Forced-exercise delays neuropathic pain in experimental diabetes: effects on voltage-activated calcium channels

Effects of swimming exercise on neuropathic pain in a rat model: role of glutamate

OBJECTIVE

The pain-reducing effects of the exercise were exerted through different mechanisms. Knowing more clear mechanisms helps to find more approach that is therapeutic. The objective of the present study is the evaluation of cerebrospinal fluid (CSF) glutamate level alteration in neuropathic pain rats and whether physical activity could modulate it.

METHODS

In the present study 104 male Wistar rats weighing 180-220 g were randomly divided into 4 groups (Sham, Sham + Exe, Neuropathy, and Neuropathy + Exe) which in turn each group subdivided into 4 groups according to time points for behavioral testing and CSF sampling (Baseline, 2 weeks, 3 weeks, and 4 weeks). To induction of neuropathy (by chronic constriction injury,), after anesthetizing with a mixture of ketamine (80 mg/kg) and xylazine (10 mg/kg), the animal's right sciatic nerve was exposed and was ligated using four movable catgut chromic suture 4/0. The exercise protocol included 25 min of daily swimming, 5 days a week for 4 weeks. Thermal hyperalgesia and mechanical tactile threshold were detected using the plantar test and Von Frey filaments, respectively. CSF glutamate level was determined using high-performance liquid chromatography.

RESULTS

Findings indicated that mechanical and thermal thresholds significantly (p < 0.01, p < 0.05 respectively) decreased in the neuropathy group against that in sham groups. On the other hand, exercise significantly increased mechanical tactile threshold (p < 0.0012) and thermal threshold (p < 0.05) compared to the neuropathy group. Moreover, CSF glutamate level prominently (p < 0.01) was increased in the neuropathy group compared to the sham group, and swimming exercise significantly (p < 0.001) reduced it.

IN CONCLUSION

The present findings provide new evidence showing that medium-intensity swimming exercise attenuates pain-like behaviors in neuropathic pain animals, which is possibly due to decreasing CSF glutamate level and its neurotransmission.

Nerve terminals in the tumor microenvironment as targets for local infiltration analgesia

Nerve terminals within the tumor microenvironment as potential pain-mitigating targets for local infiltration analgesia is relatively less explored. In this study, we examine the role of key analgesics administered as local infiltration analgesia in a model of cancer-induced bone pain (CIBP). CIBP was induced by administration of allogenic MRMT1 breast cancer cells in the proximal tibia of rats, and tumor mass characterized using radiogram, micro-CT, and histological analysis. In vitro responsiveness to key analgesics δ-opioid receptor agonist (DOPr), Ca2+ channel and TRPV1 antagonists was assessed using ratiometric Ca2+ imaging in sensory neurons innervating the tumor site. Effectiveness of locally infiltrated analgesics administered independently or in combination was assessed by quantifying evoked limb withdrawal thresholds at two distinct sites for up to 14 days. CIBP animals demonstrated DOPr, N-, and L-type and TRPV1 expression in lumbar dorsal root ganglion neurons (DRG), comparable to controls. Evoked Ca2+ transients in DRG neurons from CIBP animals were significantly reduced in response to treatment with compounds targeting DOPr, N-, L-type Ca2+ channels and TRPV1 proteins. Behaviourally, evoked hyperalgesia at the tumor site was strongly mitigated by peritumoral injection of the DOPr agonist and T-type calcium antagonist, via its activity on bone afferents. Results from this study suggest that nerve terminals at tumor site could be utilized as targets for specific analgesics, using local infiltration analgesia.

Diabetes-Induced Amplification of Nociceptive DRG Neuron Output by Upregulation of Somatic T-Type Ca2+ Channels

The development of pain symptoms in peripheral diabetic neuropathy (PDN) is associated with the upregulation of T-type Ca2+ channels (T-channels) in the soma of nociceptive DRG neurons. Moreover, a block of these channels in DRG neurons effectively reversed mechanical and thermal hyperalgesia in animal diabetic models, indicating that T-channel functioning in these neurons is causally linked to PDN. However, no particular mechanisms relating the upregulation of T-channels in the soma of nociceptive DRG neurons to the pathological pain processing in PDN have been suggested. Here we have electrophysiologically identified voltage-gated currents expressed in nociceptive DRG neurons and developed a computation model of the neurons, including peripheral and central axons. Simulations showed substantially stronger sensitivity of neuronal excitability to diabetes-induced T-channel upregulation at the normal body temperature compared to the ambient one. We also found that upregulation of somatic T-channels, observed in these neurons under diabetic conditions, amplifies a single action potential invading the soma from the periphery into a burst of multiple action potentials further propagated to the end of the central axon. We have concluded that the somatic T-channel-dependent amplification of the peripheral nociceptive input to the spinal cord demonstrated in this work may underlie abnormal nociception at different stages of diabetes development.

Swimming exercise attenuates mechanical hypersensitivity and mitigates peripheral nerve degeneration in rats with painful diabetic neuropathy (PDN)

BACKGROUND

This study aimed to assess the effectiveness of swimming exercise in alleviating mechanical hypersensitivity and peripheral nerve degeneration associated with a pre-clinical model of painful diabetic neuropathy (PDN).

METHODS

This study is a pre-clinical study conducted using the streptozocin (STZ)-induced PDN rat model. Rats were randomly allocated to three groups: a vehicle group of non-diabetic rats (Vehicle, n = 9), a group of rats with PDN (PDN, n = 8), and a group of rats with PDN that performed a swimming exercise program (PDN-SW, n = 10). The swimming exercise program included daily 30-minute swimming exercise, 5 days per week for 4 weeks. Von Frey testing was used to monitor hindpaw mechanical sensitivity over 4 weeks. Assessment of cutaneous peripheral nerve fiber integrity was performed after the 4-week study period via immunohistochemistry for protein gene product 9.5-positive (PGP9.5+) intra-epidermal nerve fiber density (IENFD) in hind-paw skin biopsies by a blinded investigator.

RESULTS

The results showed that swimming exercise mitigated but did not fully reverse mechanical hypersensitivity in rats with PDN. Immunohistochemical testing revealed that the rats in the PDN-SW group retained higher PGP9.5+ IENFD compared to the PDN group but did not reach normal levels of the Vehicle group.

CONCLUSIONS

The results of this study indicate that swimming exercise can mitigate mechanical hypersensitivity and degeneration of peripheral nerve fibers in rats with experimental PDN.

Mechanisms of exercise for diabetic neuropathic pain

Diabetic neuropathic pain (DNP) is a common disease that affects the daily lives of diabetic patients, and its incidence rate is very high worldwide. At present, drug and exercise therapies are common treatments for DNP. Drug therapy has various side effects. In recent years, exercise therapy has received frequent research and increasing attention by many researchers. Currently, the treatment of DNP is generally symptomatic. We can better select the appropriate exercise prescription for DNP only by clarifying the exercise mechanism for its therapy. The unique pathological mechanism of DNP is still unclear and may be related to the pathological mechanism of diabetic neuropathy. In this study, the mechanisms of exercise therapy for DNP were reviewed to understand better the role of exercise therapy in treating DNP.

Effect of various exercise protocols on neuropathic pain in individuals with type 2 diabetes with peripheral neuropathy: A systematic review and meta-analysis

BACKGROUND

Diabetic neuropathy is considered as the most common and alarming microvascular complication of diabetes worldwide. Despite the recent major advances, there remains a dearth in literature on effective treatment options that appropriately target the natural history of painful diabetic neuropathy.

AIMS

To review various exercise programs for neuropathic pain in type 2 diabetes individuals with diabetic peripheral neuropathy.

METHODS

An extensive literature search was performed in Scopus, Web of Science, PubMed, Science direct and ProQuest. The inclusion criteria were exercise program for neuropathic pain in type 2 diabetes individuals. Animal studies, chemotherapy, electrotherapy, yoga, behavioural and psychological approaches, and other medical interventions were excluded. A systematic strategy to conduct a review was planned, to search, screen articles and extract data by two reviewers independently.

