Spinal μ, δ and κ opioids alter chemical, mechanical and thermal sensitivities in amphibians

Previously we demonstrated the use of chemical (topical acetic acid), thermal (radiant heat) and mechanical (von Frey filament) stimuli as quantifiable behavioral response assays in northern grass frog. Rana pipiens. Furthermore, response thresholds in all of these sensory modalities are significantly elevated by systemic morphine injections, which can be antagonized by naltrexone. The present study employed these three sensory assays to assess changes in chemical, mechanical and thermal sensitivities following spinal administration of μ, δ and κ opioids. Significant elevations were observed across all three sensory modalities in each subtype category and these effects were abolished by prior systemic administration of naltrexone. However, naltrexone antagonism of morphine produced hyperalgesia in both the mechanical and thermal modalities. The results support other recent work demonstrating that the spinal site for opioid analgesia is present in amphibians and that the thermal, mechanical and acetic acid assays are measures of true nociceptive activity in the amphibian.