Early increasing-intensity treadmill exercise reduces neuropathic pain by preventing nociceptor collateral sprouting and disruption of chloride cotransporters homeostasis after peripheral nerve injury

The Role of Ion-Transporting Proteins on Crosstalk Between the Skeletal Muscle and Central Nervous Systems Elicited by Physical Exercise

A paradigm shift in the understanding of bidirectional interactions between peripheral and central nervous systems is essential for development of rehabilitation and preventive interventions based on physical exercise. Although a causal relationship has not been completely established, modulation of voltage-dependent ion channels (Ca2+, Cl-, K+, Na+, lactate-, H+) in skeletal and neuronal cells provides opportunities to maintain force production during exercise and reduce the risk of disease. However, there are caveats to consider when interpreting the effects of physical exercise on this bidirectional axis, since exercise protocol details (e.g., duration and intensity) have variable effects on this crosstalk. Therefore, an integrative perspective of the skeletal muscle and brain's communication pathway is discussed, and the role of physical exercise on such communication highway is explained in this review.

Role of Exercise on Neuropathic Pain in Preclinical Models: Perspectives for Neuroglia

The benefits of exercise on neuropathic pain (NP) have been demonstrated in numerous studies. In recent studies, inflammation, neurotrophins, neurotransmitters, and endogenous opioids are considered as the main mechanisms. However, the role of exercise in alleviating NP remains unclear. Neuroglia, widely distributed in both the central and peripheral nervous systems, perform functions such as support, repair, immune response, and maintenance of normal neuronal activity. A large number of studies have shown that neuroglia play an important role in the occurrence and development of NP, and exercise can alleviate NP by regulating neuroglia. This article reviewed the involvement of neuroglia in the development of NP and their role in the exercise treatment of NP, intending to provide a theoretical basis for the exercise treatment strategy of NP.

Beneficial Effects of Exercise in Neuropathic Pain: An Overview of the Mechanisms Involved

Neuropathic pain is a prevalent issue that often arises following injuries to the peripheral or central nervous system. Unfortunately, there is currently no definitive and flawless treatment available to alleviate this type of pain. However, exercise has emerged as a promising nonpharmacological and adjunctive approach, demonstrating a significant impact in reducing pain intensity. This is why physical therapy is considered a beneficial approach for diminishing pain and promoting functional recovery following nerve injuries. Regular physical activity exerts its hypoalgesic effects through a diverse array of mechanisms. These include inhibiting oxidative stress, suppressing inflammation, and modulating neurotransmitter levels, among others. It is possible that multiple activated mechanisms may coexist within an individual. However, the priming mechanism does not need to be the same across all subjects. Each person's response to physical activity and pain modulation may vary depending on their unique physiological and genetic factors. In this review, we aimed to provide a concise overview of the mechanisms underlying the beneficial effects of regular exercise on neuropathic pain. We have discussed several key mechanisms that contribute to the improvement of neuropathic pain through exercise. However, it is important to note that this is not an exhaustive analysis, and there may be other mechanisms at play. Our goal was to provide a brief yet informative exploration of the topic.

Postoperative early initiation of sequential exercise program in preventing persistent spinal pain syndrome type-2 after modified transforaminal lumbar interbody fusion: a prospective randomized controlled trial

PURPOSE

This prospective randomized controlled trial aimed to investigate the impact of early postoperative sequential motor control (starting first day post-operatively) and core stabilization training (starting fifth week post-operatively) compared to conventional exercise (starting fifth weeks post-operatively) on the risk of developing persistent spinal pain syndrome type-2 (PSPS-T2).

METHODS

395 patients with lumbar degenerative diseases (LDDs) undergoing modified transforaminal lumbar interbody fusion and a 12-week postoperative exercise program (sequential vs. conventional exercise: 214 vs. 181) were evaluated for low back pain (LBP) intensity, Oswestry Disability Index (ODI), pressure pain threshold (PPT), temporal summation (TS), fatty infiltration of paraspinal muscles, transversus abdominis (TrA) activation capacity and Fear-Avoidance Beliefs Questionnaire (FABQ) pre-operatively, 3 months post-operatively, and 1 year post-operatively.

RESULTS

At 3-month postoperative assessment, LBP in sequential exercise group were lower than those in conventional exercise group (P < 0.05), and sequential-exercise patients had greater local-area PPTs, lower TS, lower TrA activation capacities and less fatty infiltration of erector spinae than did the conventional-exercise patients (P < 0.05). At 1-year postoperative assessment, fewer sequential-exercise patients had PSPS-T2 compared with conventional-exercise patients (11/214 vs. 20/181; P < 0.05). LBP at rest and FABQ were lower in sequential-exercise patients than conventional-exercise patients (P < 0.05). Furthermore, both PPT and TS at 1-year postoperative assessment were associated with these measurements at 3-month postoperative assessment in patients with PSPS-T2 (P < 0.05).

CONCLUSIONS

Postoperative sequential exercise has more positive effects to avoid PSPS-T2 than conventional exercise in patients with LDDs possibly because of its advantages in improving central and peripheral sensitization.

Role of Exercise on Inflammation Cytokines of Neuropathic Pain in Animal Models

Neuropathic pain (NP) resulting from a lesion or disease of the somatosensory system can lead to loss of function and reduced life quality. Neuroinflammation plays a vital role in the development and maintenance of NP. Exercise as an economical, effective, and nonpharmacological treatment, recommended by clinical practice guidelines, has been proven to alleviate chronic NP. Previous studies have shown that exercise decreases NP by modifying inflammation; however, the exact mechanisms of exercise-mediated NP are unclear. Therefore, from the perspective of neuroinflammation, this review mainly discussed the effects of exercise on inflammatory cytokines in different parts of NP conduction pathways, such as the brain, spinal cord, dorsal root ganglion, sciatic nerve, and blood in rat/mice models. Results suggested that exercise training could modulate neuroinflammation, inhibit astrocyte glial cell proliferation and microglial activation, alter the macrophage phenotype, reduce the expression of proinflammatory cytokines, increase anti-inflammatory cytokine levels, and positively modulate the state of the immune system, thereby relieving NP.

Combined use of CLP290 and bumetanide alleviates neuropathic pain and its mechanism after spinal cord injury in rats

AIM

We aimed to explore whether the combination of CLP290 and bumetanide maximally improves neuropathic pain following spinal cord injury (SCI) and its possible molecular mechanism.

METHODS

Rats were randomly divided into five groups: Sham, SCI + vehicle, SCI + CLP290, SCI + bumetanide, and SCI + combination (CLP290 + bumetanide). Drug administration commenced on the 7th day post-injury (7 dpi) and continued for 14 days. All rats underwent behavioral assessments for 56 days to comprehensively evaluate the effects of interventions on mechanical pain, thermal pain, cold pain, motor function, and other relevant parameters. Electrophysiological assessments, immunoblotting, and immunofluorescence detection were performed at different timepoints post-injury, with a specific focus on the expression and changes of KCC2 and NKCC1 proteins in the lumbar enlargement of the spinal cord.

RESULTS

CLP290 and bumetanide alleviated SCI-associated hypersensitivity and locomotor function, with the combination providing enhanced recovery. The combined treatment group exhibited the most significant improvement in restoring Rate-Dependent Depression (RDD) levels. In the combined treatment group and the two individual drug administration groups, the upregulation of potassium chloride cotransporter 2 (K+-Cl-cotransporter 2, KCC2) expression and downregulation of sodium potassium chloride cotransporter 1 (Na+-K+-Cl-cotransporter 1, NKCC1) expression in the lumbar enlargement area resulted in a significant increase in the KCC2/NKCC1 ratio compared to the SCI + vehicle group, with the most pronounced improvement seen in the combined treatment group. Compared to the SCI + vehicle group, the SCI + bumetanide group showed no significant paw withdrawal thermal latency (PWTL) improvement at 21 and 35 dpi, but a notable enhancement at 56 dpi. In contrast, the SCI + CLP290 group significantly improved PWTL at 21 days, with non-significant changes at 35 and 56 days. At 21 dpi, KCC2 expression was marginally higher in monotherapy groups versus SCI + vehicle, but not significantly. At 56 dpi, only the SCI + bumetanide group showed a significant difference in KCC2 expression compared to the control group.

CONCLUSION

Combined application of CLP290 and bumetanide effectively increases the ratio of KCC2/NKCC1, restores RDD levels, enhances GABAA receptor-mediated inhibitory function in the spinal cord, and relieves neuropathic pain in SCI; Bumetanide significantly improves neuropathic pain in the long term, whereas CLP290 demonstrates a notable short-term effect.

