Neuroimmunomodulatory effects of morphine in Leishmania donovani-infected hamsters

Objective: The effect of morphine on host defense during Leishmania donovani infection in golden hamsters was studied. Methods: Hamsters were intracardially infected with L. donovani amastigotes and then monitored by spleen touch print microscopic examination. Morphine and naloxone were administered subcutaneously and intraperitoneally, respectively. Leukocytes were counted by a hemocytometer, and ex vivo phagocytosis was determined by the examination of stained adherent macrophages. Results: Low doses of morphine, 1.75 and 2.5 mg/kg x 2, administered subcutaneously on day 0 and day 15 significantly (p < 0.05) suppressed the infection, whereas high doses (20.0 and 50.0 mg/kg x 2) exacerbated the infection. On day 30, hamsters treated with low doses of morphine showed a significant (p < 0.05) increase in the number of circulating leukocytes and the pool size and phagocytic activity of peritoneal macrophages ex vivo; in hamsters treated with high doses, all these parameters appeared to be diminished. The bone marrow of morphine-treated hamsters showed a fall in total cellularity and no change in the number of monocytes; however, in those treated with low doses, the infection was completely eliminated by day 30, and paradoxically, a significant (p < 0.05) potentiation of infection was observed in hamsters treated with high doses. The spleens of hamsters treated with both low and high doses of morphine showed a significant (p < 0.05) decrease and increase in weight, respectively; treatment with low doses also caused an almost 2-fold increase in the percentage of monocytes. Morphine apparently exerted its protective effects via naloxone-sensitive opioid receptors; naloxone pretreatment did not affect the potentiation of infection. Conclusion: Conditional doses of morphine apparently biphasically modulated the course of L. donovani infection in hamsters, at least in part through macrophage-mediated mechanisms. Copyright (C) 2003 S. Karger AG, Basel.