The method shown in this article simulates spontaneous pain in patients with nociplastic pain using rats; the measurement with this method could be related to better translation of analgesic efficacies of therapeutic compounds between rats and humans. Nociplastic pain occurs in various disorders including fibromyalgia. Because the pain in patients occurs without an external stimulus, we assessed spontaneous pain in rats. The grimace scale, a methodology for rating facial expression, has been used for measuring spontaneous pain in animals. However, the responses in animals have been rather short-lived, and the scale has never been applied to animals exhibiting nociplastic pain. Here, we apply the rat grimace scale (RGS) to the reserpine-induced fibromyalgia-like rat, which induces nociplastic pain. The ratings of the orbital tightening, nose/cheek flattening, and changes in characteristics of ears and whiskers by three raters, who were blinded to the treatment allocated to rats, demonstrated substantial, long-lasting change in facial expression of rats. In this article, reference images for raters, and sample images used for rater training are provided. All raters independently indicated that the RGS score is significantly elevated with this methodology in reserpine-induced fibromyalgia-like rats.•The grimace scale, a method for rating facial expression, is applied to the reserpine-induced fibromyalgia-like rat, which manifests nociplastic pain.•Facial expression change in the reserpine-induced fibromyalgia-like rat is substantial and long-lasting.•Elevation of the RGS score in the reserpine-induced fibromyalgia-like rat may simulate spontaneous pain in patients with nociplastic pain.
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