Acute stress facilitates habitual behavior in female rats

Instrumental behavior can reflect the influence of goal-directed and habitual systems. Contemporary research suggests that stress may facilitate control by the habitual system under conditions where the behavior would otherwise reflect control by the goal-directed system. However, it is unclear how stress modulates the influence of these systems on instrumental responding to achieve this effect, particularly in females. Here, we examine whether a mild psychogenic stressor experienced before acquisition training (Experiment 1), or prior to the test of expression (Experiment 2) would influence goal-directed and habitual control of instrumental responding in female rats. In both experiments, rats acquired an instrumental nose-poke response for a sucrose reward. This was followed by a reinforcer devaluation phase in which half the rats in Stressed and Non-Stressed conditions received pairings of the sucrose pellet with illness induced by lithium chloride until they rejected the pellet when offered. The remaining rats received a control treatment consisting of pellets and illness on separate days (Unpaired). Control by goal-directed and habitual systems was evaluated in a subsequent nonreinforced test of nose poking. The results of Experiment 1 indicated that the Non-Stressed Paired group reduced nose-poking compared to the Unpaired controls, identifying the response as goal directed, whereas the Stressed Paired and Unpaired groups made a similar number of nose pokes identifying the response as habitual despite a similar amount of training. Results from Experiment 2 indicated habitual control of nose-poke responding was present when stress was experienced just prior to the test. Collectively, these data suggest that stress may facilitate habitual control by altering the relative influence of goal-directed and habitual processes underpinning instrumental behavior. These results may be clinically relevant for understanding the contributions of stress to dysregulated instrumental behavior in compulsive pathologies.

Levonorgestrel maintains goal-directed behavior in habit-trained intact female rats

Hormonal contraceptives are utilized by millions of women worldwide. However, it remains unclear if these powerful endocrine modulators may alter cognitive function. Habit formation involves the progression of instrumental learning as it goes from being a conscious goal-directed process to a cue-driven automatic habitual motor response. Dysregulated goal and/or habit is implicated in numerous psychopathologies, underscoring the relevance of examining the effect of hormonal contraceptives on goal-directed and habitual behavior. This study examined the effect of levonorgestrel (LNG), a widely used progestin-type contraceptive, on the development of habit in intact female rats. Rats were implanted with subcutaneous capsules that slowly released LNG over the course of the experiment or cholesterol-filled capsules. All female rats underwent operant training followed by reward devaluation to test for habit. One group of females was trained at a level that is sub-threshold to habit, while another group of females was trained to a level well over the habit threshold observed in intact females. The results reveal that all sub-threshold trained rats remained goal-directed irrespective of LGN treatment, suggesting LNG is not advancing habit formation in female rats at this level of reinforcement. However, in rats that were overtrained well above the threshold, cholesterol females showed habitual behavior, thus replicating a portion of our original studies. In contrast, LNG-treated habit-trained rats remained goal-directed, indicating that LNG impedes the development and/or expression of habit following this level of supra-threshold to habit training. Thus, LNG may offset habit formation by sustaining attentional or motivational processes during learning in intact female rats. These results may be clinically relevant to women using this type of hormonal contraceptive as well as in other progestin-based hormone therapies.

Cyclic estrogen and progesterone during instrumental acquisition contributes to habit formation in female rats

Habit formation is thought to involve two parallel processes that are mediated by distinct neural substates: one that suppresses goal-directed behavior, and one that facilitates stimulus-response (S-R) learning, which underscores habitual behavior. In previous studies we showed that habitual responding emerges early during instrumental training in gonadally-intact female, compared to male, rats. The present study aimed to determine the role of ovarian hormones during instrumental acquisition in the transition from goal-directed to habitual behavior in female rats. Ovariectomized (OVX) female rats were given subcutaneous silastic capsules that released low levels of 17-β estradiol (E2) to maintain estrogen receptor availability. Rats were assigned to one of three hormone treatment conditions: no additional hormone replacement (Control group), replacement with high E2 (High E2 group), or replacement with high E2 followed by progesterone (High E2 + P4 group). Hormone replacement occurred twice during acquisition to mimic natural hormone fluctuations. At test, the Control and High E2 groups demonstrated responding that was sensitive to devaluation by lithium chloride-induced illness, indicating goal-directed behavior. In contrast, the High E2 + P4 group exhibited a pattern of devaluation-insensitive, habitual responding, that suggested the suppression of goal-directed processes. In a follow-up experiment, similar procedures were conducted, however during acquisition, OVX rats were given cyclic high E2 plus medroxy-progesterone (MPA), a form of progesterone that does not metabolize to neuroactive metabolites. In this group, goal-directed behavior was observed. These data indicate that habit formation is not facilitated in low estrogen states, nor in the presence of cyclic high E2. However, cyclic high E2, together with progesterone during acquisition, appears to facilitate the early emergence of habitual responding. Furthermore, these data suggest that a neuroactive progesterone metabolite, like allopregnanolone, in combination with high cyclic E2, supports this phenomenon.