RESULTS

Nine out of total 5342 screened articles were identified as relevant for the comprehensive review. The studies included exercise protocols for neuropathic pain in people with type 2 diabetes mellitus. Overall, studies were of low to moderate quality evidence. The findings of this review suggested incorporating exercise program for painful diabetic neuropathy. Exercise intervention is effective in reducing the Michigan neuropathy score (Standardized Mean Difference -2.92, 95% Confidence Interval -4.49 to -1.24; participants = 114; studies = 3; I² = 88%) Conclusion: A structured exercise prescription need to be designed exclusively for neuropathic pain in population with type 2 diabetes to improve quality of life. However, there is a further need to explore exercise training to strengthen evidence using large clinical trials.

Exercise and Nutraceuticals: Eminent approach for Diabetic Neuropathy

Diabetic neuropathy is an incapacitating chronic pathological condition that encompasses a large group of diseases and manifestations of nerve damage. It affects approximately 50% of patients with diabetes mellitus. Autonomic, sensory, and motor neurons are affected. Disabilities are severe, along with poor recovery and diverse pathophysiology. Physical exercise and herbal-based therapies have the potential to decrease the disabilities associated with diabetic neuropathy. Aerobic exercises like walking, weight lifting, the use of nutraceuticals and herbal extracts are found to be effective. Literature from the public domain was studied emphasizing various beneficial effects of different exercises, use of herbal and nutraceuticals for their therapeutic action in diabetic neuropathy. Routine exercises and administration of herbal and nutraceuticals, either the extract of plant material containing the active phytoconstituent or isolated phytoconstituent at safe concentration, have been shown to have promising positive action in the treatment of diabetic neuropathy. Exercise has shown promising effects on vascular and neuronal health and has proven to be well effective in the treatment as well as prevention of diabetic neuropathy by various novel mechanisms, including herbal and nutraceuticals therapy is also beneficial for the condition. They primarily show the anti-oxidant effect, secretagogue, anti-inflammatory, analgesic, and neuroprotective action. Severe adverse events are rare with these therapies. The current review investigates the benefits of exercise and nutraceutical therapies in the treatment of diabetic neuropathy.

Effect of gold nanoparticle treated dorsal root ganglion cells on peripheral neurite differentiation

The use of gold nanoparticles (AuNps) in applications connected to the peripheral nervous system (PNS) holds much promise in terms of therapeutic and diagnostic strategies. Despite their extensive use, a clear understanding of their effects on neurons and glia in the PNS is lacking. In this study, we set out to examine the effects of AuNps on dorsal root ganglion (DRG) cells, and how such AuNp-exposed cells could in-turn affect neurite differentiation. DRG cultures were exposed to mono-dispersed spherical-shaped AuNps of diameter 24.3 ± 2.3, 109.2 ± 14.7 or 175 ± 19.2 nm at varying concentrations. Cellular uptake and viability were quantified using flow-cytometry. Neurite differentiation was quantified using neurite tracing analysis in PC-12 and DRG neurons exposed to conditioned media derived from AuNp-treated DRG cells. Both neurons and glia were found to internalize AuNps. DRG cell viability was significantly reduced upon treatment with higher concentration of 175 nm sized AuNps, while 24 nm and 109 nm sized AuNps had no effect. Further, conditioned media from AuNp-treated DRG cells produced comparable neurite outgrowth and neurite branching measurement as controls in PC-12 and DRG neurons. DRG cells were quite resilient to AuNp exposure in mild-moderate concentration. AuNp-exposed DRG cells, irrespective of size and concentration range tested, did not affect neuronal differentiation.

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.

Neuronal delivery of nanoparticles via nerve fibres in the skin

Accessing the peripheral nervous system (PNS) by topically applied nanoparticles is a simple and novel approach with clinical applications in several PNS disorders. Skin is richly innervated by long peripheral axons that arise from cell bodies located distally within ganglia. In this study we attempt to target dorsal root ganglia (DRG) neurons, via their axons by topical application of lectin-functionalized gold nanoparticles (IB4-AuNP). In vitro, 140.2 ± 1.9 nm IB4-AuNP were found to bind both axons and cell bodies of DRG neurons, and AuNP applied at the axonal terminals were found to translocate to the cell bodies. Topical application of IB4-AuNP on rat hind-paw resulted in accumulation of three to fourfold higher AuNP in lumbar DRG than in contralateral control DRGs. Results from this study clearly suggest that topically applied nanoparticles with neurotropic targeting ligands can be utilized for delivering nanoparticles to neuronal cell bodies via axonal transport mechanisms.

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.

Effect of Peritumoral Bupivacaine on Primary and Distal Hyperalgesia in Cancer-Induced Bone Pain

Background

Cancer-induced bone pain (CIBP) is a debilitating chronic pain condition caused by injury to bone nerve terminals due to primary or metastasized bone tumors. Pain manifests as enhanced sensitivity, not only over the affected bone site but also at distal areas that share common nerve innervation with the tumor. In this study, we aim to understand how tumor-induced primary and distal pain sensitivities are affected by bupivacaine-induced block of bone nerve endings in a rat model of CIBP.

Methods

MRMT-1 breast cancer cells were injected into the proximal segment of tibia in female Sprague–Dawley rats. Radiograms and micro-CT images were obtained to confirm tumor growth. Bupivacaine was injected peritumorally at day 7 or day 14 post-tumor induction, and withdrawal thresholds in response to pressure and punctate mechanical stimulus were recorded from the knee and hind-paw, respectively. Immunohistochemical studies for the determination of ATF3 and GFAP expression in DRG and spinal cord sections were performed.

Results

Rats developed primary and distal hyperalgesia after MRMT-1 administration that was sustained for 2 weeks. Peritumoral administration of bupivacaine in 7-day post-tumor-induced (PTI) rats resulted in a reversal of both primary and distal hyperalgesia for 20–30 mins. However, bupivacaine failed to reverse distal hyperalgesia in 14 day-PTI rats. ATF3 and GFAP expression were much enhanced in 14 day-PTI animals, compared to 7 day-PTI group.

Conclusion

Results from this study strongly suggest that distal hyperalgesia of late-stage CIBP demonstrates differential characteristics consistent with neuropathic pain as compared to early stage, which appears more inflammatory in nature.

Meta-Analysis of the Effect of Exercise on Neuropathic Pain Induced by Peripheral Nerve Injury in Rat Models

Background: There is accumulating evidence showing that exercise therapy may play an active role in peripheral neuropathic pain (NP). However, there have been no meta-analysis to investigate the effects of exercise on NP induced by peripheral nerve injury in rat models.

Methods: PubMed, EMBASE, and Web of Science were searched from inception to January 2019. A random-effect model was implemented to provide effect estimates for pain-related behavioral test outcome. Mean differences (MDs) with 95% confidence intervals (CIs) were calculated.

Results: Fourteen studies were included. For the mechanical withdrawal threshold, rats in the exercised group exhibited significantly higher thresholds than those in the control group, with a MD of 0.91 (95% CI 0.11–1.71), 3.11 (95% CI 1.56–4.66), 3.48 (95% CI 2.70–4.26), 4.16 (95% CI 2.53–5.79), and 5.58 (95% CI 3.44–7.73) at 1, 2, 3, 4, and 5 weeks, respectively. Additionally, thermal withdrawal latency increased in the exercised group compared with the control group, with a MD of 2.48 (95% CI 0.59–4.38), 3.57 (95% CI 2.10–5.05), 3.92 (95% CI 2.82–5.03), and 2.84 (95% CI 1.29–4.39) at 1, 2, 3, and 4 weeks, respectively. Subgroup analyses were performed for pain models, exercise start point, exercise forms, and duration, which decreased heterogeneity to some extent.