Modulation of Brain-derived Neurotrophic Factor Expression by Physical Exercise in Reserpine-induced Pain-depression Dyad in Mice

Pain accompanied by depressive symptoms is a common reason for seeking medical assistance, and many chronic pain patients experience comorbid depression. The brain-derived neurotrophic factor (BDNF) is a well-known neurotrophin expressed throughout the nervous system, playing a crucial role in neuronal growth and neuroplasticity. This study aimed to examine the effects of exercise on BDNF expression in the nervous system and reserpine (RSP)-induced pain-depression dyad. RSP (1 mg/kg) was subcutaneously administered once daily for three days in mice. The exercise was performed using a rota-rod tester for seven consecutive days following RSP administration. Pain responses were evaluated using von Frey filaments, and depression-like behaviors were assessed through forced swimming and open field tests. Immunofluorescence staining was performed to examine the changes in BDNF expression in the dorsal root ganglion (DRG), spinal cord, and hippocampus. Administration of RSP reduced mechanical paw withdrawal threshold, increased immobility time in the forced swimming test, and decreased movement in the open field test. The immunoreactivity of BDNF was increased in the DRG and spinal dorsal regions, and decreased in the hippocampus after RSP administration. Physical exercise significantly reduced the RSP-induced mechanical hypersensitivity and depression-like behaviors. In addition, exercise suppressed not only the increased expression of BDNF in the DRG and spinal dorsal regions but also the decreased expression of BDNF in the hippocampus induced by RSP administration. These findings suggest that repetitive exercise could serve as an effective and non-invasive treatment option for individuals experiencing both pain and depression by modulating BDNF expression.

Physical Exercise-Induced Activation of NRF2 and BDNF as a Promising Strategy for Ferroptosis Regulation in Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra. Ferroptosis, an iron-dependent form of regulated cell death, may contribute to the progression of PD owing to an unbalanced brain redox status. Physical exercise is a complementary therapy that can modulate ferroptosis in PD by regulating the redox system through the activation of nuclear factor (erythroid-derived 2)-like 2 (NRF2) and brain-derived neurotrophic factor (BDNF) signaling. However, the precise effects of physical exercise on ferroptosis in PD remain unclear. In this review, we explored how physical exercise influences NRF2 and BDNF signaling and affects ferroptosis in PD. We further investigated relevant publications over the past two decades by searching the PubMed, Web of Science, and Google Scholar databases using keywords related to physical exercise, PD, ferroptosis, and neurotrophic factor antioxidant signaling. This review provides insights into current research gaps and demonstrates the necessity for future research to elucidate the specific mechanisms by which exercise regulates ferroptosis in PD, including the assessment of different exercise protocols and their long-term effects. Ultimately, exploring these aspects may lead to the development of improved exercise interventions for the better management of patients with PD.

Treadmill running on neuropathic pain: via modulation of neuroinflammation

Neuropathic pain is a type of chronic pain caused by an injury or somatosensory nervous system disease. Drugs and exercise could effectively relieve neuropathic pain, but no treatment can completely stop neuropathic pain. The integration of exercise into neuropathic pain management has attracted considerable interest in recent years, and treadmill training is the most used among exercise therapies. Neuropathic pain can be effectively treated if its mechanism is clarified. In recent years, the association between neuroinflammation and neuropathic pain has been explored. Neuroinflammation can trigger proinflammatory cytokines, activate microglia, inhibit descending pain modulatory systems, and promote the overexpression of brain-derived neurotrophic factor, which lead to the generation of neuropathic pain and hypersensitivity. Treadmill exercise can alleviate neuropathic pain mainly by regulating neuroinflammation, including inhibiting the activity of pro-inflammatory factors and over activation of microglia in the dorsal horn, regulating the expression of mu opioid receptor expression in the rostral ventromedial medulla and levels of γ-aminobutyric acid to activate the descending pain modulatory system and the overexpression of brain-derived neurotrophic factor. This article reviews and summarizes research on the effect of treadmill exercise on neuropathic pain and its role in the regulation of neuroinflammation to explore its benefits for neuropathic pain treatment.

Early Intensive Neurorehabilitation in Traumatic Peripheral Nerve Injury-State of the Art

Traumatic nerve injuries are common lesions that affect several hundred thousand humans, as well as dogs and cats. The assessment of nerve regeneration through animal models may provide information for translational research and future therapeutic options that can be applied mutually in veterinary and human medicine, from a One Health perspective. This review offers a hands-on vision of the non-invasive and conservative approaches to peripheral nerve injury, focusing on the role of neurorehabilitation in nerve repair and regeneration. The peripheral nerve injury may lead to hypersensitivity, allodynia and hyperalgesia, with the possibility of joint contractures, decreasing functionality and impairing the quality of life. The question remains regarding how to improve nerve repair with surgical possibilities, but also considering electrical stimulation modalities by modulating sensory feedback, upregulation of BDNF, GFNF, TrKB and adenosine monophosphate, maintaining muscle mass and modulating fatigue. This could be improved by the positive synergetic effect of exercises and physical activity with locomotor training, and other physical modalities (low-level laser therapy, ultrasounds, pulsed electromagnetic fields, electroacupuncture and others). In addition, the use of cell-based therapies is an innovative treatment tool in this field. These strategies may help avoid situations of permanent monoplegic limbs that could lead to amputation.

Potential mechanisms of exercise for relieving inflammatory pain: a literature review of animal studies

Inflammatory pain (IP) is one of the most prevalent and intractable human conditions, and it leads to progressive dysfunction and reduced quality of life. Additionally, IP is incredibly challenging to treat successfully with drugs or surgery. The development of IP is complex and multifactorial, and peripheral and central sensitization may influence chronicity and treatment resistance in IP. Understanding the mechanisms underlying IP is vital for developing novel therapies. Strong evidence suggests that exercise can be a first-line relief for patients with IP during rehabilitation. However, the mechanisms through which exercise improves IP remain unclear. Here, we reviewed the current animal experimental evidence for an exercise intervention in IP and proposed biological mechanisms for the effects of synaptic plasticity in the anterior cingulate cortex, endocannabinoids, spinal dorsal horn excitability balance, immune cell polarization balance, cytokines, and glial cells. This information will contribute to basic science and strengthen the scientific basis for exercise therapy prescriptions for IP in clinical practice.

Transcranial direct current stimulation is more effective than pregabalin in controlling nociceptive and anxiety-like behaviors in a rat fibromyalgia-like model

OBJECTIVES

Despite the fact that fibromyalgia, a widespread disease of the musculoskeletal system, has no specific treatment, patients have shown improvement after pharmacological intervention. Pregabalin has demonstrated efficacy; however, its adverse effects may reduce treatment adherence. In this context, neuromodulatory techniques such as transcranial direct current stimulation (tDCS) may be employed as a complementary pain-relieving method. Consequently, the purpose of this study was to evaluate the effect of pregabalin and tDCS treatments on the behavioral and biomarker parameters of rats submitted to a fibromyalgia-like model.

METHODS

Forty adult male Wistar rats were divided into two groups: control and reserpine. Five days after the end of the administration of reserpine (1 mg/kg/3 days) to induce a fibromyalgia-like model, rats were randomly assigned to receive either vehicle or pregabalin (30 mg/kg) along with sham or active- tDCS treatments. The evaluated behavioral parameters included mechanical allodynia by von Frey test and anxiety-like behaviors by elevated plus-maze test (time spent in opened and closed arms, number of entries in opened and closed arms, protected head-dipping, unprotected head-dipping [NPHD], grooming, rearing, fecal boluses). The biomarker analysis (brain-derived neurotrophic factor [BDNF] and tumor necrosis factor-α [TNF-α]) was performed in brainstem and cerebral cortex and in serum.

RESULTS

tDCS reversed the reduction in the mechanical nociceptive threshold and the decrease in the serum BDNF levels induced by the model of fibromyalgia; however, there was no effect of pregabalin in the mechanical threshold. There were no effects of pregabalin or tDCS found in TNF-α levels. The pain model induced an increase in grooming time and a decrease in NPHD and rearing; while tDCS reversed the increase in grooming, pregabalin reversed the decrease in NPHD.

CONCLUSIONS

tDCS was more effective than pregabalin in controlling nociception and anxiety-like behavior in a rat model-like fibromyalgia. Considering the translational aspect, our findings suggest that tDCS could be a potential non-pharmacological treatment for fibromyalgia.

Aerobic Exercise and Neuropathic Pain: Insights from Animal Models and Implications for Human Therapy

This narrative review explores the complex relationship between aerobic exercise (AE) and neuropathic pain (NP), particularly focusing on peripheral neuropathies of mechanical origin. Pain, a multifaceted phenomenon, significantly impacts functionality and distress. The International Association for the Study of Pain's definition highlights pain's biopsychosocial nature, emphasizing the importance of patient articulation. Neuropathic pain, arising from various underlying processes, presents unique challenges in diagnosis and treatment. Our methodology involved a comprehensive literature search in the PubMed and SCOPUS databases, focusing on studies relating AE to NP, specifically in peripheral neuropathies caused by mechanical forces. The search yielded 28 articles and 1 book, primarily animal model studies, providing insights into the efficacy of AE in NP management. Results from animal models demonstrate that AE, particularly in forms like no-incline treadmill and swimming, effectively reduces mechanical allodynia and thermal hypersensitivity associated with NP. AE influences neurophysiological mechanisms underlying NP, modulating neurotrophins, cytokines, and glial cell activity. These findings suggest AE's potential in attenuating neurophysiological alterations in NP. However, human model studies are scarce, limiting the direct extrapolation of these findings to human neuropathic conditions. The few available studies indicate AE's potential benefits in peripheral NP, but a lack of specificity in these studies necessitates further research. In conclusion, while animal models show promising results regarding AE's role in mitigating NP symptoms and influencing underlying neurophysiological mechanisms, more human-centric research is required. This review underscores the need for targeted clinical trials to fully understand and harness AE's therapeutic potential in human neuropathic pain, especially of mechanical origin.

