Effects of morphine and liposomal morphine in a model of intestinal inflammation in mice.

We have investigated the antitransit effects of free and liposomal morphine in a model of intestinal inflammation. Mice received saline or croton oil orally, 3 h prior to evaluation, and gastrointestinal transit was measured 20 min afterwards. Peak/duration of effects, potency (ED50) and antagonism by naloxone and naloxone methiodide were evaluated. Peak effects occurred 30 and 40 min after administration of morphine and liposomal morphine, respectively. Encapsulated morphine had a more pronounced and prolonged effect than morphine. Comparison of the ED50S demonstrated that the potency of liposomal morphine was 3.5 times higher than that of morphine during inflammation; in addition, inflammation increased the potency of morphine and liposomal morphine, 3 and 9.2 times, respectively. The effects of morphine and liposomal morphine in croton oil-treated mice were reversed by naloxone and naloxone methiodide. The results show that during inflammation, the potency and duration of the antitransit effects of morphine are significantly enhanced by encapsulation.