Xylazine, an α 2 -adrenoceptor agonist approved for veterinary use, has increasingly been identified as an adulterant in illicit fentanyl supplies and is associated with increased overdose mortality. We previously showed that xylazine potentiates fentanyl-induced lethality in mice and that 30 mg/kg naloxone prevents mortality, whereas α 2 -adrenoceptor antagonism does not. This study examines whether opioid and adrenergic antagonists modulate the lethality, antinociceptive tolerance, and physical dependence produced by fentanyl and xylazine administered in combination. Male Swiss Webster mice received fentanyl and xylazine either at high doses to assess lethality (10 and 56 mg/kg, respectively) or at lower, analgesic doses (0.3 and 5.6 mg/kg) to evaluate antinociception, tolerance, and withdrawal. The selective kappa opioid receptor antagonist GNTI and the highly selective α 2 -adrenoceptor antagonist atipamezole were administered alone or in combination with naloxone, depending on the endpoint assessed. Antinociception over two hours was measured using the tail-flick test, and withdrawal was precipitated with different antagonists. A non-lethal dose of xylazine markedly increased fentanyl-induced lethality, an effect significantly reduced by naloxone combined with GNTI but not with atipamezole. At analgesic doses, xylazine produced mild antinociception and did not enhance fentanyl’s peak effect. Full reversal of fentanyl/xylazine antinociception required naloxone plus atipamezole. Repeated xylazine attenuated fentanyl-induced antinociceptive tolerance. Mice receiving fentanyl/xylazine displayed significantly milder naloxone-precipitated withdrawal than fentanyl alone, whereas naloxone combined with GNTI exacerbated withdrawal signs. Fentanyl–xylazine interactions are dose- and endpoint-dependent and involve distinct opioid and α 2 -adrenergic mechanisms affecting lethality, analgesia, tolerance and withdrawal. • Xylazine enhances fentanyl´s lethality; naloxone+GNTI reduce FEN/XYL mortality. • KOR antagonism (using GNTI) synergizes with naloxone to reduce FEN/XYL lethality. • Antinociception produced by fen/xyl requires opioid and α 2 -adrenergic signaling. • Xylazine dampens fentanyl tolerance and naloxone-precipitated withdrawal. • Naloxone combined with GNTI exacerbate withdrawal signs in fen/xyl-treated mice.
Albores‐García et al. (Fri,) studied this question.