Nalmefene reverses carfentanil-induced loss of righting reflex and respiratory depression in rats

Reversing the respiratory depression induced by carfentanil involves intravenous administration of naloxone or naltrexone, but this treatment has disadvantages. Hence, finding a more appropriate treatment to counter the depressive actions of carfentanil is needed. In the present study, with the naloxone as a control, we investigated the efficacy of nalmefene for countering the depressive actions of carfentanil. Rats were treated successively with carfentanil (10 μg/kg, i.v.) and nalmefene (9.4-150.0 μg/kg, i.m.), and the duration of loss of righting reflex (LORR) recorded. Respiratory parameters were measured in free-moving rats using a whole-body plethysmograph after rats were administered carfentanil (20 μg/kg, i.v.) and nalmefene (9.4-150.0 μg/kg, i.m.) sequentially. The parameters of arterial blood gases were also examined. Nalmefene (9.4-150.0 μg/kg, i.m.) treatment dose-dependently decreased the duration of carfentanil-induced LORR. The respiratory rate after 60 min of nalmefene (150.0 μg/kg, i.m.) treatment increased from 34.3±5.3 bursts/min to 117.8±18.9 bursts/min, and enhanced pause decreased from 1.1±0.1 to 0.4±0.1, and was close to those of normal rats. Furthermore, nalmefene (37.5-150.0 μg/kg) treatment could enable the PaO2, SaO2 and PaCO2 to approach normal levels 10 min (15 min after carfentanil injection) or 30 min (25 min after carfentanil injection) after injection. While, a single injection of naloxone (150.0 μg/kg, i.m.) only achieved partial remission of respiratory depression. These data suggest that nalmefene more effectively counters the depressive actions induced by carfentanil and is a more appropriate treatment to antagonize carfentanil toxicity compared with naloxone.