Effects of immunosuppression after limb fracture in mice on nociceptive, cognitive, and anxiety-related outcomes.

Introduction: Chronic pain is a common and problematic consequence of injuries with few proven methods for prevention or treatment. In addition to pain, functional limitations and neuropsychiatric changes such as cognitive impairment and anxiety worsen outcomes.

Objectives: To determine whether inhibiting activation of the adaptive immune response after limb fracture would reduce pain, functional loss, memory changes, and anxiety.

Alpha-7 Nicotinic Acetylcholine Receptor Activation Inhibits Trauma Induced Pronociceptive Autoimmune Responses.

Both autonomic nervous system dysfunction and immune system activation are characteristic of chronic pain after limb injuries. Cholinergic agonists reduce immune system activation in many settings. We hypothesized, therefore, that alpha-7 nicotinic acetylcholine receptor (α7nAChR) agonist administration would reduce nociceptive and immune changes after tibia fracture and cast immobilization in mice.

Pronociceptive autoantibodies in the spinal cord mediate nociceptive sensitization, loss of function, and spontaneous pain in the lumbar disk puncture model of chronic back pain.

Previously, we observed that B cells and autoantibodies mediated chronic nociceptive sensitization in the mouse tibia fracture model of complex regional pain syndrome and that complex regional pain syndrome patient antibodies were pronociceptive in fracture mice lacking mature B cells and antibodies (muMT). The current study used a lumbar spinal disk puncture (DP) model of low back pain in wild-type (WT) and muMT mice to evaluate pronociceptive adaptive immune responses. Spinal disks and cords were collected 3 weeks after DP for polymerase chain reaction and immunohistochemistry analyses.

Autonomic Regulation of Nociceptive and Immunologic Changes in a Mouse Model of Complex Regional Pain Syndrome.

Chronic pain frequently develops after limb injuries, and its pathogenesis is poorly understood. We explored the hypothesis that the autonomic nervous system regulates adaptive immune system activation and nociceptive sensitization in a mouse model of chronic post-traumatic pain with features of complex regional pain syndrome (CRPS). In studies sympathetic signaling was reduced using 6-hydroxydopamine (6-OHDA) or lofexidine, while parasympathetic signaling was augmented by nicotine administration.

Dimethyl Fumarate Reduces Oxidative Stress and Pronociceptive Immune Responses in a Murine Model of Complex Regional Pain Syndrome.

Background: Complex regional pain syndrome (CRPS) is a highly disabling cause of pain often precipitated by surgery or trauma to a limb. Both innate and adaptive immunological changes contribute to this syndrome. Dimethyl fumarate (DMF) works through the nuclear factor erythroid 2-related factor 2 (Nrf2) transcription factor and other targets to activate antioxidant systems and to suppress immune system activation.

IL-6 signaling mediates the germinal center response, IgM production and nociceptive sensitization in male mice after tibia fracture.

Background: Up-regulated interleukin 6 (IL-6) signaling, immune system activation, and pronociceptive autoantibodies are characteristic of complex regional pain syndrome (CRPS). IL-6 is known to promote B cell differentiation, thus we hypothesized that IL-6 signaling plays a crucial role in the development of adaptive immune responses and nociceptive sensitization in a murine tibia fracture model of CRPS.

Methods: Mice deficient in IL-6 expression (IL-6) or B cell deficient (muMT) underwent tibia fracture and 3 weeks of cast immobilization or sham injury.

Mice lacking substance P have normal bone modeling but diminished bone formation, increased resorption, and accelerated osteopenia with aging.

Substance P (SP) is a sensory neuropeptide that is expressed by the neurons innervating bone. There is considerable evidence that SP can regulate bone cell function in vitro, but it is unclear whether SP modulates bone modeling or remodeling in vivo. To answer this question we characterized the bone phenotype of mice with deletion of the Tac1 gene expressing SP.

C5a complement and cytokine signaling mediate the pronociceptive effects of complex regional pain syndrome patient IgM in fracture mice.

It has been proposed that complex regional pain syndrome (CRPS) is a posttraumatic autoimmune disease. Previously, we observed that B cells contribute to CRPS-like changes in a mouse tibia fracture model, and that early (<12 months duration) CRPS patient IgM antibodies have pronociceptive effects in the skin and spinal cord of muMT fracture mice lacking B cells. The current study evaluated the pronociceptive effects of intraplantar or intrathecal injections of early CRPS IgM (5 µg) in muMT fracture mice.

Germinal center formation, immunoglobulin production and hindlimb nociceptive sensitization after tibia fracture.

