UC Davis

In 2020, 254 people died because of opioid overdose every day in the United States. Opioid analgesics are still regarded as the gold standard for alleviating pain clinically but present significant risks. Their analgesic utility (the effect) is limited by multiple side effects, including respiratory depression, constipation, and the development of an Opioid Use Disorder (OUD). A lack of consensus on the cellular signaling events responsible for these negative side effects remains a major limitation to the development of an opioid with a reduced effect/side effect profile.All opioids exert their analgesic effects through the activation of the Gi-coupled μ- Opioid receptor (MOR). Acute action at MOR results in the reduction of neuronal excitability, reducing pain signal transmission. However, chronic use of opioid analgesics results in cellular adaptations. A central theory, biased agonism, seeks to explain the manifestation of these adaptations by comparison to endogenous signaling patterns. When MOR is activated by its endogenous ligand, a regulatory molecule called Arrestin-3 (Arr3) is recruited to the receptor, causing titration of signal transduction and subsequent internalization and recycling of the receptor. Morphine and other clinically used therapeutics do not effectively recruit Arr3. The lack of Arr3 recruitment results in receptors remaining present on the membrane without being internalized and recycled back to the membrane. Therefore, recruitment of Arr3 could be beneficial in preventing counter adaptations to chronic morphine. However, a competing theory states that Arr3 may mediate the negative side effects seen with continued opioid use. There is no consensus on the role of Arr3 in mitigating or causing the negative side effects seen in chronic opioid use. To investigate the effect of Arr3 on drug-seeking behavior, this study utilizes three different genotypes of mice with varying Arr3 recruitment profiles in response to morphine: 1) WT(poorly recruits Arr3), 2) RMOR (strongly recruits Arr3), 3) Arr3 vii -KO (does not recruit Arr3). Chapter 2 utilizes a novel oral operant self-administration paradigm for 17 weeks to model the transition from impulsive to compulsive drug use, difficulties in ceasing drug-seeking behavior, and relapse. This model shows that 38% of animals in WT and Arr3-KO genotypes exhibit compulsive drug-seeking behavior, but 0% of RMOR mice become compulsive drug-seekers. In Chapters 3-4, I investigate what underlying environmental causes explain why a subset of genetically homologous mice develop compulsive drug-seeking behavior. About 10-30% of persons prescribed opioids will develop an OUD. The explanation for why some individuals transition to misuse of opioids, placing themselves at higher risk for overdose, remains unclear. To model how individual behavior can predispose a subject to transition to compulsive drug use, a battery of anxiety measures was collected pre- and post-study to determine if baseline behavior or changes in behavior in response to drug exposure predict compulsive drug use. I determine that individual anxiety state does not influence compulsive drug-seeking behavior. In Chapter 5, I determine how changes in gut microbiome composition in response to morphine vary among individuals and how we can leverage these individual differences to develop microbial biomarkers of compulsive drug-seeking behavior.