Relationship between preferred ambient temperature and autonomic thermoregulatory function in rat.

This study was designed to elucidate the relationships between behavioral and autonomic thermoregulation in three common strains of the laboratory rat. In one experiment eight adult rats of the Sprague-Dawley (SD), Long-Evans (LE), and Fischer (FCH) strains were repeatedly placed in a longitudinal temperature gradient while their preferred ambient temperature (Ta) was recorded. The mean preferred Ta's (+/- SE) for the SD, LE, and FCH strains were 24.9 +/- 0.4, 19.8 +/- 0.3, and 23.4 +/- 0.3 degrees C, respectively. In another experiment, individual adult rats of the same strains were placed in an environmental chamber thermostabilized at 2 degrees C intervals from 14 to 36 degrees C for 90 min while metabolic rate (MR), evaporative water loss (EWL), thermal conductance (C), and colonic temperature (Tcol) were determined. All three strains exhibited a minimal MR at a Ta of 30 degrees C. As Ta decreased below 30 degrees C, MR increased in a nonlinear fashion. EWL and C were minimal at cool Ta's and increased gradually with an elevation in Ta. All three strains maintained a normal Tcol between Ta's of 14 to 30 degrees C. The FCH strain exhibited the best control of Tcol at Ta's above 30 degrees C. Generally, one would predict that the preferred Ta would be associated with minimal thermoregulatory effort. However, the rat is unusual from other rodents (e.g., mouse, hamster, and guinea pig) in that the preferred Ta is well below the lower critical Ta for elevating MR.