypothesized that the amount of MIA should be adjusted according to the osteoarthritis model under investigation. We performed radiographic and histological evaluations in rats for hip OA models induced by different doses of MIA.

METHODS

One hundred and eighty right hips of six-week-old, male Sprague-Dawley rats (n = 30 rats per group) were treated with either a single intra-articular injection of various doses of MIA (0.25, 0.5, 1.0, 2.0, and 4.0 mg) dissolved in 25 μl of sterile saline (MIA group), or with 25 μl of sterile saline alone (Sham group). Radiographic and histological evaluations of the hip joint were performed at one, two, four, eight, and 12 weeks after administration (n = 6 rats per group per time point).

RESULTS

OA changes progressed from 1 week after administration in the 1.0-mg, 2.0-mg, and 4.0-mg MIA groups. The degree of OA changes increased as the dose of MIA increased. The 0.25-mg and 0.5-mg MIA groups presented fewer OA changes than the 2.0-mg and 4.0-mg MIA groups during the entire study period (up to 12 weeks). The administration of 0.25 mg and 0.5 mg of MIA-induced both radiographic and histological OA changes in a time-dependent manner, whereas more than 2 mg of MIA provoked end-stage OA at 8 weeks after injection. Absolute, dose-dependent histopathological OA changes were observed 4 weeks after MIA administration.

CONCLUSIONS

Intra-articular MIA injection to the hip joints of rats induced diverse OA changes dose-dependently. Research for developing novel conservative treatments for hip OA and intractable pain should consider the pathological condition when determining the dose of MIA to be employed.

Joint Damage and Neuropathic Pain in Rats Treated With Lysophosphatidic Acid

Joint pain is a complex phenomenon that involves multiple endogenous mediators and pathophysiological events. In addition to nociceptive and inflammatory pain, some patients report neuropathic-like pain symptoms. Examination of arthritic joints from humans and preclinical animal models have revealed axonal damage which is likely the source of the neuropathic pain. The mediators responsible for joint peripheral neuropathy are obscure, but lysophosphatidic acid (LPA) has emerged as a leading candidate target. In the present study, male and female Wistar rats received an intra-articular injection of LPA into the right knee and allowed to recover for 28 days. Joint pain was measured by von Frey hair algesiometry, while joint pathology was determined by scoring of histological sections. Both male and female rats showed comparable degenerative changes to the LPA-treated knee including chondrocyte death, focal bone erosion, and synovitis. Mechanical withdrawal thresholds decreased by 20-30% indicative of secondary allodynia in the affected limb; however, there was no significant difference in pain sensitivity between the sexes. Treatment of LPA animals with the neuropathic pain drug amitriptyline reduced joint pain for over 2 hours with no sex differences being observed. In summary, intra-articular injection of LPA causes joint degeneration and neuropathic pain thereby mimicking some of the characteristics of neuropathic osteoarthritis.

Mini review: The role of sensory innervation to subchondral bone in osteoarthritis pain

Osteoarthritis pain is often thought of as a pain driven by nerves that innervate the soft tissues of the joint, but there is emerging evidence for a role for nerves that innervate the underlying bone. In this mini review we cite evidence that subchondral bone lesions are associated with pain in osteoarthritis. We explore recent studies that provide evidence that sensory neurons that innervate bone are nociceptors that signal pain and can be sensitized in osteoarthritis. Finally, we describe neuronal remodeling of sensory and sympathetic nerves in bone and discuss how these processes can contribute to osteoarthritis pain.

Photobiomodulation Therapy Partially Restores Cartilage Integrity and Reduces Chronic Pain Behavior in a Rat Model of Osteoarthritis: Involvement of Spinal Glial Modulation

OBJECTIVE

Chronic pain associated with osteoarthritis (OA) often leads to reduced function and engagement in activities of daily living. Current pharmacological treatments remain relatively ineffective. This study investigated the efficacy of photobiomodulation therapy (PBMT) on cartilage integrity and central pain biomarkers in adult male Wistar rats.

DESIGN

We evaluated the cartilage degradation and spinal cord sensitization using the monoiodoacetate (MIA) model of OA following 2 weeks of delayed PBMT treatment (i.e., 15 days post-MIA). Multiple behavioral tests and knee joint histology were used to assess deficits related to OA. Immunohistochemistry was performed to assess chronic pain sensitization in spinal cord dorsal horn regions. Furthermore, we analyzed the principal components related to pain-like behavior and cartilage integrity.

RESULTS

MIA induced chronic pain-like behavior with respective cartilage degradation. PBMT had no effects on overall locomotor activity, but positive effects on weight support (P = 0.001; effect size [ES] = 1.01) and mechanical allodynia (P = 0.032; ES = 0.51). Greater optical densitometry of PBMT-treated cartilage was evident in superficial layers (P = 0.020; ES = 1.34), likely reflecting the increase of proteoglycan and chondrocyte contents. In addition, PBMT effects were associated to decreased contribution of spinal glial cells to pain-like behavior (P = 0.001; ES = 0.38).

CONCLUSION

PBMT during the chronic phase of MIA-induced OA promoted cartilage recovery and reduced the progression or maintenance of spinal cord sensitization. Our data suggest a potential role of PBMT in reducing cartilage degradation and long-term central sensitization associated with chronic OA.

Dorsal Root Ganglia Macrophages Maintain Osteoarthritis Pain

Pain is the major debilitating symptom of osteoarthritis (OA), which is difficult to treat. In OA patients joint tissue damage only poorly associates with pain, indicating other mechanisms contribute to OA pain. Immune cells regulate the sensory system, but little is known about the involvement of immune cells in OA pain. Here, we report that macrophages accumulate in the dorsal root ganglia (DRG) distant from the site of injury in two rodent models of OA. DRG macrophages acquired an M1-like phenotype, and depletion of DRG macrophages resolved OA pain in male and female mice. Sensory neurons innervating the damaged knee joint shape DRG macrophages into an M1-like phenotype. Persisting OA pain, accumulation of DRG macrophages, and programming of DRG macrophages into an M1-like phenotype were independent of Nav1.8 nociceptors. Inhibition of M1-like macrophages in the DRG by intrathecal injection of an IL4-IL10 fusion protein or M2-like macrophages resolved persistent OA pain. In conclusion, these findings reveal a crucial role for macrophages in maintaining OA pain independent of the joint damage and suggest a new direction to treat OA pain.SIGNIFICANCE STATEMENT In OA patients pain poorly correlates with joint tissue changes indicating mechanisms other than only tissue damage that cause pain in OA. We identified that DRG containing the somata of sensory neurons innervating the damaged knee are infiltrated with macrophages that are shaped into an M1-like phenotype by sensory neurons. We show that these DRG macrophages actively maintain OA pain remotely and independent of joint damage. The phenotype of these macrophages is crucial for a pain-promoting role. Targeting the phenotype of DRG macrophages with either M2-like macrophages or a cytokine fusion protein that skews macrophages into an M2-like phenotype resolves OA pain. Our work reveals a mechanism that contributes to the maintenance of OA pain distant from the affected knee joint and suggests that dorsal root ganglia macrophages are a target to treat osteoarthritis chronic pain.

