Prolonged Withdrawal From Escalated Oxycodone Is Associated With Increased Expression of Glutamate Receptors in the Rat Hippocampus

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Transcriptomics in the nucleus accumbens shell reveal sex- and reinforcer-specific signatures associated with morphine and sucrose craving

Incubation of craving is a well-documented phenomenon referring to the intensification of drug craving over extended abstinence. The neural adaptations that occur during forced abstinence following chronic drug taking have been a topic of intense study. However, little is known about the transcriptomic changes occurring throughout this window of time. To define gene expression changes associated with morphine consumption and extended abstinence, male and female rats underwent 10 days of morphine self-administration. Separate drug-naive rats self-administered sucrose in order to compare opioid-induced changes from those associated with natural, non-drug rewards. After one or 30 days of forced abstinence, rats were tested for craving, or nucleus accumbens shell tissue was dissected for RNA sequencing. Morphine consumption was predictive of drug seeking after extended (30 days) but not brief (1 day) abstinence in both sexes. Extended abstinence was also associated with robust sex- and reinforcer-specific changes in gene expression, suggesting sex differences underlying incubation of morphine and sucrose seeking respectively. Importantly, these changes in gene expression occurred without re-exposure to drug-paired cues, indicating that chronic morphine causes long-lasting changes in gene expression that prime the system for increased craving. These findings lay the groundwork for identifying specific therapeutic targets for curbing opioid craving without impacting the natural reward system in males and females.