Conclusion: This meta-analysis indicated that exercise provoked an increase in mechanical withdrawal threshold and thermal withdrawal latency in animal NP models. Exercise therapy may be a promising non-pharmacologic therapy to prevent the development of NP. Further, preclinical studies focused on improving experiment design and reporting are still needed.

Randomized Controlled Trial of Physical Exercise in Diabetic Veterans With Length-Dependent Distal Symmetric Polyneuropathy

Rationale: Physical exercise is an essential adjunct to the management of patients with type 2 diabetes mellitus. Therapeutic interventions that improve blood flow to peripheral nerves, such as exercise, may slow the progression of neuropathy in the diabetic patient.

Aims: This randomized clinical trial was conducted to determine whether a structured program of aerobic, isokinetic strength, or the combination of aerobic–isokinetic strength exercise intervention alters peripheral nerve function in glycemic-controlled diabetic patients with advanced length-dependent distal symmetric polyneuropathy.

Methods: Forty-five patients with type 2 diabetes mellitus exhibiting tight glycemic control (HbA_1c intergroup range 7.2–8.0%) were randomized by block design across four experimental groups: sedentary controls (n = 12), aerobic exercise (n = 11), isokinetic strength (n = 11), or the combination of aerobic–isokinetic strength training (n = 11). Patients randomized to training groups exercised 3× per week for 12 weeks, whereas patients randomized to the sedentary control group received standard of care. To minimize attention and educational bias, all patients attended a 12-session health promotion educational series. At baseline, immediately following intervention, and again at 12-week post-intervention, detailed nerve conduction studies were conducted as a primary outcome measure. At these same intervals, all patients completed as secondary measures quantitative sensory testing, symptom-limited treadmill stress tests, and a Short-Form 36-Veterans Questionnaire (SF-36V).

Results: Of the 45 patients randomized into this study, 37 (82%) had absent sural nerve responses, 19 (42%) had absent median sensory nerve responses, and 17 (38%) had absent ulnar sensory nerve responses. By comparison, responses from tibial nerves were absent in only three (7%) subjects while responses from peroneal nerves were absent in five (11%) subjects. Eleven (92%) of 12 patients that had volunteered to be biopsied exhibited abnormal levels of epidermal nerve fiber densities. Exercise, regardless of type, did not alter sensory or motor nerve electrodiagnostic findings among those patients exhibiting measurable responses (ANOVA). There was, however, a modest (p = 0.01) beneficial effect of exercise on sensory nerve function (Fisher’s Exact Test). Importantly, the beneficial effect of exercise on sensory nerve function was enhanced (p = 0.03) during the post-intervention interval. In addition, three of six patients that had undergone exercise intervention exhibited a marked 1.9 ± 0.3-fold improvement in epidermal nerve fiber density. By comparison, none of three sedentary patients whom agreed to be biopsied a second time showed improvement in epidermal nerve fiber density. Compared to baseline values within groups, and compared with sedentary values across groups, neither aerobic, isokinetic strength, or the combination of aerobic–isokinetic strength exercise intervention altered peak oxygen uptake. Patients that underwent aerobic or the combined aerobic–isokinetic strength exercise intervention, however, demonstrated an increase in treadmill test duration that was sustained over the 12-week post-intervention period.

Conclusion: A 12-week course of physical exercise, regardless of type, does not alter sensory or motor nerve electrodiagnostic findings. In a subset of patients, a short-term structured program of aerobic exercise may selectively improve sensory nerve fiber function. Large-scale exercise lifestyle intervention trials are warranted to further evaluate the impact of aerobic exercise on sensory nerve fiber function in diabetic neuropathic patients.

Clinical Trial Registration:www.ClinicalTrials.gov, identifier NCT00955201.

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.

Effects of voluntary exercise on antiretroviral therapy-induced neuropathic pain in mice

Antiretroviral therapy (ART) often results in painful peripheral neuropathy. Given that voluntary exercise has been shown to be beneficial in terms of modulating pain-like behaviors in various animal models of peripheral neuropathy, we have investigated the effects of voluntary wheel running on neuropathic pain induced by chronic ART. We first established an animal model of peripheral neuropathy induced by chronic 2',3'-dideoxycytidine (ddC) treatment. We showed that mice receiving ddC (3 mg/kg/day) had increased mechanical and thermal sensitivity at 9 weeks after the onset of the treatment. We also found that voluntary wheel running attenuated or delayed the onset of ddC-induced peripheral neuropathy. This phenomenon was associated with the attenuation of dorsal root ganglion nociceptive neuron membrane excitability and reduction in the expression of the transient receptor potential cation channel subfamily V member 1 (TRPV1). Taken together, these results suggest that voluntary exercise is an effective strategy by which ART-induced peripheral neuropathy can be alleviated.

A Mechanism-Based Approach to Physical Therapist Management of Pain

Pain reduction is a primary goal of physical therapy for patients who present with acute or persistent pain conditions. The purpose of this review is to describe a mechanism-based approach to physical therapy pain management. It is increasingly clear that patients need to be evaluated for changes in peripheral tissues and nociceptors, neuropathic pain signs and symptoms, reduced central inhibition and enhanced central excitability, psychosocial factors, and alterations of the movement system. In this Perspective, 5 categories of pain mechanisms (nociceptive, central, neuropathic, psychosocial, and movement system) are defined, and principles on how to evaluate signs and symptoms for each mechanism are provided. In addition, the underlying mechanisms targeted by common physical therapist treatments and how they affect each of the 5 categories are described. Several different mechanisms can simultaneously contribute to a patient's pain; alternatively, 1 or 2 primary mechanisms may cause a patient's pain. Further, within a single pain mechanism, there are likely many possible subgroups. For example, reduced central inhibition does not necessarily correlate with enhanced central excitability. To individualize care, common physical therapist interventions, such as education, exercise, manual therapy, and transcutaneous electrical nerve stimulation, can be used to target specific pain mechanisms. Although the evidence elucidating these pain mechanisms will continue to evolve, the approach outlined here provides a conceptual framework for applying new knowledge as advances are made.

Swimming Training Reduces Neuroma Pain by Regulating Neurotrophins

Introduction

Neuroma formation after peripheral nerve transection leads to severe neuropathic pain in amputees. Previous studies suggested that physical exercise could bring beneficial effect on alleviating neuropathic pain. However, the effect of exercise on neuroma pain still remained unclear. In addition, long-term exercise can affect the expression of neurotrophins (NT), such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), which play key roles in nociceptor sensitization and nerve sprouting after nerve injury. Here, we investigated whether long-term swimming exercise could relieve neuroma pain by modulating NT expression.

Methods

We used a tibial neuroma transposition (TNT) rat model to mimic neuroma pain. After TNT surgery, rats performed swimming exercise for 5 wk. Neuroma pain and tactile sensitivities were detected using von Frey filaments. Immunofluorescence was applied to analyze neuroma formation. NGF and BDNF expressions in peripheral neuroma, dorsal root ganglion, and the spinal cord were measured using enzyme-linked immunosorbent assay and Western blotting.

Results

TNT led to neuroma formation, induced neuroma pain, and mechanical allodynia in hind paw. Five-week swimming exercise inhibited neuroma formation and relieved mechanical allodynia in the hind paw and neuroma pain in the lateral ankle. The analgesic effect lasted for at least 1 wk, even when the exercise ceased. TNT elevated the expressions of BDNF and NGF in peripheral neuroma, dorsal root ganglion, and the spinal cord to different extents. Swimming also decreased the elevation of NT expression.

Conclusions

Swimming exercise not only inhibits neuroma formation induced by nerve transection but also relieves pain behavior. These effects might be associated with the modulation of NT.

