The peripheral neurotransmitter that signals chromatophores is adrenergic. Usually, aggregation of pigment granules occurs in response to norepinephrine, and the transmission is regarded as α -adrenergic in nature, whereas pigment dispersion is caused via β –adrenergic receptors in chromatophores of some fish species. MSH generally disperses pigment granules within chromatophores, resulting in darkening body color. MCH is antagonistic to MSH. MSH and MCH affect directly chromatophores through their specific receptors on the cell menbrane. Recently, it was found that, in addition to the known MSH or MCH receptor, there also exists an unusual receptor that mediates pigment migration in opposite direction. That is, in some fish species, MSH aggregates pigment, and MCH causes pigment dispersion at higher doses, unlike usual cases. Melatonin secreted from the pineal gland during the night elicits pigment aggregation and dispersion through α - and β -melatonin receptors, respectively. This hormone may be responsible for circadian color changes,and for the formation of color pattern. Prolactin, the pituitary hormone, causes pigment dispersion only in erythrophores and xanthophores, suggesting the possible involvement of the peptide in nuptial coloration. The direct effect of K+ on xanthophores and erythrophores was also reported in some fish species. In addition to biogenic chemical substances, light is an important factor affecting the chromatic state of the integument. Light-sensitive chromatophores that response directly to light have lately been found in some adult fish, indicating the possible existence of visual pigment in the cell. These chromatophores may be responsible for the generation of peculiar color that may ethologically signify much. Thus, chromatophores perceive a large number of signals, resulting in numerous coloration.
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