Melanin-concentrating hormone (MCH) and α-melanocyte stimulating hormone (α-MSH) are widespread vertebrate neuropeptides. In teleost fish the peptides are involved in the hormonal control of skin pigmentation, but they have also been shown to modulate corticosteroid secretion in both fish and mammals. α-MSH has additional potent anti-inflammatory actions in mammals and both peptides stimulate leucocyte phagocytosis in rainbow trout in vitro. The effects of these peptides on phagocytosis and the release of immunomodulatory factors by rainbow trout head kidney leucocytes were investigated in vitro. Neither MCH nor α-MSH had any effect on the adherence of phagocytes to glass slides or the activity of isolated phagocytes. When added to mixed leucocyte suspensions, however, MCH (50 and 100 nM) and α-MSH (1 and 10 nM) significantly increased the percentage of cells undergoing phagocytosis and MCH (50 nM), but not α-MSH, stimulated the phagocytic index. In subsequent experiments, isolated phagocytes were exposed to supernatants derived from mixed leucocyte suspensions exposed to MCH (50 and 100 nM) and α-MSH (1 and 10 nM). Supernatants from leucocytes exposed to all doses of the peptides significantly increased the percentage phagocytosis and those from cells stimulated with MCH (100 nM) and α-MSH (1 and 10 nM) increased the phagocytic index of the phagocytes. The results suggest that cells other than phagocytes are required for MCH and α-MSH to exert their stimulatory effects on trout phagocytic cells through the release of one or more macrophage-activating factors. © 2000 Elsevier Science B.V.
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