Morphine‐tolerant longitudinal muscle strip from guinea‐pig ileum

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Abstract
Summary: Implantation of morphine pellets in guinea‐pigs produced a high degree of tolerance and dependence within 3 days. 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. 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. 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.

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