Differential Actions of Melatonin on Melanophores of the Threeline Pencilfish, Nannostomus trifasciatus(Physiology)
The longitudinally striped pattern of the threeline pencilfish, Nannostomus trifasciatus, is observed only in the daytime, and changes into a different pattern with three dark spots at night. In this study, microscopic examinations and photoelectric measurements revealed that melanophores in the greater part of the integument respond to melatonin by the aggregation of melanosomes. By contrast, larger melanophores existing in dark spots respond to the amine by the dispersal of pigment. Physiological tests indicated that the nervous mechanisms controlling these melanophores are practically identical to each other, and also with those existing in many teleost species known hitherto. It is further shown that membrane-permeating analogues of 3', 5'-cyclic nucleotides effectively disperse melanosomes in all these melanophores. We conclude that melanophores in the spots possess melatonin receptors that mediate melanosome dispersion, which we have recently described as "β-melatonin receptors" in some melanophores of another pencilfish species, N. beckfordi. Stimulation of the regulatory subunit of these receptors may be signaled via the Gs subunit, which activates the catalytic subunit, nucleotide cyclase, to increase the cytosolic concentration of cyclic nucleotide(s) as the second messenger. These results demonstrate that the threeline pencilfish affords an excellent model for studying signaling mechanisms through β-melatonin receptors.