Influence of a mu- and kappa-opioid agonist on isoflurane minimal anesthetic concentration in chickens

Kevin T. Concannon From the Department of Anatomy, Physiological Sciences, and Radiology, College of Veterinary Medicine, North Carolina State University, Raleigh NC 27606.

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John R. Dodam From the Department of Anatomy, Physiological Sciences, and Radiology, College of Veterinary Medicine, North Carolina State University, Raleigh NC 27606.

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Peter W. Hellyer From the Department of Anatomy, Physiological Sciences, and Radiology, College of Veterinary Medicine, North Carolina State University, Raleigh NC 27606.

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SUMMARY

Chickens (n = 18), ranging in age from 30 to 50 weeks and in body weight from 1.1 to 2.1 kg, were anesthetized with isoflurane. Ventilation was controlled, and temperature was maintained at 40.1 ± 1.0 C. The minimal anesthetic concentration (mac) of isoflurane was determined by use of a bracketing technique based on purposeful movement in response to a toe clamp. After determining isoflurane mac in triplicate, birds were given a mu-opioid agonist (morphine, n = 9) or a kappa-opioid agonist (U50488H, n = 9). Determination of mac was repeated after each IV administration of agonist in progressive doses of 0.1, 1.0, and 3.0 mg/kg of body weight. Heart rate and mean arterial blood pressure (MAP) were recorded immediately before and after each injection. Control mac (mean ± SEM) was 1.24 ± 0.05% and 1.05 ± 0.03% for the mu- and kappaopioid agonist groups, respectively. Morphine and U50488H caused a dose-dependent decrease in isoflurane mac in all birds. Reduction of mac from control (mean ± SEM) was 15.1 ± 2.7, 39.7 ± 3.1, and 52.4 ± 4.0% after the 3 successive doses of morphine and was 13.3 ± 3.0, 27.6 ± 3.3, and 40.8 ± 3.8% after U50488H was given. Each opioid injection resulted in significant (P ≤ 0.05, repeated measures anova) lowering of mac. Heart rate and MAP did not change significantly (P ≤ 0.05, paired Student’s t- test) after any dose of opioid. In conclusion, morphine or U50488H decreased isoflurane mac in dose-dependent manner without significant effect on heart rate and MAP.

SUMMARY

Chickens (n = 18), ranging in age from 30 to 50 weeks and in body weight from 1.1 to 2.1 kg, were anesthetized with isoflurane. Ventilation was controlled, and temperature was maintained at 40.1 ± 1.0 C. The minimal anesthetic concentration (mac) of isoflurane was determined by use of a bracketing technique based on purposeful movement in response to a toe clamp. After determining isoflurane mac in triplicate, birds were given a mu-opioid agonist (morphine, n = 9) or a kappa-opioid agonist (U50488H, n = 9). Determination of mac was repeated after each IV administration of agonist in progressive doses of 0.1, 1.0, and 3.0 mg/kg of body weight. Heart rate and mean arterial blood pressure (MAP) were recorded immediately before and after each injection. Control mac (mean ± SEM) was 1.24 ± 0.05% and 1.05 ± 0.03% for the mu- and kappaopioid agonist groups, respectively. Morphine and U50488H caused a dose-dependent decrease in isoflurane mac in all birds. Reduction of mac from control (mean ± SEM) was 15.1 ± 2.7, 39.7 ± 3.1, and 52.4 ± 4.0% after the 3 successive doses of morphine and was 13.3 ± 3.0, 27.6 ± 3.3, and 40.8 ± 3.8% after U50488H was given. Each opioid injection resulted in significant (P ≤ 0.05, repeated measures anova) lowering of mac. Heart rate and MAP did not change significantly (P ≤ 0.05, paired Student’s t- test) after any dose of opioid. In conclusion, morphine or U50488H decreased isoflurane mac in dose-dependent manner without significant effect on heart rate and MAP.

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