Comparison of rhythmic masticatory muscle activity during non-rapid eye movement sleep in guinea pigs and humans

Repetitive/rhythmic masticatory muscle activity under urethane anesthesia in guinea pigs: a descriptive pilot study

OBJECTIVES

Rhythmic/repetitive masticatory muscle activity may occur spontaneously during cyclic alternations in unconscious brain states such as sleep. An experimental model is needed to clarify these underlying mechanisms. This study investigated jaw movements and masticatory muscle activity during cyclic state alternations under urethane anesthesia.

METHODS

Cortical electroencephalography, electrocardiography, and nasal airflow were recorded simultaneously with jaw movements and jaw muscle electromyography activity in seven urethane-anesthetized male guinea pigs (420-594 g). Cortical brain states were divided into deactivated and activated states according to electroencephalogram (EEG) delta and theta powers. The respiratory and heart rates were quantified during the two cortical states. Rhythmic jaw movements (RJMs) were visually scored and the characteristics of masticatory electromyographic bursts were analyzed. Transient changes in cortical, cardiac, and respiratory activities were analyzed in association with RJMs.

RESULTS

Cortical activity and respiratory and heart rate variabilities differed significantly between activated and deactivated states. Of 321 RJMs, 290 occurred in clusters under urethane anesthesia; The majority (73.7%) were scored during the activated state. RJM episodes were associated with alternate lateral jaw excursion, predominantly with masseter muscle activation, and occasionally with tooth-grinding sounds. RJMs were preceded by decreases in EEG delta activity and transient increases in cardiac and respiratory activities.

CONCLUSIONS

Masticatory muscles may be activated repetitively and rhythmically during cyclic alternations in brain states under urethane anesthesia. Urethane-anesthetized guinea pigs are a potential experimental model for examining the mechanisms underlying the generation of RJMs in unconscious states, such as sleep bruxism.

Understanding the pathophysiology of sleep bruxism based on human and animal studies: A narrative review

BACKGROUND

Sleep bruxism (SB) is a common sleep disorder that affects approximately 20% of children and 10% of adults. It may cause orodental problems, such as tooth wear, jaw pain, and temporal headaches. However, the pathophysiological mechanisms underlying SB remain largely unknown, and a definitive treatment has not yet been established.

HIGHLIGHT

Human studies involving polysomnography have shown that rhythmic masticatory muscle activity (RMMA) is more frequent in otherwise healthy individuals with SB than in normal individuals. RMMA occurs during light non-rapid eye movement (non-REM) sleep in association with transient arousals and cyclic sleep processes. To further elucidate the neurophysiological mechanisms of SB, jaw motor activities have been investigated in naturally sleeping animals. These animals exhibit various contractions of masticatory muscles, including episodes of rhythmic and repetitive masticatory muscle bursts that occurred during non-REM sleep in association with cortical and cardiac activation, similar to those found in humans. Electrical microstimulation of corticobulbar tracts may also induce rhythmic masticatory muscle contractions during non-REM sleep, suggesting that the masticatory motor system is activated during non-REM sleep via excitatory inputs to the masticatory central pattern generator.

CONCLUSION

This review article summarizes the pathophysiology of SB and putative origin of RMMA in both human and animal studies. Physiological factors contributing to RMMA in SB have been identified in human studies and may also be present in animal models. Further research is required to integrate the findings between human and animal studies to better understand the mechanisms underlying SB.

After-effects of acute footshock stress on sleep states and rhythmic masticatory muscle activity during sleep in guinea pigs

This study investigated the effects of acute footshock stress (FS) on the occurrence of rhythmic masticatory muscle activity (RMMA) during sleep in guinea pigs. Animals were prepared for chronic recordings from electroencephalogram, electrooculogram and electromyograms of neck and masseter muscles. The signals were recorded for six hours on the two successive days: the first day with stress-free condition (non-FS condition) and the second day with acute FS (FS condition). Sleep/wake states and RMMA were scored visually. Sleep variables and the frequency of RMMA occurring during non-rapid eye movement (NREM) sleep were compared during 6-h periods between the two conditions. Compared to non-FS condition, the amount of total sleep and NREM sleep significantly reduced during 2 h following the acute FS in the FS condition. Similarly, the frequency of RMMA significantly increased during 2 h following the acute FS for the FS condition compared to non-FS condition. During 2-6 h after FS in the FS condition, sleep variables and the frequency of RMMA did not differ from those without FS in the non-FS condition. These results suggest that acute experimental stress can induce transient changes in sleep-wake states and the occurrence of RMMA in experimental animals.

Changes in cortical, cardiac, and respiratory activities in relation to spontaneous rhythmic jaw movements in ketamine-anesthetized guinea pigs

It has been reported that rhythmic jaw movements (RJMs) spontaneously occur in ketamine-anesthetized animals. The present study investigated the physiological processes that occur during the cortical, cardiac, and respiratory events which contribute to the genesis of RJMs in animals after supplemental ketamine injections. Fourteen guinea pigs were prepared to allow electroencephalographic, electrocardiographic, and electromyographic activities to be recorded from the digastric muscle, measurement of jaw movements, and nasal expiratory airflow under ketamine-xylazine anesthesia. Rhythmic jaw movements spontaneously occurred with rhythmic digastric muscle contractions, 23-29 minutes after injection of supplemental ketamine (12.5 and 25.0 mg kg^-1 , intravenously). The cycle length of RJMs did not differ significantly between the two doses of ketamine (mean±SD: 12.5 mg kg^-1 , 326.5 ± 60.0 ms; 25 mg kg^-1 , 278.5 ± 45.1 ms). Following injection of ketamine, digastric muscle activity, heart and respiratory rates, and cortical beta power significantly decreased, while cortical delta and theta power significantly increased. These changes were significantly larger in animals given 25.0 mg kg^-1 of ketamine than in those given 12.5 mg kg^-1 . With the onset of RJMs, the levels of these variables returned to pre-injection levels, regardless of the dose of ketamine administered. These results suggest that, following supplemental ketamine injections, spontaneous RJMs occur during a specific period when the pharmacological effects of ketamine wear off, and that these RJMs are characterized by stereotypical changes in cardiac, respiratory, and cortical activities.