Temperature determines the shift of thermal neutral zone and influences thermogenic capacity in striped hamsters

The thermoneutral zone (TNZ) reflects the adaptation of mammals to their natural habitat. However, it remains unclear how TNZ shifts in response to variations in ambient temperature. To test the hypothesis that ambient temperature plays a key role in determining TNZ variations between seasons, we measured metabolic rate, body temperature, and cytochrome c oxidase (COX) activity of several visceral organs in striped hamsters (Cricetulus barabensis) either acclimated to semi-natural conditions over a year, or subjected to a gradual decrease in mean temperature from 30 +/- 1 degrees C to -15 +/- 1 degrees C. The TNZ range in striped hamsters differed seasonally, with a wider TNZ and a lower lower-critical temperature in winter compared to summer. The hamsters showed a considerable leftward shift of lower-critical temperature from 30 degrees C to 20 degrees C after the ambient temperature of acclimation from 30 degrees C down to -15 degrees C, whereas the upper-critical temperature of TNZ remained fixed at 32.5 degrees C. The resting metabolic rate in thermoneutral zone (RMRt), nonshivering thermogenesis (NST), and COX activity of brown adipose tissue, liver, skeletal muscle, brain, and kidneys, increased significantly in hamsters acclimated at lower ambient temperatures. Following acute exposure to 5 degrees C and -15 degrees C, hamsters acclimated to 32.5 degrees C had significantly lower maximal NST and lower serum thyroid tri-iodothyronine (T-3) levels compared to those kept at 23 degrees C. These findings suggest that acclimation to the upper-critical temperature of TNZ impairs the hamsters' thermogenic capacity to cope with extreme cold temperature. Reduced ambient temperature was mainly responsible for the leftward shift of TNZ in striped hamsters, which reflects the adaptation to cold environments.