TLR4 Antagonism Reduces Movement-Induced Nociception and ATF-3 Expression in Experimental Osteoarthritis

Introduction

Toll-like receptor 4 (TLR4) is a pattern recognition receptor involved in the detection of pathogen-associated molecular patterns (PAMPs), but also a "danger-sensing" receptor that recognizes host-derived endogenous molecules called damage-associated molecular patterns (DAMPs). The involvement of TLR4 in rheumatic diseases is becoming evident, as well as its potential role as a target for therapeutic intervention. Moreover, increasing evidence also suggests that TLR4 is implicated in chronic pain states. Thus, in this study, we evaluated whether a systemic administration of a synthetic antagonist of TLR4 (TLR4-A1) could decrease nociception and cartilage degradation in experimental osteoarthritis (OA). Furthermore, as the activation transcription factor (ATF)-3 serves as a negative regulator for TLR4-stimulated inflammatory response, we also evaluated the effect of TLR4 inhibition on ATF-3 expression in primary afferent neurons at the dorsal root ganglia (DRG).

Methods

OA was induced in adult male Wistar rats through an intra-articular injection of 2 mg of sodium mono-iodoacetate (MIA) into the left knee. From days 14 to 28 after OA induction, animals received an intraperitoneal injection of either TLR4-A1 (10 mg/kg) or vehicle. Movement- and loading-induced nociception was evaluated in all animals, by the Knee-Bend and CatWalk tests, before and at several time-points after TLR4-A1/vehicle administration. Immunofluorescence for TLR4 and ATF-3 was performed in L3-L5 DRG. Knee joints were processed for histopathological evaluation.

Results

Administration of TLR4-A1 markedly reduced movement-induced nociception in OA animals, particularly in the Knee-Bend test. Moreover, the increase of ATF-3 expression observed in DRG of OA animals was significantly reduced by TLR4-A1. However, no effect was observed in cartilage loss nor in the neuronal cytoplasmic expression of TLR4 upon antagonist administration.

Conclusion

The TLR4 antagonist administration possibly interrupts the TLR4 signalling cascade, thus decreasing the neurotoxic environment at the joint, which leads to a reduction in ATF-3 expression and in nociception associated with experimental OA.

Reduction of knee joint load suppresses cartilage degeneration, osteophyte formation, and synovitis in early-stage osteoarthritis using a post-traumatic rat model

The purpose of this study was to clarify the histological effect of reducing the loading to knee on cartilage degeneration, osteophyte formation, and synovitis in early-stage osteoarthritis (OA) using a post-traumatic rat model. Ten male rats were randomly allocated into two experimental groups: OA induction by surgical destabilization of medial meniscus (DMM, OA group) and hindlimb suspension after OA induction by DMM (OAHS group). The articular cartilage, osteophyte formation, and synovial membrane in the medial tibiofemoral joint were analyzed histologically and histomorphometrically at 2 and 4 weeks after surgery. The histological scores and changes in articular cartilage and osteophyte formation were significantly milder and slower in the OAHS group than in the OA group. At 2 and 4 weeks, there were no significant differences in cartilage thickness and matrix staining intensity between both the groups, but chondrocytes density was significantly lower in the OA group. Synovitis was milder in OAHS group than in OA group at 2 weeks. Reducing knee joint loading inhibited histological OA changes in articular cartilage, osteophyte formation, and synovial inflammation. This result supports the latest clinical guidelines for OA treatment. Further studies using biochemical and mechanical analyses are necessary to elucidate the mechanism underlying delayed OA progression caused by joint-load reduction.

Secretome of human adipose-derived mesenchymal stem cell relieves pain and neuroinflammation independently of the route of administration in experimental osteoarthritis

OBJECTIVE

Treatment of pain associated with osteoarthritis (OA) is unsatisfactory and innovative approaches are needed. The secretome from human adipose-derived mesenchymal stem cells (hASC-Conditioned Medium, CM) has been successfully used to relieve painful symptoms in models of chronic pain. The aim of this study was to explore the efficacy of the hASC-CM to control pain and neuroinflammation in an animal model of OA.

METHODS

OA was induced in mice by intra-articular monosodium-iodoacetate (MIA) injection. Thermal hyperalgesia and mechanical allodynia were assessed. Once hypersensitivity was established (7 days after MIA), hASC-CM was injected by IA, IPL and IV route and its effect monitored over time. Neuroinflammation in nerve, dorsal root ganglia and spinal cord was evaluated measuring proinflammatory markers and mediators by RT-qPCR. Protein content analysis of secretome by Mass Spectrometry was performed.

RESULTS

A single injection with hASC-CM induced a fast and long lasting antihyperalgesic and antiallodynic effect. The IV route of administration appeared to be the most efficacious although all the treatments were effective. The effect on pain correlated with the ability of hASC-CM to reduce the neuroinflammatory condition in both the peripheral and central nervous system. Furthermore, the secretome analysis revealed 101 factors associated with immune regulation.

CONCLUSION

We suggest that hASC-CM is a valid treatment option for controlling OA-related hypersensitivity, exerting a rapid and long lasting pain relief. The mechanisms underpinning its effects are likely linked to the positive modulation of neuroinflammation in peripheral and central nervous system that sustains peripheral and central sensitization.

Undenatured type II collagen prevents and treats osteoarthritis and motor function degradation in T2DM patients and db/db mice

Osteoarthritis (OA) has been scarcely researched among patients with diabetes mellitus. This study aims to confirm the preventive and therapeutic effects of undenatured type II collagen (UC II) on OA in aging db/db mice and in patients with T2DM. Firstly, aging db/db mice were randomly assigned to three groups: the UC II intervention (UC II) group, old model (OM) group and positive control group. Meanwhile db/m mice and young db/db mice were used as the normal control and young control groups, respectively. Secondly, fifty-five T2DM patients diagnosed with knee OA were randomly assigned to two groups: UC-II and placebo control groups. After a three-month intervention in both mice and T2DM patients, the subjects' gait and physical activities were assessed and the serum biomarkers including inflammatory cytokines, oxidative stress factors and matrix metalloproteinases (MMPs) were measured. Compared with the OM group mice, those in the UC II group showed a significantly greater superiority in terms of motor functions including the movement trajectories area (163.25 ± 20.3 vs. 78.52 ± 20.14 cm²), the tremor index (0.42 vs. 1.23), standing time (left hind: 0.089 ± 0.03 vs. 0.136 ± 0.04 s), swing (right front: 0.12 ± 0.02 vs. 0.216 ± 0.02 s), stride length (right hind: 7.2 ± 0.9 vs. 5.7 ± 1.1 cm), step cycle (right hind: 0.252 ± 0.05 vs. 0.478 ± 0.11 s) and cadence (14.12 ± 2.7 vs. 7.35 ± 4.4 steps per s). In addition, the levels of IL-4, IL-10, CTX- II and TGF-β in the UC II group were 1.74, 2.23, 1.67 and 1.84 times higher than those in the OM group, respectively, while the levels of MMP-3 and MMP-13 in the UC II group were half those in the OM group. Correspondingly, UC II intervention significantly decreased the scores of pain, stiffness and physical function (p < 0.05), whereas the 6 MWT and total MET distances in the UC II group increased remarkably (p < 0.05). After a three-month period of intervention, the varus angle significantly decreased from 4.6 ± 2.0° to 3.0 ± 1.4° and the knee flexion range obviously increased from 57.9 ± 14.0° to 66.9 ± 10.4°. Importantly, the declining trend in the levels of hs-CRP and MDA and the incremental trend in the SOD level were consistent in the db/db mice and OA patients following UC II administration.

CB2 agonism controls pain and subchondral bone degeneration induced by mono-iodoacetate: Implications GPCR functional bias and tolerance development

BACKGROUND AND PURPOSE

The endocannabinoid system became a promising target for osteoarthritis (OA) treatment. Functional selectivity of cannabinoids may increase their beneficial properties while reducing side effects. The aim of the present study was to evaluate the analgesic potential of two functionally biased CB2 agonists in different treatment regimens to propose the best pharmacological approach for OA management.

