Differential effects of acute total water deprivation on voluntary exercise behavior and body mass in laboratory house mice

Various studies have demonstrated adverse effects of dehydration on human athletic and cognitive performance, but most are limited in scope. Moreover, few have studied such effects on voluntary exercise. The purpose of this study was to evaluate the effects of 24 h of water deprivation on voluntary wheel-running behavior (a model for human voluntary exercise) and body mass in laboratory house mice. To increase the probability of detecting the effects of water deprivation, we studied mice from four replicate High Runner (HR) lines that have been bred for high levels of wheel running. Both sexes of HR mice run ∼3-fold more revolutions/day than the four non-selected Control (CON) lines, have increased motivation for wheel running and an increased capacity for aerobic exercise (i.e., ability), and will run voluntarily at speeds nearer to their aerobic capacity. Adults of both sexes were given 6 days of wheel access, as in the routine selective breeding protocol. At the end of day 6, water bottles were removed for ½ of the mice and a 7th day of wheel access was allowed. As expected, body mass significantly decreased in all groups that experienced 24 h without water. As compared with days 5 and 6, wheel-running distance did not significantly change in mice from CON or HR lines with ad lib water, nor did it change in the water-deprived CON mice. However, water-deprived HR mice of both sexes significantly increased the number of revolutions run over 24 h. Given that wheel running is voluntary, presumably because it provides a rewarding experience, we speculate that the significant increase in running after water removal in HR mice may be attributable to "reward substitution," in which mice replace the reward provided by drinking water (a primary drive) with the rewards derived from running (which may also be a biological drive). Alternatively, mice may have instinctually been attempting to find water, but only those from HR lines have sufficient physical abilities to increase running in spite of adverse effects of dehydration.