Cardiorespiratory, gastrointestinal, and analgesic effects of morphine sulfate in conscious healthy horses.

The research investigates the effects of injecting morphine, a common painkiller, into the bloodstream and muscles of healthy horses, and finds that it causes minor short-term changes in behavior and bowel function without impacting the horses’ reaction to painful stimuli.

Research Methodology

• The study utilized six healthy adult horses to compare the effects of intravenous (IV) and intramuscular (IM) administration of morphine sulfate.
• The experiment was carried out with a randomized, masked crossover design. This means that each horse experienced all treatment conditions in a random order, and the researchers did not know which treatment was being applied at a given time to avoid bias.
• Multiple variables were monitored for 360 minutes following drug administration. These included heart and respiratory rates, blood pressure, rectal temperature, arterial pH, blood gas variables, intestinal motility (gut movement), and response to hot and electrically induced pain.
• The researchers also observed the horses for any adverse effects and changes in behavior.
• The concentration of morphine and its metabolites (morphine-3-glucuronide and morphine-6-glucuronide) in the plasma were measured using liquid chromatography-mass spectrometry, a technique that can identify and quantify substances in a sample.

Results & Impact

• There were no significant effects observed following the administration of a saline solution, which served as the control treatment.
• The administration of morphine, either intravenously or intramuscularly, led to minimal and short-term changes in cardiorespiratory function, intestinal motility, and behavior.
• Decreased gastrointestinal motility, or slowed gut movement, was detected 1 to 2 hours after injecting morphine into the muscles at doses of 0.05 and 0.1 mg/kg, and after an IV injection of morphine at a dose of 0.1 mg/kg.
• Importantly, morphine administration did not cause any change in the horses’ responses to painful stimuli.
• Both morphine-3-glucuronide and morphine-6-glucuronide, which are metabolites of morphine, were detected in the plasma after both IV and IM administration of morphine.
• The study concludes that clinically relevant doses of morphine sulfate cause only minor and short-term effects in healthy horses, with no alteration to their reaction to pain. This information may be important for the use of morphine as an analgesic in veterinary practice.