Repeated, High-dose Fentanyl Administration in Rats Reveals Minimal Tolerance to Unconsciousness, Bradycardia, Muscle Rigidity and Respiratory Depression

Background: Fentanyl is a synthetic opioid that is widely used in anesthesiology, but its illicit use is rapidly increasing. At high doses fentanyl induces unconsciousness and muscle rigidity, the mechanisms of which are poorly understood. Since animal models are needed to study these effects, the aim of this study was to establish a rat model of fentanyl abuse and investigate the effects of repeated high-dose fentanyl injections on loss of righting reflex, heart rate, respiratory depression, muscle, and brain activity. Methods: Male and female Sprague-Dawley rats were studied (n=40). A bolus of 100µg/kg fentanyl was administered intravenously twice a week for five consecutive weeks. Time to return of righting reflex (RORR) after fentanyl injection and changes in EMG/EEG activity as well as heart rate were analyzed. Additionally, arterial blood gas analysis for evaluation of ventilation was performed. Mixed-effect models with Dunnet’s test and effect sizes were used for statistical analysis. Results: Repeated injections resulted in a U-shaped change in time to RORR with the longest latency after the first exposure (median: 50[1 st -3 rd quartile:36-56]min) and the shortest after the fifth exposure (16[13-33]min). Following fentanyl administration, heart rate dropped immediately by 225[95%CI: 179, 271]bpm (F=3952.16, p<.001), while EMG activity increased by 291[95%CI: 212, 370]% (F=27.51, p<0.001) and PaCO2 inclined by 49.4[95%CI: 40.6, 58.2]mmHg (F=75.97, p<0.001) within 5 minutes after injection. Additionally, pH decreased by 0.48[95%CI: 0.41, 0.54] (F=142.00, p<0.01), and PaO2 decreased by 50.4[40.8, 60.0]mmHg (F=57.90, p<0.001). Repeated fentanyl exposures did not significantly affect the extent of these changes (EMG: F=1.63, p=0.237; PaCO 2 : F=1.23, p=0.312; HR: F=1.05, p=0.400; pH: F=3.05, p=0.066; pO 2 : F=3.35, p=0.052). EEG analysis revealed that repeated fentanyl exposures elicited significantly higher absolute power in frequencies >20Hz as indicated by an area under the receiver operator characteristics curve >0.7. Conclusion: We established a rodent model of repeated, high-dose fentanyl administration. Overall, significant evidence of tolerance was not observed after ten exposures of high-dose fentanyl for any of the analyzed parameters. These results suggest that tolerance does not develop for fentanyl-induced unconsciousness, muscle rigidity, or respiratory depression.