A method for discriminating between chemically induced changes in locomotor activity and whole-body oxygen consumption in mice

Abstract

Concurrent measurements of O 2 consumption and locomotor activity were made in adult female mice at 20 and 34°C. As anticipated, activity and O 2 consumption were positively correlated with a high degree of statistical significance. A 30-fold increase in spontaneous motor activity, however, was accompanied by only a 30% increase in O 2 consumption. Extrapolation of the regression relationship to zero activity provided a novel means of estimating the basal metabolic rate (BMR) of mice. That value agreed with traditional estimates based on resting or sleeping animals. Under our conditions the respiratory quotient for mice was 0.83 at 20°C and 1.0 at 34°C. Oxygen consumption averaged over all levels of spontaneous activity was 4.79 ml g −1 hr −1 at 20°C and 2.21 ml g −1 hr −1 at 34°C. These estimates are actually lower than those of others who employed physical restraint to control activity. Because the BMR of mice constitutes about 70% of the O 2 consumption at the highest levels of spontaneous motor activity, drug-induced changes in motor activity may not significantly affect O 2 consumption. Alternatively, it may be obvious that the two changes are not correlated. For example, in our hands pentobarbital decreased motor activity without affecting O 2 consumption, and amphetamine increased motor activity but decreased O 2 consumption. When the changes are in the same direction, the problem of distinguishing between a metabolic or respiratory effect of the drug and a change related to increased or decreased motor activity becomes more difficult. Analysis of covariance offers one possible approach to this problem.