Preventive Effect of Maternal Forced Exercise on Offspring Pain Perception and Intensity: The Role of 5-HT2 and D2 Receptors

BACKGROUND

Many previous studies showed that maternal forced exercise can reduce some central disorders in offsprings, but its clear mechanism remains unclear. In this study, the role of 5-HT2 and D2 receptors in neuroprotective effects of maternal forced exercise in offspring neurodevelopment and effect on some behaviors were evaluated.

MATERIALS AND METHODS

Forty-eight pregnant rats were trained by forced exercise, and some behavioral assays in their offspring were performed in the presence and absence of 5-HT2 and D2 receptor antagonists in various experimental groups.

RESULTS

Our data showed that maternal forced exercise caused increase in latency of pain perception in offsprings in hot plate test, writhing test (WT), and tail flick test. Furthermore, a decrease in intensity was shown by WT. On the other hand, treatment of mothers by forced exercise in combination with 5-HT2 and D2 receptor antagonists could inhibit these effects of forced exercise and cause disturbances in pain perception and intensity.

CONCLUSION

Our data suggested that maternal forced exercise causes protective effects on offspring pain perception and intensity, and in this effect, 5-HT2 and D2 receptors are probably involved.

Regular physical activity prevents development of chronic muscle pain through modulation of supraspinal opioid and serotonergic mechanisms

The current study shows that blockade of opioid receptors systemically in the periaqueductal gray and the rostral ventromedial medulla prevents analgesia by 8 weeks of wheel running in a chronic muscle pain model. We further show increases in serotonin transporter expression and reversal of hyperalgesia with a selective reuptake inhibitor in the rostral ventromedial medulla in the chronic muscle pain model, and exercise normalizes serotonin transporter expression.

Introduction:

It is generally believed that exercise produces its effects by activating central opioid receptors; there are little data that support this claim. The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are key nuclei in opioid-induced analgesia, and opioids interact with serotonin to produce analgesia.

Objectives:

The purpose was to examine central inhibitory mechanisms involved in analgesia produced by wheel running.

Methods:

C57/Black6 mice were given access to running wheels in their home cages before induction of chronic muscle hyperalgesia and compared with those without running wheels. Systemic, intra-PAG, and intra-RVM naloxone tested the role of central opioid receptors in the antinociceptive effects of wheel running in animals with muscle insult. Immunohistochemistry for the serotonin transporter (SERT) in the spinal cord and RVM, and pharmacological blockade of SERT, tested whether the serotonin system was modulated by muscle insult and wheel running.

Results:

Wheel running prevented the development of muscle hyperalgesia. Systemic naloxone, intra-PAG naloxone, and intra-RVM naloxone reversed the antinociceptive effect of wheel running in animals that had received muscle insult. Induction of chronic muscle hyperalgesia increased SERT in the RVM, and blockade of SERT reversed the hyperalgesia in sedentary animals. Wheel running reduced SERT expression in animals with muscle insult. The serotonin transporter in the superficial dorsal horn of the spinal cord was unchanged after muscle insult, but increased after wheel running.

Conclusion:

These data support the hypothesis that wheel running produced analgesia through central inhibitory mechanisms involving opioidergic and serotonergic systems.

Effects of Treadmill Exercise on Advanced Osteoarthritis Pain in Rats

OBJECTIVE

Exercise is commonly recommended for patients with osteoarthritis (OA) pain. However, whether exercise is beneficial in ameliorating ongoing pain that is persistent, resistant to nonsteroidal antiinflammatory drugs (NSAIDs), and associated with advanced OA is unknown.

METHODS

Rats treated with intraarticular (IA) monosodium iodoacetate (MIA) or saline underwent treadmill exercise or remained sedentary starting 10 days postinjection. Tactile sensory thresholds and weight bearing were assessed, followed by radiography at weekly intervals. After 4 weeks of exercise, ongoing pain was assessed using conditioned place preference (CPP) to IA or rostral ventromedial medulla (RVM)-administered lidocaine. The possible role of endogenous opioids in exercise-induced pain relief was examined by systemic administration of naloxone. Knee joints were collected for micro-computed tomography (micro-CT) analysis to examine pathologic changes to subchondral bone and metaphysis of the tibia.

RESULTS

Treadmill exercise for 4 weeks reversed MIA-induced tactile hypersensitivity and weight asymmetry. Both IA and RVM lidocaine D35, administered post-MIA, induced CPP in sedentary but not exercised MIA-treated rats, indicating that exercise blocks MIA-induced ongoing pain. Naloxone reestablished weight asymmetry in MIA-treated rats undergoing exercise and induced conditioned place aversion, indicating that exercise-induced pain relief is dependent on endogenous opioids. Exercise did not alter radiographic evidence of OA. However, micro-CT analysis indicated that exercise did not block lateral subchondral bone loss or trabecular bone loss in the metaphysis, but did block MIA-induced medial bone loss.

CONCLUSION

These findings support the conclusion that exercise induces pain relief in advanced, NSAID-resistant OA, likely through increased endogenous opioid signaling. In addition, treadmill exercise blocked MIA-induced bone loss in this model, indicating a potential bone-stabilizing effect of exercise on the OA joint.

A new aspect of chronic pain as a lifestyle-related disease

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Activation of mesolimbic dopamine system underlies exercise-induced hypoalgesia.

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Interaction between mesolimbic system and hypothalamus determines physical activity.

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Changing the lifestyle inactive to active may attenuate and prevent chronic pain.

Physical exercise has been established as a low-cost, safe, and effective way to manage chronic intractable pain. We investigated the underlying mechanisms of exercise-induced hypoalgesia (EIH) using a mouse model of neuropathic pain (NPP). Epigenetic changes in activated microglia and maintained GABA synthesis in the spinal dorsal horn may contribute to EIH. Voluntary exercise (VE), a strong reward for animals, also induced EIH, which may be due in part to the activation of dopamine (DA) neurons in the ventral tegmental area (VTA). VE increases the expression of pCREB in dopaminergic neurons in the VTA, which would enhance dopamine production, and thereby contributes to the activation of the mesolimbic reward system in NPP model mice. We demonstrated that neurons in the laterodorsal tegmental and pedunculopontine tegmental nuclei, a major input source of rewarding stimuli to the VTA, were activated by exercise.

Chronic pain is at least partly attributed to sedentary and inactive lifestyle as indicated by the Fear-avoidance model. Therefore, chronic pain could be recognized as a lifestyle-related disease. Physical activity/inactivity may be determined by genetic/epigenetic and neural factors encoded in our brain. The hypothalamus and reward system is closely related in the axis of food intake, energy metabolism and physical activity. Understanding the interactions between the mesolimbic DA system and the hypothalamus that sense and regulate energy balance is thus of significant importance. For example, proopiomelanocortin neurons and melanocortin 4 receptors may play a role in connecting these two systems. Therefore, in a certain sense, chronic pain and obesity may share common behavioral and neural pathology, i.e. physical inactivity, as a result of inactivation of the mesolimbic DA system. Exercise and increasing physical activity in daily life may be important in treating and preventing chronic pain, a life-style related disease.

Prior voluntary wheel running attenuates neuropathic pain

Exercise is known to exert a systemic anti-inflammatory influence, but whether its effects are sufficient to protect against subsequent neuropathic pain is underinvestigated. We report that 6 weeks of voluntary wheel running terminating before chronic constriction injury (CCI) prevented the full development of allodynia for the ∼3-month duration of the injury. Neuroimmune signaling was assessed at 3 and 14 days after CCI. Prior exercise normalized ipsilateral dorsal spinal cord expression of neuroexcitatory interleukin (IL)-1β production and the attendant glutamate transporter GLT-1 decrease, as well as expression of the disinhibitory P2X4R-BDNF axis. The expression of the macrophage marker Iba1 and the chemokine CCL2 (MCP-1), and a neuronal injury marker (activating transcription factor 3), was attenuated by prior running in the ipsilateral lumbar dorsal root ganglia. Prior exercise suppressed macrophage infiltration and/or injury site proliferation, given decreased presence of macrophage markers Iba1, iNOS (M1), and Arg-1 (M2; expression was time dependent). Chronic constriction injury-driven increases in serum proinflammatory chemokines were suppressed by prior running, whereas IL-10 was increased. Peripheral blood mononuclear cells were also stimulated with lipopolysaccharide ex vivo, wherein CCI-induced increases in IL-1β, nitrite, and IL-10 were suppressed by prior exercise. Last, unrestricted voluntary wheel running, beginning either the day of, or 2 weeks after, CCI, progressively reversed neuropathic pain. This study is the first to investigate the behavioral and neuroimmune consequences of regular exercise terminating before nerve injury. This study suggests that chronic pain should be considered a component of "the diseasome of physical inactivity," and that an active lifestyle may prevent neuropathic pain.

