Warm Water Tail Withdrawal Research Articles

In vivo determination of μ opioid receptor reserve and turnover in rhesus monkeys after irreversible blockade with clocinnamox

The two prototypical μ opioid agonists morphine and alfentanil were tested in a warm-water tail withdrawal antinociception assay performed at 45 o C, 50 o C and 55 o C in rhesus monkeys before and after administration of the irreversible opioid antagonist clocinnamox (0.1 mg/kg s.c.). Clocinnamox acutely decreased the available opioid receptors by up to 90%. Receptor numbers returned to control levels with a half-life of approximately one week. The affinity, efficacy, the maximum effect, and the stimulus-response transduction factor did not change significantly over time. The efficacy of alfentanil was always 2-3-fold higher than that for morphine, the efficacies of both agonists being higher at lower temperatures. The affinity of alfentanil was 29-fold higher than that of morphine

Effect of subtype-selective opioid receptor blockers on nitrous oxide antinociception in rats

Nitrous oxide antinociception in rats was evaluated by the warm water tail withdrawal test following central pretreatment with blockers of various opioid receptor subtypes. The analgesic dose, 50% (AD 50) value for nitrous oxide antinociception was significantly elevated by MR-2266 (which is relatively selective for κ-opioid receptors) and increased to a lesser degree by ICI-174,864 (which is selective for δ-opioid receptors). However, pretreatment with β-funaltrexamine (which is selective for μ-opioid receptors), even at extremely high doses, was ineffective in altering the AD 50 for nitrous oxide antinociception. According to these findings, nitrous oxide antinociception, as evaluated in this paradigm, appears to be mediated by κ- and possibly δ- but not μ-opioid receptors.