THE EFFECT OF REARING RAINBOW-TROUT ON BLACK OR WHITE BACKGROUNDS ON THEIR SECRETION OF MELANIN-CONCENTRATING HORMONE AND THEIR SENSITIVITY TO STRESS

Rainbow trout were reared in black or off-white coloured tanks for up to 18 months of age to achieve maximum differences in the synthesis of the neuropeptide, melanin-concentrating hormone (MCH). White-reared fish had greatly increased MCH concentrations in their pituitary glands, in their MCH perikarya and in the presumptive neuromodulatory fibres of the dorsal hypothalamus/thalamus when compared with black-reared and commercially reared trout. Following transfer to brighter white tanks, white-reared fish showed a significant increase in plasma MCH concentration and a reduction of MCH in the pituitary and MCH perikarya. The additional challenge of repeated stress further increased plasma MCH concentration in these fish and also reduced MCH in the dorsal hypothalamus/thalamus. In black-reared fish transferred to white tanks, plasma MCH concentrations were significantly raised after transfer, although they were lower after 11 days than in white-reared counterparts. Transfer from black to white background caused a fall in the MCH concentration in all regions-pituitary gland, perikarya and dorsal hypothalamus/thalamus; if transfer was accompanied by repeated stress, the hormone in the pituitary gland MCH perikarya became so depleted that plasma MCH concentrations declined. Within each experimental situation (control, background transfer and transfer with stress) there was in inverse correlation between plasma MCH concentrations of black- and white-reared fish and the cortisol concentration. MCH had no direct effect on the secretion of cortisol by interrenal tissue but incubated hypothalami, in which endogenous MCH had been immunoabsorbed, provided evidence that MCH can depress the release of corticotrophin-releasing bioactivity.