Lge. Martensson et Rgg. Andersson, Is Ca2+ the second messenger in the response to melatonin in cuckoo wrassemelanophores?, LIFE SCI, 66(11), 2000, pp. 1003-1010
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.