(a) Female mice experienced naloxone-induced withdrawal following administration of xylazine, whereas male mice only experienced withdrawal when fentanyl was provided. Female mice experienced greater symptoms when xylazine was added to fentanyl than with fentanyl alone. (c) The adrenergic antagonist atipamezole induced withdrawal in all conditions, including saline controls, by day 3 for female mice, with all 3 drug conditions experiencing greater withdrawal symptoms than saline. While male mice experienced atipamezole-induced withdrawal following xylazine or fentanyl alone, they showed reduced withdrawal symptoms following xylazine/fentanyl co-administration.

“Xylazine is an agonist at kappa opioid receptors and exhibits sex-specific responses to naloxone administration.”

Madigan L. Bedard, et al. – UNC Chapel Hill.

In this study, Bedard and colleagues found that xylazine activates kappa opioid receptors (KORs) and, in female mice, is responsive to naloxone. Xylazine pharmacology, especially in the context of opioids and naloxone, is of growing importance for public health with the increased prevalence of fentanyl being adulterated with xylazine. Since xylazine’s known receptor is the alpha-2-adrenergic receptor (α2-AR), it has been believed, even by the DEA, that xylazine overdoses would not be responsive to naloxone.

While naloxone did not induce xylazine withdrawal in male mice, it did achieve this in female mice. Xylazine also enhanced the level of naloxone-induced withdrawal symptoms of fentanyl when the two were co-administered to female mice. These sex differences are likely due to known differences in the kappa-opioid and adrenergic systems. Intriguingly, the α2-AR antagonist atipamezole induced withdrawal symptoms after female mice were given unadulterated fentanyl, suggesting an interaction between the α2-AR and KOR pathways.