Antagonism of the antinociceptive effects of fentanyl and the veterinary anesthetic xylazine in mice

A recent twist to the ongoing tragedy of the opioid epidemic is the adulteration of fentanyl with the veterinary tranquilizer xylazine, an a2-adrenoreceptor agonist. Unfortunately, our knowledge of how fentanyl and xylazine interact pharmacologically in different behavioral assays is limited. We examined fentanyl and xylazine alone, with 4 antagonists, and in multiple dose ratio combinations using a 52.5 degrees C hot-plate antinociception assay in Swiss-Webster male and female mice. Both fentanyl and xylazine produced full, dose-dependent increases in antinociception. In antagonism studies, naltrexone blocked fentanyl whereas yohimbine and the selective a2-adrenoreceptor antagonist, idazoxan, blocked the antinociceptive effects of xylazine. Naltrexone failed to inhibit xylazine antinociception and yohimbine increased or decreased the potency of fentanyl depending on the dose. Haloperidol, a D2/s1 antagonist, significantly shifted the potency of fentanyl and xylazine to the left. Overall, combining xylazine with fentanyl failed to significantly alter the potency of fentanyl although when xylazine proportion was higher, fentanyl was significantly less potent than fentanyl alone. Either naltrexone or yohimbine alone failed to block the 2.5X xylazine to fentanyl combination. However, naltrexone with either yohimbine or idazoxan blocked the antinociceptive effects of the 2.5X xylazine to fentanyl combination suggesting that both opioid and noradrenergic receptors appear involved in the antinociceptive effects of this combination. In conclusion, the antinociceptive effects of fentanyl combined with xylazine are dependent on the proportions coadministered and multiple antagonists may be required to block the effects of fentanyl adulterated with xylazine. Significance Statement: Combinations of the opioid agonist fentanyl and the veterinary tranquilizer xylazine currently appear on the illicit drug market. The experiments described here examine the pharmacology of these drugs alone and in combination using a model of antinociception in mice to better understand the underlying receptor mechanisms for fentanyl alone, xylazine alone, and fentanyl and xylazine in combination. The antinociceptive effects were predominantly a simple combination of opioid and a2-adrenoreceptor agonism. (c) 2025 American Society for Pharmacology and Experimental Therapeutics. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.