Abstract

Amphibians are commonly anesthetized by immersion with tricaine methanesulfonate (MS-222). MS-222 is a local anesthetic whose mechanism of action as a general anesthetic is unknown. Although effective as an amphibian anesthetic, MS-222 requires higher concentrations and longer induction times compared with its use in fish. Additionally, MS-222 requires the use of gram scale or pre-mixed stock solution, and buffering agent. It would therefore be useful to have a safe alternative to MS-222 for use in amphibians where the mechanism of action was known, the drug was already reconstituted, and could be administered by various routes. This project evaluated the neurosteroid, alfaxalone, as an anesthetic in bullfrogs. Eight adult bullfrogs, 593 (411–780) g, were used in a crossover design. Frogs were administered alfaxalone IM at 10, 12, 15, or, 17.5 mg/kg with a 1 week washout period. 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 25g 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 the 10, 12, 15, and 17.5 mg/kg doses the median time to recumbency was 15.4, 12.6, 12.3, and 6.6 min respectively. At the 10, 12, 15, and 17.5 mg/kg doses, median time to first limb movement was 68.5, 77.5, 89.0, and 115 min respectively. At the 10, 12, 15, and 17.5 mg/kg doses median time to recovery was 90, 68.5, 124.5, 115 min, respectively. Following induction, at 10, 12, 15, and 17.5 mg/kg doses, median HR was 42, 40, 40, and 42 respectively and median RR 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 responded to needle insertion. None of the frogs dosed by immersion became anesthetized. Intramuscular alfaxalone produced dose-dependent immobilization in frogs but did not provide sufficient anesthesia to prevent response to noxious stimuli. Immersion of frogs in alfaxalone for 30 min at 2 g/L did not produce immobilization or anesthesia.

* Presenting author