Alanna Mayberry1,2, Eamonn Duffy1,3,4,7, Johnson Ajanaku1,3,4, Laura Saba6,6, Ryan Bachtell2,3,4, Marissa Ehringer1,3,4

Chronic opioid exposure can paradoxically enhance pain sensitivity, expressed behaviorally as opioidinduced mechanical allodynia. This effect is thought to reflect maladaptive nociceptive plasticity, yet the genetic and biological determinants of vulnerability remain incompletely defined. We examined strain- and sex-dependent variation in oxycodone-induced mechanical allodynia, mass and its trajectory, and voluntary opioid intake using a genetically diverse rat panel and applied multivariate analyses to resolve latent dimensions of opioid response. Adult male and female rats (n = 410) from 15 strains of the Hybrid Rat Diversity Panel underwent intravenous oxycodone or saline self-administration. Mechanical withdrawal thresholds (von Frey testing) and body mass were assessed before and after oxycodone exposure. Univariate analyses, principal component analysis, unsupervised k-means clustering, and broad-sense heritability estimation were performed. Oxycodone self-administration produced robust mechanical allodynia and disrupted normal weight gain. Both effects were strongly strain-dependent, whereas sex significantly moderated change in body weight but not allodynia magnitude. Principal component analysis identified four orthogonal dimensions explaining 94% of total variance, dissociating baseline mechanical sensitivity from opioid-induced plasticity and global physiological state. Unsupervised clustering revealed four biologically coherent subtypes distinguished by opioid intake, severity of mechanical hypersensitivity, and body mass. Cluster membership was non-randomly distributed across strain, sex, and treatment condition. Baseline traits demonstrated moderate-to-high heritability, while opioid-induced changes showed reduced heritability, consistent with strong environmental modulation during drug exposure. Oxycodone induces genetically constrained yet heterogeneous neurobehavioral adaptations. Multivariate phenotyping reveals distinct biological routes to opioid vulnerability, underscoring the importance of systems-level approaches in addiction and pain research.

1Department of Integrative Physiology, 2 Department of Psychology and Neuroscience, 3 Institute of Behavioral Genetics, 4 University of Colorado, Boulder, Colorado, USA; 5 Department of Pharmaceutical Sciences, 6 University of Colorado Anschutz, Aurora, Colorado, USA; 7 Northeastern University, Boston, Massachusetts, USA