Light response of cultured melanophores of a teleost adult fish,Oryzias latipes

Melanin-concentrating hormone is a major substance mediating light wavelength-dependent skin color change in larval zebrafish

Melanosome dispersion is important for protecting the internal organs of fish against ultraviolet light, especially in transparent larvae with underdeveloped skin. Melanosome dispersion leads to dark skin color in dim light. Melanosome aggregation, on the other hand, leads to pale skin color in bright light. Both of these mechanisms are therefore useful for camouflage. In this study, we investigated a hormone thought to be responsible for the light wavelength-dependent response of melanophores in zebrafish larvae. We irradiated larvae using light-emitting diode (LED) lights with peak wavelengths (λ_max) of 355, 400, 476, 530, and 590 nm or fluorescent light (FL) 1-4 days post fertilization (dpf). Melanosomes in skin melanophores were more dispersed under short wavelength light (λ_max ≤ 400 nm) than under FL. Conversely, melanosomes were more aggregated under mid-long wavelength light (λ_max ≥ 476 nm) than under FL. In addition, long-term (1-12 dpf) irradiation of 400 nm light increased melanophores in the skin, whereas that of 530 nm light decreased them. In teleosts, melanin-concentrating hormone (MCH) aggregates melanosomes within chromatophores, whereas melanocyte-stimulating hormone, derived from proopiomelanocortin (POMC), disperses melanosomes. The expression of a gene for MCH was down-regulated by short wavelength light but up-regulated by mid-long wavelength light, whereas a gene for POMC was up-regulated under short wavelength light. Melanosomes in larvae (4 dpf) exposed to a black background aggregated when immersing the larvae in MCH solution. Yohimbine, an α₂-adrenergic receptor antagonist, attenuated adrenaline-dependent aggregation in larvae exposed to a black background but did not induce melanosome dispersion in larvae exposed to a white background. These results suggest that MCH plays a key role in the light wavelength-dependent response of melanophores, flexibly mediating the transmission of light wavelength information between photoreceptors and melanophores.

De novo transcriptomics reveal distinct phototransduction signaling components in the retina and skin of a color-changing vertebrate, the hogfish (Lachnolaimus maximus)

Across diverse taxa, an increasing number of photoreceptive systems are being discovered in tissues outside of the eye, such as in the skin. Dermal photoreception is believed to serve a variety of functions, including rapid color change via specialized cells called chromatophores. In vitro studies of this system among color-changing fish have suggested the use of a phototransduction signaling cascade that fundamentally differs from that of the retina. Thus, the goal of this study was to identify phototransduction genes and compare their expression in the retina and skin of a color-changing fish, the hogfish Lachnolaimus maximus. De novo transcriptomics revealed the expression of genes that may underlie distinct, yet complete phototransduction cascades in L. maximus retina and skin. In contrast to the five visual opsin genes and cGMP-dependent phototransduction components expressed in the retina of L. maximus, only a single short-wavelength sensitive opsin (SWS1) and putative cAMP-dependent phototransduction components were expressed in the skin. These data suggest a separate evolutionary history of phototransduction in the retina and skin of certain vertebrates and, for the first time, indicate an expression repertoire of genes that underlie a non-retinal phototransduction pathway in the skin of a color-changing fish.

Role of catecholamines and nitric oxide on pigment displacement of the chromatophores of freshwater snakehead teleost fish, Channa punctatus

We are reporting for the first time that the catecholamines (adrenaline and noradrenaline) inhibit the effect of nitric oxide (NO) on melanosome dispersion in freshly isolated scales of the freshwater snakehead fish, Channa punctatus. We studied the effect of NO and catecholamines on the pigment displacement by observing the changes in the melanophore index. The scales when treated with solution containing NO donor sodium nitroprusside (SNP) showed dispersion of melanosomes, whereas NO synthase blocker N-omega-Nitro-L-arginine suppresses this action of SNP. Treatment with adrenaline and noradrenaline on the isolated scales caused aggregation of melanosomes. Scales treated with solution containing catecholamines and SNP resulted in aggregation of melanosomes suggesting that catecholamines mask the effect of SNP. These results suggest that the catecholamines are inhibiting the effect of NO and causing the aggregation of the melanosomes may be via surface receptors.

