Spinal administration of selective opioid antagonists in amphibians: evidence for an opioid unireceptor
Stevens CW, Newman LC.
Department of Pharmacology and Physiology,
Oklahoma State University,
College of Osteopathic Medicine,
Tulsa 74107, USA.
Life Sci. 1999;64(10):PL125-30


In mammals, opioids act by interactions with three distinct types of receptors: mu, delta, or kappa opioid receptors. Using a novel assay of antinociception in the Northern grass frog, Rana pipiens, previous work demonstrated that selective mu, delta, or kappa opioids produced a potent antinociception when administered by the spinal route. The relative potency of this effect was highly correlated to that found in mammals. Present studies employing selective opioid antagonists, beta-FNA, NTI, or nor-BNI demonstrated that, in general, these antagonists were not selective in the amphibian model. These data have implications for the functional evolution of opioid receptors in vertebrates and suggest that the tested mu, delta, and kappa opioids mediate antinociception via a single type of opioid receptor in amphibians, termed the unireceptor.
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