Sensitization of sympathetic-adrenal medullary responses to a novel stressor in chronically stressed laboratory rats

Studies from this and other laboratories have demonstrated that sympathetic-adrenal medullary responses are reduced in laboratory rats exposed to chronic intermittent stress. To extend these findings, we have conducted three experiments in which laboratory rats were exposed daily for 26 consecutive days to the same (homotypic) stressor and then challenged with a novel (heterotypic) stressor. Each stress session lasted for 30 minutes and the following homotypic/heterotypic treatments were included: Experiment 1--restraint/footshock; Experiment 2--cold swim/restraint; and Experiment 3--footshock/cold swim. Controls were exposed to the heterotypic stressor only. Plasma levels of norepinephrine (NE) and epinephrine (EPI) were measured in blood samples taken from control and chronically stressed rats to provide an assessment of sympathetic-adrenal medullary activity. In each experiment, chronically stressed rats gained significantly less weight than handled controls. Basal plasma levels of NE and EPI were similar for chronically stressed and control rats. In contrast, plasma catecholamine responses to the heterotypic stressor were significantly greater in stressed rats compared to controls that were stressed for the first time. This pattern of exaggerated responses was especially evident for the cold swim/restraint and footshock/cold swim groups. These findings provide evidence for a sensitization of sympathetic-adrenal medullary responses to a novel stressor in animals previously exposed to chronic intermittent stress.