Hypothermia induction and recovery in free-ranging rats

1. To avoid anesthesia confounders, free-ranging rats were exposed (4 h) to cool water (CW; 10 °C; 5 cm), warm water (WW; 35 °C; 5 cm) or temperate air (TA; 25 °C) to induce hypothermia, or control for water or novel environment stress, respectively. 2. While WW and TA core temperature ( T c) and the brown adipose tissue ( T BAT)/subdermal skin ( T SDS) temperature relationship remained similar, CW hypothermia induction was variable (34.5–26.1 °C; 9–240 min) and associated with tachycardia (517.8±4.7 bpm) greater than WW (386.5±5.5 bpm) or TA (372.2±7.7 bpm) with CW T BAT (36.5±0.03 °C) elevated above CW T SDS (35.9±0.05 °C). 3. Without anesthesia to blunt thermoregulatory countermeasures to hypothermia, variable resistance to T c depression, tachycardia rather than bradycardia and BAT thermogenic responses were demonstrated.