Intermittency in Mastication and Apomorphine-Induced Gnawing

Abstract

Rhythmic behaviors like mastication, gnawing, and locomotion, are characterized by temporal segmentation or intermittency. That is, they frequently occur as a series of short bursts interrupted by pauses rather than as one long uninterrupted burst. The function of intermittency as well as the mechanisms that produce it are unknown. Biogenic amine systems may play a role in producing intermittency; however, experimental work to confirm this is only in its infancy. The current study evaluates the structure of intermittency associated with mastication and apomorphine-induced gnawing in the guinea pig. Thirteen free-roaming animals were videotaped while masticating or gnawing. Eight animals were given 0.5 mg/kg i.m. apomorphine and videotaped while gnawing. The remaining five animals received no apomorphine injections, but were taped while feeding on alfalfa pellets. Custom software was used to score instances of maximum jaw closures in videotaped mastication and gnawing sequences. The time between successive maximum jaw closures, called the interocclude interval (IOI), was calculated for all scored sequences. A cutoff IOI value of 0.26 s differentiated pauses (IOI values equal or greater than 0.26 s) from chews or gnaws (IOI values less than 0.26 s). Two or more successive chews or gnaws, without intervening pauses, defined behavior bursts. Chew, gnaw, and burst durations were quantified and compared. Chew and gnaw durations were similar. However, chewing bursts were significantly longer than gnawing bursts. The significance of these results is presented in light of previous neurophysiological work on rhythmic jaw movements and intermittency.