Evaluation of hypothermia-induced analgesia and influence of opioid antagonists in leopard frogs (Rana pipiens)

Hypothermia results in diminished voluntary muscle activity, and is frequen tly used as a means of providing deep anesthesia to ectotherms and some mam mals. In ectotherms, however, it is unclear if hypothermia produces true pa in insensation. A needle-probe thermometer was used to demonstrate in frogs (Rana pipiens) that local hypothermia (9 degrees C) could be induced by pl acement of a tourniqueted leg into ice water (6 degrees C) for 10 min in co ntrast to the contralateral nontourniqueted leg (21.8 degrees C) kept out o f ice water. Analgesia was tested by placement of dilutions of acetic acid on the rear leg. Further tests using groups of 10 frogs demonstrated that f rogs with local hypothermia tolerated greater concentrations of acetic acid (mean acetic acid test score = 11) than morphine (10 mg/kg)-treated (9.6) or nontreated (5.8) frogs. Additional studies showed that morphine analgesi a was blocked with naloxone doses as low as 0.01 mg/kg and hypothermia-indu ced analgesia at 10 mg/kg. Naltrexone blocked morphine analgesia at dosages as low as 0.01 mg/kg and hypothermia-induced analgesia at 0.10 mg/kg. In s ummary, this study demonstrates that hypothermia induces significant analge sia in an amphibian, and that this analgesia is partially blocked by naloxo ne and naltrexone, suggesting that the effect is mediated at least partiall y by opioid receptors. (C) 1999 Elsevier Science Inc.