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

Immobilization of albino rats for 2 h showed ambient temperature-dependent changes in rectal temperature, hypothermia at temperatures below 30 degrees C, and hyperthermia at 35 degrees C and above. Adrenalectomized (Adre) rats showed more hypothermia compared to sham operated controls at 25 +/- 2 degrees C. The increased hypothermia in adrenalectomized rats was reversed by 10 mg/kg IP or 100 microgram/rat ICV of hydrocortisone. Groups of rats pretreated with desmethylimipramine (DMI, 25 mg/kg IP) and 6-hydroxydopamine (6-HD, 100 microgram/rat ICV) or methyl ester of parachlorophenylalanine (ME-PCPA, 100 microgram/rat ICV for 3 days) or 5,6-dihydroxytryptamine (DHT, 75 microgram/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 and 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.