EXPERIMENTAL APPROACH

Two functionally selective CB2 agonists were administered i.p. - JWH133 (cAMP biased) and GW833972A (β-arrestin biased), in a chemically induced model of OA in rats. The drugs were tested in acute and chronic treatment regimens. Analgesic effects were assessed by pressure application measurement and kinetic weight bearing. X-ray microtomography was used for the morphometric analysis of the femur's subchondral bone tissue. Underlying biochemical changes were analysed via RT-qPCR.

KEY RESULTS

Dose-response studies established the effective dose for both JWH133 and GW833972A. In chronic treatment paradigms, JWH133 was able to elicit analgesia throughout the course of the experiment, whereas GW833972A lost its efficacy after 2 days of treatment. Later studies revealed improvement in subchondral bone architecture and decrement of matrix metalloproteinases and proinflammatory factors expression following JWH133 chronic treatment.

CONCLUSION AND IMPLICATIONS

Data presents analgesic and disease-modifying potential of CB2 agonists in OA treatment. Moreover, the study revealed more pronounced tolerance development for analgesic effects of the β-arrestin biased CB2 agonist GW833972A. These results provide a better understanding of the molecular underpinnings of the anti-nociceptive potential of CB2 agonists and may improve drug development processes for any cannabinoid-based chronic pain therapy.

Galactomannan of Delonix regia seeds reduces nociception and morphological damage in the rat model of osteoarthritis induced by sodium monoiodoacetate

This study investigated the effects of the protein-free galactomannan obtained from Delonix regia seeds (GM-DR) in an experimental osteoarthritis (OA) model. GM-DR was obtained from water-homogenized endosperms by collection of the supernatant and precipitation with ethanol. The remaining proteins in the galactomannan were removed by alkaline hydrolysis. Weight average molar mass (Mw) of the galactomannan was estimated in 5.8 × 105 g mol-1, presenting mannose:galactose ratio of 2.39:1. Rats received sodium monoiodoacetate (OA groups, 1 mg/25 μL) or saline (sham group) in the right tibio-tarsal joint. GM-DR (30-300 μg) was administered by intra-articular route at days 14 and 21 after OA induction. Hypernociception was evaluated daily by the measurement of the mechanical threshold required to cause joint flexion and paw withdrawal reflex. The 56-day animal groups were euthanized for joint histopahological analysis using the OARSI score system. Lower doses of GM-DR (30 and 100 μg) promoted antinociception from day 15 until the endpoint at day 56. Joint damage was reduced by GM-DR administration (100 μg) in OA-subjected animals, compared to the vehicle-treated OA group (5.9 ± 1.8 vs 19.0 ± 1.8, respectively, p < 0.05). Conclusion: Both antinociception and damage reduction suggest that Delonix regia galactomannan is a promising approach for osteoarthritis therapy.

Effect of intra-articular dexmedetomidine on experimental osteoarthritis in rats

Pharmacological treatment of osteoarthritis is still inadequate due to the low efficacy of the drugs used. Dexmedetomidine via the intra-articular (i.a.) route might be an option for the treatment of osteoarthritis-associated pain. The present study assessed the analgesic and anti-inflammatory effects of dexmedetomidine administered via the i.a. route in different doses in an experimental model of rat knee osteoarthritis induced with monosodium iodoacetate. Rats were allocated to four groups with 24 animals in each group. The OA (osteoarthritis), DEX-1 (dexmedetomidine in dose of 1μg/kg) and DEX-3 (dexmedetomidine in dose of 3μg/kg) groups were subjected to induction of osteoarthritis through injection of monosodium iodoacetate (MIA) via the i.a. route on the right knee; the control group was not subjected to osteoarthritis induction. Clinical assessment was performed on day 0 (before osteoarthritis induction) and then on days 5, 10, 14, 21 and 28 after induction. Treatment was performed on day 7 via the i.a. route, consisting of dexmedetomidine in doses of 1 and 3 μg/kg, while group OA received 0.9% normal saline. The animals were euthanized on days 7, 14, 21 and 28. Samples of the synovial membrane were collected for histopathological analysis, and the popliteal lymph nodes were collected for measurement of cytokines (interleukin [IL] IL-6, tumor necrosis factor alpha [TNF-α]). Dexmedetomidine (1 and 3 μg/kg) significantly reduced the animals’ weight distribution deficit during the chronic-degenerative stage of osteoarthritis and improved the pain threshold throughout the entire experiment. Histological analysis showed that dexmedetomidine did not cause any additional damage to the synovial membrane. The TNF-α levels decreased significantly in the DEX-3 group on day 28 compared with the OA group. Dexmedetomidine reduced pain, as evidenced by clinical parameters of osteoarthritis in rats, but did not have an anti-inflammatory effect on histological evaluation.

Attenuation of the Diffuse Noxious Inhibitory Controls in Chronic Joint Inflammatory Pain Is Accompanied by Anxiodepressive-Like Behaviors and Impairment of the Descending Noradrenergic Modulation

The noradrenergic system is paramount for controlling pain and emotions. We aimed at understanding the descending noradrenergic modulatory mechanisms in joint inflammatory pain and its correlation with the diffuse noxious inhibitory controls (DNICs) and with the onset of anxiodepressive behaviours. In the complete Freund's adjuvant rat model of Monoarthritis, nociceptive behaviors, DNICs, and anxiodepressive-like behaviors were evaluated. Spinal alpha2-adrenergic receptors (a2-AR), dopamine beta-hydroxylase (DBH), and noradrenaline were quantified concomitantly with a2-AR pharmacologic studies. The phosphorylated extracellular signal-regulated kinases 1 and 2 (pERK1/2) were quantified in the Locus coeruleus (LC), amygdala, and anterior cingulate cortex (ACC). DNIC was attenuated at 42 days of monoarthritis while present on days 7 and 28. On day 42, in contrast to day 28, noradrenaline was reduced and DBH labelling was increased. Moreover, spinal a2-AR were potentiated and no changes in a2-AR levels were observed. Additionally, at 42 days, the activation of ERKs1/2 was increased in the LC, ACC, and basolateral amygdala. This was accompanied by anxiety- and depressive-like behaviors, while at 28 days, only anxiety-like behaviors were observed. The data suggest DNIC is attenuated in prolonged chronic joint inflammatory pain, and this is accompanied by impairment of the descending noradrenergic modulation and anxiodepressive-like behaviors.

Analgesic Effect of Duloxetine on an Animal Model of Monosodium Iodoacetate-Induced Hip Osteoarthritis

We investigated the efficacy of duloxetine on hyperalgesia, histopathological and radiographic findings, pain-related sensory innervation of dorsal-root ganglia (DRG), and spinal changes in a rat model of induced hip osteoarthritis (OA). The right hip joints of male Sprague-Dawley rats (n = 6 rats/group) in the Sham group were injected with 25 μl of sterile saline and 25 μl of sterile saline with 2 mg of monosodium iodoacetate (MIA) were injected to the MIA + Vehicle and MIA + Duloxetine groups. We injected duloxetine 20 mg/kg intraperitoneally in the MIA + Duloxetine group 28 days after injection, whereas rats in the MIA + Vehicle group were injected with 0.5 ml of 20% dimethyl sulfoxide. We assessed hyperalgesia, histopathological changes, immunoreactive (-ir) neurons for calcitonin gene-related peptide and activating transcription factor 3 in DRG, and immunoreactive neurons for ionized-calcium-binding adaptor molecule 1 (Iba1) in the dorsal horn of the spinal cord. MIA administration into the hip joint let to mechanical hyperalgesia of the ipsilateral hind paw (p < 0.05). A single injection of duloxetine significantly attenuated it in induced hip OA (p < 0.05) and suppressed the number of Iba1-ir microglia of the ipsilateral dorsal horn (p < 0.05). These results suggest that a single injection of duloxetine suppressed mechanical hyperalgesia and may influence the expression of Iba1 in the microglia of the ipsilateral dorsal horn in the MIA-induced hip OA. This finding implies the inhibitory effects of duloxetine against neuropathic pain, which may lead to a change of microglial activities. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:422-430, 2020.

