Behavioral flexibility and gut microbiome as potential predictors of oral oxycodone self-administration.

Prescription opioids such as oxycodone have been widely used in the United States and have contributed to the ongoing opioid epidemic. While many individuals limit use to prescribed contexts, a subset transitions to misuse and, in some cases, to illicit opioid use. Identifying behavioral and biological factors that predict this vulnerability is critical for improving prevention and intervention strategies. Here, we investigated whether individual differences in behavioral flexibility and gut microbiome composition are associated with future oxycodone intake using a translationally relevant model of oral oxycodone self-administration in male and female Long-Evans rats. We established a model in which distinct intake phenotypes emerged, characterized by animals with high versus low oxycodone consumption. Behavioral flexibility, assessed using a contingency degradation task, was associated with oxycodone intake, identifying it as a potential behavioral biomarker of vulnerability. In parallel, oral oxycodone exposure altered gut microbiome composition, and microbiome features were associated with both behavioral flexibility and drug-taking behavior. These findings support a framework in which individual differences in opioid intake arise from the interaction of pre-existing behavioral traits and biological states, including gut microbiome composition which provides a foundation for identifying predictive biomarkers and developing individualized strategies to mitigate risk for opioid misuse.