The relationship between the Rat Grimace Scale and mechanical hypersensitivity testing in three experimental pain models

Background The assessment of facial expressions associated with pain has been used to evaluate pain in humans and has recently found application in non‐human mammals. These so called ‘grimace scales’ have the potential to be developed into a widely accepted non‐invasive method of measuring pain in laboratory rodents. Currently, common methodologies to assess pain rely on nociceptive tests that assess stimulus evoked withdrawal responses. These tests, however, are limited to the assessment of a reflexive response without an affective component. This study aimed to use the recently developed Rat Grimace Scale (RGS) and assess its relationship with a conventional nociceptive test (the application of von Frey filaments). Methods Fifty‐two adult, male Wistar rats were randomized to one of five treatment groups: intraplantar carrageenan, intraplantar complete Freund's adjuvant (CFA), plantar incision, anaesthetic control and saline injection control. The RGS and response to mechanical hypersensitivity testing was evaluated at predetermined time points before and after treatment until withdrawal responses returned to baseline levels. Results The RGS score significantly increased in all pain models. The peak RGS score also coincided with the development of paw hypersensitivity. However, mechanical hypersensitivity persisted after RGS scores returned to baseline. Conclusion This study confirms that the three pain models induce pain in rodents and showed that peak pain coincided with peak mechanical hypersensitivity. However, mechanical hypersensitivity remained once pain subsided, mimicking the human experience of CFA injection. These findings further our understanding of the roles of, and relationship between, these assays in the assessment of nociception and pain.