Possible involvement of cone opsins in distinct photoresponses of intrinsically photosensitive dermal chromatophores in tilapia Oreochromis niloticus

Dermal specialized pigment cells (chromatophores) are thought to be one type of extraretinal photoreceptors responsible for a wide variety of sensory tasks, including adjusting body coloration. Unlike the well-studied image-forming function in retinal photoreceptors, direct evidence characterizing the mechanism of chromatophore photoresponses is less understood, particularly at the molecular and cellular levels. In the present study, cone opsin expression was detected in tilapia caudal fin where photosensitive chromatophores exist. Single-cell RT-PCR revealed co-existence of different cone opsins within melanophores and erythrophores. By stimulating cells with six wavelengths ranging from 380 to 580 nm, we found melanophores and erythrophores showed distinct photoresponses. After exposed to light, regardless of wavelength presentation, melanophores dispersed and maintained cell shape in an expansion stage by shuttling pigment granules. Conversely, erythrophores aggregated or dispersed pigment granules when exposed to short- or middle/long-wavelength light, respectively. These results suggest that diverse molecular mechanisms and light-detecting strategies may be employed by different types of tilapia chromatophores, which are instrumental in pigment pattern formation.

The signaling pathway in photoresponses that may be mediated by visual pigments in erythrophores of Nile tilapia

The ability to increase the synthesis or vary the distribution of pigment in response to light is an important feature of many pigment cells. Unlike other light-sensitive pigment cells, erythrophores of Nile tilapia change the direction of pigment migration depending on the peak wavelength of incident light: light near 365, 400 or 600 nm induces pigment aggregation, while dispersion occurs in response to light at 500 nm. How these phenomena are achieved is currently unknown. In the present study, the phototransduction involved in the pigment dispersion caused by light at 500 nm or the aggregation by light at 600 nm was examined, using pertussis toxin, cholera toxin, blockers of ion channels, various chemicals affecting serial steps of signaling pathways and membrane-permeable cAMP analog. The results show that light-induced bidirectional movements in tilapia erythrophores may be controlled by cytosolic cAMP levels via Gi- or Gs-type G proteins. In addition, RT-PCR demonstrated for the first time the expression of mRNAs encoding red and green opsins in tilapia fins, only where erythrophores exist. Here, we suggest that multiple cone-type visual pigments may be present in the erythrophores, and that unique cascades in which such opsins couple to Gi or Gs-type G proteins are involved in the photoresponses in these pigment cells. Thus, tilapia erythrophore system seems to be a nice model for understanding the photoresponses of cells other than visual cells.

Direct Effects of Visible and UVA Light on Pigment Migration in Erythrophores of Nile Tilapia

Erythrophores derived from Nile tilapia (Oreochromis niloticus) are sensitive to visible light of defined wavelengths in primary culture in the same manner as erythrophores in the skin. Cultured erythrophores aggregate their pigment in response to light with peak wavelengths near 400 or 600 nm, while dispersion is caused by light near 500 nm. In this study, we report that ultraviolet A (UVA) with a peak wavelength near 365 nm also induces pigment aggregation in erythrophores in the skin and in primary culture. The responses of erythrophores in the skin or in culture depend on the light intensity, although the photo-sensitivity differs among individual cells. From the results, we conclude that the action of visible light and UVA light on tilapia erythrophores is direct, and that multiple types of visual pigments may coexist in individual erythrophores.

beta-Adrenergic receptor subtypes in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus

The subtype of beta-adrenergic receptors in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus was studied. Pigment of denervated melanophores in isolated, split caudal fins was preliminarily aggregated by incubating the specimens in a physiological saline containing 10 microM phentolamine and 30-100 microM verapamil or 2-10 nM melatonin, and the responses of the melanophores to a beta-adrenergic agonist added to the incubating medium were recorded photoelectrically. The beta-adrenergic agonists noradrenaline, adrenaline, isoproterenol, salbutamol and, dobutamine were all effective in evoking a dispersion of melanophore pigment in the presence of phentolamine and verapamil or melatonin. The pigment-dispersing effect of noradrenaline (beta 1-selective agonist) was inhibited by metoprolol (beta 1-selective antagonist), propranolol,- and butoxamine. Whereas, the effect of salbutamol (beta 2-selective agonist) was hardly inhibited by metoprolol, though it was considerably inhibited by propranolol and ICI-118551. It was estimated that beta 1- and beta 2-adrenergic receptors coexist at ratios of 8.6:91.4, in the melanophore of Tridentiger trigonocephalus, and 25:75, in the melanophore of Chasmichthys gulosus, through the analyses of Hofstee plots of the effects of the beta-adrenergic drugs. It was suggested that the relation between the pigment-dispersing effect of a beta-adrenergic agonist on the melanophores and the concentration of the drug follows mass action kinetics, when the effect is mainly caused by the activation of beta 2-adrenergic receptors of the melanophores. However, when it is mainly caused by the activation of beta 1-adrenergic receptors of the melanophores, the relation does not follow mass action kinetics.

