Brad Balderson1 , Narayan Pokhrel2 , Arnav Gurha2 , Yanning Zuo2 , Benjamin Johnson2 , Olivier George2 , Abraham A. Palmer2,3 , Graham McVicker1 , Francesca Telese2

The nucleus accumbens (NAc) is a key subcortical brain structure that regulates reward and is involved in addiction. However, the molecular basis of individual differences in addiction phenotypes are not well characterized. To dissect the molecular basis of oxycodone addiction, we used an intravenous self administration assay (IVSA) to measure oxycodone addiction-like behaviours in outbred Heterogeneous Stock (HS) rats. NAc tissues were collected from 85 HS rats after 5 weeks of abstinence from oxycodone. We obtained whole genome sequencing, and also 10X multi-ome from 500,000 single-cell nuclei. Latent factor analysis on gene expression and chromatin accessibility across 14 NAc cell types identified a molecular pattern of changes significantly associated with oxycodone in-take (factor of oxycodone, FOXY). A major driver of FOXY was the abundance of Drd3-expressing medium spiny neurons (MSNs), with higher abundance associated with increased oxycodone in-take. We also identified gene expression and chromatin accessibility changes in Grm8-expressing MSNs and Chat expressing interneurons as a major component of FOXY. These cell types exclusively express the opioid receptor Oprm1 in the NAc. To evaluate if FOXY was genetically driven, we called cis-acting eQTLs and caQTLs across all cell types, and trained Predixscan models to predict gene expression and chromatin accessibility for identified eGenes and caPeaks in each cell type. These cis-variant predicted expressions indicated 18% of the variance in FOXY could be explained by genetic differences between individuals, and this variance was marginally predictive of oxycodone-intake (R=0.18, p=0.14). Overall, we identified a pattern of molecular and cell abundance changes in the NAc associated with oxycodone in-take, and at least 18% of the variance in this pattern was explained by genetic differences between individuals.

1 Salk Institute for Biological Studies, Integrative Biology Laboratory, La Jolla, CA 2 Department of Psychiatry, University of California San Diego, La Jolla, CA, USA 3 Institute for Genomic Medicine, University of California, San Diego, La Jolla, CA, USA