The effects of aerobic exercise on neuroimmune responses in animals with traumatic peripheral nerve injury: a systematic review with meta-analyses

BACKGROUND

Increasing pre-clinical evidence suggests that aerobic exercise positively modulates neuroimmune responses following traumatic nerve injury. However, meta-analyses on neuroimmune outcomes are currently still lacking. This study aimed to synthesize the pre-clinical literature on the effects of aerobic exercise on neuroimmune responses following peripheral nerve injury.

METHODS

MEDLINE (via Pubmed), EMBASE and Web of Science were searched. Controlled experimental studies on the effect of aerobic exercise on neuroimmune responses in animals with a traumatically induced peripheral neuropathy were considered. Study selection, risk of bias assessment and data extraction were performed independently by two reviewers. Results were analyzed using random effects models and reported as standardized mean differences. Outcome measures were reported per anatomical location and per class of neuro-immune substance.

RESULTS

The literature search resulted in 14,590 records. Forty studies were included, reporting 139 comparisons of neuroimmune responses at various anatomical locations. All studies had an unclear risk of bias. Compared to non-exercised animals, meta-analyses showed the following main differences in exercised animals: (1) in the affected nerve, tumor necrosis factor-α (TNF-α) levels were lower (p = 0.003), while insulin-like growth factor-1 (IGF-1) (p < 0.001) and Growth Associated Protein 43 (GAP43) (p = 0.01) levels were higher; (2) At the dorsal root ganglia, brain-derived neurotrophic factor (BDNF)/BDNF mRNA levels (p = 0.004) and nerve growth factor (NGF)/NGF mRNA (p < 0.05) levels were lower; (3) in the spinal cord, BDNF levels (p = 0.006) were lower; at the dorsal horn, microglia (p < 0.001) and astrocyte (p = 0.005) marker levels were lower; at the ventral horn, astrocyte marker levels (p < 0.001) were higher, and several outcomes related to synaptic stripping were favorably altered; (4) brainstem 5-HT2A receptor levels were higher (p = 0.001); (5) in muscles, BDNF levels (p < 0.001) were higher and TNF-α levels lower (p < 0.05); (6) no significant differences were found for systemic neuroimmune responses in blood or serum.

CONCLUSION

This review revealed widespread positive modulatory effects of aerobic exercise on neuroimmune responses following traumatic peripheral nerve injury. These changes are in line with a beneficial influence on pro-inflammatory processes and increased anti-inflammatory responses. Given the small sample sizes and the unclear risk of bias of the studies, results should be interpreted with caution.

Does irisin has neuroprotective effect against diabetes induced neuropathy in male rats?

We aimed to investigate the contribution of irisin in the neuroprotective process of exercise training in diabetic rats. Serum irisin levels, thermal and mechanical pain thresholds and intracellular calcium ([Ca²⁺]_i) levels in sensory neurons were measured at different time intervals during the eight weeks of exercise sessions for the control, non-exercise diabetics (3 groups) and exercise performing (low and high intensity groups) diabetic rats (n = 7-10 for all groups). Non-exercise diabetic groups were treated with irisin in different doses (1, 10 and 20 µg/kg respectively). Recovered pain thresholds at the end of the exercise sessions (p < .05), higher serum irisin levels that compared to control and diabetics (p < .05) and insignificant mean [Ca2⁺]_i peak amplitudes in sensory neurons (p > .05) obtained from experiments. Furthermore, irisin injection decreased the thermal pain threshold of diabetics only at 60th minutes (p < .05). Irisin may have a role in the neuroprotective effect of exercise training.

Proteomic Analysis of the Spinal Dorsal Horn in Mice with Neuropathic Pain After Exercise

Purpose

Neuropathic pain (NP) is a chronic pain state with a complex etiology that currently lacks effective treatment in clinical practice. Studies have found that exercise training can alleviate NP hyperalgesia, but the specific mechanism remains unclear. Here, we sought to identify proteins and signaling pathways critical for mediating the effects of treadmill training on NP in a mouse model of spared nerve injury (SNI).

Methods

We used Tandem Mass Tag (TMT) technology for proteins and signaling pathways identification. Functional enrichment analyses were conducted using DAVID and Metascape software. Ingenuity pathway analysis was used to conduct functional annotation and analyze alterations in canonical pathways and molecular networks. Reverse transcription quantitative PCR (RT-qPCR) was used to confirm the results of proteomics analysis.

Results

A total of 270 differentially expressed proteins were screened in the detrained and trained groups (P ≤0.05). Enrichment and ingenuity pathway analysis revealed the effects of treadmill training on autophagy, cAMP-mediated signaling, calcium signaling and NP signaling in dorsal horn nerves. Treadmill training reduced the expression of Akt3, Atf2, Gsk3b, Pik3c3, Ppp2ca, and Sqstm1, and increased the expression of Pik3cb in the autophagic pathway.

Conclusion

Our results suggest that treadmill training may alleviate nociceptive hyperalgesia in NP mice by modulating the autophagic pathway, providing unique mechanistic insights into the analgesic effects of exercise.

The Role of Physical Exercise and Rehabilitative Implications in the Process of Nerve Repair in Peripheral Neuropathies: A Systematic Review

BACKGROUND

The various mechanisms involved in peripheral nerve regeneration, induced by exercise and electrical nerve stimulation, are still unclear.

OBJECTIVE

The aim of this review was to summarize the influence of physical exercise and/or electrical stimulation on peripheral nerve repair and regeneration and the variation of impact of intervention depending on timing, as well as kind and dosage of the intervention. A literature survey was conducted on PubMed, Scopus, and Web of Science, between February 2021 to July 2021, with an update in September 2022.

METHODOLOGY

The literature search identified 101,386 articles with the keywords: "peripheral nerve" OR "neuropathy" AND "sprouting" OR "neuroapraxia" OR "axonotmesis" OR "neurotmesis" OR "muscle denervation" OR "denervated muscle" AND "rehabilitation" OR "physical activity" OR "physical exercise" OR "activity" OR "electrical stimulation". A total of 60 publications were included. Eligible studies were focused on evaluating the process of nerve repair (biopsy, electromyographic parameters or biomarker outcomes) after electrical stimulation or physical exercise interventions on humans or animals with peripheral sensory or motor nerve injury.

SYNTHESIS

This study shows that the literature, especially regarding preclinical research, is mainly in agreement that an early physical program with active exercise and/or electrical stimulation promotes axonal regenerative responses and prevents maladaptive response. This was evaluated by means of changes in electrophysiological recordings of CMAPs for latency amplitude, and the sciatic functional index (SFI). Furthermore, this type of activity can cause an increase in weight and in muscle fiber diameter. Nevertheless, some detrimental effects of exercising and electrical stimulation too early after nerve repair were recorded.

CONCLUSION

In most preclinical studies, peripheral neuropathy function was associated with improvements after physical exercise and electrical stimulation. For humans, too little research has been conducted on this topic to reach a complete conclusion. This research supports the need for future studies to test the validity of a possible rehabilitation treatment in humans in cases of peripheral neuropathy to help nerve sprouting.

Influence of routine exercise on the peripheral immune system to prevent and alleviate pain

Routine physical activity reduces the onset of pain and exercise is a first line treatment for individuals who develop chronic pain. In both preclinical and clinical research regular exercise (routine exercise sessions) produces pain relief through multiple mechanisms such as alterations in the central and peripheral nervous system. More recently, it has been appreciated that exercise can also alter the peripheral immune system to prevent or reduce pain. In animal models, exercise can alter the immune system at the site of injury or pain model induction, in the dorsal root ganglia, and systemically throughout the body to produce analgesia. Most notably exercise shows the ability to dampen the presence of pro-inflammatory immune cells and cytokines at these locations. Exercise decreases M1 macrophages and the cytokines IL-6, IL-1β, and TFNα, while increasing M2 macrophages and the cytokines IL-10, IL-4, and IL-1ra. In clinical research, a single bout of exercise produces an acute inflammatory response, however repeated training can lead to an anti-inflammatory immune profile leading to symptom relief. Despite the clinical and immune benefits of routine exercise, the direct effect of exercise on immune function in clinical pain populations remains unexplored. This review will discuss in more detail the preclinical and clinical research which demonstrates the numerous ways through which multiple types of exercise alter the peripheral immune system. This review closes with the clinical implications of these findings along with suggestions for future research directions.