Emerging evidence suggests that Complex Regional Pain Syndrome (CRPS) is in part a post-traumatic autoimmune disease mediated by an adaptive immune response after limb injuries. We previously observed in a murine tibial fracture model of CRPS that pain-related behaviors were dependent upon adaptive immune mechanisms including the neuropeptide-dependent production of IgM for 5 months after injury. However, the time course of induction of this immune response and the demonstration of germinal center formation in lymphoid organs has not been evaluated.

Complex regional pain syndrome patient immunoglobulin M has pronociceptive effects in the skin and spinal cord of tibia fracture mice.

It has been proposed that complex regional pain syndrome (CRPS) is a post-traumatic autoimmune disease. Previously, we observed that B cells are required for the full expression of CRPS-like changes in a mouse tibia fracture model and that serum immunoglobulin M (IgM) antibodies from fracture mice have pronociceptive effects in muMT fracture mice lacking B cells. The current study evaluated the pronociceptive effects of injecting CRPS patient serum or antibodies into muMT fracture mice by measuring hind paw allodynia and unweighting changes.

Sex differences in the temporal development of pronociceptive immune responses in the tibia fracture mouse model.

Previously, distinct sex differences were observed in the pronociceptive role of spinal immune cells in neuropathic and inflammatory mouse pain models. Both peripheral and central innate and adaptive immune changes contribute to sensitization in the tibia fracture rodent model of complex regional pain syndrome, and the current study evaluated sex differences in the development of pronociceptive immune responses after fracture. At 4 and 7 weeks after fracture, the analgesic effects of a microglia inhibitor were tested in male and female mice, and polymerase chain reaction was used to measure inflammatory mediator expression in skin and spinal cord.

Morphine Exacerbates Postfracture Nociceptive Sensitization, Functional Impairment, and Microglial Activation in Mice.

Background: Emerging evidence suggests that opioid use immediately after surgery and trauma may worsen outcomes. In these studies, the authors aimed to determine whether morphine administered for a clinically relevant time period (7 days) in a tibia fracture orthopedic surgery model had adverse effects on postoperative recovery.

Methods: Mice were given morphine twice daily for 7 days after unilateral tibial fracture and intramedullary pin fixation to model orthopedic surgery and limb trauma.

Autoinflammatory and autoimmune contributions to complex regional pain syndrome.

Complex regional pain syndrome (CRPS) is a highly enigmatic syndrome typically developing after injury or surgery to a limb. Severe pain and disability are common among those with chronic forms of this condition. Accumulating evidence suggests that CRPS may involve both autoinflammatory and autoimmune components.

Exercise Reverses Nociceptive Sensitization, Upregulated Neuropeptide Signaling, Inflammatory Changes, Anxiety, and Memory Impairment in a Mouse Tibia Fracture Model.

What We Already Know About This Topic: WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: This study tested the hypothesis that ad lib running wheel exercise in a tibia fracture model of complex regional pain syndrome can reverse hindlimb nociceptive sensitization and inflammation in mice.

Methods: Three weeks after tibia fracture, the cast was removed and hindlimb von Frey thresholds and unweighting were tested; the mice were then randomized to either ad lib access to a running wheel for 4 weeks or no wheel access. After 4 weeks the behavioral testing was repeated and then skin, sciatic nerve, and spinal cord tissues collected for polymerase chain reaction and enzyme immunoassay measurements of neuropeptide and inflammatory mediator levels.

Oxidative Stress Contributes to Fracture/Cast-Induced Inflammation and Pain in a Rat Model of Complex Regional Pain Syndrome.

Unlabelled: Clinical evidence suggests that vitamin C (Vit C) may protect against the development of complex regional pain syndrome (CRPS) after fracture or surgery. Tibia fracture followed by 4 weeks of cast immobilization (fracture/cast) in rats results in nociceptive, vascular, and bone changes resembling clinical CRPS. In this study, fracture/cast rats were treated with the oxidative stress inhibitors Vit C, N-acetyl cysteine, or 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl to examine their effects on CRPS-related nociceptive and vascular changes.

The Rodent Tibia Fracture Model: A Critical Review and Comparison With the Complex Regional Pain Syndrome Literature.

Unlabelled: Distal limb fracture is the most common cause of complex regional pain syndrome (CRPS), thus the rodent tibia fracture model (TFM) was developed to study CRPS pathogenesis. This comprehensive review summarizes the published TFM research and compares these experimental results with the CRPS literature. The TFM generated spontaneous and evoked pain behaviors, inflammatory symptoms (edema, warmth), and trophic changes (skin thickening, osteoporosis) resembling symptoms in early CRPS.

