A MELATONIN BINDING-SITE MODULATES THE ALPHA(2)-ADRENOCEPTOR

The melanophores of the cuckoo wrasse (Labrus ossifagus L., a teleost fish) can aggregate and disperse their pigment granules. This migratio n is controlled by sympathetic nerves and a postsynaptic alpha(2)-adre noceptor. Melatonin was discovered because of its ability to aggregate pigment granules, hence we used the cuckoo wrasse melanophore as a mo del for studying the effect of melatonin at a cellular level. We found that melatonin had no aggregating effect; instead the hormone enhance d the actions of several alpha(2)-agonists, such as noradrenaline, med etomedine and clonidine. When the melanophores were pre-aggregated in vitro by use of the alpha 2-agonist B-HT 920, the aggregation was not augmented after the addition of melatonin. Instead the pre-aggregated granules were dispersed. This suggests that melatonin has affinity for an alpha(2)-adrenoceptor site that can modulate the effect of alpha(2 )-adrenoceptor agonists.