Behavioral effects of chronic stress in Carioca High- and Low-conditioned Freezing rats

Chronic unpredictable mild stress (CUMS) is a widely used model to study
stress-coping strategies in rodents. Different factors have been shown to
influence whether animals adopt passive or active coping responses to
CUMS. Individual adaptation and susceptibility to the environment seem to
play a critical role in this process. To further investigate this
relationship, we examined the effects of CUMS on Carioca high- and
low-conditioned freezing rats (CHF and CLF, respectively), bidirectional
lines of animals selected for high and low freezing in response to
contextual cues that were previously associated with footshocks. For this
purpose, the behavior of CHF and CLF animals was evaluated in the
contextual fear conditioning, open field, elevated T maze, and forced
swimming tests before and after 21 days of CUMS. Animals kept under
standard housing conditions were used as controls. For all tests, CHF rats
were more susceptible to the effects of CUMS compared to CLF and control
groups. CHF animals exposed to CUMS displayed a reduction in freezing
behavior, decreased number of entries and time spent in the center of the
open field, greater latencies to become immobile, and increased avoidance
and escaping behaviors in the elevated T maze. Overall, these findings
support the hypothesis that a heightened susceptibility to the environment
exerts a strong influence on coping responses to chronic stress.