Large-scale behavioral characterization of oxycodone self-administration in heterogeneous stock rats reveals initial analgesic effects are associated with addiction-like behaviors - PubMed (original) (raw)
. 2026 May;51(6):1074-1083.
doi: 10.1038/s41386-026-02348-8. Epub 2026 Jan 30.
Giordano de Guglielmo 2, Lieselot L G Carrette 2, Sierra Simpson 2, Jenni Kononoff 3, Adam Kimbrough 2, Lauren C Smith 2, Kokila Shankar 2 3, Alicia Avelar 2, Dana Conlisk 3, Molly Brennan 2, Lani Tieu 2, Sharona Sedighim 2, Brent Boomhower 2, Lisa Maturin 2, McKenzie J Fannon 2, Angelica Martinez 2, Caitlin Crook 2, Selen Dirik 2, Nathan Velarde 2, Paul Schweitzer 2, Selene Bonnet-Zahedi 2, Elizabeth Sneddon 2, Sonja Plasil 2, Alex A Morgan 2, Dyar N Othman 2, Benjamin Sichel 2, Beverly Peng 2, Apurva S Chitre 2, Oksana Polesskaya 2, Justin Lau 2, Ashley Vang 2, Leah C Solberg Woods 4, Abraham A Palmer 2 5, Olivier George 6
Affiliations
- PMID: 41617925
- PMCID: PMC13125645
- DOI: 10.1038/s41386-026-02348-8
Large-scale behavioral characterization of oxycodone self-administration in heterogeneous stock rats reveals initial analgesic effects are associated with addiction-like behaviors
Marsida Kallupi et al. Neuropsychopharmacology. 2026 May.
Abstract
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.
© 2026. The Author(s).
Conflict of interest statement
Competing interests: The authors declare no competing interests.
Figures
Fig. 1. Individual differences in addiction-behaviors in HS rats following intravenous oxycodone self-administration.
A Timeline of the behavioral paradigms. B Number of oxycodone infusions (150 μg/kg/infusion) during the first hour of self-administration under ShA (2 h/day), and LgA (12 h/day) conditions across 14 cohorts (N = 542). Data show significant escalation of intake from day 4 of LgA onward (***p < 0.001 vs. first LgA session, one-way ANOVA with Bonferroni post hoc comparisons). C Violin plot depicting the number of oxycodone infusions during PR testing after ShA and LgA phases (N = 542, ***p < 0.001, paired _t_-test). D Development of tolerance to oxycodone’s analgesic effects. Tail withdrawal thresholds (in seconds) measured via the tail immersion test at baseline (BSL), after three oxycodone infusions pre-extended-access self-administration (Oxy-pre-SA), and post-extended-access self-administration (Oxy post-SA) (***p < 0.001 vs BSL and ###p < 0.001 vs Oxy-pre-SA, one-way ANOVA with Bonferroni post hoc comparisons). E Development of withdrawal-induced hyperalgesia. Data represent percentage change from baseline in paw withdrawal force (g) measured via the von Frey test before and after extended-access oxycodone self-administration (***p < 0.001 vs BSL, paired _t_-test).
Fig. 2. Sex differences in addiction-like behaviors.
A Number of oxycodone infusions (150 μg/kg/infusion) during the first hour of short-access (ShA; 2 h/day) and long-access (LgA; 12 h/day) oxycodone self-administration in male and female HS rats (N = 542). Females exhibited greater escalation from day 6 of LgA onward (***p < 0.01 vs LgA 1 and ##p < 0.01 vs males, two-way ANOVA with Bonferroni post hoc tests). B Violin plot depicting the number of oxycodone infusions under progressive ratio (PR) testing after ShA and LgA phases in male and female HS rats (N = 542; ***p < 0.01 vs ShA, ##p < 0.01 vs males, two-way ANOVA with Bonferroni post hoc tests). C Development of tolerance to oxycodone’s analgesic effects. Tail withdrawal thresholds (in seconds) measured via the tail immersion test at baseline (BSL), after two oxycodone infusions pre-extended-access self-administration (Oxy-pre-SA), and post- extended-access self-administration (Oxy post-SA) in male and female HS rats (N = 542, ***p < 0.001 vs BSL and ###p < 0.001 vs Oxy-pre-SA, one-way ANOVA with Bonferroni post hoc tests). D Development of withdrawal-induced hyperalgesia in male and female HS rats. Data represent percent change from baseline in paw withdrawal force (g) measured via the von Frey test before and after extended-access oxycodone self-administration (N = 542; ***p < 0.001 vs BSL, paired _t_-test).
