Distance-based Permutation of Inter-Meal Differences as a Sensitive Test of Temporomandibular Joint Nociception in Rats

We compare nonparametric permutation method using intra-meal rate as endpoint with existing ANOVA method that uses average daily meal duration as an endpoint for detection of chronic pain in Sprague-Dawley rats. Nociception following bilateral temporomandibular joint (TMJ) injection of high dose of Complete Freunds Adjuvant (CFA, 250 μg/50 μL per side) could be detected in young adult male Sprague-Dawley rats using average daily meal durations as a measure of nociception for up to 19 days (Kramer, Kerins, Schneiderman & Bellinger, Physiology & Behavior, 99, 2010; 669) using ANOVA and multiple comparison range tests. In this study, we reanalyzed the data using a nonparametric permutation procedure based on absolute differences between intra-meal feeding rate curves. In addition to that experiment, we injected bilaterally the TMJ of naive rats with either a low-dose CFA (15 μg/50 μL per side, n = 6) or saline (50 μL of 0.9%, n = 4) and monitored the animals for 7 days. The permutation test of the intra-meal feeding rate detected the presence of nociception in the high-dose CFA treatment group for up to 40 days or twice as long as when using ANOVA on average daily meal durations. The permutation method also detected the low-dose CFA-induced nociception with ten times lower p-values and for several days longer than ANOVA of changes in meal durations. CFA-induced injury resulted in even reduction in intra-meal feeding rate and lengthening of the meals in both high- and low-dose CFA-injected animals. The rate analysis also showed when the rats first started a meal they were experiencing the same level of nociception as at the end of the meal. This demonstrated that intra-meal chewing itself did not alter the level of nociception. These results suggest that permutation tests based on differences in intra-meal feeding rates can be used as a sensitive test to determine and study the temporal patterns of TMJ nociception.