Incline treadmill exercise suppresses pain hypersensitivity associated with the modulation of pro-inflammatory cytokines and anti-inflammatory cytokine in rats with peripheral nerve injury

We aimed to investigate the impact of 3 weeks of incline treadmill exercise (TE) on withdrawal responses elicited by thermal and mechanical stimuli, and on anti-inflammatory cytokine (interleukin-10, IL-10) and pro-inflammatory cytokines (IL-6 and tumor necrosis factor-alpha [TNF-α]) expression in the sciatic nerve of rats underwent chronic constriction injury (CCI). Group 1 received a sham-operation where the sciatic nerve was exposed but not ligated, while Group 2 underwent a sham-operation followed by exercising on an 8%-incline treadmill (TE8). Group 3 underwent only the CCI procedure, and Groups 4 and 5 underwent the CCI procedure followed by exercising on an 0%-incline treadmill (TE0) and TE8, respectively. Mechanical and thermal sensitivity and protein expression of IL-10, IL-6 and TNF-α were evaluated on postoperative days 12 and 26. Among the five groups, Group 5 displayed the least weight gain. Compared with Group 3, Group 5 had smaller decreases in mechanical withdrawal thresholds and heat withdrawal latencies. The CCI rats who received TE at 8% incline showed the downregulation of TNF-α and IL-6 in their sciatic nerves on postoperative days 12 and 26, as was found in the Group 3 rats. TE at 8% incline also prevented the downregulation of IL-10 in their sciatic nerves on postoperative day 12. The results demonstrated that increased incline improves the anti-nociceptive effects of treadmill running. Inclined exercise reduces the levels of pro-inflammatory cytokines and increases the level of an anti-inflammatory cytokine.

Modest Amounts of Voluntary Exercise Reduce Pain- and Stress-Related Outcomes in a Rat Model of Persistent Hind Limb Inflammation

Aerobic exercise improves outcomes in a variety of chronic health conditions, yet the support for exercise-induced effects on chronic pain in humans is mixed. Although many rodent studies have examined the effects of exercise on persistent hypersensitivity, the most used forced exercise paradigms that are known to be highly stressful. Because stress can also produce analgesic effects, we studied how voluntary exercise, known to reduce stress in healthy subjects, alters hypersensitivity, stress, and swelling in a rat model of persistent hind paw inflammation. Our data indicate that voluntary exercise rapidly and effectively reduces hypersensitivity as well as stress-related outcomes without altering swelling. Moreover, the level of exercise is unrelated to the analgesic and stress-reducing effects, suggesting that even modest amounts of exercise may impart significant benefit in persistent inflammatory pain states.

PERSPECTIVE

Modest levels of voluntary exercise reduce pain- and stress-related outcomes in a rat model of persistent inflammatory pain, independently of the amount of exercise. As such, consistent, self-regulated activity levels may be more relevant to health improvement in persistent pain states than standardized exercise goals.

Physical Training and Activity in People With Diabetic Peripheral Neuropathy: Paradigm Shift

Diabetic peripheral neuropathy (DPN) occurs in more than 50% of people with diabetes and is an important risk factor for skin breakdown, amputation, and reduced physical mobility (ie, walking and stair climbing). Although many beneficial effects of exercise for people with diabetes have been well established, few studies have examined whether exercise provides comparable benefits to people with DPN. Until recently, DPN was considered to be a contraindication for walking or any weight-bearing exercise because of concerns about injuring a person's insensitive feet. These guidelines were recently adjusted, however, after research demonstrated that weight-bearing activities do not increase the risk of foot ulcers in people who have DPN but do not have severe foot deformity. Emerging research has revealed positive adaptations in response to overload stress in these people, including evidence for peripheral neuroplasticity in animal models and early clinical trials. This perspective article reviews the evidence for peripheral neuroplasticity in animal models and early clinical trials, as well as adaptations of the integumentary system and the musculoskeletal system in response to overload stress. These positive adaptations are proposed to promote improved function in people with DPN and to foster the paradigm shift to including weight-bearing exercise for people with DPN. This perspective article also provides specific assessment and treatment recommendations for this important, high-risk group.

Exercise prevents development of autonomic dysregulation and hyperalgesia in a mouse model of chronic muscle pain

Chronic musculoskeletal pain (CMP) conditions, like fibromyalgia, are associated with widespread pain and alterations in autonomic functions. Regular physical activity prevents the development of CMP and can reduce autonomic dysfunction. We tested if there were alterations in autonomic function of sedentary mice with CMP, and whether exercise reduced the autonomic dysfunction and pain induced by CMP. Chronic musculoskeletal pain was induced by 2 intramuscular injections of pH 5.0 in combination with a single fatiguing exercise task. A running wheel was placed into cages so that the mouse had free access to it for either 5 days or 8 weeks (exercise groups) and these animals were compared to sedentary mice without running wheels. Autonomic function and nociceptive withdrawal thresholds of the paw and muscle were assessed before and after induction of CMP in exercised and sedentary mice. In sedentary mice, we show decreased baroreflex sensitivity, increased blood pressure variability, decreased heart rate variability, and decreased withdrawal thresholds of the paw and muscle 24 hours after induction of CMP. There were no sex differences after induction of the CMP in any outcome measure. We further show that both 5 days and 8 weeks of physical activity prevent the development of autonomic dysfunction and decreases in withdrawal threshold induced by CMP. Thus, this study uniquely shows the development of autonomic dysfunction in animals with chronic muscle hyperalgesia, which can be prevented with as little as 5 days of physical activity, and suggest that physical activity may prevent the development of pain and autonomic dysfunction in people with CMP.

Therapeutic Ultrasound and Treadmill Training Suppress Peripheral Nerve Injury-Induced Pain in Rats

BACKGROUND

Although evidence suggests that therapeutic ultrasound (TU) in combination with treadmill training (TT) suppresses nerve injury-associated pain, the molecular mechanisms for this action are not clear.

OBJECTIVE

The purpose of this research was to study the possible beneficial effects of TU and TT, alone and in combination, on 2 clinical indicators of neuropathic pain and correlate these findings with changes in inflammatory mediators within the spinal cord. Our experimental model used the well-known chronic constriction injury (CCI) of the rat sciatic nerve.

DESIGN

This was an experimental study.

METHODS

Each group contained 10 rats. Group 1 underwent only the CCI procedure. Group 2 underwent a sham operation where the sciatic nerve was exposed but not ligated. Group 3 had the sham operation followed by both TT and TU. Groups 4, 5, and 6 underwent the CCI procedure followed by TT alone, TU alone, and both the TT and TU interventions, respectively. Heat and mechanical sensitivity, interleukin-6 (IL-6), interleukin-10 (IL-10), and ionized calcium binding adaptor molecule 1 (Iba1) were evaluated.

RESULTS

Compared with group 1 animals, TT or TU, or both, produced smaller decreases in mechanical withdrawal threshold and heat withdrawal latencies. The combination of TT and TU was more effective than either treatment alone. In addition, rats that received these treatments did not express the upregulation of IL-6 and Iba1 in their spinal cords on postoperative days 14 and 28, as was found in the group 1 animals.

