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

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Abstract
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. After 12 h of acclimation to the new Ta (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 Ta of 22 °C showed transient increases in core temperature (Tc) on the 1st test day, compared with responses of the free-moving group, but little or no differences appeared thereafter. At an Ta of 32 °C, unrestrained rats lay prostrate on their backs and had decreased Tcs on each testing day compared with those of both restrained groups. Tcs 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 Ta and that restraint during the initial exposure disrupts this regulation. Tcs 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.

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