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

The effect of acute and chronic stress on the phagocytic activity of p utative macrophages from the rainbow trout. Oncorhynchus mykiss has be en assessed, using an in vitro phagocytic index, in which the average number of engulfed yeast cells in a population of phagocytes is determ ined. An injection stress given under light anaesthesia, or a longer n oise stress combined with confinement, both significantly reduced, wit hin 3 h, the level of phagocytic activity of macrophages from the sple en and pronephros. Daily injection stress over six days had a lesser e ffect on the proportion of phagocytically active cells even though pla sma cortisol levels were equally raised. Daily dexamethasone injection depressed the proportion of phagocytically active cells more than sal ine injection. In these in vivo experiments, it was not possible to de termine 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 wi thin a 3h period but both alpha- and beta-adrenergic agonists (10 muM) were usually depressive. It is proposed that the autonomic nervous sy stem may be an early regulator of macrophage phagocytosis following st ress and that corticosteroids only exert their suppressive effect on m acrophage activity in the longer term.