Large-scale behavioral characterization of oxycodone self-administration in heterogeneous stock rats reveals initial analgesic effects are associated with addiction-like behaviors

Family and twin studies indicate that 20-60% of the vulnerability to opioid use disorder (OUD) is influenced by genetic factors, but the specific genes driving addiction-like behaviors, including sensitivity to opioid analgesia, tolerance, dependence, and escalation of oxycodone self-administration, remain unidentified, limiting precision medicine approaches. To address this, we phenotyped over 500 heterogeneous stock (HS) rats, an outbred population with high genetic diversity, to characterize traits associated with OUD vulnerability and resilience. Rats self-administered oxycodone (150 µg/kg/infusion) in short-access (2 h/day, 4 days) followed by long-access (12 h/day, 14 days) sessions. We assessed motivation for oxycodone using progressive ratio testing, withdrawal-induced hyperalgesia with von Frey tests, and tolerance to oxycodone's analgesic effects via tail immersion tests. Large cohorts (n = 46-60) and Z-score normalization minimized cohort-specific effects. An Addiction Index, derived from averaging Z-scores of escalation, motivation, tolerance, and hyperalgesia, revealed significant individual variability. Rats with severe addiction-like behaviors displayed higher initial analgesia, greater escalation, and more pronounced tolerance compared to resilient rats. Females showed increased escalation and motivation compared to males, but similar tolerance and hyperalgesia. Principal component analysis confirmed the Addiction Index's validity, accounting for 40% of behavioral variance. This high-throughput phenotyping in HS rats, leveraging their genetic diversity, provides a robust framework for genome-wide association studies to identify gene variants linked to OUD vulnerability, offering translational potential for discovering novel therapeutic targets and advancing pharmacogenetic strategies for OUD treatment.