Is Ca2+ the second messenger in the response to melatonin in cuckoo wrasse melanophores?
Pigment aggregation in melanophores of Labrus ossifagus is controlled by an alpha2-adrenoceptor and is somehow modulated by melatonin. The signal transduction mechanisms seem to involve both an attenuation of cAMP and an increase in intracellular Ca2+, inhibiting protein kinase A or activating a phosphatase, respectively. These effects result in dephosphorylation, which in turn induces aggregation. Various alpha2-adrenoceptor agonists attenuate cAMP levels or increase the concentration of intracellular Ca2+. Noradrenaline, for example, lowers cAMP but does not affect the calcium signal whereas B-HT 920, an alpha2-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 experiments with BAPTA/AM, an intracellular calcium chelator, which counteracts the aggregation induced by B-HT 920. Interestingly, the very potent alpha2-adrenoceptor agonist medetomidine apparently activates both signal transduction pathways, which could explain its high efficacy in producing aggregation. Melatonin itself does not cause pigment aggregation, but it potentiates noradrenaline-induced aggregation. It has been suggested that melatonin receptors and alpha2-adrenoceptors follow the same signal transduction pathway, i.e. an attenuation of cAMP. In our experiments, melatonin did not reduce cAMP levels; instead it appears to increase Ca2+ concentration, since melatonin-potentiated aggregation was inhibited by BAPTA/AM. Thus, aggregation amplified by melatonin is probably not mediated by a further decrease in cAMP, but by the same signal transduction mechanism as B-HT 920, i.e. an increase in Ca2+. This further strengthens 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 alpha2-adrenoceptor.
Mårtensson
L G
LG
Department of Zoology, Zoophysiology, Göteborg University, Sweden. lena.martensson@zool.gu.se
Andersson
R G
RG
D000818:Animals; D001381:Azepines / Q000494:pharmacology; D001717:Birds / Q000502:physiology*; D002118:Calcium / Q000502:physiology*; D000242:Cyclic AMP / Q000502:physiology; D020926:Medetomidine / Q000494:pharmacology; D008547:Melanophores / Q000187:drug effects / Q000502:physiology*; D008550:Melatonin / Q000494:pharmacology / Q000502:physiology*; D009638:Norepinephrine / Q000494:pharmacology / Q000502:physiology; D015290:Second Messenger Systems; D015398:Signal Transduction; D015016:Yohimbine / Q000494:pharmacology
D001381:Azepines; D015016:Yohimbine; C024763:talipexole; D000242:Cyclic AMP; D008550:Melatonin; D020926:Medetomidine; D002118:Calcium; D009638:Norepinephrine