Emotional stress and brux-like activity of the masseter muscle in rats

The aim of this study was to further clarify the relationship between emotional stress and bruxism. In experiment 1, 60 male 9-week-old Wistar rats were divided into four groups: the emotionally stressed (ES), the emotionally non-stressed (NS), the electrically foot-shocked (FSd), and the non-foot-shocked (NSd). ES rats were confined in a communication box for one hour a day to observe the emotional responses of neighbouring FSd rats. On days 0, 1, 4, 8, and 12, the electromyographic activity of the ES and NS rats' left masseter muscles was recorded for one hour, three hours after confinement in the communication box. Brux-like activity appeared in the masseter muscle of the ES group on days 1, 4, 8, and 12, but not in the NS group. In experiment 2, 36 male Wistar rats, 9 weeks old, were divided into three groups: emotionally stressed rats treated with an anti-anxiety drug (DES), emotionally stressed rats treated with saline as a vehicle (VES), and 24 FSd rats. Stress and EMG procedures were the same as those in experiment 1. Brux-like episodes decreased in DES rats from day 1 and significant differences were found on days 4 (P < 0.01), 8 (P < 0.05), and 12 (P < 0.05), when compared with the VES group. These findings suggest that emotional stress induces brux-like activity in the masseter muscle of rats, which was reduced with anti-anxiety drugs.