Is Ca2+ the second messenger in the response to melatonin in cuckoo wrassemelanophores?

Pigment aggregation in melanophores of Labrus ossifagus is controlled by an alpha(2)-adrenoceptor and is somehow modulated by melatonin. The signal tr ansduction mechanisms seem to involve both an attenuation of cAMP and an in crease in intracellular Ca2+, inhibiting protein kinase A or activating a p hosphatase, respectively. These effects result in dephosphorylation, which in turn induces aggregation. Various alpha(2)-adrenoceptor agonists attenua te cAMP levels or increase the concentration of intracellular Ca2+. Noradre naline, for example, lowers cAMP but does not affect the calcium signal whe reas B-HT 920, an alpha(2)-adrenoceptor specific agonist, does not induce a cAMP decrease but does appear to induce an increase in intracellular Ca2+. This later inference is drawn from experimens with BAPTA/AM, an intracellu lar calcium chelator, which counteracts the aggregation induced by B-HT 920 . Interestingly, the very potent alpha(2)-adrenoceptor agonist medetomidine apparently activates both signal transduction pathways, which could explai n its high efficacy in producing aggregation. Melatonin itself does not cau se pigment aggregation, but it potentiates noradrenaline-induced aggregatio n. It has been suggested that melatonin receptors and alpha(2)-adrenoceptor s follow the same signal transduction pathway, i.e. an attenuation of cAMP. In our experiments, melatonin did not reduce cAMP levels; instead it appea rs to increase Ca2+; concentration, since melatonin-potentiated aggregation was inhibited by BAPTA/AM. Thus, aggregation amplified by melatonin is pro bably not mediated by a further decrease in cAMP, but by the same signal tr ansduction mechanism as B-HT 920, i.e. an increase in Ca2+. This further st rengthens the suggestion that melatonin and B-HT 920 bind to the same site, but it is unclear if that particular site is on the melatonin receptor or the alpha(2)-adrenoceptor.