The role of KCC2 and NKCC1 in spinal cord injury: From physiology to pathology

The balance of ion concentrations inside and outside the cell is an essential homeostatic mechanism in neurons and serves as the basis for a variety of physiological activities. In the central nervous system, NKCC1 and KCC2, members of the SLC12 cation-chloride co-transporter (CCC) family, participate in physiological and pathophysiological processes by regulating intracellular and extracellular chloride ion concentrations, which can further regulate the GABAergic system. Over recent years, studies have shown that NKCC1 and KCC2 are essential for the maintenance of Cl^- homeostasis in neural cells. NKCC1 transports Cl^- into cells while KCC2 transports Cl^- out of cells, thereby regulating chloride balance and neuronal excitability. An imbalance of NKCC1 and KCC2 after spinal cord injury will disrupt CI^- homeostasis, resulting in the transformation of GABA neurons from an inhibitory state into an excitatory state, which subsequently alters the spinal cord neural network and leads to conditions such as spasticity and neuropathic pain, among others. Meanwhile, studies have shown that KCC2 is also an essential target for motor function reconstruction after spinal cord injury. This review mainly introduces the physiological structure and function of NKCC1 and KCC2 and discusses their pathophysiological roles after spinal cord injury.

Effect of Physiotherapeutic Interventions on Biomarkers of Neuropathic Pain: A Systematic Review of Preclinical Literature

The purpose of this systematic review was to evaluate the effects of physiotherapeutic interventions on biomarkers of neuropathic pain in preclinical models of peripheral neuropathic pain (PNP). The search was performed in Pubmed, Web of Science, EMBASE, Cochrane, Cinhal, Psycinfo, Scopus, Medline, and Science Direct. Studies evaluating any type of physiotherapy intervention for PNP (systemic or traumatic) were included. Eighty-one articles were included in this review. The most common PNP model was chronic constriction injury, and the most frequently studied biomarkers were related to neuro-immune processes. Exercise therapy and Electro-acupuncture were the 2 most frequently studied physiotherapy interventions while acupuncture and joint mobilization were less frequently examined. Most physiotherapeutic interventions modulated the expression of biomarkers related to neuropathic pain. Whereas the results seem promising; they have to be considered with caution due to the high risk of bias of included studies and high heterogeneity of the type and anatomical localization of biomarkers reported. The review protocol is registered on PROSPERO (CRD42019142878). PERSPECTIVE: This article presents the current evidence about physiotherapeutic interventions on biomarkers of neuropathic pain in preclinical models of peripheral neuropathic pain. Existing findings are reviewed, and relevant data are provided on the effectiveness of each physiotherapeutic modality, as well as its certainty of evidence and clinical applicability.

The timing point of exercise intervention regulates neuropathic pain-related molecules in the ipsilateral dorsal root ganglion neurons after sciatic nerve injury

The purpose of this study was to determine whether the timing of tread-mill exercise application can control expression levels of neuropathic pain- and regeneration-related proteins in the ipsilateral lumbar 4 (L4) to 6 (L6) dorsal root ganglion cells (DRG) after sciatic nerve injury (SNI). The experimental rats were randomly divided into five groups: the normal control, SNI+sedentary (IS), exercise+SNI (EI), SNI+exercise (IE), exercise+SNI+exercise (EIE) groups. The rats in exercise groups per-formed treadmill exercise at a speed of 8 m/min for 30 min once a day during 14 days before and/or after SNI. For investigating the expression of specific neuropathic pain and regeneration-related proteins in DRG, we prepared L4 to L6 DRG in the ipsilateral side. In the quantitative analysis, growth associated protein 43 (GAP-43) and brain-derived neurotrophic factor levels were further increased in the ipsilateral DRG at all treadmill exercise groups than those in IS group. In the histological findings, GAP-43 was qualitatively increased IE and EIE groups than IS group at DRG. Wnt3a and β-catenin were dramatically downregulated in EIE and IE groups than IS groups. In addition, nuclear factor kappa-light-chain-enhancer of activated B cells and tumor necrosis factor-α were significantly decreased in IE and EIE groups than IS group in the ipsilateral DRG. Our findings suggested novel information that regular low-intensity exercise before and/or after SNI might be a therapeutic and preventive approaches for relieving neuropathic pain and improving axonal elongation after peripheral nerve injury.

Early Intervention of Cold-Water Swimming on Functional Recovery and Spinal Pain Modulation Following Brachial Plexus Avulsion in Rats

Brachial plexus avulsion (BPA) causes peripheral nerve injury complications with motor and sensory dysfunction of the upper limb. Growing evidence has shown an active role played by cold-water swimming (CWS) in alleviating peripheral neuropathic pain and functional recovery. This study examined whether CWS could promote functional recovery and pain modulation through the reduction of neuroinflammation and microglial overactivation in dorsal horn neurons at the early-stage of BPA. After BPA surgery was performed on rats, they were assigned to CWS or sham training for 5 min twice a day for two weeks. Functional behavioral responses were tested before and after BPA surgery, and each week during training. Results after the two-week training program showed significant improvements in BPA-induced motor and sensory loss (p < 0.05), lower inflammatory cell infiltration, and vacuole formation in injured nerves among the BPA-CWS group. Moreover, BPA significantly increased the expression of SP and IBA1 in dorsal horn neurons (p < 0.05), whereas CWS prevented their overexpression in the BPA-CWS group. The present findings evidenced beneficial rehabilitative effects of CWS on functional recovery and pain modulation at early-stage BPA. The beneficial effects are partially related to inflammatory suppression and spinal modulation. The synergistic role of CWS combined with other management approaches merits further investigation.

Functional Neurorehabilitation in Dogs with an Incomplete Recovery 3 Months following Intervertebral Disc Surgery: A Case Series

Simple Summary

A non-invasive neurorehabilitation multimodal protocol (NRMP) may be applicable to chronic T3-L3 dogs 3 months after undergoing surgery for acute Intervertebral Disc Disease (IVDD) Hansen type I; this protocol has been shown to be safe, feasible, and potentially effective at improving ambulation in both open field score (OFS) 0 and OFS 1 dogs. The specific sample population criteria limit the number of dogs included, mainly due to owners withdrawing over time. Thus, the present case series study aimed to demonstrate that an NRMP could contribute to a functional treatment possibly based on synaptic and anatomic reorganization of the spinal cord.

Abstract

This case series study aimed to evaluate the safety, feasibility, and positive outcome of the neurorehabilitation multimodal protocol (NRMP) in 16 chronic post-surgical IVDD Hansen type I dogs, with OFS 0/DPP− (n = 9) and OFS 1/DPP+ (n = 7). All were enrolled in the NRMP for a maximum of 90 days and were clinically discharged after achieving ambulation. The NRMP was based on locomotor training, functional electrical stimulation, transcutaneous electrical spinal cord stimulation, and 4-aminopyridine (4-AP) pharmacological management. In the Deep Pain Perception (DPP)+ dogs, 100% recovered ambulation within a mean period of 47 days, reaching OFS ≥11, which suggests that a longer period of time is needed for recovery. At follow-up, all dogs presented a positive evolution with voluntary micturition. Of the DPP− dogs admitted, all achieved a flexion/extension locomotor pattern within 30 days, and after starting the 4-AP, two dogs were discharged at outcome day 45, with 78% obtaining Spinal Reflex Locomotion (SRL) and automatic micturition within a mean period of 62 days. At follow-up, all dogs maintained their neurological status. After the NRMP, ambulatory status was achieved in 88% (14/16) of dogs, without concurrent events. Thus, an NRMP may be an important therapeutic option to reduce the need for euthanasia in the clinical setting.

Effect of treadmill exercise on pain-related Wnt/β-catenin signaling pathway in dorsal root ganglion neurons at the early phase regeneration of the injured sciatic nerve

The purpose of this study was to determine whether treadmill walking exercise can improve mechanical allodynia through regulation of Wnt/β-catenin signaling in dorsal root ganglion (DRG) neurons at the early stage of regeneration after sciatic nerve injury (SNI). The experimental rats were divided into seven groups: the normal control, sedentary groups for 3-, 7-, and 14-day post crush (dpc), and exercise group for 3, 7, and 14 dpc. The rats in exercise groups performed treadmill walking exercise at a speed of 8 m/min for 20 min once a day according to experiment duration. For evaluating neuropathic pain-like behavior after SNI, the mechanical allodynia was examined by von Frey apparatus. And the expression levels of pain-related protein were identified in the cytoplasm or nucleus of DRG neurons using Western blot techniques. Mechanical allodynia was significantly ameliorated in the exercise group at 7 and 14 dpc. Treadmill exercise further decreased Wnt3a expression at 3, 7, and 14 dpc compared to in the sedentary group. Also, phosphorylated-low-density lipoprotein receptor 6 was decreased in exercise groups at 3 and 14 dpc. Beta-catenin was significantly decreased in exercise groups at 3 and 14 dpc compared to sedentary groups as well as treadmill exercise decreased translocation of β-catenin towards the nucleus of DRG neurons at 14 dpc. Our findings indicate that treadmill walking exercise may be an important regulator of neuropathic pain after peripheral nerve injury through delayed Wnt/β-catenin signaling pathway in DRG neurons.

