Immobilization stress in rats: Effect on rectal temperature and possible role of brain monoamines in hypothermia

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Abstract
Immobilization of albino rats for 2 h showed ambient temperature-dependent changes in rectal temperature, hypothermia at temperatures below 30° C, and hyperthermia at 35° C and above. Adrenalectomized (Adre) rats showed more hypothermia compared to sham operated controls at 25±2° C. The increased hypothermia in adrenalectomized rats was reversed by 10 mg/kg IP or 100 μg/rat ICV of hydrocortisone. Groups of rats pretreated with desmethylimipramine (DMI, 25 mg/kg IP) and 6-hydroxydopamine (6-HD, 100 μg/rat ICV) or methyl ester of parachlorophenylalanine (ME-PCPA, 100 μg/rat ICV for 3 days) or 5,6-dihydroxytryptamine (DHT, 75 μg/rat ICV) showed significantly less hypothermia at the end of 2 h of immobilization. Applying analysis of variance test, the hypothermia in Adre, ME-PCPA and DHT groups, was found to be not significantly different from their respective control groups between 0 to 45 min of immobilization but was significantly different between 45 to 120 min of immobilization. DMI-6-HD group however, showed significant difference between 0–45 min only and not between 45–120 min of immobilization. The results suggest that the early phase of immobilization induced hypothermia between 0–45 min is dopamine and the late phase of hypothermia between 45–120 min is 5-hydroxytryptamine mediated.