Time-dependent changes in cognitive flexibility performance during intermittent social stress: Relevance for motivation and reward-seeking behavior

Choice impulsivity after repeated social stress is associated with increased perineuronal nets in the medial prefrontal cortex

Repeated stress can predispose to substance abuse. However, behavioral and neurobiological adaptations that link stress to substance abuse remain unclear. This study investigates whether intermittent social defeat (ISD), a stress protocol that promotes drug-seeking behavior, alters intertemporal decision-making and cortical inhibitory function in the medial prefrontal cortex (mPFC). Male long evans rats were trained in a delay discounting task (DDT) where rats make a choice between a fast (1 s) small reward (1 sugar pellet) and a large reward (3 sugar pellets) that comes with a time delay (10 s or 20 s). A decreased preference for delayed rewards was used as an index of choice impulsivity. Rats were exposed to ISD and tested in the DDT 24 h after each stress episode, and one- and two-weeks after the last stress episode. Immunohistochemistry was performed in rat's brains to evaluate perineuronal nets (PNNs) and parvalbumin GABA interneurons (PV) labeling as markers of inhibitory function in mPFC. ISD significantly decreased the preference for delayed large rewards in low impulsive, but not high impulsive, animals. ISD also increased the density of PNNs in the mPFC. These results suggest that increased choice impulsivity and cortical inhibition predispose animals to seek out rewards after stress.

Sex differences in cognitive flexibility are driven by the estrous cycle and stress-dependent

Stress is associated with psychiatric disorders such as post-traumatic stress disorder, major depressive disorder, anxiety disorders, and panic disorders. Women are more likely to be diagnosed with these stress-related psychiatric disorders than men. A key phenotype in stress-related psychiatric disorders is impairment in cognitive flexibility, which is the ability to develop new strategies to respond to different patterns in the environment. Because gonadal hormones can contribute to sex differences in response to stress, it is important to consider where females are in their cycle when exposed to stress and cognitive flexibility testing. Moreover, identifying neural correlates involved in cognitive flexibility could not only build our understanding of the biological mechanisms behind this crucial skill but also leads to more targeted treatments for psychiatric disorders. Although previous studies have separately examined sex differences in cognitive flexibility, stress effects on cognitive flexibility, and the effect of gonadal hormones on cognitive flexibility, many of the findings were inconsistent, and the role of the estrous cycle in stress-induced impacts on cognitive flexibility is still unknown. This study explored potential sex differences in cognitive flexibility using an operant strategy shifting-paradigm after either control conditions or restraint stress in freely cycling female and male rats (with estrous cycle tracking in the female rats). In addition, we examined potential neural correlates for any sex differences observed. In short, we found that stress impaired certain aspects of cognitive flexibility and that there were sex differences in cognitive flexibility that were driven by the estrous cycle. Specifically, stress increased latency to first press and trials to criterion in particular tasks. The female rats demonstrated more omissions and perseverative errors than the male rats; the sex differences were mostly driven by proestrus female rats. Interestingly, the number of orexinergic neurons was higher in proestrus female rats than in the male rats under control conditions. Moreover, orexin neural count was positively correlated with number of perseverative errors made in cognitive flexibility testing. In sum, there are sex differences in cognitive flexibility that are driven by the estrous cycle and are stress-dependent, and orexin neurons may underlie some of the sex differences observed.

Intermittent social stress produces different short- and long-term effects on effort-based reward-seeking behavior

Previous studies show that intermittent social defeat (ISD) stress increases self-administration of psychostimulants, which suggests that ISD promotes reward-seeking behavior and, ultimately, increases vulnerability to develop drug abuse. The present study investigates whether ISD alters cost/benefit evaluations to promote reward-seeking behavior and whether these alterations are time-dependent. Male rats performed two different tasks that assessed their motivation to seek and consume food rewards. An effort-discounting task in which rats chose between less and more effortful options (i.e., 1 lever-press versus 2, 5, 10 or 20 lever-presses) associated with low- and high-reward (i.e., 1 sugar pellet versus 3 sugar pellets), respectively; and a progressive ratio task in which rats had to increase their effort (more lever presses) to obtain a sugar pellet. ISD consisted of exposing animals to social defeat once every three days for ten days (4 stress episodes). Rats were tested 24-48 h after stress episodes, and 1 week and 6 weeks after the last stress episode. In the effort-discounting task, stressed animals showed a decrease in their preference for high rewards associated with more effort (i.e., 10 and 20 lever-presses). These effects were transient and not maintained one week after stress. In the progressive ratio task, stressed animals showed an increase in the number of lever presses to obtain rewards that emerged six weeks after the last stress episode. These results suggest different short- and long-term effects on the motivation for rewards after ISD and indicate temporal dynamic adaptations in the function of the brain reward system.