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In this two-part study, the potency of two analgesics on nociception was assessed in African Clawed Frogs (ACFs). First, three different Pain Stimuli (PSs) were evaluated in the frogs during Tricaine Methanesulfonate (MS222) anaesthesia. Using the most effective PS from the preliminary study, the analgesic effects of three different doses of fentanyl and butorphanol were examined in frogs under MS222 anaesthesia. Comparing the three different PSs (5% acetic acid onto skin, toe pinch by a clamp and pull on the ovaries), continuous Blood Pressure (BP) and Heart Rate (HR) recordings of the frogs indicated a sharp and reproducible increase in both parameters in response to acetic acid. That result clearly indicated an increased nociception during MS222 anaesthesia. Therefore, MS222 alone does not provide sufficient analgesia for painful interventions in ACFs. From all tested analgesic groups only 5 mg/kg butorphanol showed a short lasting decreased BP and HR response. In contrast, neither lower dosed butorphanol nor fentanyl in general reduced BP and HR response to a PS, only producing considerable side effects on the haemodynamic system. These findings argue against using fentanyl as an analgesic in ACFs. Butorphanol significantly reduced the nociception in the high dose group. However, considering its limited duration of action and potential adverse effects, further analgesics (e.g., ketamine and metamizole) should be evaluated to improve intraoperative analgesia when using MS222 anaesthesia in ACFs.

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