The trait 'pessimism' does not interact with cognitive flexibility but makes rats more vulnerable to stress-induced motivational deficits: Results from the attentional set-shifting task.

In the present study, we have investigated the effects of the traits 'optimism' and 'pessimism' on cognitive flexibility in an animal model of depression based on chronic restraint stress. For this, first, we trained and tested the rats in a series of ambiguous-cue interpretation (ACI) tests, which allowed us to classify them as 'optimistic' or 'pessimistic'. Subsequently, we re-trained and re-tested the animals in the Attentional Set Shifting Task (ASST), which allowed evaluation of the differences between 'optimists' and 'pessimists' in terms of cognitive flexibility. Finally, we subjected half of the 'optimistic' and half of the 'pessimistic' rats to chronic (2 weeks) restraint stress and assessed the interaction between cognitive judgement bias and stress in the ASST. Although we did not observe statistically significant effects of the investigated traits and stress on cognitive flexibility, the 'pessimistic' animals subjected to chronic restraint stress showed significantly longer latencies to approach experimental rewards than their 'optimistic' conspecifics. This effect may indicate a stress-induced motivational deficit that is specific to 'pessimistic' animals. The results of the present study, along with our previous reports, indicate that the trait 'pessimism' determines animals' vulnerability to stress.