Dynamics of stress activation in rats exposed to repeated stress conditions

Abstract The aim of the study was to analyze the dynamics of stress-specific indices of the body functional state, i.e. the level of catecholamines in blood plasma and spectral indices of heart rate variability (HRV) in repeated immobilization stress for identifying the effects of habituation. Materials and Methods: The jugular veins of the rats (Long-Evans, n=6) were catheterized before the experiment. For immobilization, the animals were fixed mildly in a special hammock for 30 min daily during 5 days. In the process of rat immobilization as well as for 30 min before and 60 min after immobilization under the normal conditions in the home cage, ECG was noninvasively recorded for a subsequent HRV analysis based on the spectral indices. On experimental days 1, 3, and 5, blood was collected 30 min before immobilization (stage 1), 5 min after it (stage 2), at the end of immobilization (stage 3), and 30 min after the end of the immobilization period (stage 4) to test the concentration of catecholamines in blood plasma. Results: The effects of habituation to immobilization were found in the dynamics of catecholamine levels in blood plasma, heart rate, and HRV. The concentration of adrenaline and noradrenaline increased statistically significantly in the period of immobilization on the first experimental day and did not change significantly on the following days. Heart rate decreased significantly at stages 3 and 4 of the experiment (free behavior in the home cage after immobilization) on day 5 and remained at the initial high level on experimental days 1 and 3. The total power of the HRV spectrum statistically significantly decreased in the immobilization period on all experimental days. On day 1, HRV remained reduced till stage 4 of the experiment. On days 3 and 5, the total power of the HRV spectrum was restored after immobilization up to the initial level by stage 4 of the experiment. Conclusions: Five-day exposure of the animals to the repeated immobilization stress in a special hammock designed for their fixation in electrophysiological experiments resulted in habituation which is demonstrated by the dynamics of catecholamine level in the blood and HRV. The data obtained showed that the effects of habituation are displayed not only by the reduction of the stress activation intensity but by the decrease of its duration as well