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Papers of the Week

Papers: 14 Mar 2020 - 20 Mar 2020

Animal Studies

2020 Mar 13


Physical disuse contributes to widespread chronic mechanical hyperalgesia, tactile allodynia, and cold allodynia through neurogenic inflammation and spino-parabrachio-amygdaloid pathway activation.


Ohmichi Y, Ohmichi M, Tashima R, Osuka K, Fukushige K, Kanikowska D, Fukazawa Y, Yawo H, Tsuda M, Naito M, Nakano T
Pain. 2020 Mar 13.
PMID: 32187118.


Physical disuse could lead to a state of chronic pain typified by complex regional pain syndrome type I due to fear of pain through movement (kinesiophobia) or inappropriate resting procedures. However, the mechanisms by which physical disuse is associated with acute/chronic pain and other pathological signs remain unresolved. We have previously reported that inflammatory signs, contractures, disuse muscle atrophy, spontaneous pain-like behaviors, and chronic widespread mechanical hyperalgesia based on central plasticity occurred after 2-weeks of cast immobilization in chronic post-cast pain (CPCP) rat model. In the present study, we also demonstrated dystrophy-like changes, both peripheral nociceptive signals and activation of the central pain pathway in CPCP rats. This was done by the following methods: (1) vascular permeability (Evans blue dye) and inflammatory- and oxidative stress-related messenger RNA (mRNA) changes (real-time quantitative polymerase chain reaction); (2) immunofluorescence of pERK and/or c-Fos expression in the spino-parabrachio-amygdaloid pathway; and (3) blockade of nociceptive-related signals using sciatic nerve block (SNB). Furthermore, we demonstrated tactile allodynia using an optogenetic method in a transgenic rat line (W-TChR2V4), cold allodynia using the acetone test, and activation of dorsal horn neurons in the chronic phase associated with chronic mechanical hyperalgesia using c-Fos immunofluorescence. In addition, we showed that nociceptive signals in the acute phase are involved in chronic pathological pain-like behaviors by studying the effects of SNB. Thus, we conclude that physical disuse contributes to dystrophy-like changes, spontaneous pain-like behavior, and chronic widespread pathological pain-like behaviors in CPCP rats after 2 weeks of cast immobilization.