An immunological study of the secretory activity of neurons producing melanin-concentrating hormone in a teleost

The melanin-concentrating hormone is a general vertebrate neurosecretory peptide which, in bony fish, serves as a neurohypophysial hormone influencing pigmentary changes in response to background colour. Young carp were reared for six months in white- or black-coloured tanks to determine how this would influence the development of the neurons producing the peptide. Cytological criteria and radioimmunoassay of tissue extracts showed that the background markedly influenced the synthetic activity of these neurons. In carp reared in black tanks, the perikarya were small and poorly granulated, with small nuclei and often undetectable nucleoli. Transfer of such fish to a white tank for six days caused no significant change in hormone content but cytological criteria suggested an increased activity of some of the neurons. In fish reared on a white background, over 50% of these neurons showed a greatly enhanced synthetic activity, while radioimmunoassays showed significantly higher concentrations of immunoreactive peptide in their hypothalami but not in their pituitary glands. After such fish were moved to black tanks for six days, the neuropeptide content of the hypothalamus and pituitary gland was significantly increased. Histologically, this was reflected in the amount of immunostainable granulation in both sites but cell nuclear size was not decreased. These changes are interpreted in terms of changes of hormone synthesis and release. The observations provide evidence that the activity of many but not necessarily all of the neurons producing melanin-concentrating hormone in the carp hypothalamus is controlled by background colour. © 1988