Locomotor deficits induced by lumbar muscle inflammation involve spinal microglia and are independent of KCC2 expression in a mouse model of complete spinal transection

Spinal cord injury (SCI) is associated with damage to musculoskeletal tissues of the spine. Recent findings show that pain and inflammatory processes caused by musculoskeletal injury mediate plastic changes in the spinal cord. These changes could impede the adaptive plastic changes responsible for functional recovery. The underlying mechanism remains unclear, but may involve the microglia-BDNF-KCC2 pathway, which is implicated in sensitization of dorsal horn neurons in neuropathic pain and in the regulation of spinal excitability by step-training. In the present study, we examined the effects of step-training and lumbar muscle inflammation induced by complete Freund's adjuvant (CFA) on treadmill locomotion in a mouse model of complete spinal transection. The impact on locomotor recovery of each of these interventions alone or in combination were examined in addition to changes in microglia and KCC2 expression in the dorsal and ventral horns of the sublesional spinal cord. Results show that angular motion at the hip, knee and ankle joint during locomotion were decreased by CFA injection and improved by step-training. Moreover, CFA injection enhanced the expression of the microglial marker Iba1 in both ventral and dorsal horns, with or without step-training. However, this change was not associated with a modulation of KCC2 expression, suggesting that locomotor deficits induced by inflammation are independent of KCC2 expression in the sublesional spinal cord. These results indicate that musculoskeletal injury hinders locomotor recovery after SCI and that microglia is involved in this effect.

Exercise and Neuropathic Pain: A General Overview of Preclinical and Clinical Research

Neuropathic pain is a disease of the somatosensory system that is characterized by tingling, burning, and/or shooting pain. Medication is often the primary treatment, but it can be costly, thus there is an interest in understanding alternative low-cost treatments such as exercise. The following review includes an overview of the preclinical and clinical literature examining the influence of exercise on neuropathic pain. Preclinical studies support the hypothesis that exercise reduces hyperalgesia and allodynia in animal models of neuropathic pain. In human research, observational studies suggest that those who are more physically active have lower risk of developing neuropathic pain compared to those who are less active. Exercise studies suggest aerobic exercise training (e.g., 16 weeks); a combination of aerobic and resistance exercise training (e.g., 10–12 weeks); or high-intensity interval training (e.g., 15 weeks) reduces aspects of neuropathic pain such as worst pain over the past month, pain over the past 24 h, pain scores, or pain interference. However, not all measures of pain improve following exercise training (e.g., current pain, heat pain threshold). Potential mechanisms and future directions are also discussed to aid in the goal of understanding the role of exercise in the management of neuropathic pain. Future research using standardized methods to further understanding of the dose of exercise needed to manage neuropathic pain is warranted.

tDCS and exercise improve anxiety-like behavior and locomotion in chronic pain rats via modulation of neurotrophins and inflammatory mediators

Anxiety disorders cause distress and are commonly found to be comorbid with chronic pain. Both are difficult-to-treat conditions for which alternative treatment options are being pursued. This study aimed to evaluate the effects of transcranial direct current stimulation (tDCS), treadmill exercise, or both, on anxiety-like behavior and associated growth factors and inflammatory markers in the hippocampus and sciatic nerve of rats with neuropathic pain. Male Wistar rats (n = 216) were subjected to sham-surgery or sciatic nerve constriction for pain induction. Fourteen days following neuropathic pain establishment, either bimodal tDCS, treadmill exercise, or a combination of both was used for 20 min a day for 8 consecutive days. The elevated plus-maze test was used to assess anxiety-like behavior and locomotor activity during the early (24 h) or late (7 days) phase after the end of treatment. BDNF, TNF-ɑ, and IL-10 levels in the hippocampus, and BDNF, NGF, and IL-10 levels in the sciatic nerve were assessed 48 h or 7 days after the end of treatment. Rats from the pain groups developed an anxiety-like state. Both tDCS and treadmill exercise provided ethological and neurochemical alterations induced by pain in the early and/or late phase, and a modest synergic effect between tDCS and exercise was observed. These results indicate that non-invasive neuromodulatory approaches can attenuate both anxiety-like status and locomotor activity and alter the biochemical profile in the hippocampus and sciatic nerve of rats with neuropathic pain and that combined interventions may be considered as a treatment option.

Mechanism of exercise-induced analgesia: what we can learn from physically active animals

Physical activity has become a first-line treatment in rehabilitation settings for individuals with chronic pain. However, research has only recently begun to elucidate the mechanisms of exercise-induced analgesia. Through the study of animal models, exercise has been shown to induce changes in the brain, spinal cord, immune system, and at the site of injury to prevent and reduce pain. Animal models have also explored beneficial effects of exercise through different modes of exercise including running, swimming, and resistance training. This review will discuss the central and peripheral mechanisms of exercise-induced analgesia through different modes, intensity, and duration of exercise as well as clinical applications of exercise with suggestions for future research directions.

ASICs are required for immediate exercise-induced muscle pain and are downregulated in sensory neurons by exercise training

Exercise training is an effective therapy for many pain-related conditions, and trained athletes have lower pain perception compared with unconditioned people. Some painful conditions, including strenuous exercise, are associated with elevated levels of protons, metabolites, and inflammatory factors, which may activate receptors and/or ion channels, including acid-sensing ion channels (ASICs), on nociceptive sensory neurons. We hypothesized that ASICs are required for immediate exercise-induced muscle pain (IEIP) and that exercise training diminishes IEIP by modulating ASICs within muscle afferents. We found high-intensity interval training (HIIT) reduced IEIP in C57BL/6 mice and diminished ASIC mRNA levels in lumber dorsal root ganglia, and this downregulation of ASICs correlated with improved exercise capacity. Additionally, we found that ASIC3 -/- mice did not develop IEIP; however, the exercise capacity of ASIC3 -/- was similar to wild-type mice. These results suggest that ASICs are required for IEIP and that diminishment of IEIP after exercise training correlates with downregulation of ASICs in sensory neurons.NEW & NOTEWORTHY Exercise performance can be limited by the sensations of muscle fatigue and pain transmitted by muscle afferents. It has been proposed that exercise training abrogates these negative feedback signals. We found that acid-sensing ion channels (ASICs) are required for immediate exercise-induced muscle pain (IEIP). Moreover, exercise training prevented IEIP and was correlated with downregulation of ASICs in sensory neurons.

The etiological contribution of GABAergic plasticity to the pathogenesis of neuropathic pain

Neuropathic pain developing after peripheral or central nerve injury is the result of pathological changes generated through complex mechanisms. Disruption in the homeostasis of excitatory and inhibitory neurons within the central nervous system is a crucial factor in the formation of hyperalgesia or allodynia occurring with neuropathic pain. The central GABAergic pathway has received attention for its extensive distribution and function in neural circuits, including the generation and development of neuropathic pain. GABAergic inhibitory changes that occur in the interneurons along descending modulatory and nociceptive pathways in the central nervous system are believed to generate neuronal plasticity, such as synaptic plasticity or functional plasticity of the related genes or proteins, that is the foundation of persistent neuropathic pain. The primary GABAergic plasticity observed in neuropathic pain includes GABAergic synapse homo- and heterosynaptic plasticity, decreased synthesis of GABA, down-expression of glutamic acid decarboxylase and GABA transporter, abnormal expression of NKCC1 or KCC2, and disturbed function of GABA receptors. In this review, we describe possible mechanisms associated with GABAergic plasticity, such as central sensitization and GABAergic interneuron apoptosis, and the epigenetic etiologies of GABAergic plasticity in neuropathic pain. Moreover, we summarize potential therapeutic targets of GABAergic plasticity that may allow for successful relief of hyperalgesia from nerve injury. Finally, we compare the effects of the GABAergic system in neuropathic pain to other types of chronic pain to understand the contribution of GABAergic plasticity to neuropathic pain.

Impact of Therapeutic Interventions on Pain Intensity and Endogenous Pain Modulation in Knee Osteoarthritis: A Systematic Review and Meta-analysis

OBJECTIVE

To study the impact of therapeutic interventions on pain analgesia and endogenous pain modulation in knee osteoarthritis (KOA).

DESIGN

Systematic review and meta-analysis.

METHODS

We searched for KOA randomized clinical trials and observational studies with data on therapeutic interventions comparing pain intensity, temporal summation (TS), and conditioned pain modulation (CPM) scores relative to control. These data were pooled as Hedge's g. To study the relationship between pain intensity and TS/CPM, we performed metaregression with 10,000 Monte-Carlo permutations.

