INVESTIGATING THE RESPIRATORY CONSEQUENCES OF FENTANYL AND XYLAZINE POLY-DRUG ADMINISTRATION ON ADULT CD1 MICE USING UNRESTRAINED WHOLE-BODY PLETHYSMOGRAPHY

Xylazine, an α₂‑adrenoceptor agonist and sedative, has increasingly been used as a fentanyl adulterant, raising significant public health concerns about its effects on ventilatory control. We investigated how xylazine and fentanyl, individually and combined, alter ventilatory patterns in adult male and female CD1 mice. We also assessed the rescue potential of the antagonists atipamezole (for xylazine) and naloxone (for fentanyl). Ventilatory parameters were measured by unrestrained whole‑body plethysmography, following xylazine (3, 10 mg/kg), fentanyl (0.05, 0.15, 1.35 mg/kg), and reversal agents (1 mg/kg and 3 mg/kg respectively). Both fentanyl and xylazine independently produced robust ventilatory depression through distinct phases of breathing: 1.35mg/kg fentanyl prolonged inspiratory time (Ti) by 167%, whilst 10mg/kg xylazine increased expiratory time (Te) by 197%. Consequently, ventilation rate was reduced (p<0.05). At these doses, fentanyl and xylazine induced changes in the Ti/Te ratio by +65% and -30.5%, and the ratio of peak inspiratory flow and peak expiratory flow (PIF/PEF) by -42% and -23% respectively, suggesting opposing mechanisms of ventilatory depression. Fentanyl and xylazine co-administration produced equal effect sizes on Ti and Te (d=0.05), but a significant reduction in PIF/PEF ratio emerged (p<0.01), reflecting a greater effect on inspiration. Only combined naloxone-atipamezole treatment restored ventilation rate to baseline following co‑drug exposure (p<0.01). All interventions improved Ti/Te and PIF/PEF ratios, especially the naloxone and combined treatments (p<0.001). These findings highlight the clinical complexity of xylazine-fentanyl co-use; with ongoing work using an in situ model (the working heart-brain preparation) to identify central mechanisms underpinning their divergent effects.