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

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.

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 power. 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.

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.

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.