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Rationale According to psychological theories, cognitive distortions play a pivotal role in the aetiology and recurrence of mood disorders. Although clinical evidence for the coexistence of depression and altered sensitivity to performance feedback is relatively coherent, we still do not know whether increased or decreased sensitivity to positive or negative feedback is associated with ‘pro-depressive’ profile in healthy subjects. Objective Our research has been designed to answer this question, and here, we present the first steps in that direction. Methods Using a rat version of the probabilistic reversal-learning (PRL) paradigm, we evaluated how sensitivity to negative and positive feedback influences other cognitive processes associated with mood disorders, such as motivation in the progressive ratio schedule of reinforcement (PRSR) paradigm, hedonic status in the sucrose preference (SP) test, locomotor and exploratory activity in the open field (OF) test, and anxiety in the light/dark box (LDB) test. Results The results of our study demonstrated for the first time that in rodents, sensitivity to negative and positive feedback could be considered a stable and enduring behavioural trait. Importantly, we also showed that these traits are independent of each other and that trait sensitivity to positive feedback is associated with cognitive flexibility in the PRL test. The computational modelling results also revealed that in animals classified as sensitive to positive feedback, the α learning rates for both positive and negative reward prediction errors were higher than those in animals classified as insensitive. We observed no statistically significant interactions between sensitivity to negative or positive feedback and the parameters measured in the PRSR, SP, OF or LDB tests. Conclusions Further studies using animal models of depression based on chronic stress should reveal whether sensitivity to feedback is a latent trait that when interacts with stressful life events, could produce correlates of depressive symptoms in rats.

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Although the cognitive theory has implicated judgment bias in various psychopathologies, its role in decision making under risk remains relatively unexplored. In the present study, we assessed the effects of cognitive judgment bias on risky choices in rats. First, we trained and tested the animals on the rat version of the probability-discounting (PD) task. During discrete trials, the rats chose between two levers; a press on the "small/certain" lever always resulted in the delivery of one reward pellet, whereas a press on the "large/risky" lever resulted in the delivery of four pellets. However, the probability of receiving a reward from the "large/risky" lever gradually decreased over the four trial blocks. Subsequently, the rats were re-trained and evaluated on a series of ambiguous-cue interpretation (ACI) tests, which permitted their classification according to the display of "optimistic" or "pessimistic" traits. Because dopamine (DA) has been implicated in both: risky choices and optimism, in the last experiment, we compared the reactivity of the dopaminergic system in the "optimistic" and "pessimistic" animals using the apomorphine (APO; 2 mg/kg s.c.) sensitivity test. We demonstrated that as risk increased, the proportion of risky lever choices decreased significantly slower in "optimists" compared with "pessimists" and that these differences between the two groups of rats were associated with different levels of dopaminergic system reactivity. Our findings suggest that cognitive judgment bias, risky decision-making and DA are linked, and they provide a foundation for further investigation of the behavioral traits and cognitive processes that influence risky choices in animal models.

Background: Pessimistically biased judgment has been associated with the aetiology and recurrence of depressive disorder in humans. In the present studies we investigated if biased judgments, measured as stable and enduring behavioural traits (optimism/pessimism), could determine vulnerability of laboratory rats to acute antidepressant treatments. Methods: For this, initially, in a series of ambiguous-cue interpretation tests, we identified animals displaying 'pessimistic' and 'optimistic' traits. Subsequently, we tested, in a fully randomized Latin-square design, how these traits interacted with sensitivity of rats to the effects of 5 different (agomelatine, escitalopram, mirtazapine, venlafaxine and clomipramine) antidepressant drugs. The effects of antidepressant treatments were measured in the probabilistic reversal-learning paradigm, which allowed assessing the sensitivity of animals to positive and negative feedback, and in the progressive ratio schedule of reinforcement paradigm, which allowed assessment of approach and avoidance motivation. Results: We report significant differences between 'optimistic' and 'pessimistic' rats in vulnerability to antidepressant treatments in both applied behavioural paradigms. Conclusions: The results of our studies are discussed in terms of neurobiological mechanisms of the observed effects and their possible implications for establishing novel ways of treatment based on previous cognitive assessments.

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