The effects of stress, background colour and steroid hormones on the lymphocytes of rainbow trout (Oncorhynchus Mykiss)

Abstract

This study investigated the effect that adapting Rainbow trout to black or white backgrounds has on their stress and immune responses. Experiments in vivo showed that stressed fish, adapted to black backgrounds, had higher plasma cortisol levels and a suppressed immune system compared to white-adapted fish. Thus, stress reduced antibody production, induced lymphocytopenia and inhibited the ability oflymphocytes to grow in vitro. These effects were always more pronounced in black-adapted trout. It is argued that white-adapted fish are less susceptible to the effects of stress because of the neuromodulatory influence of the neuropeptide, melanin-concentrating hormone (MCH). In fish reared from eggs on dark-or light-coloured backgrounds, differences between black and white groups were not so evident. In some cases, the effects seen in adapted fish became reversed when reared fish were used. It is suggested that homeostatic mechanisms counteract the modulatory actions of MCR in reared fish. In experiments using radiolabelled thymidine to monitor lymphocyte growth in vitro, MCH enhanced both T and B cell-like proliferation. The peptides also modulated the action of corticosteroids on lymphocyte growth and was found to reduce, but not prevent, the inhibitory influence of cortisol. Melanocyte stimulating hormone, an antagonist of MeR, had no effect on lymphocyte growth at concentrations normally found in fish plasma. The major reproductive steroids of trout were tested for their ability to influence lymphocyte growth in vitro. The results were variable, some steroids were predominantly stimulatory (e.g. oestradiol), some inhibitory (e.g. ketotestosterone), while others had mixed actions (e.g. 17a-hydroxy 20b-dihydroxyprogesterone). These observations are discussed in relation to the normal plasma levels of reproductive steroids found at different stages in the life cycle of trout and the possible effects these steroids have on fish immunity.