Bondy, E. O. | bioRxiv Science

bioRxiv2025

Utility of the Recombinase Driver CX3CR1::Cre Rat Strain to Evaluate Microglial Contributions to Nicotine Addiction

Smoking remains a leading preventable cause of death, and nicotine is the primary substance responsible for maintaining use of tobacco products. Preclinical rodent models have shown that neuroimmune signaling is dysregulated by nicotine self-administration (SA) within the nucleus accumbens core (NAcore), which is a key region within the mesolimbic brain reward pathway. Microglia are the resident brain immune cell and prior studies have shown that they play an important role in nicotine-related behaviors. However, while there are transgenic mouse lines that allow for specific evaluations of microglia to neurobiology and behavior, there are fewer tools available for rats as a model species thus limiting our ability to evaluate specific contributions of microglia to nicotine SA. Using transgenic rats expressing Cre under the control of the CX3CR1 promoter bred on a Long Evans (LE) background, we show that NAcore microglia can be specifically transduced with Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). We further show that CX3CR1::Cre rats readily self-administer nicotine, and display a characteristic extinction curve that is not different from outbred LE or Cre-negative littermates. Together, these validation studies lay the foundation for future use of this transgenic rat line to evaluate the specific contributions of microglia in the brain to neurobehavioral underpinnings of nicotine addiction.

neuroscience↗

bioRxiv2023

Xylazine co-self-administration suppresses fentanyl consumption during self-administration and induces a unique sex-specific withdrawal syndrome that is not altered by naloxone in rats

Prescription and illicit opioid use are a public health crisis, with the landscape shifting to fentanyl use. Since fentanyl is 100-fold more potent than morphine, its use is associated with a higher risk of fatal overdose that can be remediated through naloxone (Narcan) administration. However, recent reports indicate that xylazine, an anesthetic, is increasingly detected in accidental fentanyl overdose deaths. Anecdotal reports suggest that xylazine may prolong the fentanyl "high", alter the onset of fentanyl withdrawal, and increase resistance to naloxone-induced reversal of overdose. To date no preclinical studies have evaluated the impacts of xylazine on fentanyl self-administration (SA; 2.5 g/kg/infusion) or withdrawal to our knowledge. We established a rat model of xylazine/fentanyl co-SA and withdrawal and evaluated outcomes as a function of biological sex. When administered alone, chronic xylazine (2.5 mg/kg, IP) induced unique sex-specific withdrawal symptomatology whereby females showed delayed onset of signs and a possible enhancement of sensitivity to the motor-suppressing effects of xylazine. Xylazine reduced fentanyl consumption both male and female rats regardless of whether it was experimenter-administered or added to the intravenous fentanyl product (0.05. 0.10, and 0.5 mg/kg/infusion) when compared to fentanyl SA alone. Interestingly, this effect was dose-dependent when self-administered intravenously. Naloxone (0.1 mg/kg, SC) did not increase somatic signs of fentanyl withdrawal, regardless of the inclusion of xylazine in the fentanyl infusion in either sex; however, somatic signs of withdrawal were higher across timepoints in females after xylazine/fentanyl co-SA regardless of naloxone exposure as compared to females following fentanyl SA alone. Together, these results indicate that xylazine/fentanyl co-SA dose-dependently suppressed fentanyl intake in both sexes, and induced a unique withdrawal syndrome in females which was not altered by acute naloxone treatment.

animal behavior and cognition↗