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Advisory Committee Chair

Jeremy Day

Advisory Committee Members

Aurelio Galli

Robert Sorge

Document Type

Thesis

Date of Award

2023

Degree Name by School

Master of Science (MS) Heersink School of Medicine

Abstract

Opioid-related overdose deaths have increased drastically in the past three decades, and especially since the beginning of the COVID-19 pandemic, due to the addictive, analgesic, and euphoric properties of the drugs. To produce these effects, opioids bind to the mu opioid receptor (MOR) in GABAergic neurons. The MOR is encoded by the Oprm1 gene and is expressed in multiple brain regions that regulate reward and motivation, such as the nucleus accumbens (NAc). Previous studies of Oprm1 knockdown in medium spiny neurons (MSNs) of the mouse NAc have shown that these neuronal populations are involved in reinforcing reward-context pairing as well as voluntary reward seeking and consumption. While recent single nucleus RNA sequencing (snRNA-seq) studies in rat, primate, and human NAc have detected subpopulations of NAc neurons that express high levels of Oprm1 mRNA, the identity and function of these unique populations is unknown. To investigate these populations in the rat brain, we performed differential expression and co-expression analysis using published snRNA-seq datasets from the rat NAc. This analysis revealed that one Oprm1+ neuronal subpopulation, which contained the highest expression of the MOR in the NAc, was marked selectively by colocalization of the genes Grm8 (encoding a metabotropic glutamate receptor) and Chst9 (encoding a carbohydrate sulfotransferase). Notably, this colocalization was also observed in published human and primate snRNA-seq studies, meaning this novel population may be conserved across species. To begin determining whether these cells play a functional role in opioid reward processing, we validated their existence and MSN identity, mapped their spatial localization, assessed their transcriptional activation profile in response to opioid administration, and created a molecular tool for tracing their projection targets. Consistent with our hypotheses, these cells exhibit colocalization of Grm8, significantly high amounts of Oprm1, and Chst9, localize on the outer border of the NAc shell region, and are not transcriptionally activated in response to opioid administration. These findings will advance our understanding of the neural mechanisms through which opioids exert their effects, and propel further study into this poorly described neuronal population.

Available for download on Thursday, May 09, 2024

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