Grading facial expression is a sensitive means to detect grimace differences in orofacial pain in a rat model

Abstract

Although pre-clinical models of pain are useful for defining relationships between biological mechanisms and pain, common methods testing peripheral sensitivity do not translate to the human pain experience. Facial grimace scales evaluate affective pain levels in rodent models by capturing and scoring spontaneous facial expression. But, the Rat Grimace Scale (RGS) has not assessed the common disorder of temporomandibular joint (TMJ) pain. A rat model of TMJ pain induced by jaw loading (1 hr/day for 7 days) was used to investigate the time course of RGS scores and compare them between different loading magnitudes with distinct peripheral sensitivity profiles (0N–no sensitivity, 2N–acute sensitivity, 3.5N–persistent sensitivity). In the 3.5N group, RGS is elevated over baseline during the loading period and one day after loading and is correlated with peripheral sensitivity (ρ = −0.48, p = 0.002). However, RGS is not elevated later when that group exhibits peripheral sensitivity and moderate TMJ condylar cartilage degeneration. Acutely, RGS is elevated in the 3.5N loading group over the other loading groups (p < 0.001). These findings suggest that RGS is an effective tool for detecting spontaneous TMJ pain and that spontaneous pain is detectable in rats that develop persistent TMJ sensitivity, but not in rats with acute resolving sensitivity.

Evoked and spontaneous pain assessment during tooth pulp injury

“…To our knowledge, this is the first assessment of MGS following tooth pulp exposure injury, and somewhat surprisingly the first for facial Von Frey in mouse with this model as well. MGS and the rat equivalent RGS are significantly elevated following other types of dental pain, including tooth movement 35 and mechanical load injury to the temporomandibular joint (TMJ) 36 , but in both of these cases, the elevation in score is transient, likely only corresponding with the presence of acute mechanical load. Our elevation in score does not subside, possibly due to the more invasive nature of the injury and the fact that ongoing inflammation is not being treated.…”

Influence of tramadol on functional recovery of acute spinal cord injury in rats

“…Despite the apparent lack of applicability of the rat grimace scale for animal care staff in evaluating pain levels on a day-to-day basis, the use of facial coding to detect pain has clinical utility in refining our understanding of acute pain [29][30][31][32][33][34] . This study confirms the importance of applying a pain scale in spinal cord injury procedures reaffirming that cited by Sperry et al 33 , as a useful modality to predict long periods in clinical pain models. Although the non-invasive collection of rat grimace scale data is simple to execute, its scoring can be a labor-intensive process 17 .…”

Evoked and spontaneous pain assessment during tooth pulp injury

“…To our knowledge, this is the first assessment of MGS following tooth pulp exposure injury, and somewhat surprisingly the first for facial Von Frey in mouse with this model as well. MGS and the rat equivalent RGS are significantly elevated following other types of dental pain, including tooth movement 32 and mechanical load injury to the temporomandibular joint (TMJ) 33 , but in both of these cases, the elevation in score is transient, likely only corresponding with the presence of acute mechanical load. Our elevation in score does not subside, possibly due to the more invasive nature of the injury and the fact that ongoing inflammation is not being treated.…”

“…The Mouse Grimace Scale is a scoring system developed in the laboratory of Jeff Mogil to objectively evaluate pain-like facial expressions following experimental procedures 13 , which has been adopted for many trigeminal pain models 33,34,36 , but not yet used to evaluate tooth pain in rodents. Mice (6/treatment group) were acclimated in the chambers at least twice prior to baseline testing, and were in the chamber for 10 minutes before recording began each day.…”