Establishing Preclinical Withdrawal Syndrome Symptomatology Following Heroin Self-Administration in Male and Female Rats

Opioid use disorder (OUD) is a significant health problem, and understanding mechanisms of various aspects of OUD including drug use and withdrawal is important. Preclinical models provide an ideal opportunity to evaluate mechanisms underlying opioid withdrawal. Current models are limited by their reliance upon forced opioid administration, focus on the acute (and not protracted) syndrome, and exclusion of females. In this study, male and female rats self-administered heroin (maintenance dose of 12.5 μg/kg/infusion) opioid withdrawal following abrupt discontinuation was measured. In Phase 1, acute withdrawal symptoms were rated in male and female rats at 0, 16, 48, and 72 hrs following the last self-administration session. Total somatic signs increased until 48 hrs (predominantly in females), and heroin intake positively correlated with total somatic signs at the 48 and 72 hr timepoints. Measures of hyperactivity and anxiety-like behavior increased by 16 and 48 hrs, respectively. In Phase 2, symptoms were assessed at baseline, acute, and protracted (168 and 312 hrs after self-administration) timepoints in a subset of male and female rats from Phase 1. The total number of somatic signs did not differ across timepoints, though females displayed significantly higher body temperature at all timepoints compared to males, indicating sex-specific protracted withdrawal symptomatology. These data provide a thorough characterization of rodent opioid withdrawal symptomatology following self-administration and abrupt discontinuation that serve as a foundation for future studies designed to mimic the human experience, and demonstrate the importance of characterizing acute and protracted withdrawal with sex-specificity in preclinical models of opioid self-administration.