Fentanyl reinforcement history has sex-specific effects on multi-step decision-making.

It is commonly thought that drug addiction involves a transition to habitual control of action, where the choice to consume drugs becomes automatized and reflects a failure to deliberate over possible negative outcomes. Determining whether the pursuit of addictive drugs is habitual is hampered by a lack of behavior assessments suitable for use during a bout of actual drug seeking. Therefore, to understand how variable histories of drug reinforcement might affect goal-directed and habitual pursuit of drug, we trained rats to perform a multi-step decision-making task to earn oral fentanyl and sucrose rewards following extensive pretraining with either fentanyl or sucrose. Importantly, this task allowed for independent measurements of goal-directed and habitual choice characteristics during online pursuit of rewards, and habitual choice could be further categorized into perseverative and reward-guided components. Chronic fentanyl led to a bias for reward-guided habitual choice specifically in females, and a high degree of perseveration in both sexes. These behavioral changes after chronic fentanyl pretraining generalized across fentanyl and sucrose seeking. In contrast, acute fentanyl selectively increased perseveration in females, and blunted the gradual within-session improvement in goal-directed choice in both sexes. These results show that chronic fentanyl reinforcement promotes habits that generalize across drug and non-drug reward seeking, and that female rats are especially susceptible to habitual control induced by both chronic and acute fentanyl reinforcement.