RESULTS

We reviewed 11 studies (559 participants). On studying all the interventions together, we found no significant changes in pain modulation, TS, or CPM. Our findings show that this lack of difference is likely because surgical and nonsurgical interventions resulted in contrary effects. Metaregression significantly correlated pain reduction with normalization of TS and CPM.

CONCLUSIONS

We demonstrate an association between pain reduction and TS/CPM normalization. Though we cannot directly compare these interventions, the results allow us to draw hypotheses on potential practice schemas. Recovering defective endogenous pain modulation mechanisms may help establish long-term analgesia. However, to validate these paradigms as robust clinical biomarkers, further investigation into their mechanisms would be necessary. The registration number for this review is CRD42017072066.

Mesenchymal stem cells and treadmill training enhance function and promote tissue preservation after spinal cord injury

Spinal cord injury (SCI) is considered a serious neurological disorder that can lead to severe sensory, motor and autonomic deficits. In this work, we investigated whether cell therapy associated with physical activity after mouse SCI could promote morphological and functional outcomes, using a lesion model established by our group. Mesenchymal stem cells (8 × 10⁵ cells/2 µL) or DMEM (2 µL), were injected in the epicenter of the lesion at 7 days after SCI, and the mice started a moderate treadmill training 14 days after injury. Functional assessments were performed weekly up to 8 weeks after injury when the morphological analyses were also performed. Four injured groups were analyzed: DMEM (SCI plus DMEM injection), MSCT (SCI plus MSC injection), DMEM + TMT (SCI plus DMEM injection and treadmill training) and MSCT + TMT (SCI plus MSC injection and treadmill training). The animals that received the combined therapy (MSCT + TMT) were able to recover and maintained the better functional results throughout the analyzed period. The morphometric analysis from MSCT + TMT group evidenced a larger spared white matter area and a higher number of preserved myelinated fibers with the majority of them reaching the ideal G-ratio values, when compared to other groups. Ultrastructural analysis from this group, using transmission electron microscopy, showed better tissue preservation with few microcavitations and degenerating nerve fibers. Also, this group exhibited a significantly higher neurotrophin 4 (NT4) expression as compared to the other groups. The results provided by this study support the conclusion that the association of strategies is a potential therapeutic approach to treat SCI, with the possibility of translation into the clinical practice.

Involvement of Spinal Cannabinoid CB2 Receptors in Exercise-Induced Antinociception

Muscle pain affects approximately 11-24% of the global population. Several studies have shown that exercise is a non-pharmacological therapy to pain control. It has been suggested that the endocannabinoid system is involved in this antinociceptive effect. However, the participation of this pathway is unclear. The present study aimed to investigate whether spinal cannabinoid CB₂ receptors participate in the exercise-induced antinociception. The inflammatory muscle pain model was induced by the intramuscular injection of carrageenan. Tactile allodynia and thermal hyperalgesia were determined with the von Frey filaments and hot-plate tests. C57BL/6J female mice underwent a swimming training protocol that lasted 3 weeks. This protocol of exercise reduced carrageenan-induced tactile allodynia and thermal hyperalgesia and this effect was prevented by the cannabinoid CB₂ receptors inverse agonist AM630 and potentiated by MAFP (inhibitor of the enzyme that metabolizes endocannabinoids) and minocycline (microglia inhibitor). In addition, exercise increased the endocannabinoid anandamide levels and cannabinoid CB₂ receptors expression whereas it reduced Iba1 (microglial marker) protein expression as well as pro-inflammatory cytokines (TNF-α and IL-1β) in the spinal cord of mice with inflammatory muscle pain. Swimming training also reduced muscle temperature of carrageen-treated animals. The present study suggests that activation of spinal cannabinoid CB₂ receptors and reduction of activated microglia are involved in exercise-induced antinociception.

Roles of glial ion transporters in brain diseases

Glial ion transporters are important in regulation of ionic homeostasis, cell volume, and cellular signal transduction under physiological conditions of the central nervous system (CNS). In response to acute or chronic brain injuries, these ion transporters can be activated and differentially regulate glial functions, which has subsequent impact on brain injury or tissue repair and functional recovery. In this review, we summarized the current knowledge about major glial ion transporters, including Na⁺ /H⁺ exchangers (NHE), Na⁺ /Ca²⁺ exchangers (NCX), Na⁺ -K⁺ -Cl^- cotransporters (NKCC), and Na⁺ -HCO₃ ^- cotransporters (NBC). In acute neurological diseases, such as ischemic stroke and traumatic brain injury (TBI), these ion transporters are rapidly activated and play significant roles in regulation of the intra- and extracellular pH, Na⁺ , K⁺ , and Ca²⁺ homeostasis, synaptic plasticity, and myelin formation. However, overstimulation of these ion transporters can contribute to glial apoptosis, demyelination, inflammation, and excitotoxicity. In chronic brain diseases, such as glioma, Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS), glial ion transporters are involved in the glioma Warburg effect, glial activation, neuroinflammation, and neuronal damages. These findings suggest that glial ion transporters are involved in tissue structural and functional restoration, or brain injury and neurological disease development and progression. A better understanding of these ion transporters in acute and chronic neurological diseases will provide insights for their potential as therapeutic targets.

Voluntary wheel running reveals sex-specific nociceptive factors in murine experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an inflammatory, neurodegenerative autoimmune disease associated with sensory and motor dysfunction. Although estimates vary, ∼50% of patients with MS experience pain during their disease. The mechanisms underlying the development of pain are not fully understood, and no effective treatment for MS-related pain is available. Previous work from our laboratory demonstrated that voluntary exercise (wheel running) can reduce nociceptive behaviours at the disease onset in female mice with experimental autoimmune encephalomyelitis (EAE), an animal model used to study the immunopathogenesis of MS. However, given the established sex differences in the underlying mechanisms of chronic pain and MS, we wanted to investigate whether wheel running would also be effective at preventing nociceptive behaviours in male mice with EAE. C57BL/6 mice of both sexes were given access to running wheels for 1 hour/day until the disease onset, when nociceptive behaviour was assessed using von Frey hairs. Daily running effectively reduced nociceptive behaviour in female mice, but not in male mice. We explored the potential biological mechanisms for these effects and found that the reduction in nociceptive behaviour in female mice was associated with reduced levels of inflammatory cytokines from myelin-reactive T cells as well as reduced dorsal root ganglia excitability as seen by decreased calcium responses. These changes were not seen in male mice. Instead, running increased the levels of inflammatory cytokines and potentiated Ca responses in dorsal root ganglia cells. Our results show that voluntary wheel running has sex-dependent effects on nociceptive behaviour and inflammatory responses in male and female mice with EAE.

Spinal cord repair: advances in biology and technology

Individuals with spinal cord injury (SCI) can face decades with permanent disabilities. Advances in clinical management have decreased morbidity and improved outcomes, but no randomized clinical trial has demonstrated the efficacy of a repair strategy for improving recovery from SCI. Here, we summarize recent advances in biological and engineering strategies to augment neuroplasticity and/or functional recovery in animal models of SCI that are pushing toward clinical translation.

Interleukin-4 mediates the analgesia produced by low-intensity exercise in mice with neuropathic pain

Peripheral nerve injury (PNI) activates the immune system, resulting in increased proinflammatory cytokines at the site of injury and in the spinal cord dorsal horn. Exercise modulates the immune system promoting an anti-inflammatory phenotype of macrophages in uninjured muscle, and increases in anti-inflammatory cytokines can promote healing and analgesia. We proposed that PNI will decrease, and treadmill exercise will increase, release of anti-inflammatory cytokines at the site of injury and in the spinal cord. We show that 2 weeks of treadmill exercise improves neuropathic pain behaviors in mice: mechanical hyperalgesia, escape and avoidance behavior, and spontaneous locomotor activity. Peripheral nerve injury reduced anti-inflammatory cytokines (interleukin-4 [IL-4], IL-1ra, and IL-5) at the site of nerve injury and in the spinal dorsal horn, whereas exercise restored IL-4, IL-1ra, and IL-5 concentrations to preinjury levels. IL4 mice and mice treated with IL-4 antibody did not develop analgesia to treadmill exercise. Using immunohistochemical staining of the sciatic nerve, treadmill exercise increased the percentage of M2 macrophages (secretes anti-inflammatory cytokines) and decreased M1 macrophages (secretes proinflammatory cytokines) when compared with sedentary mice. The increased M2 and decreased M1 macrophages in exercised mice did not occur in IL-4 mice. In the spinal cord, PNI increased glial cell activation, brain-derived neurotrophic factor and β-nerve growth factor levels, and decreased IL-4 and IL-1ra levels, whereas treadmill exercise suppressed glial cells activation (Glial Fibrillary Acidic Protein and Iba1 immunoreactivity), reduced brain-derived neurotrophic factor and β-nerve growth factor, and increased IL-4, IL-1ra, and IL-5 concentrations. Our results suggest that IL-4 mediates the analgesia produced by low-intensity exercise by modulating peripheral and central neuroimmune responses in mice with neuropathic pain.

