The catecholamines, adrenalin, dopamine and noradrenalin induce differential aggregation of melanophores in black-adapted winter flounder, Pseudopleuronectes americanus, paralleling patterning responses to albedo change. These differential responses to catecholamines suggest that the patterning mechanism in this species is largely dependent on a balance between neural aggregating and dispersing elements. The α-adrenoceptor agonist phenylephrine evokes paling in all pattern components in blackadapted flounder, whilst the α-adrenoceptor antagonist phentolamine darkens white-adapted flounders. The α-adrenoceptor agonist isoproterenol and the α-adrenoceptor antagonist propranalol have no effect on chromatophores of white-adapted flounder, but induce pallor in blackadapted flounder, which is interpreted as non specific. Noradrenalin elicits patterning responses in chromatically decentralized flounder, although the duration of pallor is shorter. The xanthophores, which are dispersed by a pituitary factor, will aggregate in response to high catecholamine doses.

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