Voluntary Exercise Training: Analysis of Mice in Uninjured, Inflammatory, and Nerve-Injured Pain States

RESEARCH ARTICLE Voluntary Exercise Training: Analysis of Mice in Uninjured, Inflammatory, and NerveInjured Pain States Tayler D. Sheahan1,2, Bryan A. Copits1, Judith P. Golden1, Robert W. Gereau, IV1,2* 1 Washington University Pain Center and Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri, United States of America, 2 Washington University Program in Neuroscience, Washington University School of Medicine, St. Louis, Missouri, United States of America * Abstract Published: July 21, 2015 Both clinical and animal studies suggest that exercise may be an effective way to manage inflammatory and neuropathic pain conditions. However, existing animal studies commonly use forced exercise paradigms that incorporate varying degrees of stress, which itself can elicit analgesia, and thus may complicate the interpretation of the effects of exercise on pain. We investigated the analgesic potential of voluntary wheel running in the formalin model of acute inflammatory pain and the spared nerve injury model of neuropathic pain in mice. In uninjured, adult C57BL/6J mice, 1 to 4 weeks of exercise training did not alter nociceptive thresholds, lumbar dorsal root ganglia neuronal excitability, or hindpaw intraepidermal innervation. Further, exercise training failed to attenuate formalin-induced spontaneous pain. Lastly, 2 weeks of exercise training was ineffective in reversing spared nerve injuryinduced mechanical hypersensitivity or in improving muscle wasting or hindpaw denervation. These findings indicate that in contrast to rodent forced exercise paradigms, short durations of voluntary wheel running do not improve pain-like symptoms in mouse models of acute inflammation and peripheral nerve injury. Copyright: © 2015 Sheahan et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Introduction OPEN ACCESS Citation: Sheahan TD, Copits BA, Golden JP, Gereau RW, IV (2015) Voluntary Exercise Training: Analysis of Mice in Uninjured, Inflammatory, and Nerve-Injured Pain States. PLoS ONE 10(7): e0133191. doi:10.1371/journal.pone.0133191 Editor: David D McKemy, University of South California, UNITED STATES Received: March 24, 2015 Accepted: June 23, 2015 Data Availability Statement: All relevant data are within the paper. Funding: RWG was supported by National Institutes of Neurological Disorders and Stroke R01 NS48602 (nih.gov). BAC was supported by Washington University in St. Louis W.M. Keck Fellowship in Molecular Medicine (http://www.wmkeck.org) and National Institutes of General Medical Sciences TR32 GM108539 (nih.gov). The funders had no role in study design, data collection and analysis, decision to publish, or in preparation of the manuscript. Chronic pain is a debilitating condition that effects over 100 million Americans and has an annual cost of $635 billion in the form of health care expenses and productivity loss [1,2]. Clinical studies suggest aerobic exercise is an effective, non-invasive approach to managing ongoing inflammatory and neuropathic pain conditions, as well as musculoskeletal disorders [3–7]. Similarly, exercise improves pain-like symptoms in rodent inflammatory and neuropathic pain models [8–20]. The effect of exercise on pain has been primarily evaluated using forced exercise paradigms. For example, recent studies have demonstrated that forced treadmill running following peripheral nerve injury reverses nerve injury-induced thermal and mechanical hypersensitivity in rats and mice [9–11,21,22]. Extended swimming has also been shown to both attenuate the PLOS ONE | DOI:10.1371/journal.pone.0133191 July 21, 2015 1 / 21 Effects of Voluntary Exercise on Pain Competing Interests: The authors have declared that no competing interests exist. development of and reverse nerve injury-induced thermal and mechanical hypersensitivity in rodents, as well as formalin-induced spontaneous pain in rats [8,14,20]. As acknowledged in these studies and others, forced exercise may elicit both acute and chronic stress responses that in turn can produce analgesia [23–28]. Although most forced exercise studies have evaluated nociceptive thresholds after exercise-induced acute stress responses have resolved, prolonged forced exercise gives rise to chronic stress responses in some cases [23,25,26]. Voluntary exercise may also elicit a stress response [24]; however, numerous studies demonstrate anxiolytic effects of prolonged voluntary wheel running in cases of mild to moderate stress [29–33]. One way to minimize potential complications of stress dependent effects on pain is to use voluntary exercise paradigms. Voluntary wheel running has been shown to be effective in delaying decreases in muscle withdrawal thresholds and increased paw withdrawal frequency in mouse models of chronic muscle pain [13]. Similarly, in a high-fat model of prediabetic neuropathy, voluntary wheel running reverses mechanical and visceral hypersensitivity [17]. While these studies support that voluntary wheel running improves pain-like behavior in rodents, reports utilizing voluntary exercise paradigms in injured and uninjured pain states are limited. To investigate whether repeated voluntary exercise sessions alter basal nociception in rodents, we determined whether voluntary wheel running changes nociceptive thresholds, sensory neuron excitability, or skin innervation in the absence of injury. We also investigated if voluntary exercise training alters acute inflammatory pain responses as well as the hypersensitivity, muscle wasting, or skin denervation induced by the spared nerve injury (SNI) model of neuropathic pain. We report that voluntary wheel running did not alter nociceptive thresholds in uninjured mice, and was ineffective in attenuating acute inflammatory or nerve-injury induced pain. Materials and Methods Ethics Statement The entire study was carried out in accordance to the guidelines of the Washington University in St. Louis Department of Comparative Medicine (DCM). The protocol was approved by the Animal Studies Committee of DCM (Protocol Number: 20130147). Animals Adult C57BL/6J male mice bred in house or obtained from Jackson Labs were housed and cared for in compliance with the Animal Studies Committee of Washington University in St. Louis. Mice were housed on a 12/12 hour (hr) light/dark cycle and had ad libitum access to food and water while in their home cages. Behavioral experiments were initiated on 7–10 week old mice. Over the course of testing, injury (if applicable), and exercise training, mice reached a maximum age of 14 weeks. At the conclusion of behavioral testing, mice were sacrificed using a rodent ketamine euthanasia cocktail. Exercise paradigm During exercise training, mice were placed into individual cages with low-profile wireless runn (...truncated)