A method to estimate the potency of the mu-component of an opioid having mixed mu-, and kappa- and/or delta-agonist activities.

The SC administration of either typical mu-agonists such as morphine, pethidine, fentanyl and levorphanol or a mixed mu- and delta-agonist like [D-Ala2, D-Leu5]-enkephalin to 10-day-old rats produced loss of righting reflex. Additionally, the loss of righting reflex induced by these opioid agonists was antagonized by naloxone, an opioid antagonist having a preference for mu-receptors, but by neither nor-binaltorphimine nor naltrindole, a specific kappa- or delta-antagonist, respectively, indicating that the loss of righting reflex was produced by the interaction of an opioid with mu-receptors. Moreover, the potency of each opioid agonist relative to that of morphine estimated by the present in vivo method was similar to that determined by the traditional in vitro isolated preparation. In contrast to mu-agonists, neither typical kappa-agonists such as U-50, 488H, ketocyclazocine, pentazocine and butorphanol, nor a selective delta-agonist like [D-Pen2, D-Pen5]-enkephalin affected the righting reflex of 10-day-old rats, indicating that mu-agonists, but neither kappa- nor delta-agonists, produced the naloxone-reversible loss of righting reflex in infant rats. By employing the present in vivo method to estimate the mu-agonist activity of an opioid with mixed agonist activities, it was indicated that the mu-agonist activity of ethylketocyclazocine, which had been employed as a representative kappa-agonist, was essentially the same as that of morphine, a representative mu-agonist.