May a sigma-1 antagonist improve neuropathic signs induced by cisplatin and vincristine in rats?

BACKGROUND

The antineoplastic drugs cisplatin and vincristine induce peripheral neuropathies. The sigma-1 receptor (σ1R) is expressed in areas of pain control, and its blockade with the novel selective antagonist MR-309 has shown efficacy in nociceptive and neuropathic pain models. Our goal was to test whether this compound reduces neuropathic signs provoked by these antitumoural drugs.

METHODS

Rats were treated with cisplatin or vincristine to induce neuropathies. The effects of acute or repeated administration of MR-309 were tested on mechanical and thermal sensitivity, electrophysiological activity of Aδ-primary afferents in the rat skin-saphenous nerve preparation, and gastrointestinal or cardiovascular functions.

RESULTS

Rats treated with antitumourals developed tactile allodynia, while those treated with vincristine also developed mechanical hyperalgesia. These in vivo modifications correlated with electrophysiological hyperactivity (increased spontaneous activity and hyperresponsiveness to innocuous and noxious mechanical stimulation). Animals treated with cisplatin showed gastrointestinal impairment and those receiving vincristine showed cardiovascular toxicity. A single dose of MR-309 strongly reduced both nociceptive behaviour and electrophysiological changes. Moreover, its concomitant administration with the antitumourals blocked the development of neuropathic symptoms, thus restoring mechanical sensitivity, improving the impairment of feeding behaviour and gastrointestinal transit in the cisplatin-treated group along with ameliorating the altered vascular reactivity recorded in rats treated with vincristine.

CONCLUSION

σ1R antagonist, MR-309, reduces sensorial and electrophysiological neuropathic signs in rats treated with cisplatin or vincristine and, in addition, reduces gastrointestinal and cardiovascular side effects.

SIGNIFICANCE

σ1R antagonism could be an interesting and new option to palliate antitumoural neuropathies.

Role of Noradrenergic Inputs From Locus Coeruleus on Changes Induced on Axotomized Motoneurons by Physical Exercise

Physical rehabilitation is one of the cornerstones for the treatment of lesions of the nervous system. After peripheral nerve injuries, activity dependent therapies promote trophic support for the paralyzed muscles, enhance axonal growth and also modulate the maladaptive plastic changes induced by the injury at the spinal level. We have previously demonstrated that an intensive protocol of treadmill running (TR) in rats reduces synaptic stripping on axotomized motoneurons, preserves their perineuronal nets (PNN) and attenuates microglia reactivity. However, it is not clear through which mechanisms exercise is exerting these effects. Here we aimed to evaluate if activation of the locus coeruleus (LC), the noradrenergic center in the brain stem, plays a role in these effects. Since LC is strongly activated during stressful situations, as during intensive exercise, we selectively destroyed the LC by administering the neurotoxin DPS-4 before injuring the sciatic nerve of adult rats. Animals without LC had increased microglia reactivity around injured motoneurons. In these animals, an increasing intensity protocol of TR was not able to prevent synaptic stripping on axotomized motoneurons and the reduction in the thickness of their PNN. In contrast, TR was still able to attenuate microglia reactivity in DSP-4 treated animals, thus indicating that the noradrenergic projections are important for some but not all the effects that exercise induces on the spinal cord after peripheral nerve injury. Moreover, animals subjected to treadmill training showed delayed muscle reinnervation, more evident if treated with DSP-4. However, we did not find differences in treated animals regarding the H/M amplitude ratio, which increased during the first stages of regeneration in all injured groups.

Chronic electrical stimulation reduces hyperalgesia and associated spinal changes induced by peripheral nerve injury

OBJECTIVES

We aimed to investigate if different protocols of electrical stimulation following nerve injury might improve neuropathic pain outcomes and modify associated plastic changes at the spinal cord level.

MATERIALS AND METHODS

Adult rats were subjected to sciatic nerve transection and repair, and distributed in four groups: untreated (SNTR, n = 12), repeated acute electrical stimulation (rAES, 50 Hz, one hour, n = 12), chronic electrical stimulation (CES, 50 Hz, one hour, n = 12), and increasing-frequency chronic electrical stimulation (iCES, one hour, n = 12) delivered during two weeks following the lesion. The threshold of nociceptive withdrawal to mechanical stimuli was evaluated by means of a Von Frey algesimeter during three weeks postlesion. Spinal cord samples were processed by immunohistochemistry for labeling glial cells, adrenergic receptors, K⁺ -Cl^- cotransporter 2 (KCC2) and GABA.

RESULTS

Acute electrical stimulation (50 Hz, one hour) delivered at 3, 7, and 14 days induced an immediate increase of mechanical pain threshold that disappeared after a few days. Chronic electrical stimulation given daily reduced mechanical hyperalgesia until the end of follow-up, being more sustained with the iCES than with constant 50 Hz stimulation (CES). Chronic stimulation protocols restored the expression of β2 adrenergic receptor and of KCC2 in the dorsal horn, which were significantly reduced by nerve injury. These treatments decreased also the activation of microglia and astrocytes in the dorsal horn.

CONCLUSION

Daily electrical stimulation, especially if frequency-patterned, was effective in ameliorating hyperalgesia after nerve injury, and partially preventing the proinflammatory and hyperalgesic changes in the dorsal horn associated to neuropathic pain.

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. A similar protocol was used in fracture mice treated with exercise for 4 weeks, and then the running wheel was removed for 2 weeks. Memory and anxiety were measured in both groups with use of open-field, zero-maze, and novel-objects recognition assays.

RESULTS

At 7 weeks postfracture the mice with no wheel access exhibited hindlimb allodynia and unweighting, anxiety, memory loss, upregulated spinal neuropeptide signaling, and increased hind paw and spinal inflammatory mediator expression, but the postfracture mice allowed to exercise for 4 weeks exhibited none of these changes (n = 12/cohort). When exercise was stopped for 2 weeks after 4 weeks of running, hindlimb allodynia and unweighting were rekindled, and this nociceptive sensitization was associated with increased sciatic nerve neuropeptide levels and hind paw skin interleukin 6 and nerve growth factor expression (n = 12/cohort).

CONCLUSIONS

Daily exercise reversed nociceptive sensitization, inflammation, anxiety, and memory loss after tibia fracture.

Exercise-Induced Changes to the Macrophage Response in the Dorsal Root Ganglia Prevent Neuropathic Pain after Spinal Cord Injury

Spinal cord injury (SCI) induces neuropathic pain that is refractory to treatment. Central and peripheral immune responses to SCI play critical roles in pain development. Although immune responses in the dorsal horn have been implicated in SCI-pain, immune mechanisms in the periphery, especially in the dorsal root ganglia (DRG), where nociceptor cell bodies reside, have not been well studied. Exercise is an immunomodulator, and we showed previously that early exercise after SCI reduces pain development. However, the mechanisms of exercise-mediated pain reduction are not understood. Therefore, we examined the 1) underlying immune differences in the spinal cord and DRG between rats with and without pain and 2) immunomodulatory effects of exercise in pain reduction. Rats were subjected to a unilateral contusion at C5 and tested for pain development using von Frey and mechanical conflict-avoidance paradigms. A subgroup of rats was exercised on forced running wheels starting at 5 days post-injury for 4 weeks. We observed greater microglial activation in the C7-C8 dorsal horn of rats with SCI-induced pain compared to rats with normal sensation, and early exercise reduced this activation independently of pain behavior. Further, abnormal pain sensation strongly correlated with an increased number of DRG macrophages. Importantly, exercise-treated rats that maintain normal sensation also have a lower number of macrophages in the DRG. Our data suggest that macrophage presence in the DRG may be an important effector of pain development, and early wheel walking exercise may mediate pain prevention by modulating the injury-induced macrophage response in the DRG. Further supportive evidence demonstrated that rats that developed pain despite exercise intervention still displayed a significantly elevated number of macrophages in the DRG. Collectively, these data suggest that macrophage presence in the DRG may be an amenable cellular target for future therapies.

The Impact of Exercise in Rodent Models of Chronic Pain

PURPOSE OF REVIEW

Physical activity is increasingly recommended for chronic pain. In this review, we briefly survey recent, high-quality meta-analyses on the effects of exercise in human chronic pain populations, followed by a critical discussion of the rodent literature.