LIMITATIONS

These experimental findings may not be generalizable to humans.

CONCLUSIONS

The combination of TU and TT reduces neuropathic pain more than either modality alone. This beneficial effect appears related to downregulation of proinflammatory IL-6 and Iba1, while upregulating the anti-inflammatory IL-10.

Emerging Relationships between Exercise, Sensory Nerves, and Neuropathic Pain

The utilization of physical activity as a therapeutic tool is rapidly growing in the medical community and the role exercise may offer in the alleviation of painful disease states is an emerging research area. The development of neuropathic pain is a complex mechanism, which clinicians and researchers are continually working to better understand. The limited therapies available for alleviation of these pain states are still focused on pain abatement and as opposed to treating underlying mechanisms. The continued research into exercise and pain may address these underlying mechanisms, but the mechanisms which exercise acts through are still poorly understood. The objective of this review is to provide an overview of how the peripheral nervous system responds to exercise, the relationship of inflammation and exercise, and experimental and clinical use of exercise to treat pain. Although pain is associated with many conditions, this review highlights pain associated with diabetes as well as experimental studies on nerve damages-associated pain. Because of the global effects of exercise across multiple organ systems, exercise intervention can address multiple problems across the entire nervous system through a single intervention. This is a double-edged sword however, as the global interactions of exercise also require in depth investigations to include and identify the many changes that can occur after physical activity. A continued investment into research is necessary to advance the adoption of physical activity as a beneficial remedy for neuropathic pain. The following highlights our current understanding of how exercise alters pain, the varied pain models used to explore exercise intervention, and the molecular pathways leading to the physiological and pathological changes following exercise intervention.

Painful Diabetic Neuropathy: Prevention or Suppression?

Pain-sensing sensory neurons (nociceptors) of the dorsal root ganglia (DRG) and dorsal horn (DH) can become sensitized (hyperexcitable) in response to pathological conditions such as diabetes, which in turn may lead to the development of painful peripheral diabetic neuropathy (PDN). Because of incomplete knowledge about the mechanisms underlying painful PDN, current treatment for painful PDN has been limited to somewhat nonspecific systemic drugs that have significant side effects or potential for abuse. Recent studies have established that several ion channels in DRG and DH neurons are dysregulated and make a previously unrecognized contribution to sensitization of pain responses by enhancing excitability of nociceptors in animal models of type 1 and type 2 PDN. Furthermore, it has been reported that targeting posttranslational modification of nociceptive ion channels such as glycosylation and methylglyoxal metabolism can completely reverse mechanical and thermal hyperalgesia in diabetic animals with PDN in vivo. Understanding details of posttranslational regulation of nociceptive channel activity may facilitate development of novel therapies for treatment of painful PDN. We argue that pharmacological targeting of the specific pathogenic mechanism rather than of the channel per se may cause fewer side effects and reduce the potential for drug abuse in patients with diabetes.

Improvements in impaired GABA and GAD65/67 production in the spinal dorsal horn contribute to exercise-induced hypoalgesia in a mouse model of neuropathic pain

Background

Physical exercise effectively attenuates neuropathic pain, and multiple events including the inhibition of activated glial cells in the spinal dorsal horn, activation of the descending pain inhibitory system, and reductions in pro-inflammatory cytokines in injured peripheral nerves may contribute to exercise-induced hypoalgesia. Since fewer GABAergic hypoalgesic interneurons exist in the dorsal horn in neuropathic pain model animals, the recovery of impaired GABAergic inhibition in the dorsal horn may improve pain behavior. We herein determined whether the production of gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD) in the dorsal horn is restored by treadmill running and contributes to exercise-induced hypoalgesia in neuropathic pain model mice. C57BL/6 J mice underwent partial sciatic nerve ligation (PSL). PSL-Runner mice ran on a treadmill at 7 m/min for 60 min/day, 5 days/week, from two days after PSL.

Results

Mechanical allodynia and heat hyperalgesia developed in PSL-Sedentary mice but were significantly attenuated in PSL-Runner mice. PSL markedly decreased GABA and GAD65/67 levels in neuropils in the ipsilateral dorsal horn, while treadmill running inhibited these reductions. GABA+ neuronal nuclei+ interneuron numbers in the ipsilateral dorsal horn were significantly decreased in PSL-Sedentary mice but not in PSL-Runner mice. Pain behavior thresholds positively correlated with GABA and GAD65/67 levels and GABAergic interneuron numbers in the ipsilateral dorsal horns of PSL-Sedentary and -Runner mice.

Conclusions

Treadmill running prevented PSL-induced reductions in GAD65/67 production, and, thus, GABA levels may be retained in interneurons and neuropils in the superficial dorsal horn. Therefore, improvements in impaired GABAergic inhibition may be involved in exercise-induced hypoalgesia.

Histone Acetylation in Microglia Contributes to Exercise-Induced Hypoalgesia in Neuropathic Pain Model Mice

Physical exercise can attenuate neuropathic pain (NPP), but the exact mechanism underlying exercise-induced hypoalgesia (EIH) remains unclear. Recent studies have shown that histone hyperacetylation via pharmacological inhibition of histone deacetylases in the spinal cord attenuates NPP, and that histone acetylation may lead to the production of analgesic factors including interleukin 10. We intended to clarify whether histone acetylation in microglia in the spinal dorsal horn contributes to EIH in NPP model mice. C57BL/6J mice underwent partial sciatic nerve ligation (PSL) and PSL- and sham-runner mice ran on a treadmill at a speed of 7 m/min for 60 min/d, 5 days per week, from 2 days after the surgery. PSL-sedentary mice developed mechanical allodynia and heat hyperalgesia, but such behaviors were significantly attenuated in PSL-runner mice. In immunofluorescence analysis, PSL surgery markedly increased the number of histone deacetylase 1-positive/CD11b-positive microglia in the ipsilateral superficial dorsal horn, and they were significantly decreased by treadmill-running. Moreover, the number of microglia with nuclear expression of acetylated H3K9 in the ipsilateral superficial dorsal horn was maintained at low levels in PSL-sedentary mice, but running exercise significantly increased them. Therefore, we conclude that the epigenetic modification that causes hyperacetylation of H3K9 in activated microglia may play a role in producing EIH.

PERSPECTIVE

This article presents the importance of epigenetic modification in microglia in producing EIH. The current research is not only helpful for developing novel nonpharmacological therapy for NPP, but will also enhance our understanding of the mechanisms and availability of exercise in our daily life.

Delayed Exercise Is Ineffective at Reversing Aberrant Nociceptive Afferent Plasticity or Neuropathic Pain After Spinal Cord Injury in Rats

Neuropathic pain is a debilitating consequence of spinal cord injury (SCI) that correlates with sensory fiber sprouting. Recent data indicate that exercise initiated early after SCI prevents the development of allodynia and modulated nociceptive afferent plasticity. This study determined if delaying exercise intervention until pain is detected would similarly ameliorate established SCI-induced pain. Adult, female Sprague-Dawley rats with a C5 unilateral contusion were separated into SCI allodynic and SCI non-allodynic cohorts at 14 or 28 days postinjury when half of each group began exercising on automated running wheels. Allodynia, assessed by von Frey testing, was not ameliorated by exercise. Furthermore, rats that began exercise with no allodynia developed paw hypersensitivity within 2 weeks. At the initiation of exercise, the SCI Allodynia group displayed marked overlap of peptidergic and non-peptidergic nociceptive afferents in the C7 and L5 dorsal horn, while the SCI No Allodynia group had scant overlap. At the end of 5 weeks of exercise both the SCI Allodynia and SCI No Allodynia groups had extensive overlap of the 2 c-fiber types. Our findings show that exercise therapy initiated at early stages of allodynia is ineffective at attenuating neuropathic pain, but rather that it induces allodynia-aberrant afferent plasticity in previously pain-free rats. These data, combined with our previous results, suggest that there is a critical therapeutic window when exercise therapy may be effective at treating SCI-induced allodynia and that there are postinjury periods when exercise can be deleterious.

