Effect of chronic emotional stress on habituation processes in open field in adult rats.

In our previous study, repeated stress in the neonatal period was found to slow habituation in the open field in adult rats. The objective of the present study was to investigate how chronic stress can affect habituation processes in the open field when occurring in adulthood. Animals were exposed to 1 week of immobilization on metal boards followed by 1 week of hypokinesis. After the stress procedure, rats were tested in the open field (once daily in 6-min sessions for four consecutive days). Immediately after the last open-field test, animals were decapitated. The rapidity of between- and within-session habituation was lower than in control rats. However, this lowering failed to be statistically significant compared with control rats. On the other hand, time latency to step down from an elevated platform was significantly increased in stress-exposed animals. Four days after the last stressful event, corticotropin-releasing hormone mRNA levels in the paraventricular nucleus were significantly increased, indicating a long-term activation of the hypothalamic-pituitary-adrenocortical axis. The results suggest that, in contrast to neonatal stress exposure, chronic emotional stress in adult rats does not represent a risk factor for the alteration of habituation processes.