Yoko Miyashita

Receptor mechanisms in fish chromatophores—I. alpha nature of adrenoceptors mediating melanosome aggregation in guppy melanophores

Abstract 1. Either electrical nervous stimulation or adrenergic monoamines having alpha effects caused melanosome aggregation in guppy melanophores. 2. Some beta agonistic drugs were also found to be pigment-aggregating. 3. Various alpha adrenergic antagonists eliminated these melanin-aggregating effects. 4. On the other hand, beta adrenergic blocking agents were not effective in blocking these effects. 5. Postsynaptic adrenoceptors in melanophores which mediate pigment aggregation appear to be alpha adrenergic.

Receptor mechanisms in fish chromatophores—II. Evidence for beta adrenoceptors mediating melonosome dispersion in guppy melanophores

1. In vitro methods for assessing melanin dispersion in melanophores are described, in which split fin preparations of the guppy, Lebistes reticulatus, were employed. 2. Acetylcholine and some current neurotransmitter suspects were ineffective in dispersing the pigment. 3. While exhibiting a melanin-aggregating action in higher concentrations, sympathomimetic monoamines in lower concentrations were found to disperse melanosomes. Such an action was more noticeable with beta stimulating drugs, some of them being solely pigment-dispersing. 4. These monoamine-induced dispersion responses were antagonized by beta adrenergic blocking agents. 5. Beta adrenoceptors in the melanophore appear to take part in the melanosome dispersion response of the cell.

Receptor mechanisms in fish chromatophores. VI: Adenosine receptors mediate pigment dispersion in guppy and catfish melanophores

Using the guppy, Lebistes reticulatus, and the siluroid catfish, Parasilurus asotus , the effects of purine and pyrimidine derivatives on the movement of melanophores were studied. All the substances tested did not aggregate pigment within melanophores. Adenosine and adenine nucleotides were very effective in dispersing melanosomes within the cell, although adenine itself lacked such action. Derivatives of other purines than adenine and of pyrimidines did not disperse melanosomes. The pigment dispersion induced by adenine derivatives was specifically antagonized by methylxanthines. It was concluded that adenosine receptors are present on the melanophore membrane, which take part in the darkening reaction of fishes.

Receptor mechanisms in fish chromatophores—V. MSH disperses melanosomes in both dermal and epidermal melanophores of a catfish (Parasilurus asotus)

1. Synthetic alpha-MSH was tested for its action on dermal and epidermal melanophores of the siluroid catfish. Parasilurus asotus. 2. MSH did not produce melanosome aggregation in these melanophores. 3. It was, however, very effective in bringing about pigment dispersion in dermal melanophores. 4. Epidermal melanophores were also responsive to MSH, although the rate and the degree of the response was smaller than those of the dermal melanophores. 5. A possible role of MSH in skin color changes in vivo is discussed.

Receptor mechanisms in fish chromatophores—IV. Effects of melatonin and related substances on dermal and epidermal melanophores of the siluroid, Parasilurus asotus

Abstract 1. Melatonin was extremely effective in inducing melanosome aggregation in both dermal and epidermal melanophores of the siluroid catfish, Parasilurus asotus. 2. While precursors of melatonin, 5-hydroxytryptamine and N-acetyl-5-hydroxytryptamine, were without effects, its possible metabolites, 5-methoxytryptamine and 6-hydroxymelatonin, showed moderate action to aggregate the pigment. 3. Muscarinic cholinolytic agents, which were complete blockers of the melanin-aggregating nervous effect, had no influence in melatonin action. 4. Also, neither alpha nor beta adrenolytic agents showed an inhibitory effect on the melatonin action. 5. The conclusion is that the melanin-aggregating action of melatonin may be mediated by its specific receptors in the melanophores, and that the 5-methoxy group on the indole ring may be essential for its action.

Receptor mechanisms in fish chromatophores--III. Neurally controlled melanosome aggregation in a siluroid (Parasilurus asotus) is strangely mediated by cholinoceptors.

Abstract 1. Electrical nervous stimulation elicited melanosome aggregation within both dermal and epidermal melanophores in isolated skin pieces of the siluroid catfish Parasilurus asotus . 2. Catecholamines failed to bring about pigment aggregation. 3. On the other hand, acetylcholine, methacholine and carbamylcholine strongly aggregated the melanosomes. 4. Atropine and scopolamine abolished the effects of both nervous stimulation and cholinomimetic agents. 5. An anticholinesterase, physotigmine, augmented these melanin-aggregating effects. 6. It was concluded that in this siluroid nervously evoked pigment aggregation within melanophores is mediated by cholinoceptors of a muscarinic type.