The effect of repeated periods of restraint and continued exposure to various ambient temperatures on the core temperature responses of drug-naive rats

Male, Sprague–Dawley rats, maintained at an ambient temperature (Ta = 22 °C) were placed in an environmental room at an Ta of 14, 22, or 32 °C and colonic temperatures were measured while the rats were restrained or free moving.

Abstract

Male, Sprague–Dawley rats, maintained at an ambient temperature (T a = 22 °C) were placed in an environmental room at an T a of 14, 22, or 32 °C. After 12 h of acclimation to the new T a (first testing) and 3 and 6 days following (2nd and 3rd test days), colonic temperatures were measured while the rats were restrained (Plexiglas holders or surgical towels) or free moving. Restrained rats kept at an T a of 22 °C showed transient increases in core temperature (T c ) on the 1st test day, compared with responses of the free-moving group, but little or no differences appeared thereafter. At an T a of 32 °C, unrestrained rats lay prostrate on their backs and had decreased T c s on each testing day compared with those of both restrained groups. T c s on the 1st test day of restrained rats at 4 °C fell markedly and some rats died; the free-moving group maintained normal body temperatures and it was noted that shivering, piloerection, and huddling occurred. By the last test day, temperature responses of all cold-acclimated groups were similar. The results indicate that behavioural thermoregulation is important for the maintenance of normal body temperatures of rats first exposed to extreme T a and that restraint during the initial exposure disrupts this regulation. T c s are less affected by restraint on prolonged exposure to the cold as other mechanisms (i.e. nonshivering thermogenesis) appear to have been stimulated to a sufficient degree to offset the increased heat loss of restraint immobilization.