The Pharmacology of Pain Associated With the Monoiodoacetate Model of Osteoarthritis

The high incidence of osteoarthritis (OA) in an increasingly elderly population anticipates a dramatic rise in the number of people suffering from this disease in the near future. Because pain is the main reason patients seek medical help, effective pain management-which is currently lacking-is paramount to improve the quality of life that OA sufferers desperately seek. Good animal models are, in this day and age, fundamental tools for basic research of new therapeutic pathways. Several animal models of OA have been characterized, but none of them reproduces entirely all symptoms of the disease. Choosing between different animal models depends largely on which aspect of OA one aims to study. Here, we review the current understanding of the monoiodoacetate (MIA) model of OA. MIA injection in the knee joint leads to the progressive disruption of cartilage, which, in turn, is associated with the development of pain-like behavior. There are several reasons why the MIA model of OA seems to be the most adequate to study the pharmacological effect of new drugs in pain associated with OA. First, the pathological changes induced by MIA share many common traits with those observed in human OA (Van Der Kraan et al., 1989; Guingamp et al., 1997; Guzman et al., 2003), including loss of cartilage and alterations in the subchondral bone. The model has been extensively utilized in basic research, which means that the time course of pain-related behaviors and histopathological changes, as well as pharmacological profile, namely of commonly used pain-reducing drugs, is now moderately understood. Also, the severity of the progression of pathological changes can be controlled by grading the concentration of MIA administered. Further, in contrast with other OA models, MIA offers a rapid induction of pain-related phenotypes, with the cost-saving consequence in new drug screening. This model, therefore, may be more predictive of clinical efficacy of novel pharmacological tools than other chronic or acute OA models.

Joint unloading inhibits articular cartilage degeneration in knee joints of a monosodium iodoacetate-induced rat model of osteoarthritis

OBJECTIVE

The aim of the study was to examine how mechanical unloading affects articular cartilage degeneration in the patellofemoral (PF) and tibiofemoral (TF) joints of a monosodium iodoacetate (MIA)-induced rat model of osteoarthritis (OA).

DESIGN

The study involved 60 male rats. OA was induced by intra-articular injecting MIA into both knee joints. All animals were equally divided into two groups: sedentary (SE) and hindlimb unloading (HU) groups. Histopathological changes in the articular cartilage of the PF and TF joints were evaluated using the Osteoarthritis Research Society International (OARSI) score and modified Mankin score at 2 and 4 weeks after MIA injection.

RESULTS

In the SE and HU groups, representative histopathological changes in OA were detected in the PF and TF joints. The OARSI and modified Mankin scores for the PF and TF joints tended to increase over time after the injection of 0.2 mg or 1.0 mg of MIA in the SE and HU groups. Both the scores for the HU group were significantly lower than those for the SE group [OARSI score: P < 0.0001 (1.0-mg injection at 4 weeks); modified Mankin score: P = 0.0116 (0.2-mg injection at 4 weeks); P = 0.0004 and < 0.0001 (1.0-mg injection at 2 and 4 weeks, respectively)].

CONCLUSION

This study revealed new histological evidence that indicates that unloading condition suppresses articular cartilage degeneration and is beneficial in many areas of basal and clinical research involving OA.

Sensitivity of functional targeted neuropeptide evaluation in testing pregabalin analgesic efficacy in a rat model of osteoarthritis pain

The monosodium iodoacetate (MIA)-induced joint degeneration in rats is the most used animal model to screen analgesic drugs to alleviate osteoarthritis (OA) pain. This study aimed to evaluate the analgesic efficacy of pregabalin (PGB) in an MIA-induced OA model in rodents by using functional and neuroproteomic pain assessment methods. Treatment group included PGB in curative intent over 9 days compared to gold standard therapy (positive controls) and placebo (negative control). Functional assessments of pain (quantitative sensory testing and operant test) were performed concomitantly with spinal neuropeptides quantification. At day 21 post-OA induction, PGB in MIA rats reduced tactile allodynia (P = 0.028) and improved the place escape/avoidance behaviour (P = 0.04) compared to values recorded at last time-point before initiating analgesic therapy. All spinal neuropeptide concentrations, such as substance P, calcitonin gene-related peptide, bradykinin and somatostatin, came back to normal (non-affected) rat values, compared to their increase observed in MIA rats receiving the placebo (P < 0.0001). Initiated 13 days after chemical OA induction, repeated medication with PGB provided analgesia according to quantitative sensory testing, operant test and targeted neuropeptides pain assessment methods. This report highlights the interest of using reliable and sensitive methods like targeted neuropeptide quantification to detect the analgesic effects of a test article with concomitant functional assessments of pain when studying OA pain components.

Gait Assessment of Pain and Analgesics: Comparison of the DigiGait™ and CatWalk™ Gait Imaging Systems

Investigation of pain requires measurements of nociceptive sensitivity and other pain-related behaviors. Recent studies have indicated the superiority of gait analysis over traditional evaluations (e.g., skin sensitivity and sciatic function index [SFI]) in detecting subtle improvements and deteriorations in animal models. Here, pain-related gait parameters, whose criteria include (1) alteration in pain models, (2) correlation with nociceptive threshold, and (3) normalization by analgesics, were identified in representative models of neuropathic pain (spared nerve injury: coordination data) and inflammatory pain (intraplantar complete Freund’s adjuvant: both coordination and intensity data) in the DigiGait™ and CatWalk™ systems. DigiGait™ had advantages in fixed speed (controlled by treadmill) and dynamic SFI, while CatWalk™ excelled in intrinsic velocity, intensity data, and high-quality 3D images. Insights into the applicability of each system may provide guidance for selecting the appropriate gait imaging system for different animal models and optimization for future pain research.

Changes in proinflammatory cytokines, neuropeptides, and microglia in an animal model of monosodium iodoacetate-induced hip osteoarthritis

The aim of this study was to investigate the local production of proinflammatory cytokines, pain-related sensory innervation of dorsal-root ganglia (DRG), and spinal changes in a rat model of induced hip osteoarthritis (OA). Seventy-five Sprague-Dawley rats were used, including 25 controls and 50 injected into the right hip joints (sham group, injected with 25 µl of sterile saline: N = 25; and monosodium iodoacetate (MIA) group, injected with 25 µl of sterile saline with 2 mg of MIA: N = 25). We measured the local production of TNF-α, immunoreactive (-ir) neurons for calcitonin gene-related peptide (CGRP), and growth associated protein-43 (GAP-43) in DRG, and immunoreactive neurons for ionized-calcium-binding adaptor molecule-1 (Iba-1) in the dorsal horn of spinal cord, on post-induction days 7, 14, 28, 42, and 56 (N = 5 rats/group/time point). For post-induction days 7-42, the MIA group presented significantly elevated concentrations of TNF-α than the other groups (p < 0.01), and a higher expression of CGRP-ir in FG-labeled DRG neurons than the sham group (p < 0.01). MIA rats also presented significantly more FG-labeled GAP-43-ir DRG neurons than the sham group on post-induction days 28, 42, and 56 (p < 0.05), and a significantly higher number of Iba-1-ir microglia in the ipsilateral dorsal horn than the other groups, on post-induction days 28, 42, and 56. The results suggest that in rat models, pain-related pathologies due to MIA-induced hip OA, originate from inflammation caused by cytokines, which leads to progressive, chronic neuronal damage that may cause neuropathic pain. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2978-2986, 2018.

