Regulation of melanophore responsiveness in the background-adapted medaka, Oryzias latipes: change in the intracellular signaling system.

The responsiveness of melanophores of the medaka fish (wild type, Oryzias latipes) to a neurotransmitter and hormones is changed differentially after long-term adaptation to a black or white background. In the present study, we further examined whether this phenomenon involved some change in the intracellular signaling system. Using a permeabilized melanophore model, in which pigment granules could be dispersed by exogenously applied cAMP, the requirement of cAMP for pigment-dispersing reaction was revealed to be higher in melanophores of fish adapted to a black background (B cells) than in those of white background-adapted fish (W cells). Specific inhibitors of cAMP-dependent protein kinase (PKA) and cyclic nucleotide phosphodiesterase did not reduce the difference in the pigment dispersion level between B and W cells. A similar result was obtained with the free catalytic subunit of PKA. In contrast, the inhibition of protein phosphatase activity by okadaic acid diminished the difference in the responsiveness between B and W cells. These results suggest that the activity of protein phosphatase in B cell is higher than that in W cells, and that the change in the melanophore responsiveness by long-term chromatic adaptation to a background involves the change in the enzyme activity in the intracellular signaling system.