Mechanism of Signal Transduction in the Chromatophore Membrane

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The concept that membranous phosphatidyl-inositol 1, 4, 5-bisphosphate (PIP_2) is hydrolyzed by phospholipase C to produce inositol 1, 4, 5-trisphosphate (IP_3), which in turn acts on IP_3 receptors on the intracellular Ca^<2+> storage compartment to release the ions has been demonstrated in a few effector cells. In this profect, the sequence has firstly been shown in the chromatophore system. The apparatus which enables us to measure the chromatophore responses and the intracellular Ca^<2+> concentration at the same time was invented by remodelling a commercial industrial microscope system. By applying this to a tilapia melanophore in culture, a quick Ca^<2+> increase followed by melanosome aggregation was demonstrated in response to norepinephrine stimulation. The present results showing the involvement of the IP_3 system strongly suggest that alpha_1-adrenergic receptors are involved in the pigment aggregation in the chromatophores. In practice, both alpha_1 and alpha_2 systems may be operating in fish chromatophores, and the ratios of their incidences may vary among species and also among tissues in a particular species. It is also assumed that alpha_2 system which involves cyclic AMP takes part in relatively slow movement of pigment, whereas alpha_1 system with the IP_3 changes has role in more rapid component of the response. In silurid catfishes, the pigment aggregation in response to sympathetic stimulation is mediated by acetylcholine receptors of the muscarinic type. Working on the translucent glass catfish, we characterized the receptors to be of the M3 type. Significant progresses have also been made withthe studies on motile iridophores. Among them, is a novel aspect on the change of reflective color from dendritic iridophores of the dark sleeper goby.