Light-induced pigment aggregation in xanthophores of the medaka, Oryzias latipes

The response mechanism of medaka xanthophores to light was examined at the cellular level. Innervated and denervated xanthophores of adult medakas responded to light (9,000 lux) within 30 sec by pigment aggregation, and this aggregation was not mediated through alpha-adrenoceptors on the cell membrane. Maximum sensitivity to light was at wavelengths of 410-420 nm, and the direct effect of light was reversible. Xanthophore responsiveness to light in summer was higher than that in winter. Ca2+ and calmodulin were not involved in the response, but rather, an important role for cAMP and phosphodiesterase (PDE) was suggested. It seems likely that photoreception by visual pigment which is sensitive to light at wavelengths of 410-420 nm increases PDE activity, probably via a G-protein, such as occurs with visual cells in the retina, which causes a decrease in levels of cytosolic cAMP, in turn leading to pigment aggregation within medaka xanthophores.

Action of light on frog pigment cells in culture

Solar radiation induces numerous biologic effects in skin but the mechanism underlying these responses is poorly understood. To study the etiology of these phenomena, we investigated the effect of light on cultured Xenopus laevis melanophores. Visible light stimulated a marked increase in intracellular cAMP levels within the first minute of irradiation. This light-induced elevation in cAMP was blocked by melatonin and was not seen in fibroblasts irradiated in a similar manner. These data show that the photoresponse of pigment cells from amphibian skin can be mediated by a cAMP-dependent mechanisms and suggest that a unique member of the rhodopsin family is involved in this process.

Dermal and epidermal chromatophores of the Antarctic teleost Trematomus bernacchii

The physiological response and ultrastructure of the pigment cells of Trematomus bernacchii, an Antarctic teleost that lives under the sea ice north of the Ross Ice Shelf, were studied. In the integument, two types of epidermal chromatophores, melanophores and xanthophores, were found; in the dermis, typically three types of chromatophores--melanophores, xanthophores, and iridophores--were observed. The occurrence of epidermal xanthophore is reported for the first time in fish. Dermal melanophores and xanthophores have well-developed arrays of cytoplasmic microtubules. They responded rapidly to epinephrine and teleost melanin-concentrating hormone (MCH) with pigment aggregation and to theophylline with pigment dispersion. Total darkness elicited pigment aggregation in the majority of dermal xanthophores of isolated scales, whereas melanophores remained dispersed under both light and dark conditions. Pigment organelles of epidermal and dermal xanthophores that translocate during the pigmentary responses are carotenoid droplets of relatively large size. Dermal iridophores containing large reflecting platelets appeared to be immobile.

A sensitive bioassay for melanotropic hormones using isolated medaka melanophores

Melanophore-stimulating hormones (MSHs) from chum salmon cause pigment dispersion in isolated melanophores of medaka, a teleost. The in vitro medaka melanophore bioassay that responded to light with pigment dispersion and to the dark with pigment aggregation was utilized for measuring the activity of melanotropic hormones. alpha-MSH I was the most potent melanophore-dispersing agent tested. The minimal dose for the induction of pigment dispersion was 10(-15) M alpha-MSH I, 10(-13) M N-des-acetyl(Ac)-alpha-MSH, and 10(-11) M beta-MSH I, respectively. The melanosome-dispersing activity of beta-MSH I was enhanced about 40% by salmon N-acetyl-endorphin I (N-Ac-EP). The results suggest that N-Ac-EP may act as an enhancer for the activity of certain MSHs. The present bioassay provides a unique method for determining the biological activity of melanotropic peptides.

Spectral sensitivity of melanophores of a freshwater teleost, Zacco temmincki

1. The melanophores of a freshwater teleost, Zacco temmincki, responded to changes in illumination: in darkness the melanophores induced a melanosome aggregation and when subjected to light they caused a melanosome dispersion. 2. Using monochromatic light, the spectral sensitivity of the melanophores was examined. 3. The melanophores showed a different sensitivity to light between 400 and 600 nm with a maximum at about 525 nm. 4. The action spectrum closely resembled a porphyropsin absorbance curve, suggesting a porphyropsin or similar photopigment is active in the melanophore light response of Zacco temmincki.