Adrenergic nerves and the alpha 2-adrenoceptor system regulating melanosome aggregation within fish melanophores

Nervous control of melanosome aggregation of melanophores of Labrus ossifagus was studied by electrical field stimulation of isolated scales. Field stimulation elicited a rapid melanosome aggregation which was reversible upon interruption of the stimuli. The response was blocked by guanethidine indicating that adrenergic nerves were stimulated. The melanophore receptors which mediate the nerve-controlled pigment aggregation were characterized to be of the alpha 2-adrenoceptor subtype, since they were inhibited by yohimbine but not by prazosin. Stimulation of the alpha 2-adrenoceptors by noradrenaline was associated with a significant reduction of the cyclic AMP content of the melanophores. It is suggested that adrenergic nerves control melanosome aggregation via alpha 2-adrenoceptors and that these receptors are coupled to the adenylate cyclase-cyclic AMP system of the melanophores.