Morphine-tolerant longitudinal muscle strip from guinea-pig ileum

1. Implantation of morphine pellets in guinea-pigs produced a high degree of tolerance and dependence within 3 days.2. The contractions of the longitudinal muscle induced by electrical stimulation of the myenteric plexus-longitudinal muscle preparations obtained from tolerant animals were less depressed by morphine than the contractions evoked in preparations from non-tolerant animals.3. Naloxone did not alter the size of the evoked twitch but antagonized the depressant action of morphine in tolerant and in non-tolerant animals. When given to tolerant guinea-pigs, naloxone caused an increase in intestinal activity in vivo.4. The contractile response of the longitudinal muscle to acetylcholine was the same in preparations obtained from tolerant and non-tolerant animals. Electrically evoked contractions of the myenteric plexus-longitudinal muscle preparations from tolerant animals showed reduced sensitivity to the depressant effects of adrenaline, isoprenaline, and particularly, dopamine.