Xylazine potentiates the interoceptive effects of fentanyl in male and female rats.

Rationale: Xylazine, a sedative typically used in veterinary medicine, has been increasingly detected as an adulterant in the unregulated opioid supply and present in opioid overdose deaths. Therefore, xylazine-adulterated fentanyl is a growing public health concern. People who use drugs have reported that xylazine changes and prolongs the effects of fentanyl.

Objectives: We used standard operant drug discrimination procedures to better understand how xylazine impacts the discriminative stimulus/interoceptive effects of fentanyl.

Methods: Male and female Long-Evans rats (n=23) were trained to discriminate fentanyl (0.032 mg/kg intraperitoneal) such that one lever was reinforced with sucrose on days when fentanyl was administered, and the other lever was reinforced when vehicle was administered. Once rats met testing criteria, we tested a dose range of fentanyl to confirm discriminative stimulus control, then we tested if xylazine alone produced fentanyl-like effects and if the addition of xylazine to fentanyl impacted fentanyl interoceptive effects.

Results: Stimulus control was confirmed, as rats showed increased percent responses on the fentanyl-appropriate lever as well as decreased response rates for increasing doses of fentanyl. Xylazine alone did not substitute for the stimulus effects of fentanyl but produced similar response rate reductions as fentanyl alone. Xylazine co-administered with fentanyl potentiated the stimulus effects of lower doses of fentanyl in both males and females and potentiated response rate reductions.

Conclusions: These results indicate that xylazine enhances the interoceptive effects of fentanyl, which may inform clinical research about xylazine-adulterated fentanyl.