DENERVATION OF PIGMENT-CELLS LEAD TO A RECEPTOR THAT IS ULTRASENSITIVE TO MELATONIN AND NORADRENALINE

Pigment granule aggregation and dispersal can be studied in the melano phores of isolated scales from the cuckoo wrasse (Labrus ossifagus L.) . Stimulation of a melanophore alpha(2)-adrenoceptor or the sympatheti c nerve innervating the cell causes pigment aggregation. When the stim ulation ceases, the pigment granules disperse throughout the cell. Stu dying this migration has been a useful tool In pharmacological researc h, particularly in investigations of the alpha(2)-adrenoceptor. Denerv ation of melanophores creates a receptor that is ultrasensitive to nor adrenaline and melatonin. After three to four weeks of isolation, the denervated melanophores exhibit a 10(9)-fold increase in sensitivity. The efficacy of melatonin is increased from a negligible pigment-aggre gation ability to the level of a full agonist. The melatonin-induced a ggregation can, however, be counteracted by the alpha(2)-adrenoceptor antagonist yohimbine, but not by alpha(1)-adrenoceptor antagonist praz osin, indicating that the ultrasensitive receptor possesses alpha(2)-a drenoceptor features. Consequently, we conclude that the ultrasensitiv e receptor may represent an alpha(2)-adrenoceptor that has, due to den ervation of the melanophore, become sensitive to melatonin.