Differences in arginine vasotocin gene transcripts and cortisol secretion in trout with high or low endogenous melanin-concentrating hormone secretion

Previous studies on trout suggest that melanin-concentrating hormone (MCH) acts at both hypothalamic and pituitary levels to restrain the release of a drenocorticotropic hormone and hence cortisol during stress. Using in situ hybridization, the present work examined whether high rates of MCH secretio n were associated with changes in the synthesis of arginine vasotocin (AVT) , one of the corticotropin secretogogues. It also examined whether high end ogenous MCH secretion restrains cortisol secretion during intense as well a s mild stress, and how exogenous MCH affects the rise in plasma cortisol fo llowing injection stress. Trout were reared in black- or white-coloured tan ks for 1 year or more to achieve maximal differences in MCH secretion. Foll owing a mild stress, cortisol secretion was greater in black-reared fish wi th low MCH secretion which is in line with previous findings but, following a more severe stress, plasma cortisol concentrations were similar in the t wo groups. Injection of MCH into black-adapted fish restrained the stress-i nduced rise in plasma cortisol concentration during the first hour but did not affect final cortisol values, In two separate experiments, AVT mRNA lev els were significantly lower in the hypothalamus of black-reared fish. Poss ible explanations for this include a greater negative-feedback restraint by cortisol, which is likely to rise higher in black-adapted fish during the moderate, daily stresses of aquarium life; or the possibility that exposure to a white background may be psychologically stressful, stimulating AVT tr anscription, The possibility that MCH directly stimulates AVT transcription cannot be excluded but seems less likely, The results suggest that while M CH may restrain the release of hypothalamopituitary stress hormones under m oderately stressful conditions, it does not restrain AVT synthesis.