Interactions between cardiorespiratory effects of fentanyl and xylazine and their reversal in male and female Sprague-Dawley rats.

Fentanyl continues to drive overdose deaths in the United States; however, the use of fentanyl adulterated with xylazine, an α₂ adrenergic receptor (α₂AR) agonist, results in severe overdoses that are at least partially resistant to reversal by naloxone. The present study used collar-based pulse oximetry to characterize the cardiorespiratory effects of fentanyl and xylazine, administered alone and in combination, as well as the ability of naloxone and atipamezole to reverse these effects in male and female rats. Administered alone, fentanyl and xylazine dose dependently decreased heart rate, blood oxygenation (SpO₂), and respiratory rate. Dose addition analyses indicated that fixed-ratio (3:1, 1:1, and 1:3) mixtures of fentanyl and xylazine exhibited additive interactions for all 3 cardiorespiratory endpoints, although a subadditive interaction was observed for the effects of the 1:1 mixture of fentanyl and xylazine on SpO₂. Naloxone reversed the cardiorespiratory effects of fentanyl but was slower and less effective at reversing the effects of mixtures of fentanyl and xylazine. Atipamezole reversed the cardiorespiratory effects of xylazine but was ineffective at reversing the effects of mixtures of fentanyl and xylazine. Combining naloxone with atipamezole sped the recovery of SpO₂ but induced a transient rebound tachycardia followed by sustained bradycardia, raising concerns about cardiovascular instability. These studies provide direct evidence of additive and subadditive interactions between the cardiorespiratory effects of fentanyl and xylazine and an inability of naloxone to fully reverse the cardiorespiratory effects of mixtures fentanyl and xylazine, highlighting the need for novel treatment strategies to reverse overdoses involving opioids and α₂AR agonists. SIGNIFICANCE STATEMENT: This study demonstrates that the cardiorespiratory effects of fentanyl and xylazine exhibit additive or subadditive interactions in rats and that current reversal agents, including naloxone, are insufficient to fully reverse their combined effects. These findings highlight the urgent need for improved strategies to treat overdoses involving opioids adulterated with α₂ adrenergic receptor agonists, such as xylazine.