A black background facilitates the response to stress in teleosts.

This work examines the difference in responsiveness to stress which characterizes fish adapted to white and black backgrounds. Trout were maintained in black or white tanks for 2 weeks and then subjected to intermittent intense or moderate noise stress for periods between 1 h and 5 days, or to the stress of being injected daily with a large volume of liquid for 3 days. Plasma cortisol concentrations increased more readily and to a greater extent in fish from black tanks in response to moderate stress or brief intense stress. Dexamethasone suppressed the stress-induced rise of cortisol in white-adapted fish but was only partially effective in trout from black backgrounds. These differences in plasma cortisol between black- and white-adapted fish can be related to the different titres of plasma ACTH, apparently derived from the pars distalis. Removal of the neurointermediate lobe (NIL) from black-adapted eels markedly depressed the normal rise in plasma cortisol elicited by noise stress. It is suggested that products from the NIL may modulate the stress response of the hypothalamo-pituitary axis although other routes through which background colour could affect the pituitary responsiveness to stress are also considered. In several cases, stress also enhanced the secretion of MSH from the NIL.

Evidence for the participation of a melanin-concentrating hormone in physiological colour change in the eel.

The hormonal and nervous control of colour change in the eel has been investigated. The only bioactive forms of MSH found in eel pituitary extracts or secreted by eel pituitary cultures were forms of alpha-MSH; no beta-MSH was detected. After transfer of eels from a black to a white background, the melanin concentration in skin melanophores was accompanied by a rapid decline in plasma alpha-MSH titres. Hypophysectomy resulted in melanin concentration, and pituitary extracts injected into hypophysectomized eels caused melanin dispersion. This effect was eliminated if the pituitary extracts were first incubated with a specific alpha-MSH antiserum or if the antiserum was injected into the hypophysectomized eel. However, injection of alpha-MSH antiserum into intact, black-adapted eels failed to result in melanin concentration although the same antiserum was effective in causing pallor in black-adapted toads. Partially purified preparations of teleost melanin-concentrating hormone (MCH), free from catecholamines, induced melanin concentration when injected into black-adapted eels and this effect was significantly potentiated by injections of alpha-MSH antiserum. The denervation of melanophores on the pectoral fin had only a slight effect on the responses of the melanophores to humoral agents. It is concluded that the control of physiological colour change in the eel is largely hormonal, and involves the antagonistic effects of alpha-MSH and a melanin-concentrating agent which is probably MCH.