Mouse preferential incising force orientation changes during jaw closing muscle hyperalgesia and is sex dependent

• Published in: Physiology & behavior 2016-12, Vol.167, p.132-144
• Main Authors: Widmer, C.G, Morris-Wiman, J
• Format: Article
• Language: English
• Subjects: Acids - toxicity; Analysis of Variance; Animals; Bite Force; cages; Central pattern generator; Disease Models, Animal; Electromyography; Female; females; head; Hyperalgesia - chemically induced; Hyperalgesia - physiopathology; Incising forces; Jaw - physiopathology; Male; males; Mastication; Mastication - physiology; Masticatory Muscles - physiology; Mice; muscles; Myalgia - chemically induced; Myalgia - complications; Pain; Psychiatry; Sex Characteristics; somatosensory disorders; statistical models
• ISSN: 0031-9384; 1873-507X
• DOI: 10.1016/j.physbeh.2016.09.005

Abstract Introduction Mouse incising is controlled by a central pattern generator and this activity can change in the presence of pain. The incising frequency and maximum force generation decreases with pain. In this study, we used repetitive acidic injections in the left masseter muscle of male and female mice to determine differences between baseline and jaw muscle pain conditions and the effect of sex on preferential incising direction. Methods A within subject design was used to evaluate data previously acquired using multi-axis force data (X, Y and Z) from the 4th baseline recording day and day 7 post-injection (day of maximal pain response) for each mouse of each sex. A total of 34 female and male (age 3–9 months) CD-1 mice were evaluated. After mathematically rotating the X and Y axes to align the Y axis to be parallel to the wire struts of the cage top, data were analyzed to determine incising direction preference during baseline (non-pain) and pain (day 7) conditions and between sex. Radar plots of X-Y, X-Z and Y-Z axes depicted the average direction of incising preference between baseline and pain conditions for each sex. Statistical differences among groups were tested using a mixed model ANOVA. Results Similar to previous findings, female mice had a more robust difference in incising direction preference when comparing male and female pain conditions and this was most evident in the X-Z axes. The incising frequencies most commonly affected were 5.3, 6.2 and 7.6 Hz. Male mice varied little in their incising direction preference between the baseline and pain conditions. In addition, statistical comparison of ratios of the percent of time spent incising in the Z versus X axes for each incising frequency found that the incising preference was not different when comparing 5.3 and 7.6 Hz frequencies. Finally, female mice used a novel approach to minimize pain while incising by rotating their head and body nearly 180 degrees while males did not use this strategy as frequently. Conclusions The preferred incising direction changes in a jaw muscle pain condition and this information can be used to further characterize functional pain in the masticatory muscle system. The changes were dependent on the incising frequency generated by the central pattern generator for incising.