Electroacupuncture Potentiates Cannabinoid Receptor-Mediated Descending Inhibitory Control in a Mouse Model of Knee Osteoarthritis

Knee osteoarthritis (KOA) is a highly prevalent, chronic joint disorder, which can lead to chronic pain. Although electroacupuncture (EA) is effective in relieving chronic pain in the clinic, the involved mechanisms remain unclear. Reduced diffuse noxius inhibitory controls (DNIC) function is associated with chronic pain and may be related to the action of endocannabinoids. In the present study, we determined whether EA may potentiate cannabinoid receptor-mediated descending inhibitory control and inhibit chronic pain in a mouse model of KOA. We found that the optimized parameters of EA inhibiting chronic pain were the low frequency and high intensity (2 Hz + 1 mA). EA reversed the reduced expression of CB1 receptors and the 2-arachidonoylglycerol (2-AG) level in the midbrain in chronic pain. Microinjection of the CB1 receptor antagonist AM251 into the ventrolateral periaqueductal gray (vlPAG) can reversed the EA effect on pain hypersensitivity and DNIC function. In addition, CB1 receptors on GABAergic but not glutamatergic neurons are involved in the EA effect on DNIC function and descending inhibitory control of 5-HT in the medulla, thus inhibiting chronic pain. Our data suggest that endocannabinoid (2-AG)-CB1R-GABA-5-HT may be a novel signaling pathway involved in the effect of EA improving DNIC function and inhibiting chronic pain.

Peripheral Mechanisms Contributing to Osteoarthritis Pain

PURPOSE OF REVIEW

Osteoarthritis (OA) is the most common form of arthritis and a major source of pain and disability worldwide. OA-associated pain is usually refractory to classically used analgesics, and disease-modifying therapies are still lacking. Therefore, a better understanding of mechanisms and mediators contributing to the generation and maintenance of OA pain is critical for the development of efficient and safe pain-relieving therapies.

RECENT FINDINGS

Both peripheral and central mechanisms contribute to OA pain. Clinical evidence suggests that a strong peripheral nociceptive drive from the affected joint maintains pain and central sensitization associated with OA. Mediators present in the OA joint, including nerve growth factor, chemokines, cytokines, and inflammatory cells can contribute to sensitization. Furthermore, structural alterations in joint innervation and nerve damage occur in the course of OA. Several interrelated pathological processes, including joint damage, structural reorganization of joint afferents, low-grade inflammation, neuroplasticity, and nerve damage all contribute to the pain observed in OA. It can be anticipated that elucidating exactly how these mechanisms are operational in the course of progressive OA may lead to the identification of novel targets for intervention.

Do Neuroendocrine Peptides and Their Receptors Qualify as Novel Therapeutic Targets in Osteoarthritis?

Joint tissues like synovium, articular cartilage, meniscus and subchondral bone, are targets for neuropeptides. Resident cells of these tissues express receptors for various neuroendocrine-derived peptides including proopiomelanocortin (POMC)-derived peptides, i.e., α-melanocyte-stimulating hormone (α-MSH), adrenocorticotropin (ACTH) and β-endorphin (β-ED), and sympathetic neuropeptides like vasoactive intestinal peptide (VIP) and neuropeptide y (NPY). Melanocortins attained particular attention due to their immunomodulatory and anti-inflammatory effects in several tissues and organs. In particular, α-MSH, ACTH and specific melanocortin-receptor (MCR) agonists appear to have promising anti-inflammatory actions demonstrated in animal models of experimentally induced arthritis and osteoarthritis (OA). Sympathetic neuropeptides have obtained increasing attention as they have crucial trophic effects that are critical for joint tissue and bone homeostasis. VIP and NPY are implicated in direct and indirect activation of several anabolic signaling pathways in bone and synovial cells. Additionally, pituitary adenylate cyclase-activating polypeptide (PACAP) proved to be chondroprotective and, thus, might be a novel target in OA. Taken together, it appears more and more likely that the anabolic effects of these neuroendocrine peptides or their respective receptor agonists/antagonists may be exploited for the treatment of patients with inflammatory and degenerative joint diseases in the future.

Mechanosensitive ion channels in articular nociceptors drive mechanical allodynia in osteoarthritis

OBJECTIVE

Osteoarthritis (OA) is a disabling and highly prevalent condition affecting millions worldwide. Pain is the major complaint of OA patients and is presently inadequately managed. It manifests as mechanical allodynia, a painful response to innocuous stimuli such as joint movement. Allodynia is due in part to the sensitization of articular nociceptors to mechanical stimuli. These nociceptors respond to noxious mechanical stimuli applied to their terminals via the expression of depolarizing high-threshold mechanosensitive ion channels (MSICs) that convert painful mechanical forces into electrical signals. In this study, we examined the contribution of MSICs to mechanical allodynia in a mouse model of OA.

METHOD

Sodium mono-iodoacetate (MIA) was injected in the left knee of adult male Trpv1:Cre; GFP mice. Primary mechanical allodynia was monitored using the knee-bend test. Single-channel patch clamp electrophysiology was performed on visually-identified knee-innervating nociceptors. Dorsal horn neuronal activation was assessed by Fos immunoreactivity.

RESULTS

In examining the gating properties of MSICs of naïve and OA mice, we discovered that their activation threshold is greatly reduced, causing their opening at significantly lower stimuli intensities. Consequently, nociceptors are activated by mild mechanical stimuli. These channels are reversibly inhibited by the selective MSIC inhibitor GsMTx4, and the intra-articular injection of this peptide significantly reduced the activation of dorsal horn nociceptive circuits and primary mechanical allodynia in OA mice.

CONCLUSIONS

These results suggest that MSICs are sensitized during OA and directly contribute to mechanical allodynia. They therefore represent potential therapeutic targets in the treatment of OA pain.

Glial activation in the collagenase model of nociception associated with osteoarthritis

Background

Experimental osteoarthritis entails neuropathic-like changes in dorsal root ganglia (DRG) neurons. Since glial activation has emerged as a key player in nociception, being reported in numerous models of neuropathic pain, we aimed at evaluating if glial cell activation may also occur in the DRG and spinal cord of rats with osteoarthritis induced by intra-articular injection of collagenase.

Methods

Osteoarthritis was induced by two injections, separated by three days, of 500 U of type II collagenase into the knee joint of rats. Movement-induced nociception was evaluated by the Knee-Bend and CatWalk tests during the following six weeks. Glial fibrillary acidic protein (GFAP) expression in satellite glial cells of the DRG was assessed by immunofluorescence and Western Blot analysis; the pattern of GFAP and activating transcription factor-3 (ATF-3) expression was also compared through double immunofluorescence analysis. GFAP expression in astrocytes and IBA-1 expression in microglia of the L3–L5 spinal cord segments was assessed by immunohistochemistry and Western Blot analysis. The effect of the intrathecal administration of fluorocitrate, an inhibitor of glial activation, on movement-induced nociception was evaluated six weeks after the first collagenase injection.

Results

GFAP expression in satellite glial cells of collagenase-injected animals was significantly increased six weeks after osteoarthritis induction. Double immunofluorescence showed GFAP upregulation in satellite glial cells surrounding ATF-3-positive neurons. In the spinal cord of collagenase-injected animals, an ipsilateral upregulation of GFAP and IBA-1 was also observed. The inhibition of glial activation with fluorocitrate decreased movement- and loading-induced nociception.

