Alfaxalone anesthesia in bullfrogs (Lithobates catesbeiana) by injection or immersion

This project evaluated alfaxalone, a neurosteroid, as an anesthetic in bullfrogs. Eight adult bullfrogs (Lithobates catesbeiana), averaging 593 g (411-780 g) each, were used in a crossover design. Frogs were administered alfaxalone i.m. at 10, 12, 15, or 17.5 mg/kg with a 1-wk washout. Following injection, time to recumbency, first limb movement following induction, and recovery were recorded. Respiratory rate was recorded following injection and then every 15 min following induction. Heart rate was assessed via Doppler every 15 min following induction. At 20 and 40 min, a 25-ga needle was inserted in a thigh muscle to assess response to noxious stimuli. Frogs were also immersed in 2 g/L of alfaxalone for up to 30 min and similarly assessed. At dosages of 10, 12, 15, and 17.5 mg/kg, the median time to recumbency was 15.4, 12.6, 12.3, and 6.6 min, respectively. At dosages of 10, 12, 15, and 17.5 mg/kg, median time to first limb movement was 68.5, 77.5, 89.0, and 115 min, respectively. At dosages of 10, 12, 15, and 17.5 mg/kg, median time to recovery was 90, 68.5, 124.5, 115 min, respectively. Following induction, at 10, 12, 15, and 17.5 mg/kg, median heart rate was 42, 40, 40, and 42, respectively; and median respiratory rate was 44, 36, 29, and 35, respectively. Following administration of 10, 12, 15, and 17.5 mg/ kg, 8/8, 6/8, 7/8, and 8/8 frogs, respectively, responded to needle insertion. None of the frogs dosed by immersion became anesthetized. Intramuscular alfaxalone produced immobilization in frogs but did not provide sufficient anesthesia to prevent response to noxious stimuli. Alfaxalone immersion at 2 g/L for 30 min did not produce immobilization or anesthesia.