Opioid antinociception in amphibians.

Systemic and spinal administration of opioids produces a behaviorally defined antinociception in a variety of mammalian models. Although endogenous opioid peptides and opioid binding sites are ubiquitous throughout phylogeny, little attention has been paid to the function of endogenous opioid system(s) or development of nociceptive models in nonmammalian species. Recent work has shown that the amphibian, Rana pipiens, provides an appropriate model for assessment of opioid antinociception and that endogenous opioid systems may likewise modulate the central processing of noxious information in amphibians as well as mammals.