In vitro modulation of fish phagocytic cells by β-endorphin

The activation of rainbow trout, Oncorhynchus mykiss, and carp, Cyprinus carpio, phagocytic cells by synthetic chum salmon, O. keta, β-endorphin was analysed in vitro. Rainbow trout head kidney leukocytes were cultured in RPMI 1640 medium containing 1, 10, 50 or 100 ng ml−1of chum salmon β-endorphin and the production of superoxide anion was measured via the reduction of nitroblue tetrazolium (NBT) in vitro. Macrophages incubated with 10 ng ml−1up to 100 ng ml−1of β-endorphin showed an increase in their production of superoxide anion in comparison with control macrophages which were cultured without hormone. β-endorphin also increased the production of superoxide anion in phagocytic cells prepared from kidney of carp. This stimulation was inhibited by naloxone. Phagocytic cells treated with β-endorphin also displayed increased phagocytic activity and phagocytic index. These results showed that β-endorphin in lower vertebrates activates the function of phagocytic cells in vitro.