Conclusion

Collagenase-induced knee osteoarthritis leads to the development of nociception associated with movement of the affected joint and to the activation of glial cells in both the DRG and the spinal cord. Inhibition of glial cell activation by fluorocitrate decreases these osteoarthritis-associated nociceptive behaviours. These results suggest that glial cell activation may play a role in the development of chronic pain in this experimental model of osteoarthritis.

Prophylactic inhibition of neutrophil elastase prevents the development of chronic neuropathic pain in osteoarthritic mice

Background

A subset of osteoarthritis (OA) patients experience joint pain with neuropathic characteristics. Mediators such as neutrophil elastase, a serine proteinase, may be released during acute OA inflammatory flares. We have previously shown that local administration of neutrophil elastase causes joint inflammation and pain via activation of proteinase-activated receptor-2 (PAR2). The aim of this study was to examine the contribution of endogenous neutrophil elastase and PAR2 to the development of joint inflammation, pain, and neuropathy associated with monoiodoacetate (MIA)-induced experimental OA.

Methods

MIA (0.3 mg/10 μl) was injected into the right knee joint of male C57BL/6 mice (20–34 g). Joint inflammation (edema, leukocyte kinetics), neutrophil elastase proteolytic activity, tactile allodynia, and saphenous nerve demyelination were assessed over 14 days post-injection. The effects of inhibiting neutrophil elastase during the early inflammatory phase of MIA (days 0 to 3) were determined using sivelestat (50 mg/kg i.p.) and serpinA1 (10 μg i.p.). Involvement of PAR2 in the development of MIA-induced joint inflammation and pain was studied using the PAR2 antagonist GB83 (5 μg i.p. days 0 to 1) and PAR2 knockout animals.

Results

MIA caused an increase in neutrophil elastase proteolytic activity on day 1 (P < 0.0001), but not on day 14. MIA also generated a transient inflammatory response which peaked on day 1 (P < 0.01) then subsided over the 2-week time course. Joint pain appeared on day 1 and persisted to day 14 (P < 0.0001). By day 14, the saphenous nerve showed signs of demyelination. Early treatment with sivelestat and serpinA1 blocked the proteolytic activity of neutrophil elastase on day 1 (P < 0.001), and caused lasting improvements in joint inflammation, pain, and saphenous nerve damage (P < 0.05). MIA-induced synovitis was reversed by early treatment with GB83 and attenuated in PAR2 knockout mice (P < 0.05). PAR2 knockout mice also showed reduced MIA-induced joint pain (P < 0.0001) and less nerve demyelination (P = 0.81 compared to saline control).

Conclusions

Neutrophil elastase and PAR2 contribute significantly to the development of joint inflammation, pain, and peripheral neuropathy associated with experimental OA, suggesting their potential as therapeutic targets.

Pain-related behavior and the characteristics of dorsal-root ganglia in a rat model of hip osteoarthritis induced by mono-iodoacetate

The principal aim of this study was to clarify the time course of pain-related behavior and pain-related sensory innervation in a rat model of hip osteoarthritis (OA) induced by intra-articular injection of mono-iodoacetate (MIA). Using 6-week-old male Sprague Dawley rats, 25 μl of sterile saline of 1% Fluoro-Gold solution (FG) (control group; n = 30) and 25 μl of sterile saline of 1% FG with 2 mg of MIA (MIA group; n = 30) was injected into the right hip joints. Gait function was evaluated using a CatWalk system after 7, 14, 28, 42, and 56 days (n = 5, respectively). Neurons in the dorsal root ganglion (DRG) between L1 and L5 were immunostained for calcitonin gene-related peptide (CGRP) and activating transcription factor-3 (ATF3). Gait analysis revealed the mean six parameters of hind paws at all time points were significantly lower in the MIA group (p = 0.05). The number of CGRP-immunoreactive (-IR) DRG neurons was significantly increased on days 7, 14, 28, and 42 peaking at 14 days in the MIA group. By contrast, expression of ATF3-IR in FG-labeled DRG neurons was significantly increased on days 42 and 57. The FG-labeled DRG neurons were distributed between L1 and L5, mainly at the L4 level. Pain-related behavior indicated by gait disturbance was observed in a MIA model of hip OA in rat. Early elevation of CGRP expression and late expression of ATF-3 were demonstrated in DRG neurons, possibly reflecting inflammatory pain and neuropathic pain in hip OA. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1424-1430, 2017.

Repair of cartilage defects in osteoarthritis rats with induced pluripotent stem cell derived chondrocytes

Background

The incapacity of articular cartilage (AC) for self-repair after damage ultimately leads to the development of osteoarthritis. Stem cell-based therapy has been proposed for the treatment of osteoarthritis (OA) and induced pluripotent stem cells (iPSCs) are becoming a promising stem cell source.

Results

Three steps were developed to differentiate human iPSCs into chondrocytes which were transplanted into rat OA models induced by monosodium iodoacetate (MIA). After 6 days embryonic body (EB) formation and 2 weeks differentiation, the gene and protein expression of Col2A1, GAG and Sox9 has significantly increased compare to undifferentiated hiPSCs. After 15 weeks transplantation, no immune responses were observed, micro-CT showed gradual engraftment and the improvement of subchondrol plate integrity, and histological examinations demonstrated articular cartilage matrix production.

Conclusions

hiPSC could be an efficient and clinically translatable approach for cartilage tissue regeneration in OA cartilages.

Bone Injury and Repair Trigger Central and Peripheral NPY Neuronal Pathways

Bone repair is a specialized type of wound repair controlled by complex multi-factorial events. The nervous system is recognized as one of the key regulators of bone mass, thereby suggesting a role for neuronal pathways in bone homeostasis. However, in the context of bone injury and repair, little is known on the interplay between the nervous system and bone. Here, we addressed the neuropeptide Y (NPY) neuronal arm during the initial stages of bone repair encompassing the inflammatory response and ossification phases in femoral-defect mouse model. Spatial and temporal analysis of transcriptional and protein levels of NPY and its receptors, Y1R and Y2R, reported to be involved in bone homeostasis, was performed in bone, dorsal root ganglia (DRG) and hypothalamus after femoral injury. The results showed that NPY system activity is increased in a time- and space-dependent manner during bone repair. Y1R expression was trigged in both bone and DRG throughout the inflammatory phase, while a Y2R response was restricted to the hypothalamus and at a later stage, during the ossification step. Our results provide new insights into the involvement of NPY neuronal pathways in bone repair.

Monoiodoacetic acid induces arthritis and synovitis in rats in a dose- and time-dependent manner: proposed model-specific scoring systems

OBJECTIVE

In a rat monoiodoacetic acid (MIA)-induced arthritis model, the amount of MIA commonly used was too high, resulting in rapid bone destruction. We examined the effect of MIA concentrations on articular cartilage and infrapatellar fat pad (IFP). We also established an original system for "macroscopic cartilage and bone score" and "IFP inflammation score" specific to the rat MIA-induced arthritis model.

DESIGN

Male Wistar rats received a single intra-articular injection of MIA in the knee. The amount of MIA was 0.1, 0.2, 0.5, and 1 mg respectively. Articular cartilage was evaluated at 2-12 weeks. IFP was also observed at 3-14 days.

RESULTS

Macroscopically, low MIA doses induced punctate depressions on the cartilage surface, and cartilage erosion proceeded slowly over 12 weeks, while higher MIA doses already induced cartilage erosion at 2 weeks, followed by bone destruction. MIA macroscopic cartilage and bone score, OARSI histological score, and Mankin score increased in a dose- and time-dependent manner. The IFP inflammation score peaked at 5 days in low dose groups, then decreased, while in high dose groups, the IFP score continued to increase over 14 days due to IFP fibrosis.

