Dose- and time-dependent biphasic response to morphine on intestinal migrating myoelectric complex

Area of Science:

• Gastroenterology
• Pharmacology
• Physiology

Background:

• The migrating myoelectric complex (MMC) is crucial for gastrointestinal motility.
• Morphine, an opioid analgesic, is known to affect gastrointestinal function.
• Understanding morphine's precise effects on MMC cycling is important for managing related side effects.

Purpose of the Study:

• To investigate the biphasic response of canine migrating myoelectric complex (MMC) cycling to intravenous morphine infusion.
• To determine the influence of morphine dosage and administration timing on MMC cycle parameters.
• To explore the receptor mechanisms involved in morphine's effects on MMC.

Main Methods:

• Seven conscious dogs underwent intravenous infusions of morphine at varying rates (20–1000 µg/kg/hr).
• Infusions were initiated at different phases of the MMC cycle (0% or 40%).
• Naloxone administration and surgical interventions (truncal vagotomy, splanchnectomy) were used to probe receptor pathways.

Main Results:

• Morphine infusion, particularly when started at 40% of the MMC cycle, prematurely initiated the subsequent MMC cycle in a dose-dependent manner.
• Higher morphine doses (500–1000 µg/kg/hr) completely inhibited MMC cycling post-initiation.
• Morphine also dose-dependently induced phase III activity and its effects were not abolished by vagotomy or splanchnectomy, but were blocked by naloxone.

Conclusions:

• Morphine exerts a biphasic effect on canine MMC cycling, acting as both an excitant and an inhibitor.
• The observed effects are dose-dependent and influenced by the timing of administration within the MMC cycle.
• Evidence suggests that these responses are mediated via peripheral mu opiate receptors.