Effect of environment on the long-term consequences of chronic pain

Much evidence from pain patients and animal models shows that chronic pain does not exist in a vacuum but has varied comorbidities and far-reaching consequences. Patients with long-term pain often develop anxiety and depression and can manifest changes in cognitive functioning, particularly with working memory. Longitudinal studies in rodent models also show the development of anxiety-like behavior and cognitive changes weeks to months after an injury causing long-term pain. Brain imaging studies in pain patients and rodent models find that chronic pain is associated with anatomical and functional alterations in the brain. Nevertheless, studies in humans reveal that lifestyle choices, such as the practice of meditation or yoga, can reduce pain perception and have the opposite effect on the brain as does chronic pain. In rodent models, studies show that physical activity and a socially enriched environment reduce pain behavior and normalize brain function. Together, these studies suggest that the burden of chronic pain can be reduced by nonpharmacological interventions.

Forced Exercise Preconditioning Attenuates Experimental Autoimmune Neuritis by Altering Th1 Lymphocyte Composition and Egress

A short-term exposure to moderately intense physical exercise affords a novel measure of protection against autoimmune-mediated peripheral nerve injury. Here, we investigated the mechanism by which forced exercise attenuates the development and progression of experimental autoimmune neuritis (EAN), an established animal model of Guillain–Barré syndrome. Adult male Lewis rats remained sedentary (control) or were preconditioned with forced exercise (1.2 km/day × 3 weeks) prior to P2-antigen induction of EAN. Sedentary rats developed a monophasic course of EAN beginning on postimmunization day 12.3 ± 0.2 and reaching peak severity on day 17.0 ± 0.3 (N = 12). By comparison, forced-exercise preconditioned rats exhibited a similar monophasic course but with significant (p < .05) reduction of disease severity. Analysis of popliteal lymph nodes revealed a protective effect of exercise preconditioning on leukocyte composition and egress. Compared with sedentary controls, forced exercise preconditioning promoted a sustained twofold retention of P2-antigen responsive leukocytes. The percentage distribution of pro-inflammatory (T_h1) lymphocytes retained in the nodes from sedentary EAN rats (5.1 ± 0.9%) was significantly greater than that present in nodes from forced-exercise preconditioned EAN rats (2.9 ± 0.6%) or from adjuvant controls (2.0 ± 0.3%). In contrast, the percentage of anti-inflammatory (T_h2) lymphocytes (7–10%) and that of cytotoxic T lymphocytes (∼20%) remained unaltered by forced exercise preconditioning. These data do not support an exercise-inducible shift in T_h1:T_h2 cell bias. Rather, preconditioning with forced exercise elicits a sustained attenuation of EAN severity, in part, by altering the composition and egress of autoreactive proinflammatory (T_h1) lymphocytes from draining lymph nodes.

Moderate exercise training attenuates inflammatory mediators in DRG of Type 1 diabetic rats

Painful neuropathy is a long-term and difficult to treat complication of diabetes that affects 25% of diabetic patients and interferes with their quality of life. Unfortunately, available medical treatments are relatively ineffective due to dependency and addiction. Emerging research indicates that moderate-to-vigorous physical activity provides health-related benefits. However, adequate data is not available to determine whether regular exercise would prevent or delay the development of painful neuropathy in subjects with Type 1 diabetes. This study demonstrates the significance of moderate exercise in the amelioration of pain in animals with Type 1 diabetes after 6weeks of exercise paradigm. After initial acclimatization, streptozotocin-diabetic animals were placed in motorized running wheels for 60minperday, for five days a week for 6weeks starting at one week after diabetes. A growing body of evidence suggests that the release of proinflammatory cytokines plays an important role in the development and persistence of pain. This study demonstrates that moderate exercise increases the expression of inhibitory neurotransmitter enkephalin and also reduces the presence of a number of proinflammatory cytokines in the dorsal root ganglia (DRG), subsequently impeding the development of neuropathy along with a decrease in the voltage gated ion channels in the DRG. Overall, the study suggests that exercise may provide an alternate route of treatment of painful neuropathy in Type 1 diabetic subjects by decreasing the use of pain medications, thereby providing a more useful and efficient way for pain management.

Exercise-induced modulation of pain in adults with and without painful diabetic neuropathy

The purpose of this study was to examine exercise-induced pain modulation in diabetic adults with painful diabetic neuropathy (PDN) compared to diabetic adults without PDN. Eighteen adults diagnosed with type 2 diabetes with and without PDN (mean age of 49 years) completed 2 sessions. During the familiarization session, participants completed questionnaires, were familiarized with the pain testing protocols, and completed maximal isometric contractions. During the exercise session, experimental pain testing was completed before and following exercise consisting of 3 minutes of isometric exercise performed at 25% maximal voluntary contraction. Ratings of perceived exertion and muscle pain were assessed every 30 seconds during exercise. Results indicated that ratings of perceived exertion and muscle pain during exercise were significantly higher (P < .05) for diabetic adults with PDN versus diabetic adults without PDN. Diabetic adults with PDN did not experience changes in thermal pain ratings following exercise, whereas diabetic adults without PDN reported significantly lower pain ratings following exercise. It is concluded that diabetic adults with PDN experienced high levels of muscle pain during exercise and a lack of exercise-induced hypoalgesia following exercise, in comparison to diabetic adults without PDN, who experienced lower levels of muscle pain during exercise and a hypoalgesic response following exercise.

PERSPECTIVE

Very little research has been conducted examining the impact of exercise on pain modulation in diabetic adults with PDN. This study provides support that adults with PDN exhibit exercise-induced endogenous pain modulatory system dysfunction.

Benefits of exercise intervention in reducing neuropathic pain

Peripheral neuropathy is a widespread and potentially incapacitating pathological condition that encompasses more than 100 different forms and manifestations of nerve damage. The diverse pathogenesis of peripheral neuropathy affects autonomic, motor and/or sensory neurons, and the symptoms that typify the condition are abnormal cutaneous sensation, muscle dysfunction and, most notably, chronic pain. Chronic neuropathic pain is difficult to treat and is often characterized by either exaggerated responses to painful stimuli (hyperalgesia) or pain resulting from stimuli that would not normally provoke pain (allodynia). The objective of this review is to provide an overview of some pathways associated with the development of peripheral neuropathy and then discuss the benefits of exercise interventions. The development of neuropathic pain is a highly complex and multifactorial process, but recent evidence indicates that the activation of spinal glial cells via the enzyme glycogen synthase kinase 3 and increases in the production of both pro-inflammatory cytokines and brain derived neurotropic factor are crucial steps. Since many of the most common causes of peripheral neuropathy cannot be fully treated, it is critical to understand that routine exercise may not only help prevent some of those causes, but that it has also proven to be an effective means of alleviating some of the condition’s most distressing symptoms. More research is required to elucidate the typical mechanisms of injury associated with peripheral neuropathy and the exercise-induced benefits to those mechanisms.

Targeting of CaV3.2 T-type calcium channels in peripheral sensory neurons for the treatment of painful diabetic neuropathy

Pain-sensing sensory neurons (nociceptors) of the dorsal root ganglion (DRG) can become sensitized (hyperexcitable) in response to pathological conditions such as diabetes, which in turn may lead to the development of painful peripheral diabetic neuropathy (PDN). Because of insufficient knowledge about the mechanisms for this hypersensitization, current treatment for painful PDN has been limited to somewhat nonspecific systemic drugs having significant side effects or potential for abuse. Recent studies have established that the CaV3.2 isoform of T-channels makes a previously unrecognized contribution to sensitization of pain responses by enhancing excitability of nociceptors in animal models of type 1 and type 2 PDN. Furthermore, it has been reported that the glycosylation inhibitor neuraminidase can inhibit the native and recombinant CaV3.2 T-currents in vitro and completely reverse mechanical and thermal hyperalgesia in diabetic animals with PDN in vivo. Understanding details of posttranslational regulation of nociceptive channel activity via glycosylation may facilitate development of novel therapies for treatment of painful PDN. Pharmacological targeting the specific pathogenic mechanism rather than the channel per se may cause fewer side effects and reduce the potential for drug abuse in patients with diabetes.