CONCLUSIONS

Punctate depressions, cartilage erosion, and bone destruction were observed in the MIA-induced arthritis model. The macroscopic cartilage and bone scoring enabled the quantification of cartilage degeneration and demonstrated that MIA-induced arthritis progressed in a dose- and time-dependent manner. IFP inflammation scores revealed that 0.2 mg MIA induced reversible synovitis, while 1 mg MIA induced fibrosis of the IFP body.

Concurrent validity of different functional and neuroproteomic pain assessment methods in the rat osteoarthritis monosodium iodoacetate (MIA) model

Background

Lack of validity in osteoarthritis pain models and assessment methods is suspected. Our goal was to 1) assess the repeatability and reproducibility of measurement and the influence of environment, and acclimatization, to different pain assessment outcomes in normal rats, and 2) test the concurrent validity of the most reliable methods in relation to the expression of different spinal neuropeptides in a chemical model of osteoarthritic pain.

Methods

Repeatability and inter-rater reliability of reflexive nociceptive mechanical thresholds, spontaneous static weight-bearing, treadmill, rotarod, and operant place escape/avoidance paradigm (PEAP) were assessed by the intraclass correlation coefficient (ICC). The most reliable acclimatization protocol was determined by comparing coefficients of variation. In a pilot comparative study, the sensitivity and responsiveness to treatment of the most reliable methods were tested in the monosodium iodoacetate (MIA) model over 21 days. Two MIA (2 mg) groups (including one lidocaine treatment group) and one sham group (0.9 % saline) received an intra-articular (50 μL) injection.

Results

No effect of environment (observer, inverted circadian cycle, or exercise) was observed; all tested methods except mechanical sensitivity (ICC <0.3), offered good repeatability (ICC ≥0.7). The most reliable acclimatization protocol included five assessments over two weeks. MIA-related osteoarthritic change in pain was demonstrated with static weight-bearing, punctate tactile allodynia evaluation, treadmill exercise and operant PEAP, the latter being the most responsive to analgesic intra-articular lidocaine. Substance P and calcitonin gene-related peptide were higher in MIA groups compared to naive (adjusted P (adj-P) = 0.016) or sham-treated (adj-P = 0.029) rats. Repeated post-MIA lidocaine injection resulted in 34 times lower downregulation for spinal substance P compared to MIA alone (adj-P = 0.029), with a concomitant increase of 17 % in time spent on the PEAP dark side (indicative of increased comfort).

Conclusion

This study of normal rats and rats with pain established the most reliable and sensitive pain assessment methods and an optimized acclimatization protocol. Operant PEAP testing was more responsive to lidocaine analgesia than other tests used, while neuropeptide spinal concentration is an objective quantification method attractive to support and validate different centralized pain functional assessment methods.

Cutaneous tissue damage induces long-lasting nociceptive sensitization and regulation of cellular stress- and nerve injury-associated genes in sensory neurons

Tissue damage is one of the major etiological factors in the emergence of chronic/persistent pain, although mechanisms remain enigmatic. Using incision of the back skin of adult rats as a model for tissue damage, we observed sensitization in a nociceptive reflex enduring to 28days post-incision (DPI). To determine if the enduring behavioral changes corresponded with a long-term impact of tissue damage on sensory neurons, we examined the temporal expression profile of injury-regulated genes and the electrophysiological properties of traced dorsal root ganglion (DRG) sensory neurons. The mRNA for the injury/stress-hub gene Activating Transcription Factor 3 (ATF3) was upregulated and peaked within 4 DPI, after which levels declined but remained significantly elevated out to 28 DPI, a time when the initial incision appears healed and tissue-inflammation largely resolved. Accordingly, stereological image analysis indicated that some neurons expressed ATF3 only transiently (mostly medium-large neurons), while in others it was sustained (mostly small neurons), suggesting cell-type-specific responses. In retrogradely-traced ATF3-expressing neurons, Calcium/calmodulin-dependent protein kinase type IV (CAMK4) protein levels and isolectin-B4 (IB4)-binding were suppressed whereas Growth Associated Protein-43 (GAP-43) and Neuropeptide Y (NPY) protein levels were enhanced. Electrophysiological recordings from DiI-traced sensory neurons 28 DPI showed a significant sensitization limited to ATF3-expressing neurons. Thus, ATF3 expression is revealed as a strong predictor of single cells displaying enduring pain-related electrophysiological properties. The cellular injury/stress response induced in sensory neurons by tissue damage and indicated by ATF3 expression is positioned to contribute to pain which can occur after tissue damage.

Intra-articular injection of mono-iodoacetate induces osteoarthritis of the hip in rats

Background

The mechanism for hip pain has been unclear because of a lack of experimental animal models. We aimed to establish an intra-articular injection technique to the rat hip and to document the effect of intra-articular mono-iodoacetate (MIA) injection to the rat hip with radiography and histology.

Methods

Using 60 6-week-old male Sprague Dawley rats, 25 μl of sterile saline (control group; n = 30) and 25 μl of sterile saline with 2 mg of MIA (MIA group; n = 30) was injected into the right hip joints via posterior approach using a 27G needle. The animals were examined with X-ray and histology 7, 14, 28, 42, and 56 days later (MIA group [n = 6] and control group [n = 6], respectively).

Results

The MIA group showed progressive radiographic changes to the hip joint during the experimental period, whereas the control group maintained a normal appearance. The microanatomic appearance was consistent with X-ray images of progressive destruction in the MIA group and normal tissue in the control group. Osteoarthritic (OA) changes became apparent at 42 and 56 days in the MIA group.

Conclusions

We established an intra-articular injection technique to the rat hip, creating a hip OA model in the rat by intra-articular injection of MIA.

The effects of intra-articular resiniferatoxin on monosodium iodoacetate-induced osteoarthritic pain in rats

This study was performed to investigate whether an intra-articular injection of transient receptor potential vanilloid 1 (TRPV1) receptor agonist, resiniferatoxin (RTX) would alleviate behavioral signs of arthritic pain in a rat model of osteoarthritis (OA). We also sought to determine the effect of RTX treatment on calcitonin gene-related peptide (CGRP) expression in the spinal cord. Knee joint inflammation was induced by intra-articular injection of monosodium iodoacetate (MIA, 8 mg/50 µl) and weight bearing percentage on right and left hindpaws during walking, paw withdrawal threshold to mechanical stimulation, and paw withdrawal latency to heat were measured to evaluate pain behavior. Intra-articular administration of RTX (0.03, 0.003 and 0.0003%) at 2 weeks after the induction of knee joint inflammation significantly improved reduction of weight bearing on the ipsilateral hindlimb and increased paw withdrawal sensitivity to mechanical and heat stimuli. The reduction of pain behavior persisted for 3~10 days according to each behavioral test. The MIA-induced increase in CGRP immunoreactivity in the spinal cord was decreased by RTX treatment in a dose-dependent manner. The present study demonstrated that a single intra-articular administration of RTX reduced pain behaviors for a relatively long time in an experimental model of OA and could normalize OA-associated changes in peptide expression in the spinal cord.

Deficits in spontaneous burrowing behavior in the rat bilateral monosodium iodoacetate model of osteoarthritis: an objective measure of pain-related behavior and analgesic efficacy

OBJECTIVE

To characterize deficits in burrowing behavior - an ethologically-relevant rodent behavior - in the monosodium iodoacetate (MIA) rat model of osteoarthritis (OA), and the sensitivity of these deficits to reversal by analgesic drugs of both prototypical and novel mechanisms of action. A second objective was to compare the burrowing assay to a spontaneous locomotor activity (sLA) assay.

METHOD

Male Wistar Han rats (200-220 g) received intrarticular (i.a.) injections of MIA or saline for sham animals. A deficit in the amount of sand burrowed from steel tubes filled with 2.5 kg of sand was used as a measure of pain-related behavior, and sensitivity to reversal of these deficits by analgesic drugs was assessed in bilaterally MIA-injected rats.

