THE EFFECT OF VARIOUS STRESSES, CORTICOSTEROIDS AND ADRENERGIC AGENTS ON PHAGOCYTOSIS IN RAINBOW-TROUT ONCORHYNCHUS-MYKISS

The effect of acute and chronic stress on the phagocytic activity of putative macrophages from the rainbow trout. Oncorhynchus mykiss has been assessed, using an in vitro phagocytic index, in which the average number of engulfed yeast cells in a population of phagocytes is determined. An injection stress given under light anaesthesia, or a longer noise stress combined with confinement, both significantly reduced, within 3 h, the level of phagocytic activity of macrophages from the spleen and pronephros. Daily injection stress over six days had a lesser effect on the proportion of phagocytically active cells even though plasma cortisol levels were equally raised. Daily dexamethasone injection depressed the proportion of phagocytically active cells more than saline injection. In these in vivo experiments, it was not possible to determine whether stress and steroids depressed the phagocytic activity of individual macrophages or caused the active macrophages to migrate out of the spleen and pronephros. Administration of cortisol (200 nM) to trout macrophages in vitro failed to depress phagocytic activity within a 3h period but both alpha- and beta-adrenergic agonists (10 muM) were usually depressive. It is proposed that the autonomic nervous system may be an early regulator of macrophage phagocytosis following stress and that corticosteroids only exert their suppressive effect on macrophage activity in the longer term.