Behavioral thermoregulation in obese and lean Zucker rats in a thermal gradient

Abstract

Genetically obese Zucker (Z) rats have been reported to display a body core temperature (Tb) that is consistently below that of their lean littermates. We asked the question whether the lower Tb was a result of deficits in thermoregulation or a downward resetting of the set point for Tb. For a period of 45 consecutive hours, lean and obese Z rats were free to move within a thermal gradient with an ambient temperature (Ta) range of 15–35°C, while subjected to a 12:12-h light-dark cycle. Tb was measured using a miniature radio transmitter implanted within the peritoneal cavity. Oxygen consumption (V˙o 2) was measured using an open flow technique. Movements and most frequently occupied position in the gradient (preferred Ta) were recorded using a series of infrared phototransmitters. Obese Z rats were compared with lean Z rats matched for either age (A) or body mass (M). Our results show that obese Z rats have a lower Tb[37.1 ± 0.1°C (SD) vs. 37.3 ± 0.1°C, P< 0.001] and a lower V˙o 2 (25.3 ± 1.9 ml · kg−1 · h−1) than lean controls [33.1 ± 3.7 (A) and 33.9 ± 3.9 (M) ml · kg−1 · h−1, P < 0.001]. Also, the obese Z rats consistently chose to occupy a cooler Ta [20.9 ± 0.6°C vs. 22.7 ± 0.6°C (A) and 22.5 ± 0.7°C (M), P < 0.001] in the thermal gradient. This suggests a lower set point for Tb in the obese Z rat, as they refused the option to select a warmer Ta that might allow them to counteract any thermoregulatory deficiency that could lead to a low Tb. Although all rats followed a definite circadian rhythm for both Tb and V˙o 2, there was no discernible circadian pattern for preferred Ta in either obese or lean rats. Obese Z rats tended to show a far less definite light-dark activity cycle compared with lean rats.