Different effects of novel stressors on sympathoadrenal system activation in rats exposed to long-term immobilization.

Activation of the sympathoadrenal system, evaluated by plasma levels of epinephrine (E) and norepinephrine (NE) after exposure of rats to various stressors, is well documented. However, response of rats exposed long-term to a homotypic stressor and then exposed once to a heterotypic novel stressor is poorly understood. In the present study, we examined changes in plasma levels of catecholamines (CA) and corticosterone (COR) of rats after a single (2-h) or long-term repeated immobilization (41 times, 2 h daily) and in rats adapted to long-term immobilization exposed once to the novel stress of cold exposure or insulin or 2-deoxy-D-glucose (2DG) administration. Long-term immobilization produced a significant elevation of basal plasma COR but not NE and E levels. Long-term immobilized rats exposed to insulin or 2DG showed significant elevation of plasma CA and COR levels in comparison to the administration to control rats. Exposure of long-term immobilized and control rats to cold stress increased plasma NE and COR, whereas plasma E was not significantly changed. The exposure of long-term immobilized rats to a further single immobilization (2 h) increased plasma CA levels, but, in naive control rats, the single immobilization produced more pronounced increases. These data suggest that rats exposed to homotypic long-term immobilization are able to respond to heterotypic stressors by higher activation of the sympathoadrenal system as compared with the control, previously unstressed rats. Reduced plasma CA levels in long-term immobilized rats exposed to homotypic stressor are most probably due to an adaptation at the level of brain regulatory centers.