RESULTS

Bilateral MIA injections induced a significant impairment of burrowing behavior, which was concentration-dependent. The temporal pattern of the deficits was biphasic: a large deficit at 3 days post-injection, resolving by day 14 and returning at the 21 and 28 day time points. At the 3 day time point ibuprofen, celecoxib and an anti-nerve growth factor (NGF) monoclonal antibody (mAb) were able to significantly reinstate burrowing behavior, whereas the fatty acid amide hydrolase (FAAH) inhibitor PF-04457845 and morphine displayed no reversal effect. Morphine impaired burrowing behavior at 3 mg/kg in sham animals. Deficits in rearing frequency in the locomotor activity assay proved irreversible by analgesics.

CONCLUSION

Burrowing behavior provides an objective, non-reflexive read-out for pain-related behavior in the MIA model that has predictive validity in detecting analgesic efficacy of nonsteroidal anti-inflammatory drugs (NSAIDs) and an anti-NGF mAb.

Injury of primary afferent neurons may contribute to osteoarthritis induced pain: an experimental study using the collagenase model in rats

OBJECTIVE

Pain exacerbated by movement and loading on the joint is the major symptom of osteoarthritis (OA), but the mechanisms of chronic pain in this pathology are still poorly understood. Using the intra-articular (i.a.) injection of collagenase in the knee of rats as a model of OA, we aimed at evaluating whether injury of sensory neurons may contribute to the development of OA-associated nociception.

DESIGN

OA was induced by i.a. injection of collagenase into the left knee joint of adult male Wistar rats. Histopathological changes and movement and loading-induced nociception were assessed for 6 weeks. A time-course analysis of the expression of the neuronal injury markers activating transcription factor-3 (ATF-3) and neuropeptide Y (NPY) and of the neuropeptide SP in the dorsal root ganglion (DRG) was performed. Gabapentin's effect on nociception was evaluated, as well as the expression of the α2δ-1 voltage-gated calcium channel subunit.

RESULTS

Collagenase induced the development of OA-like histopathological changes and of movement-induced nociception. Altered expression of ATF-3, NPY and SP was observed in the DRG, correlating with the degree of articular degeneration after 6 weeks of disease progression. Repeated administration of gabapentin reversed the nociceptive responses 6 weeks after the induction of OA. α2δ-1 was upregulated in the DRG.

CONCLUSION

By inducing nociceptive behaviours associated with relevant joint structural changes, the i.a. injection of collagenase presents itself as a pertinent model for the study of OA pain. The findings in this study support the hypothesis that injury of sensory neurons innervating OA joints may be a significant element in the mechanisms of OA-associated pain.

Quantitative Gait Analysis Detects Significant Differences in Movement between Osteoarthritic and Nonosteoarthritic Guinea Pig Strains before and after Treatment with Flunixin Meglumine

A computer-aided gait analysis system was used to contrast two guinea pig strains with differing propensity for osteoarthritis (OA), with/without administration of a nonsteroidal anti-inflammatory drug. Walking speed and static/dynamic gait parameters were determined at baseline. Flunixin meglumine was given and animals were evaluated 4, 24, and 72 hours after treatment. Body weight was compared using unpaired t-tests. Knee joints were histologically evaluated using species-specific criteria; indices were analyzed using one-way ANOVA, Kruskal-Wallis test, followed by Dunn's multiple comparisons. A generalized linear model followed by Tukey's posttests juxtaposed gait parameters; walking speed was a covariate for other outcome measures. Body weight was not different between strains; OA-prone animals demonstrated more progressive chondropathy. At baseline, OA-prone animals had slower walking speeds, narrower hind limb bases of support, shorter stride lengths, and slower limb swing speeds relative to OA-resistant animals. These differences were not detected 4 or 24 hours after treatment. By 72 hours, OA-prone animals had returned to baseline values. These findings indicate a distinct voluntary gait pattern in a rodent model of bilateral primary OA, modification of which may allow rapid screening of novel therapies. Flunixin meglumine temporarily permitted OA-prone animals to move in a manner that was analogous to OA-resistant animals.

Intra-articular injection of collagenase in the knee of rats as an alternative model to study nociception associated with osteoarthritis

INTRODUCTION

Animal models currently used in osteoarthritis-associated pain research inadequately reproduce the initiating events and structural pathology of human osteoarthritis. Conversely, intra-articular injection of collagenase is a structurally relevant model, as it induces articular degeneration both by digesting collagen from cartilage and by causing articular instability, thereby reproducing some of the main events associated with osteoarthritis onset and development. Here, we evaluated if the intra-articular injection of collagenase can be an alternative model to study nociception associated with osteoarthritis.

METHODS

Osteoarthritis was induced by two intra-articular injections of either 250 U or 500 U of collagenase into the left knee joint of adult male Wistar rats. A six weeks time-course assessment of movement- and loading-induced nociception was performed by the Knee-Bend and CatWalk tests. The effect of morphine, lidocaine and diclofenac on nociceptive behaviour was evaluated in animals injected with 500 U of collagenase. Joint histopathology was scored for both doses throughout time. The expression of transient receptor potential vanilloid 1 (TRPV1) in ipsilateral dorsal root ganglia (DRG) was evaluated.

RESULTS

An increase in nociceptive behaviour associated with movement and loading of affected joints was observed after intra-articular collagenase injection. With the 500 U dose of collagenase, there was a significant correlation between the behavioural and the histopathological osteoarthritis-like structural changes developed after six weeks. One week after injection of 500 U collagenase, swelling of the injected knee and inflammation of the synovial membrane were also observed, indicating the occurrence of an early inflammatory reaction. Behavioural changes induced by the 500 U dose of collagenase were overall effectively reversed by morphine and lidocaine. Diclofenac was effective one week after injection. TRPV1 expression increased six weeks after 500 U collagenase injection.

CONCLUSION

We conclude that the intra-articular injection of 500 U collagenase in the knee of rats can be an alternative model for the study of nociception associated with osteoarthritis, since it induces significant nociceptive alterations associated with relevant osteoarthritis-like joint structural changes.

Differences in structural and pain phenotypes in the sodium monoiodoacetate and meniscal transection models of osteoarthritis

OBJECTIVES

To characterize differences in joint pathology and pain behavior between two rat models of osteoarthritis (OA) in order to inform selection of animal models for interventional studies.

METHOD

Knee OA was induced in Sprague Dawley rats by either meniscal transection (MNX) or intra-articular injection of monosodium iodoacetate (MIA). Controls were subjected to sham surgery or saline-injection. In a separate experiment, a single intra-articular injection of triamcinolone acetonide was administered 14 days after MNX or MIA arthritis induction. Pain behavior and joint pathology were quantified.

RESULTS

Both models displayed synovial inflammation, chondropathy and osteophytosis. Chondropathy scores increased with time similarly in the two models. Inflammation and osteophyte scores were greater in MNX model compared to the MIA model. At day 49, the MNX model exhibited a greater number of channels crossing the osteochondral junction compared to all other groups. The MNX model exhibited greater weight bearing asymmetry compared to the MIA model, whereas the MIA model displayed more consistent hindpaw allodynia. Triamcinolone attenuated weight bearing asymmetry and distal allodynia to control levels in the MNX model, but distal allodynia was unaltered in the MIA model.

CONCLUSIONS

The comparison of the two models of OA in rats, using identical assessment tools has demonstrated that although both models display features of OA, there are differences between the models which may represent different aspects of human OA. Thus, model selection should be based on the pathological aspects of OA under investigation.