Reciprocal changes in sensitivity to MCH and noradrenaline after denervation of teleost melanophores

Melanosome and erythrosome positioning regulates cAMP-induced movement in chromatophores from spotted triplefin, Grahamina capito

This study investigated regulation of uniform positioning of melanosomes and erythrosomes in chromatophores from spotted triplefin Grahamina capito from New Zealand, by modulating levels of intracellular cAMP. Elevated cAMP levels, caused by forskolin treatment, inhibited aggregation and induced rapid dispersion of melanosomes and erythrosomes. The dispersing organelles moved to and accumulated at the cell periphery, leading to an abnormal hyperdispersed state with a melanosome- or erythrosome-depleted cell center. Minutes after hyperdispersion, these organelles reversed direction and moved towards the center again to finally distribute throughout the cells. When chromatophores with initially dispersed melanosomes or erythrosomes were treated with forskolin, no hyperdispersion was seen, but the erythrosomes aggregated slowly. Disassembly of actin by latrunculin resulted in a similar but constant hyperdispersed melanosome and erythrosome distribution. The results show that cAMP not only disperses but also aggregates melanosomes and erythrosomes, and that it is the intracellular position of these organelles that determine the directionality of the cAMP-induced movement. To ascertain the even distribution in the dispersed state, regulatory components associated with the actin cytoskeleton in the cell periphery might modify activity of cytoplasmic dynein or kinesin upon contact with dispersing melanosomes or erythrosomes.

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.

Denervation of pigment cells lead to a receptor that is ultrasensitive to melatonin and noradrenaline

Pigment granule aggregation and dispersal can be studied in the melanophores of isolated scales from the cuckoo wrasse (Labrus ossifagus L.). Stimulation of a melanophore alpha2-adrenoceptor or the sympathetic nerve innervating the cell causes pigment aggregation. When the stimulation ceases, the pigment granules disperse throughout the cell. Studying this migration has been a useful tool in pharmacological research, particularly in investigations of the alpha2-adrenoceptor. Denervation of melanophores creates a receptor that is ultrasensitive to noradrenaline and melatonin. After three to four weeks of isolation, the denervated melanophores exhibit a 10(9)-fold increase in sensitivity. The efficacy of melatonin is increased from a negligible pigment-aggregation ability to the level of a full agonist. The melatonin-induced aggregation can, however, be counteracted by the alpha2-adrenoceptor antagonist yohimbine, but not by alpha1-adrenoceptor antagonist prazosin, indicating that the ultrasensitive receptor possesses alpha2-adrenoceptor features. Consequently, we conclude that the ultrasensitive receptor may represent an alpha2-adrenoceptor that has, due to denervation of the melanophore, become sensitive to melatonin.

Cloning and expression of a fish alpha 2-adrenoceptor

1. Pigment granule aggregation in specialized cells (melanophores) from the skin of teleost fishes has been shown to be mediated by receptors with an alpha 2-adrenoceptor pharmacology. We now report the cloning of the alpha 2-F, a fish skin alpha 2-receptor from the cuckoo wrasse (Labrus ossifagus). 2. Degenerate oligonucleotides corresponding to conserved regions of the human alpha 2-adrenoceptor subtypes were used in a polymerase chain reaction (PCR) with cDNA prepared from mRNA isolated subtypes were used in a polymerase chain reaction (PCR) with cDNA prepared from mRNA isolated from the skin of the cuckoo wrasse. An 876 base pair (bp) product was obtained that was homologous with that of the human alpha 2-adrenoceptor and was used to screen a genomic library from the cuckoo wrasse. 3. A clone (pTB17BS) consisting of approximately 5 kb of genomic DNA was obtained which contained the nucleotide sequence of the initial PCR product. In addition, it contained an open reading frame that encoded a protein of 432 amino acids and approximately 2 kb of 5'untranslated sequence. The deduced amino acid sequence of this protein showed 47-57% identity with the human alpha 2-adrenoceptors and thus appeared to encode a fish alpha 2-adrenoceptor. 4. In the 5'-untranslated region of the gene, nucleotide sequences were present suggesting that transcription of the alpha 2-F might be regulated by cyclic AMP, calcium and/or steroids. 5. The alpha 2-F was expressed in COS-7 cells and radioligand binding studies were performed with [3H]-rauwolscine. The binding was of high affinity and it was saturable with a KD of 0.8 +/- 0.1 nM and a Bmax of 5.7 +/- 1.0 pmol mg-1 of protein.6. Competition curves for the displacement of specific [3H]-rauwolscine binding showed the following order of potency: for agonists, medetomidine > clonidine >p-aminoclonidine> B-HT 920> (- )-noradrenaline;for antagonists, rauwolscine > atipamezole > yohimbine > phentolamine > prazosin.7. These results show that alpha2-F has characteristics of both the human alpha2-CIO and alpha2-C4 and that it might represent an ancestral alpha2-adrenoceptor subtype.