Temporal change in the occlusal vertical dimension and its involvement in modulation of jaw movement in bite-reduced animals.

The occlusal vertical dimension (OVD) in guinea pigs is maintained by tooth eruption and grinding. It has been reported that the experimentally raised OVD recovers to the innate OVD within a few days in guinea pigs. However, the mechanisms underlying OVD adjustment are not entirely understood. This study thus aimed to clarify whether the experimentally reduced OVD would recover. Bite-reduced guinea pigs were created by applying bilateral intermaxillary elastics for 10 days. Guinea pigs without elastics were used as a control. The OVD after removal of the elastics in the experimental group was compared with that of the control group. Jaw movement during chewing was also compared between the experimental and control groups. After removal of the elastics, the experimentally reduced OVD did not recover fully and a significant difference was observed between the experimental and control groups for up to 25 days during the recording period. The minimum closed position during chewing was significantly higher in the experimental group than in the control group, whereas the maximum open position was no different between the groups. The present findings indicated that the experimentally reduced OVD could not be fully recovered, suggesting that reduction of the OVD may have limited influence on jaw movement.

Alteration of masticatory muscle EMG activities during chewing after a reversible bite-raising in guinea pigs.

Previous studies have investigated the effects of increasing the occlusal vertical dimension (OVD) with an oral appliance on masticatory muscle EMG activity during oral behaviours in humans and animals. The present study investigated whether a short-term and reversible increase in OVD, followed by a reduction in OVD to the normal level, resulted in a time-correlated change in the EMG activities of the masseter and digastric muscles during chewing. To do this, a guinea pig model in which an increased OVD was established with natural tooth contacts was used. In the control group, in which no bite-raising treatment was applied, OVD gradually increased with a natural growth during the experimental period whilst the masseter and digastric EMG activities, burst duration, and chewing rhythm were unchanged. When the increase in OVD was established in the bite-raised group, the EMG activities of the masseter and digastric muscles were significantly increased by 88.6 and 55.2% from those before bite-raising treatment, respectively. However, during the following 11 days, the increased EMG activities of both muscles did not show changes associated with the subsequent decrease in the OVD to a normal level. The burst durations of both muscles and chewing rhythm were not significantly affected by the change in OVD during the experimental period. Within the limited recording period of the study, the return of OVD from increased to normal levels did not reverse the increased chewing-related masticatory muscle EMG activity that was induced by the bite-raising treatment.

Heterogeneous activity level of jaw-closing and -opening muscles and its association with arousal levels during sleep in the guinea pig.

Exaggerated jaw motor activities during sleep are associated with muscle symptoms in the jaw-closing rather than the jaw-opening muscles. The intrinsic activity of antagonistic jaw muscles during sleep remains unknown. This study aims to assess the balance of muscle activity between masseter (MA) and digastric (DG) muscles during sleep in guinea pigs. Electroencephalogram (EEG), electroocculogram, and electromyograms (EMGs) of dorsal neck, MA, and DG muscles were recorded with video during sleep-wake cycles. These variables were quantified for each 10-s epoch. The magnitude of muscle activity during sleep in relation to mean EMG activity of total wakefulness was up to three times higher for MA muscle than for DG muscle for nonrapid eye movement (NREM) and rapid-eye-movement (REM) sleep. Although the activity level of the two jaw muscles fluctuated during sleep, the ratio of activity level for each epoch was not proportional. Epochs with a high activity level for each muscle were associated with a decrease in deltaEEG power and/or an increase in heart rate in NREM sleep. However, this association with heart rate and activity levels was not observed in REM sleep. These results suggest that in guinea pigs, the magnitude of muscle activity for antagonistic jaw muscles is heterogeneously modulated during sleep, characterized by a high activity level in the jaw-closing muscle. Fluctuations in the activity are influenced by transient arousal levels in NREM sleep but, in REM sleep, the distinct controls may contribute to the fluctuation. The above intrinsic characteristics could underlie the exaggeration of jaw motor activities during sleep (e.g., sleep bruxism).

Temporal alteration of chewing jaw movements after a reversible bite-raising in guinea pigs.

Previous studies analysed the temporal changes of occlusal vertical dimension (OVD) with natural molar tooth contacts in bite-raised guinea pigs and found that the OVD is strictly maintained in the growing guinea pigs. Since the OVD is considered to be the influencing factor for performing mastication, we hypothesised that a control of the OVD is integrated with maintenance of an appropriate chewing behavior. In young adult guinea pigs with or without bite-raised treatment, the OVD was measured by micro-CT and stable chewing movements were recorded during 11 days after the removal of the bite-raising appliance. In control animals, the OVD increased developmentally up to 10.5+/-6.0% for 11 days. In the bite-raised animals, the increased OVD was increased by 19.1+/-6.7% with the bite-raising appliance, and the OVD rapidly decreased to the same level as that of controls within 4 days after the removal of the appliance. During chewing on the first day after the removal of the appliance, the most closed position of the jaw was lower in bite-raised animals than in controls while the most opened position did not differ between the two groups, i.e., the jaw gape was decreased. These changes disappeared when OVD returned to the same level as that of controls. Any other variables for movements (e.g., rhythm) did not differ between the both groups throughout the experimental period. These results suggest that there is a robust association between an innate OVD and a centrally-programmed pattern of chewing movements for functional integrity.

Muscle activities are differently modulated between masseter and neck muscle during sleep-wake cycles in guinea pigs.

Sleep bruxism is a sleep-related movement disorder characterized by an exaggerated jaw motor activity during sleep. Currently, the magnitude of jaw motor activation in normal sleep remains poorly understood. In this study, we aim to assess the state-dependent changes in the magnitude of electromyographic activities of the jaw-closing masseter muscle in comparison with those of a neck muscle (specifically, the obliquus capitis) during sleep-wake cycles in guinea pigs. These electromyographic activities were integrated for 10-s epochs during wakefulness, non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The masseter activity per epoch was found to be five times lower in both sleep stages while the neck muscle activity also decreased to 30% in NREM sleep and was lowest (16%) in REM sleep. In the periods without motor activity, masseter tone did not differ between the three states, whereas neck muscle tone decreased from wakefulness to NREM sleep and further to REM sleep. Moreover, in the epochs with masseter activation, the neck muscle activity did not increase during sleep. These results suggest that masseter activity decreases but is occasionally activated during sleep, and that state-dependent changes in electromyographic activity can be differently modulated in time and intensity between the masseter and the obliquus capitis.