Pro-nociceptive role of the activation of mineralocorticoid receptor in the pathogenesis of painful diabetic neuropathy

Patients with diabetic neuropathy may develop severe pain which persists over several years, resulting from impaired nerve endings in the skin, which originate the pain signals of diabetic neuropathy. Inflammatory processes have been implicated in the genesis and maintenance of chronic pain. The activation of the mineralocorticoid receptor (MR) is believed to promote classical inflammation features such as high levels of oxidative metabolites and proinflammatory cytokines, tissue destruction. Selectively blocking MR's can prevent the development of pain behaviors induced by neuroinflammation. Since proinflammatory cytokines and mediators were found to have increased in diabetic skin, we propose MR activation may play a pro-nociceptive role in diabetic neuropathy through local inflammation of the skin. Research methods examining MR overexpression in normal skin and selectively blocking MR in the diabetic skin are useful in identifying whether MR activation may bring cutaneous nerve insult and to explain whether MR activation is involved in the progression of painful diabetic neuropathy. If proven, this hypothesis would indicate the MR may potentially act as a novel target for pain therapeutics.

Regular physical activity prevents development of chronic pain and activation of central neurons

Chronic musculoskeletal pain is a significant health problem and is associated with increases in pain during acute physical activity. Regular physical activity is protective against many chronic diseases; however, it is unknown if it plays a role in development of chronic pain. The current study induced physical activity by placing running wheels in home cages of mice for 5 days or 8 wk and compared these to sedentary mice without running wheels in their home cages. Chronic muscle pain was induced by repeated intramuscular injection of pH 4.0 saline, exercise-enhanced pain was induced by combining a 2-h fatiguing exercise task with a low-dose muscle inflammation (0.03% carrageenan), and acute muscle inflammation was induced by 3% carrageenan. We tested the responses of the paw (response frequency) and muscle (withdrawal threshold) to nociceptive stimuli. Because the rostral ventromedial medulla (RVM) is involved in exercise-induced analgesia and chronic muscle pain, we tested for changes in phosphorylation of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor in the RVM. We demonstrate that regular physical activity prevents the development of chronic muscle pain and exercise-induced muscle pain by reducing phosphorylation of the NR1 subunit of the NMDA receptor in the central nervous system. However, regular physical activity has no effect on development of acute pain. Thus physical inactivity is a risk factor for development of chronic pain and may set the nervous system to respond in an exaggerated way to low-intensity muscle insults.

Specific functioning of Cav3.2 T-type calcium and TRPV1 channels under different types of STZ-diabetic neuropathy

Streptozotocin (STZ)-induced type 1 diabetes in rats leads to the development of peripheral diabetic neuropathy (PDN) manifested as thermal hyperalgesia at early stages (4th week) followed by hypoalgesia after 8weeks of diabetes development. Here we found that 6-7 week STZ-diabetic rats developed either thermal hyper- (18%), hypo- (25%) or normalgesic (57%) types of PDN. These developmentally similar diabetic rats were studied in order to analyze mechanisms potentially underlying different thermal nociception. The proportion of IB4-positive capsaicin-sensitive small DRG neurons, strongly involved in thermal nociception, was not altered under different types of PDN implying differential changes at cellular and molecular level. We further focused on properties of T-type calcium and TRPV1 channels, which are known to be involved in Ca(2+) signaling and pathological nociception. Indeed, TRPV1-mediated signaling in these neurons was downregulated under hypo- and normalgesia and upregulated under hyperalgesia. A complex interplay between diabetes-induced changes in functional expression of Cav3.2 T-type calcium channels and depolarizing shift of their steady-state inactivation resulted in upregulation of these channels under hyper- and normalgesia and their downregulation under hypoalgesia. As a result, T-type window current was increased by several times under hyperalgesia partially underlying the increased resting [Ca(2+)]i observed in the hyperalgesic rats. At the same time Cav3.2-dependent Ca(2+) signaling was upregulated in all types of PDN. These findings indicate that alterations in functioning of Cav3.2 T-type and TRPV1 channels, specific for each type of PDN, may underlie the variety of pain syndromes induced by type 1 diabetes.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Synaptic transmission at parasympathetic neurons of the major pelvic ganglion from normal and diabetic male mice

Bladder and erectile dysfunction are common urologic complications of diabetes and are associated with reduced parasympathetic autonomic control. To determine whether disruption of ganglionic neurotransmission contributes to the loss of function, we investigated synaptic transmission at parasympathetic, major pelvic ganglion (MPG) neurons in control and chronically (20 wk) diabetic mice. In contrast to what has been reported for sympathetic neurons, diabetes did not cause an interruption of synaptic transmission at parasympathetic MPG neurons from streptozotocin-treated C57BL/6J (STZ) or db/db mice. Cholinergically mediated excitatory postsynaptic potentials (EPSPs) were suprathreshold during 5-s trains of 5-, 10-, and 20-Hz stimuli. Asynchronous neurotransmitter release, observed as miniature EPSPs (mEPSPs) during and after stimulation, permitted quantitative assessment of postganglionic, cholinergic receptor sensitivity. mEPSP amplitude following tetanic stimulation (recorded at -60 mV) was reduced in STZ (4.95 ± 0.4 vs. 3.71 ± 0.3 mV, P = 0.03), but not db/db mice. The number of posttetanic mEPSPs was significantly greater in db/db mice at all frequencies tested. Assessment of basic electrophysiological properties revealed that parasympathetic MPG neurons from db/db mice had less negative membrane potentials, lower input resistances, and shorter afterhyperpolarizations relative to their control. MPG neurons from STZ had longer afterhyperpolarizations but were otherwise similar to controls. Membrane excitability, measured by the membrane responsiveness to long-duration (1 s), suprathreshold depolarizing pulses, was unchanged in either model. The present study indicates that, while parasympathetic neurotransmission at the MPG is intact in chronically diabetic mice, obese, type 2 diabetic animals exhibit an altered presynaptic regulation of neurotransmitter release.

Prolonged nerve blockade delays the onset of neuropathic pain

Aberrant neuronal activity in injured peripheral nerves is believed to be an important factor in the development of neuropathic pain. Pharmacological blockade of that activity has been shown to mitigate the onset of associated molecular events in the nervous system. However, results in preventing onset of pain behaviors by providing prolonged nerve blockade have been mixed. Furthermore, the experimental techniques used to date to provide that blockade were limited in clinical potential in that they would require surgical implantation. To address these issues, we have used liposomes (SDLs) containing saxitoxin (STX), a site 1 sodium channel blocker, and the glucocorticoid agonist dexamethasone to provide nerve blocks lasting ~1 wk from a single injection. This formulation is easily injected percutaneously. Animals undergoing spared nerve injury (SNI) developed mechanical allodynia in 1 wk; nerve blockade with a single dose of SDLs (duration of block 6.9 ± 1.2 d) delayed the onset of allodynia by 2 d. Treatment with three sequential SDL injections resulting in a nerve block duration of 18.1 ± 3.4 d delayed the onset of allodynia by 1 mo. This very prolonged blockade decreased activation of astrocytes in the lumbar dorsal horn of the spinal cord due to SNI. Changes in expression of injury-related genes due to SNI in the dorsal root ganglia were not affected by SDLs. These findings suggest that formulations of this kind, which could be easy to apply clinically, can mitigate the development of neuropathic pain.