Behavioral analysis of opiate-mediated inhibition in the early chick embryo

Morphine (opiate agonist) produced a dose-dependent decrease in the spontaneous motility of 5- and 9-day chick embryos. Naloxone (opiate antagonist) appeared to reverse competitively the inhibition of motility caused by morphine. The effects of morphine on spontaneous motility in 5-day embryos were also reversed stereospecifically by the opiate antagonist pairs WIN 44441-3/WIN 44441-2 and levallorphan/dextrallorphan. Levorphanol (opiate agonist) also produced a dose-dependent decrease in the motility of 5-day embryos while its inactive (+)-isomer, dextrophan, was not effective. Etorphine (opiate agonist) was more than 1000-fold more effective than morphine in inhibiting the motility of 5-day embryos. The effectiveness of several opiate agonists and antagonists on the spontaneous motility of 5-day embryos was similar to their effectiveness in radioligand-binding studies on isolated membrane receptors from either adult mammalian brain or ileum. Levorphanol was more effective than dextrophan and etorphine was substantially more effective than morphine in decreasing the spontaneous motility of 4-day embryos. WIN 44441-3 was more effective than WIN 44441-2 in reversing the inhibition of motility in 4-day embryos caused by morphine. Morphine inhibited spontaneous hind-limb motility in both thoracic spinal and sham-operated 7-day embryos; the inhibition of motility caused by morphine was reversed by WIN 44441-3 in both thoracic spinal and sham-operated 7-day embryos. [Leu5]enkephalin-like immunoreactivity in the lumbar spinal cord was concentrated in the superficial laminae of the dorsal horn and along the midline rostral to the central canal. A lesser concentration of immunoreactive processes occurred in the medial and lateral motor columns where labelled varicosities appeared to contact motoneurons. Opiate receptors appear to be present at least as early as day 5 (and perhaps as early as day 4) in the chick embryo. Opiate receptors appear to be present in the lumbar spinal cord of the chick embryo at least as early as day 7. The structural requirements for ligand binding to opiate receptors in the 5-day chick embryo are similar to the requirements for ligand binding to opiate receptors in the adult.