Neuropeptide regulation of adaptive immunity in the tibia fracture model of complex regional pain syndrome.

Background: Both dysfunctional neuropeptide signaling and immune system activation are characteristic of complex regional pain syndrome (CRPS). Unknown is whether substance P (SP) or calcitonin gene-related peptide (CGRP) support autoantibody production and, consequently, nociceptive sensitization.

Methods: These experiments involved the use of a well-characterized tibia fracture model of CRPS.

Passive transfer autoimmunity in a mouse model of complex regional pain syndrome.

It has been proposed that complex regional pain syndrome (CRPS) is a posttraumatic autoimmune disease, and we previously observed that B cells are required for the full expression of CRPS-like changes in a mouse tibia fracture CRPS model. The current study used the mouse model to evaluate the progression of postfracture CRPS-like changes in wild-type (WT) and muMT fracture mice lacking B cells and antibodies. The pronociceptive effects of injecting WT fracture mouse serum antibodies into muMT fracture mice were also evaluated.

Identification of KRT16 as a target of an autoantibody response in complex regional pain syndrome.

Objective: Using a mouse model of complex regional pain syndrome (CRPS), our goal was to identify autoantigens in the skin of the affected limb.

Methods: A CRPS-like state was induced using the tibia fracture/cast immobilization model. Three weeks after fracture, hindpaw skin was homogenized, run on 2-d gels, and probed by sera from fracture and control mice.

Bisphosphonates Inhibit Pain, Bone Loss, and Inflammation in a Rat Tibia Fracture Model of Complex Regional Pain Syndrome.

Background: Bisphosphonates are used to prevent the bone loss and fractures associated with osteoporosis, bone metastases, multiple myeloma, and osteogenesis deformans. Distal limb fractures cause regional bone loss with cutaneous inflammation and pain in the injured limb that can develop into complex regional pain syndrome (CRPS). Clinical trials have reported that antiresorptive bisphosphonates can prevent fracture-induced bone loss, inhibit serum inflammatory cytokine levels, and alleviate CRPS pain.

Acute versus chronic phase mechanisms in a rat model of CRPS.

Background: Tibia fracture followed by cast immobilization in rats evokes nociceptive, vascular, epidermal, and bone changes resembling complex regional pain syndrome (CRPS). In most cases, CRPS has three stages. Over time, this acute picture, allodynia, warmth, and edema observed at 4 weeks, gives way to a cold, dystrophic but still painful limb.

Differential Efficacy of Ketamine in the Acute versus Chronic Stages of Complex Regional Pain Syndrome in Mice.

Background: Complex regional pain syndrome (CRPS) is a painful, disabling, and often chronic condition, where many patients transition from an acute phase with prominent peripheral neurogenic inflammation to a chronic phase with evident central nervous system changes. Ketamine is a centrally acting agent believed to work through blockade of N-methyl-D- aspartate receptors and is being increasingly used for the treatment of refractory CRPS, although the basis for the drug's effects and efficacy at different stages of the syndrome remains unclear.

Methods: The authors used a mouse model of CRPS (n = 8 to 12/group) involving tibia fracture/cast immobilization to test the efficacy of ketamine (2 mg kg day; 7 days) or vehicle infusion during acute (3 weeks after fracture) and chronic (7 weeks after fracture) stages.

Sex differences in a Murine Model of Complex Regional Pain Syndrome.

Complex Regional Pain Syndrome (CRPS) is a major cause of chronic pain after surgery or trauma to the limbs. Despite evidence showing that the prevalence and severity of many forms of chronic pain, including CRPS, differ between males and females, laboratory studies on sex-related differences in animal models of CRPS are not available, and the impact of sex on the transition from acute to chronic CRPS pain and disability are unexplored. Here we make use of a tibia fracture/cast mouse model that recapitulates the nociceptive, functional, vascular, trophic, inflammatory and immune aspects of CRPS.

Facilitated spinal neuropeptide signaling and upregulated inflammatory mediator expression contribute to postfracture nociceptive sensitization.

Tibia fracture induces exaggerated substance P (SP) and calcitonin gene-related peptide (CGRP) signaling and neuropeptide-dependent nociceptive and inflammatory changes in the hind limbs of rats similar to those seen in complex regional pain syndrome. Inflammatory changes in the spinal cord contribute to nociceptive sensitization in a variety of animal pain models. This study tested the hypothesis that fracture-induced exaggerated neuropeptide signaling upregulates spinal inflammatory mediator expression, leading to postfracture hind limb nociceptive sensitization.