Dawkins, A. J. | bioRxiv Science

bioRxiv2025

Repeated fentanyl abstinence intensifies opioid withdrawal and induces a proinflammatory state in striatal microglia

Opioid withdrawal is a serious obstacle to self-initiated abstinence, and previous experiences of opioid withdrawal may exacerbate the severity of subsequent incidences. To study the impact of repeated opioid withdrawal episodes, we compared male and female mice after one or five cycles of fentanyl exposure and withdrawal. We selectively expressed hemagglutinin-tagged ribosomes (RiboTag) in microglia of transgenic mice to immunoprecipitate and sequence RNA actively undergoing translation (the "translatome") from striatal microglia during fentanyl withdrawal. Key changes were confirmed by RTqPCR of RiboTag RNA. Repeated bouts of fentanyl treatment and withdrawal impacted striatal microglia much more than a single cycle of fentanyl followed by withdrawal. Multiple withdrawal cycles reduced ramification of microglial processes, suggesting a more reactive cell state, and induced more severe behavioral withdrawal signs in mice. Five cycles of fentanyl exposure and withdrawal increased the expression of gene networks associated with innate immunity signaling. Indeed, 100% of the genes associated with the "microglia core sensome", were upregulated after five cycles of withdrawal. Together these results suggest that mouse striatal microglia initiate a proinflammatory response following five, but not one, opioid exposure and withdrawal experiences and suggest that drug therapies targeting microglial innate immune responses may mitigate the severe withdrawal associated with repeated opioid tolerance and withdrawal. Significance statementRepeated cycles of fentanyl administration and withdrawal caused worsened behavioral signs of withdrawal in mice. This is the first such study to directly examine the effects of repeated opioid withdrawal on mouse behavior. We also found that repeated opioid withdrawal increased the expression of RNAs related to the proinflammatory "microglia core sensome". Furthermore, microglia in the mouse striatum were present at a higher density with reduced ramification, which suggests that multiple opioid withdrawal experiences cause a significant change to microglial signaling-state.

neuroscience↗

bioRxiv2023

Rapid appearance of negative emotion during oral fentanyl self-administration in male and female rats

Opioid use disorder has become an epidemic in the United States, fueled by the widespread availability of fentanyl, which produces rapid and intense euphoria followed by severe withdrawal and emotional distress. We developed a new preclinical model of fentanyl seeking in outbred male and female rats using volitional oral self-administration that can be readily applied in labs without intravascular access. Using a traditional two lever operant procedure, rats learned to take oral fentanyl vigorously, escalated intake across sessions, and readily reinstated responding to conditioned cues after extinction. Oral self-administration also revealed individual and sex differences that are essential to studying substance use risk propensity. During a behavioral economics task, rats displayed inelastic demand curves and maintained stable intake across a wide range of fentanyl concentrations. Oral SA was also neatly patterned, with distinct " loading" and " maintenance" phases of responding within each session. Using our software DeepSqueak, we analyzed thousands of ultrasonic vocalizations (USVs), which are innate expressions of current emotional state in rats. Rats produced 50 kHz USVs during loading then shifted quickly to 22 kHz calls despite ongoing maintenance oral fentanyl taking, reflecting a transition to negative reinforcement. Using fiber photometry, we found that the lateral habenula differentially processed drug-cues and drug-consumption depending on affective state, with potentiated modulation by drug cues and consumption during the negative affective maintenance phase. Together, these results indicate a rapid progression from positive to negative reinforcement occurs even within an active drug taking session, revealing a within-session opponent process. Significance StatementThe United States opioid epidemic is defined by rampant and treatment resistant fentanyl use. Better understanding of neural substrates underlying this phenomenon is essential to slowing the opioid crisis. Intravenous and vapor self-administration (SA) are the standard models for studying fentanyl use in rodents, however they many carry pragmatic downsides. Here, we used a novel oral fentanyl self-administration model that provides key translational and technical benefits and can be readily applied in other labs to study the neurobiology of fentanyl SA. This method captured individual and sex differences necessary for studying substance use risk propensity and uncovered a rapid shift in affective state in rats, suggesting and shift from positive to negative reinforcement within each fentanyl taking session.

neuroscience↗