Characterization of mechanical withdrawal responses and effects of μ-, δ- and κ-opioid agonists in normal and μ-opioid receptor knockout mice

Perry N. Fuchs, Carolina Roza, Ichiro Sora, George Uhl, Srinivasa N. Raja

Research output: Contribution to journalArticlepeer-review


Clinical and experimental observations suggest that opiates can exert different influences on the perception of stimuli from distinct sensory modalities. Thermally-induced nociception is classically responsive to opiate agonists. μ-Opioid receptor-deficient transgenic mice are more sensitive to thermal nociceptive stimuli and morphine fails to attenuate the nociceptive responses to thermal stimuli in these animals. To enhance our understanding of opiate influences on mechanical sensitivity, we have examined withdrawal responses to a sequence of ascending forces of mechanical stimuli in mice with normal (wild type), half-normal (heterozygous) and absent (homozygous) μ-opioid receptor levels. We report data from mice examined without drug pretreatment or following pretreatment with morphine, the selective κ- opioid agonist, U50488H, and the selective δ-opioid agonist, DPDPE. Saline- pretreated mice of each genotype displayed similar, monotonically increasing frequency of withdrawal responses to the graded stimuli. Subcutaneously administered morphine produced a dose-dependent reduction in withdrawal responses in wild type and heterozygous mice, but had no significant effect in homozygous mice. Intraventricular administration of DPDPE also reduced the frequency of paw withdrawal (FPW) in wild type mice, but not in homozygous mice. In contrast, systemic U50488H produced a dose-dependent attenuation of paw withdrawal in both wild type and homozygous mice. These findings suggest that (1) interactions of endogenous peptides with μ-opioid receptors may not play a significant role in the response to mechanical stimuli in drug-free animals, and (2) deficiency of μ-opioid receptors has no functional consequence on the response to the prototypical κ-opioid receptor agonist, but decreases responses to the prototypical μ- and δ-opioid receptor agonists.

Original languageEnglish (US)
Pages (from-to)480-486
Number of pages7
JournalBrain research
Issue number2
StatePublished - Mar 13 1999


  • Antinociception
  • Morphine
  • Opioid receptor
  • Transgenic mouse

ASJC Scopus subject areas

  • Neuroscience(all)
  • Molecular Biology
  • Clinical Neurology
  • Developmental Biology


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