Evidence for the presence of novel β-melatonin receptors along with classical α-melatonin receptors in the fishRasbora daniconius(Ham.)

Abstract

The effects of melatonin (MT) were examined on the isolated scale melanophores from dorso-lateral (D-L) and band regions of a tropical fish Rasbora daniconius. Our study primarily aimed for further depiction of the signaling receptors involved in MT mediated pigment translocations in the fish. Melanophore Size Index (MSI) was employed as a recording parameter for the responses of melanophores to MT and various antagonists. MT has induced aggregation as well as dispersion in D-L region and aggregation in band region melanophores during summer season. During winter, MT-induced responses were only of aggregatory type in D-L region, while in the band region there was an increase in the sensitivity. The responses of the melanophores to MT were reversible. The aggregation of innervated melanophores induced by MT on the D-L and band regions was partially mediated through the neurotransmitters released under the influence of MT and partially by the specific MT receptors. Luzindole and K185 have completely blocked the aggregatory responses of D-L and band region melanophores. Aggregatory receptors may be of the conventional α-MT type. Dispersion of D-L and band region melanophores induced by MT in the presence of various antagonists and on denervated band region could be the result of activation of β-MT receptors of dispersive nature. Presence of α and β MT receptors is thus indicated in this fish melanophores.

New Insights Into the Evolutionary History of Melatonin Receptors in Vertebrates, With Particular Focus on Teleosts

“…We found different sets of mtnr genes expressed in the integument of teleost species: mtnr1b and mtnr1d in sole ( Solea senegalensis ) ( 94 ), mtnr1a and mtnr1d in mudskipper ( Boleophthalmus pectinirostris) ( 95 ), mtnr1aa α, mtnr1aa β, mtnr1ab , and mtnt1b in Atlantic salmon ( 10 ) and mtnr1b, mtnr1c , and mtnr1d in goldfish ( 8 ). Melatonin induces hypopigmentation in several ostariophysian species, by stimulating aggregation of melanophores partially through stimulation of Mtnr1a and Mtnr1b ( 111 – 113 ). In mammals, both MTNR1A and MTNR1B are expressed in the skin with different distributions depending of the species [for review see ( 114 )].…”

Intertidal gobies acclimate rate of luminance change for background matching with shifts in seasonal temperature

“…During winter months, however, increases in melatonin concentrations are positively correlated with melanophore aggregation, and therefore, phenotype brightening (Ovais et al, 2015). The average higher luminance values observed in the cold-exposed fish may be explained by melatonin induced melanophore aggregation in cold temperatures, simulating winter.…”

Thermal adaptation and acclimation in an intertidal goby (Bathygobius cocosensis)

“…During summer months, increased concentrations of melatonin have an inverse aggregation response, whereas higher concentrations of melatonin correlates with lower melanophore aggregation and phenotype darkening. During winter months, however, increases in melatonin concentrations are positively correlated with melanophore aggregation, and therefore, phenotype brightening (Ovais et al 2015). The average higher luminance values observed in the cold exposed fish may be explained by melatonin induced melanophore aggregation in cold temperatures, simulating winter.…”

“…This may be a result of long-term exposure to different seasonal thermal conditions and may be been driven by increase concentration of melatonin. The effect melatonin has on melanophore aggregation or dispersion has been shown to be seasonally dependent in a tropical reef fish Rasbora daniconius (Ovais et al 2015). During summer months, increased concentrations of melatonin have an inverse aggregation response, whereas higher concentrations of melatonin correlates with lower melanophore aggregation and phenotype darkening.…”

“…The average higher luminance values observed in the cold exposed fish may be explained by melatonin induced melanophore aggregation in cold temperatures, simulating winter. The effect melatonin has on the aggregation or dispersal of melanosomes however, is highly species dependent (Nishi and Fujii 1992;Ovais et al 2015).…”