Individual differences in behavioral effects of xylazine and opioid-xylazine mixtures in male rats.

Rationale: Recent work in animals suggests xylazine neither enhances the rewarding effects nor intake of fentanyl. Anecdotal evidence from people who use drugs indicates some individuals prefer fentanyl adulterated with xylazine. Systematic examination of pharmacological interactions between xylazine and opioids is needed to understand the disparate findings between preclinical studies and human reports. Objectives: This study examined behavioral interactions between xylazine and opioids in rats to investigate the pharmacology underlying an emerging trend in drug use. Methods: The sedative, hypothermic, subjective, and respiratory effects of xylazine and opioid-xylazine mixtures were examined in male rats. Locomotor activity was measured in an open field, and body temperature changes were measured with a rectal probe. Rats were trained to discriminate 0.04 mg/kg fentanyl, 0.02 mg/kg fentanyl, or 1.5 mg/kg xylazine from saline and were probed with fentanyl, xylazine, or both to observe whether the drug(s) generalized with the training dose. Whole body plethysmography was used to assess the effects of xylazine on respiration. Results: Xylazine depressed locomotor activity and core body temperature, but considerable variability between subjects was observed. In some subjects, xylazine fully substituted for fentanyl, and prolonged the subjective effects of fentanyl. Doses of 1 and 1.78 mg/kg xylazine only partially generalized to the training dose of 1.5 mg/kg xylazine. Xylazine exacerbated the respiratory depressant effects of opioids, and atipamezole reversed the xylazine enhancement of morphine-induced respiratory depression. Conclusions: Individual differences were observed in multiple behavioral measures following xylazine administration and may recapitulate the divisiveness of xylazine reported in people who use drugs.