Morphine modulation of peritoneal inflammation in Atlantic salmon and CB6 mice.

Peritoneal inflammation is a convenient model for comparisons of modulatory effects of morphine in phylogenetically distant vertebrates. Both in salmon and mice morphine injected intraperitoneally together with an irritant (thioglycollate) significantly inhibits inflammation as estimated by the number of peritoneal leukocytes. The low number of exudate cells in morphine-treated animals seems to be compensated by their high activity, as evidenced by the enhanced phorbol myristate acetate-induced respiratory burst. The morphine-inhibited influx of leukocytes into the irritated peritoneal cavity correlates with the morphine-lowered level of plasma chemotactic factors both in fish and mice. It implies that morphine impairs the level of plasma chemotactic factor either directly (affecting their release from the resident peritoneal cells) or indirectly (decreasing the number of inflammatory leukocytes by inhibition of their migration from hemopoietic sites). The inhibitory effects of morphine on both the cell number and chemoattractant level are completely reversed by the naltrexone pretreatment, which implicates the involvement of opioid receptors.