The role of behavioral thermoregulation as a thermoeffector during prolonged hypoxia in the rat

Abstract

Exposing rodents to insults such as hypoxia and chemical toxicants induces regulated hypothermia, characterized by a preference for cooler ambient temperatures (T_a) and reduced core temperature (T_c). This study clarified the efficacy of behavioral thermoregulation to regulate T_c when subjected to a steady-state hypoxic insult. T_c, selected T_a (ST_a), heart rate (HR), and motor activity (MA) were monitored by radiotelemetry in rats housed in a temperature gradient. A 6 to 6.5 h period of hypoxia (6.9% O₂)was induced by diluting the air entering the gradient with nitrogen. During the first hour of hypoxia T_c decreased by 2.5°C while ST_a decreased from 30 to 24.5°C. As hypoxia progressed, ST_a rebounded slightly to 26–27°C while T_c remained between 34–35°C. HR increased gradually from 300 to 375 beats min⁻¹ during hypoxia. MA increased transiently but then remained near baseline throughout the remainder of the exposure period. Exposure to hypoxia in a gradient maintained at 31.5°C blocked the hypothermic response, showing that preference for cooler T_aS during hypoxia is critical for regulating T_c. Calorimetric measurements showed that exposure to hypoxia at a T_a of 22°C led to a transient rise in dry heat loss, suppression in metabolic rate, and reduction in T_c. At a warmer T_a of 32°C, hypoxia led to a suppression in heat loss along with an elevation in CO₂ production and T_c. Overall, the data show that behavioral and autonomic thermoeffectors serve to regulate T_c at a hypothermic level throughout a prolonged period of hypoxic stress.