Alpha-melanocyte stimulating hormone (α-MSH) and melanin-concentrating hormone (MCH) stimulate phagocytosis by head kidney leucocytes of rainbow trout (Oncorhynchus mykiss) in vitro

Alpha-melanocyte stimulating hormone (α-MSH) and melanin-concentrating hormone (MCH) are two peptides with antagonistic roles in the regulation of skin pigmentation in teleost fish. Both have also been implicated in the modulation of the stress response via the hypothalamo-pituitary-adrenal (HPA) axis in fish and other vertebrates. Alpha-MSH is also known to be a potent immunomodulatory peptide in mammals, while both hormones have been shown to influence the immune responses of trout in vitro. Head kidney phagocytes (macrophages and neutrophils) were exposed to α-MSH at concentrations of 0·05 to 10 nM in vitro for 60 min and the phagocytic activity of glass-adherent cells was assessed microscopically. At 1, 5 and 10 nM, α-MSH significantly increased the percentage of phagocytes that ingested heat-killed yeast cells. Percentage phagocytosis was also significantly increased when cells were exposed to 0·1, 1 and 10 nM des-acetyl-α-MSH. When α-MSH and MCH were added to cells together, at concentrations of 1 nM and 50 nM respectively, the stimulatory effects of both hormones were diminished. The results suggest that these peptides may play an immunomodulatory role in the fish immune system.