Simultaneous measurement of preferred ambient temperature and metabolism in rats.

A recent study from this laboratory found that rats placed in a temperature gradient preferred ambient temperatures (Ta) that were significantly below the lower critical Ta of the thermoneutral zone for elevating metabolic rate (MR). To further evaluate the interaction between preferred Ta and MR in the rat, a system was developed where preferred Ta, MR (i.e., O2 consumption), and activity could be automatically monitored in the unrestrained rat. Two groups of male Sprague-Dawley rats, a light-weight group and a heavy-weight group, were placed in a longitudinal temperature gradient for 90 min. These animals were further divided into two groups. One group was permitted free run in the gradient with a temperature range of 7-40 degrees C, whereas another group was restricted to the warm end of the gradient where the coolest available temperature was approximately equal to the lower critical Ta (29 degrees C). The preferred Ta and MR data were statistically analyzed over the last 20 min of the experimental run. Rats allowed to run freely in the gradient selected Ta of 17-20 degrees C and had MR significantly higher than animals restricted to a range of Ta no cooler than the lower critical Ta. Animals that selected the cooler Ta were much more active than those forced to remain at the warmer Ta. In conclusion, the interaction between behavioral and autonomic thermoregulation in the rat is unique compared with other rodent species such as the mouse, hamster, and guinea pig, which prefer Ta associated with thermoneutrality.