Respiratory depressant interactions of fentanyl and xylazine and their reversal by naloxone and yohimbine in mice.

BACKGROUND

Opioid misuse remains a critical public health crisis in the U.S., causing over 82,000 opioid-related overdose deaths in 2023. Fentanyl, a potent synthetic opioid acting via μ-opioid receptors (MOR), poses unique challenges for overdose reversal due to its high potency and rapid onset. Increasingly, illicit fentanyl is adulterated with xylazine, an alpha-2 adrenergic receptor agonist (α2AR) used in veterinary medicine. This combination, known as "tranq dope," is associated with polydrug dependence and enhanced respiratory depression that may be more resilient to naloxone reversal.

METHODS

Respiratory depression was assessed via whole-body plethysmography in adult male C57BL/6J mice. Dose-response curves were generated for fentanyl (0.01-10 mg/kg) and xylazine (0.3-10 mg/kg) over an 80-min recording period. A dose of 0.3 mg/kg for each agonist, which reduced minute volume (MVb) by ∼50% at T = 20, was used for antagonist reversal testing with naloxone (0.1-10 mg/kg) and yohimbine (1-10 mg/kg). Lastly, the agonists were co-administered and naloxone, yohimbine, or their combination were tested for their ability to reverse respiratory depression.

RESULTS

Fentanyl reduced MVb by lowering frequency (Freq) and increasing tidal volume (TVb). Naloxone reversed fentanyl-induced MVb depression. Yohimbine transiently worsened fentanyl-induced depression. Xylazine reduced both TVb and Freq, which was reversed by yohimbine but not naloxone. The fentanyl-xylazine combination was reversed by naloxone, but not yohimbine. When compared to fentanyl-xylazine controls, the combination of naloxone and yohimbine produced a faster reversal than naloxone alone.

CONCLUSION

Naloxone attenuated fentanyl-xylazine respiratory depression, but yohimbine co-treatment accelerated recovery, highlighting potential for α2AR antagonists as adjunctive therapy in suspected fentanyl-xylazine overdose.