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. 2017 Aug 21;2(5):e618. doi: 10.1097/PR9.0000000000000618

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Copyright © 2017 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of The International Association for the Study of Pain.

This is an open access article distributed under the Creative Commons Attribution License 4.0 (CCBY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Figure 2. — Central opioid receptors in periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) mediate the analgesia produced by wheel running. Muscle withdrawal thresholds for animals treated with naloxone systemically (A and B), into the PAG (C and D) and into the RVM (E and F) for both the ipsilateral and contralateral sides. All animals were given running wheels for 8 weeks before induction of the chronic muscle pain model. Thirty minutes after injection of naloxone systemically, or into the PAG or RVM, there was a significant decrease in the withdrawal thresholds of the muscle (*, P < 0.05) when compared with animals that received vehicle. Missed injection sites at the level of the PAG or RVM show significantly greater withdrawal thresholds compared with those injected with naloxone into the PAG or RVM (+, P < 0.05). Data are the mean ± SEM.