Characterisation of nociception and inflammation observed in a traumatic muscle injury model in rats

Muscle pain is the most prevalent type of pain in the world, but treatment remains ineffective. Objective: Therefore, this study characterised the nociception and inflammation in a traumatic muscle injury model in rats Methods: A single blunt trauma impact on the right gastrocnemius muscle of male Wistar rats. Procedures were approved by the Institutional Committee for Animal Use of the Federal University of Santa Maria (#6579280218/2018). Animals were divided into four groups (sham/no treatment; sham/diclofenac 1%; injury/no treatment; injury/diclofenac 1%) and the topical treatment with cream of 1% monosodium diclofenac (applied at 2, 6, 12, 24, and 46 h after muscle injury; 200 mg/muscle) was used as an anti-inflammatory control. Nociception (mechanical and cold allodynia, or nociceptive score) and locomotor activity were evaluated at 26 and 48 h after injury. Also, inflammatory and oxidative parameters were evaluated in gastrocnemius muscle and the creatine kinase (CK) activity and lactate levels in plasma and serum, respectively. Results: Muscle injury caused mechanical and cold allodynia, and increased nociceptive scores, without inducing locomotor impairment. This model also increased the inflammatory cells infiltration (seen by myeloperoxidase and Nacetyl-β-D-glucosaminidase activities and histological procedure), nitric oxide, IL- 1β, IL-6, and dichlorofluorescein levels in muscle samples; and CK activity and lactate levels in serum. The treatment with 1% monosodium diclofenac reduced inflammatory cells infiltration, dichlorofluorescein, and lactate levels. Conclusion: In this view, we characterised the traumatic muscle injury as a reproducible model of muscle pain, which make it possible to evaluate promising antinociceptive and anti-inflammatory therapies.