Gnawing behavior of a mouse in a narrow cylinder: a simple system for the study of muscle activity, fatigue, and stress.

When a mouse is put into a cylinder too narrow for it to turn (its front end being blocked with a thin plastic strip), the mouse gnaws away the plastic to escape. Hence, the weight reduction in the plastic can be used as an index of 'gnawing activity (GA).' GA was high at first, but decreased with time. Training augmented GA, but not the activity of the histamine-forming enzyme (histidine decarboxylase [HDC]: a proposed marker of muscle fatigue) in the masseter muscle. In trained mice, GA was higher at night than in the daytime, and was decreased by starvation. In mice prevented from reaching the strip, the elevation of serum cortisol was greater than that seen in mice able to gnaw at it. As such gnawing is a form of voluntary behavior, these results suggest that our experimental system may be useful for (i) the quantitative study of voluntary muscle activity associated with physical or mental fatigue or motivation, and (ii) the study of an animal's response to stress when it has, or alternatively does not have, an apparent way of escape.