Activity of mu- and delta-selective opioid agonists in the guinea pig ileum preparation: differentiation into peptide and nonpeptide classes with beta-funaltrexamine.

Previous studies have demonstrated that pretreatment of guinea pig longitudinal muscle-myenteric plexus ileal preparations with the highly selective noncompetitive mu antagonist beta-funaltrexamine (beta-FNA) causes an increase in the Ke value for the interaction of morphine with naloxone, suggesting that beta-FNA inactivates those receptors at which morphine interacts in the guinea pig ileum. The effect is selective for mu receptors since beta-FNA has no effect upon the interaction of naloxone with the kappa agonist nalorphine. In the present study, it was found that although beta-FNA attenuated the effects of morphine and other morphine-like agonists at mu receptors in the guinea pig longitudinal muscle-myenteric plexus ileal preparation, the mu-mediated actions of delta-selective peptide agonists and mu-selective peptide agonists were not completely attenuated by beta-FNA pretreatment. These data suggest that morphine-like mu agonists and other mu-selective and delta-selective peptide agonists in the guinea-pig ileum preparation either interact with similar opioid receptors but in a distinguishable manner or interact with different populations of opioid receptors or the peptides studied had greater intrinsic activity than the nonpeptides.