Dynamic Changes of Behavioral Despair, HPA Axis Activity, and Hippocampal Neurogenesis in Male Rats Induced by Social Defeat Stress.

BACKGROUND

Psychosocial stress factors, such as threat and defeat, are major risk factors for the development of depression. The precise mechanisms underlying stress-induced depression are not clearly understood because the stress response in the brain varies in a stress-frequency-dependent manner. In the current research milieu on the pathogenesis of depression, the focus is on depression-like behavioral phenotype, hypothalamic-pituitary-adrenal (HPA) axis, and hippocampal neurogenesis. However, most studies have evaluated the symptomatic features of depression at certain time points after exposure to psychosocial stress. Here, we examined the frequency-dependent effects of psychosocial stress on depression-related features in rats.

METHODS

In the present study, different frequencies (one, two, three, or four times) of psychosocial stress were applied to 19 male Sprague-Dawley rats using a resident/intruder paradigm. Subsequently, the rats were subjected to a stress reactivity test to evaluate HPA axis activity, following which assessments of immobility behavior in the forced swimming test (FST) and adult neurogenesis were conducted.

RESULTS

One-time stressed rats showed a decrease in immobility behavior in the FST and the amount of doublecortin (DCX)-positive cells. Two-time stress caused hypoactivity of the HPA axis. In contrast, immobility behavior and HPA axis activity were increased after four-time stress exposure, but the number of DCX-positive cells was decreased.

CONCLUSIONS

Our findings suggest that psychosocial stress produces a biphasic effect on the symptoms of depression in a stress-frequency-dependent manner, which could provide insights to facilitate further pathogenesis research on depression.