Fig. 3. Construction and visualization of the Addiction Index for oxycodone addiction-like behaviors.
A–D _Z_-scores for individual measures in the HS rat population (n = 542). A Escalation of oxycodone intake (average of last 3 days of long-access [LgA] self-administration), B motivation (progressive ratio [PR] breakpoint post-LgA, C withdrawal-induced hyperalgesia (percent reduction in von Frey test thresholds), and D tolerance (difference in tail immersion test responses pre- and post-LgA). E Addiction Index, calculated as the mean of the four _Z_-scores, categorizing rats into quartiles: severe (red), moderate (orange), mild (yellow), and resilient (green). The scatter plot depicts individual rats along the principal analysis, highlighting resilient (green) and severe (red) groups. F Addiction Index distribution for individual rats, showing constituent _Z_-scores and classifications by vulnerability (resilient vs. severe) and sex (male vs. female).
Fig. 4. Differential vulnerability to oxycodone addiction-like behaviors in HS rats.
A Oxycodone infusions during short-access (ShA; 2 h/day) and long-access (LgA:12/day) self-administration in resilient, mild, moderate, and severe groups (***p < 0.001). B Motivation assessed by progressive ratio (PR) testing post- ShA and post-LgA, showing infusions per session (***p < 0.001 vs ShA, #p < 0.05 and ###p < 0.001 vs immediate lower group). C Tolerance to oxycodone’s analgesic effects measured by tail immersion test at baseline (BSL), pre-ShA (15 min post-oxycodone, 150 μg/kg/infusion × 2), and post- LgA (12 h withdrawal, post-oxycodone). Data show tail withdrawal (***p < 0.001 vs BSL and ###p < 0.001 vs Oxy-pre-SA, @p < 0.05 vs immediate lower group). D Withdrawal-induced hyperalgesia assessed by von Frey test, expressed as percent change in paw withdrawal force from baseline at 10–12 h post-LgA (***p < 0.001 vs BSL; #p < 0.05, ##p < 0.01, and ###p < 0.001 vs. immediate lower group). E Linear regression analysis correlating baseline oxycodone-induced analgesia (tail immersion) with the final Addiction Index for all subjects (n = 542). Each dot represents an individual animal. The analysis reveals a highly significant positive correlation (r = 0.2426, p < 0.0001), identifying high initial physiological sensitivity as a risk factor for the development of the severe addiction phenotype.
Fig. 5. Individual drug self-administration trajectories reveal distinct cluster-specific acquisition patterns across sexes.
Longitudinal oxycodone self-administration data for A males and B females. Thin semi-transparent lines represent the daily infusion counts for individual rats, color-coded by their designated cluster (red = severe, orange = moderate, yellow = mild, green = resilient). Thick solid lines with shaded bands represent the Mean ± SEM for each cluster. The visualization highlights the extensive individual variability within the heterogeneous stock (HS) population and confirms that the identified clusters capture distinct acquisition trajectories that are masked by the total population average.
Fig. 6. Unsupervised Bayesian Stochastic Block Model (BSBM) clustering validates the behavioral phenotypes.
A Uniform Manifold Approximation and Projection (UMAP) plot visualizing the data structure of the total population (n = 542). Points are colored by the four clusters identified via the unsupervised BSBM algorithm. B Behavioral heatmap profiling the four unsupervised clusters. Rows represent _Z_-score normalized behavioral features; columns represent the clusters. The color scale indicates deviation from the population mean. The analysis confirms that Cluster 1 aligns with the severe phenotype, while Cluster 4 aligns with the resilient phenotype. C Sankey diagram illustrating the concordance between the original behavioral phenotypes (Left) and the unsupervised BSBM clusters (Right). The flow width represents the number of animals. A Chi-square test indicates a significant dependence between the manual and unsupervised classification methods (_χ_2 = 863.1, df = 9, p < 0.0001).
References
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