RECENT FINDINGS

Most meta-analytical studies on the effects of exercise in human chronic pain populations describe moderate improvements in various types of chronic pain, despite substantial variability in the outcomes reported in the primary literature. The most consistent findings suggest that while greater adherence to exercise programs produces better outcomes, there is minimal support for the superiority of one type of exercise over another. The rodent literature similarly suggests that while regular exercise reduces hypersensitivity in rodent models of chronic pain, exercise benefits do not appear to relate to either the type of injury or any particular facet of the exercise paradigm. Potential factors underlying these results are discussed, including the putative involvement of stress-induced analgesic effects associated with certain types of exercise paradigms. Exercise research using rodent models of chronic pain would benefit from increased attention to the role of stress in exercise-induced analgesia, as well as the incorporation of more clinically relevant exercise paradigms.

Sensorimotor Activity Partially Ameliorates Pain and Reduces Nociceptive Fiber Density in the Chronically Injured Spinal Cord

A large proportion of patients suffering from spinal cord injury (SCI) develop chronic central neuropathic pain. Previously, we and others have shown that sensorimotor training early after SCI can prevent the development of mechanical allodynia. To determine whether training initiated in the subchronic/chronic phase remains effective, correlates of below-level neuropathic pain were analyzed in the hindpaws 5-10 weeks after a moderate T11 contusion SCI (50 kDyn) in adult female C57BL/6 mice. In a comparison of SCI and sham mice 5 weeks post-injury, about 80% of injured animals developed mechanical hypersensitivity to light mechanical stimuli, whereas testing of noxious stimuli revealed hypo-responsiveness. Thermal sensitivity testing showed a decreased response latency after injury. Without intervention, mechanical and thermal hyper-responsiveness were evident until the end of the experiment (10 weeks). In contrast, treadmill training (2 × 15 min/day; 5 × /week) initiated 6 weeks post-injury resulted in partial amelioration of pain behavior and this effect remained stable. Analysis of calcitonin gene-related peptide (CGRP)-labeled fibers in lamina III-IV of the lumbar dorsal horn revealed an increase in labeling density after SCI. This was not due to changes in the number or size distribution of CGRP-labeled lumbar dorsal root ganglion neurons. Treadmill training reduced the CGRP-labeling density in the spinal cord of injured mice, whereas the density of non-peptidergic isolectin-B4 (IB4)⁺ fibers showed no changes in lamina IIi and a slight reduction of sparse IB4 labeling in laminae III-IV. Thus, sensorimotor activity initiated in the subchronic/chronic phase of SCI remains effective in ameliorating pain behavior and influencing structural changes of the nociceptive system.

Spatial distribution affects the role of CSPGs in nerve regeneration via the actin filament-mediated pathway

CSPGs are components of the extracellular matrix in the nervous system, where they serve as cues for axon guidance during development. After a peripheral nerve injury, CSPGs switch roles and become axon inhibitors and become diffusely distributed at the injury site. To investigate whether the spatial distribution of CSPGs affects their role, we combined in vitro DRG cultures with CSPG stripe or coverage assays to simulate the effect of a patterned substrate or dispersive distribution of CSPGs on growing neurites. We observed neurite steering at linear CSPG interfaces and neurite inhibition when diffused CSPGs covered the distal but not the proximal segment of the neurite. The repellent and inhibitory effects of CSPGs on neurite outgrowth were associated with the disappearance of focal actin filaments on growth cones. The application of an actin polymerization inducer, jasplakinolide, allowed neurites to break through the CSPG boundary and grow on CSPG-coated surfaces. The results of our study collectively reveal a novel mechanism that explains how the spatial distribution of CSPGs determines whether they act as a cue for axon guidance or as an axon-inhibiting factor. Increasing our understanding of this issue may promote the development of novel therapeutic strategies that regulate the spatial distributions of CSPGs to use them as an axon guidance cue.

Reconstruction of a 10-mm-long median nerve gap in an ischemic environment using autologous conduits with different patterns of blood supply: A comparative study in the rat

The aim of this study was to evaluate in the Wistar rat the efficacy of various autologous nerve conduits with various forms of blood supply in reconstructing a 10-mm-long gap in the median nerve (MN) under conditions of local ischemia. A 10-mm-long median nerve defect was created in the right arm. A loose silicone tube was placed around the nerve gap zone, in order to simulate a local ischemic environment. Rats were divided in the following experimental groups (each with 20 rats): the nerve Graft (NG) group, in which the excised MN segment was reattached; the conventional nerve flap (CNF) and the arterialized neurovenous flap (ANVF) groups in which the gap was bridged with homonymous median nerve flaps; the prefabricated nerve flap (PNF) group in which the gap was reconstructed with a fabricated flap created by leaving an arteriovenous fistula in contact with the sciatic nerve for 5 weeks; and the two control groups, Sham and Excision groups. In the latter group, the proximal stump of the MN nerve was ligated and no repair was performed. The rats were followed for 100 days. During this time, they did physiotherapy. Functional, electroneuromyographic and histological studies were performed. The CNF and ANVF groups presented better results than the NG group in the following assessments: grasping test, nociception, motor stimulation threshold, muscle weight, and histomorphometric evaluation. Radial deviation of the operated forepaw was more common in rats that presented worse results in the other outcome variables. Overall, CNFs and ANVFs produced a faster and more complete recovery than NGs in the reconstruction of a 10-mm-long median nerve gap in an ischemic environment in the Wistar rat. Although, results obtained with CNFs were in most cases were better than ANVFs, these differences were not statistically significant for most of the outcome variables.

The effect of exercise frequency on neuropathic pain and pain-related cellular reactions in the spinal cord and midbrain in a rat sciatic nerve injury model

Background

Exercise regimens are established methods that can relieve neuropathic pain. However, the relationship between frequency and intensity of exercise and multiple cellular responses of exercise-induced alleviation of neuropathic pain is still unclear. We examined the influence of exercise frequency on neuropathic pain and the intracellular responses in a sciatic nerve chronic constriction injury (CCI) model.

Materials and methods

Rats were assigned to four groups as follows: CCI and high-frequency exercise (HFE group), CCI and low-frequency exercise (LFE group), CCI and no exercise (No-Ex group), and naive animals (control group). Rats ran on a treadmill, at a speed of 20 m/min, for 30 min, for 5 (HFE) or 3 (LFE) days a week, for a total of 5 weeks. The 50% withdrawal threshold was evaluated for mechanical sensitivity. The activation of glial cells (microglia and astrocytes), expression of brain-derived neurotrophic factor (BDNF) and μ-opioid receptor in the spinal dorsal horn and endogenous opioid in the midbrain were examined using immunohistochemistry. Opioid receptor antagonists (naloxone) were administered using intraperitoneal injection.

Results

The development of neuropathic pain was related to the activation of glial cells, increased BDNF expression, and downregulation of the μ-opioid receptor in the ipsilateral spinal dorsal horn. In the No-Ex group, neuropathic pain showed the highest level of mechanical hypersensitivity at 2 weeks, which improved slightly until 5 weeks after CCI. In both exercise groups, the alleviation of neuropathic pain was accelerated through the regulation of glial activation, BDNF expression, and the endogenous opioid system. The expression of BDNF and endogenous opioid in relation to exercise-induced alleviation of neuropathic pain differed in the HFE and LFE groups. The effects of exercise-induced alleviation of mechanical hypersensitivity were reversed by the administration of naloxone.

Conclusion

The LFE and HFE program reduced neuropathic pain. Our findings indicated that aerobic exercise-induced alleviated neuropathic pain through the regulation of glial cell activation, expression of BDNF in the ipsilateral spinal dorsal horn, and the endogenous opioid system.

Anion exchanger 3 in dorsal root ganglion contributes to nerve injury-induced chronic mechanical allodynia and thermal hyperalgesia

Objective

To determine the role of anion exchanger 3 (AE3) in dorsal root ganglion (DRG) in nerve injury-induced chronic nociception in the rat.

Methods

Spared nerve injury (SNI) was used to induce neuropathic pain. Von Frey filaments and Hargreaves test were used to assess tactile allodynia and thermal hyperalgesia, respectively. Drugs were given by intrathecal administration. Western blotting was used to determine AE3 expression in DRG.

Key findings

SNI produced long-lasting mechanical allodynia and thermal hyperalgesia. AE3 was found in DRG of sham-operated rats. SNI enhanced baseline AE3 expression in L4 and L5 DRGs at days 7 and 14, respectively. In contrast, SNI did not affect AE3 expression in L6 DRG. AE3 expression returned to baseline levels 21 days after SNI. Intrathecal 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) (5–50 μg) pretreatment prevented SNI-induced allodynia and, at a lesser extent, hyperalgesia. Moreover, DIDS (50 μg) reduced SNI-induced AE3 upregulation in L4, but not L5, DRGs. Intrathecal DIDS (5–50 μg) or anti-AE3 antibody (1 μg), but not vehicle, post-treatment (6 days) partially reversed SNI-induced allodynia and hyperalgesia. DIDS or anti-AE3 antibody post-treatment diminished SNI-induced AE3 upregulation in L4 and L5 DRGs.

Conclusions

Data suggest that AE3 is present in DRG and contributes to mechanical allodynia and thermal hyperalgesia in neuropathic rats.