THE INFLUENCE OF REPEATED STRESS ON THE RELEASE OF MELANIN-CONCENTRATING HORMONE IN THE RAINBOW-TROUT

Melanin-concentrating hormone (MCH) is a neuro-hypophysial peptide that induces pigmentary pallor in teleosts and which is released when the fish are placed on a white background. An additional effect of the peptide is the depression of ACTH and hence cortisol secretion during moderate stress. The present work on rainbow trout shows that plasma MCH concentrations, while unaffected by a single stress, are raised by repeated stress (1 ml saline injected i.p. without anaesthesia) and remain high for several hours thereafter. The response to stress is observed only in white-adapted fish and not in fish kept in black-coloured tanks, when MCH release in normally low. Plasma concentrations of MCH vary diurnally but stress induces an equivalent incremental rise in plasma MCH, whether administered in the middle or towards the end of the photophase. The stress-induced rise in MCH concentrations is prevented by treatment with dexamethasone. The results support the suggestion that the modulatory effect of MCH on the hypothalamopituitary-interrenal axis of fish might be enhanced under conditions of stress.