Abstract
Opioid receptor ligands have been shown to elicit antinociception in mammals through three distinct types of receptors designated as μ, δ and κ. These opioid receptors have been characterized and cloned in several mammalian species. Radioligand binding techniques were employed to characterize the sites of opioid action in the amphibian, Rana pipiens. Naloxone is a general opioid receptor antagonist which has not been characterized in R. pipiens. Kinetic analyses of [ 3H]naloxone in the amphibian yielded a K D of 6.84 nM while the experimentally derived K D value from saturation experiments was found to be 7.11 nM. Density data were also determined from saturation analyses which yielded a B max of 2170 fmol/mg. Additionally, K i values were calculated in competition studies for various unlabelled μ-, δ- and κ-opioid receptor ligands to isolate their site of action. Highly selective antagonists for μ-, δ- and κ-opioid receptors yielded nearly identical K i values against [ 3H]naloxone.
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