Differential sensitivity of antinociceptive tests to opioid agonists and partial agonists

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Abstract
1 The antinociceptive activity of a range of opioid agonists and agonist-antagonist analgesics was determined in mice by use of the 55°C hot plate and abdominal constriction assays. 2 Opioid agonists were approximately 10 times more effective in the abdominal constriction assay. 3 The agonist-antagonists produced analgesia only in the abdominal constriction assay, and antagonized the antinociceptive action of opioid agonists in the 55°C hot plate test. 4 These differences were shown to be attributable to the different levels of stimulus employed in the two tests. 5 By comparing the antagonist potencies of the agonist-antagonists in the 55°C hot plate test with their antinociceptive ED50 values in the abdominal constriction assay, an index of intrinsic activity was calculated.