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

Exploring the mechanisms and impacts of melatonin on fish colouration

The pineal hormone melatonin is a multi-functional molecule with a recognized role in pigment aggregation in chromatophores, mediating its actions through binding to subtypes of its specific receptors. Since its discovery, melatonin has been known to be responsible for pigment aggregation towards the cell centre in fishes, including their embryos, as an adaptation to reduced light and thus results in pale body colouration. Diversity exists in the sensitivity of melanophores towards melatonin at interspecies, intraspecific levels, seasons, and amongst chromatophores at different regions of the animal body. In most of the fishes, melatonin leads to their skin paling at night. It is indicated that the melatonin receptors have characteristically maintained to show the same aggregating effects in fishes and other vertebrates in the evolutionary hierarchy. However, besides this aggregatory effect, melatonin is also responsible for pigment dispersion in certain fishes. Here is the demand in our review to explore further the nature of the dispersive behaviour of melatonin through the so-called β-melatonin receptors. It is clear that the pigment translocations in lower vertebrates under the effect of melatonin are mediated through the melatonin receptors coupled with other hormonal receptors as well. Therefore, being richly supplied with a variety of receptors, chromatophores and melanocytes can be used as in vitro test models for pharmacological applications of known and novel drugs. In this review, we present diverse effects of melatonin on chromatophores of fishes in particular with appropriate implications on most of the recent findings.

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

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.

Postsynaptic alpha 2-adrenoceptors mediate melanosome aggregation in melanophores of the white-spotted rabbitfish (Siganus canaliculatus)

The present investigation was undertaken to study the nature of neuro-melanophore junction in the white-spotted rabbit fish Siganus canaliculatus. In vitro experiments using split fin preparation indicated that melanophores of S. canaliculatus are highly responsive to potassium ions and adrenergic agonists. Potassium ions and the adrenergic agonists induced prompt melanosome aggregation that could be competitively blocked by yohimbine (alpha-2 specific adrenergic antagonist) and phentolamine (non-specific alpha adrenergic antagonist). The melanophore responses to repeated potassium stimulation (up to 20 stimuli) did not show any sign of fatigue. However, statistically significant enhancement was observed in responses to potassium that followed the first five stimulations. Adrenergic agonists acted in a time and concentration-dependent manner and their relative potency had the following rank order: clonidine (alpha-2 specific agonist) > norepinephrine (non-specific adrenergic agonist) > phenylephrine (alpha-1 specific agonist) > methoxamine (alpha-1-specific agonist). Yohimbine exerted a more potent inhibiting effect on norepinephrine induced melanosome aggregation compared to phentolamine. Prazosine (alpha-1 specific antagonist) had no effect on such aggregation. Chemically denervated melanophores displayed hypersensitivity to alpha-adrenergic agonists but were refractive to potassium ion stimulation. The refractivity of denervated melanophores to potassium indicates the effect of potassium ion is not direct on melanophores but it is rather through depolarization effect of potassium on the neuro-melanophore peripheral sympathetic fibers and hence release of norepinephrine. In denervated melanophores, similar to intact melanophores, only phentolamine and yohimbine but not prazosine, significantly inhibited melanosome aggregation effect of norepinephrine, indicating that norepinephrine effect is through postsynaptic alpha-2 adrenoceptors. The present data demonstrate that the nature of melanophore innervation in this teleost is adrenergic and neuro-melanophore signals mediating melanosome aggregation are transmitted through alpha-2 postsynaptic adrenoceptors.

Some sweet and bitter tastants stimulate inhibitory pathway of adenylyl cyclase via melatonin and alpha 2-adrenergic receptors in Xenopus laevis melanophores

The sweeteners saccharin, D-tryptophan, and neohesperidin dihydrochalcone (NHD) and the bitter tastant cyclo(Leu-Trp) stimulated concentration-dependent pigment aggregation in a Xenopus laevis melanophore cell line similar to melatonin. Like melatonin, these tastants inhibited (by 45-92%) cAMP formation in melanophores; pertussis toxin pretreatment almost completely abolished the tastant-induced cAMP inhibition, suggesting the involvement of the inhibitory pathway (Gi) of adenylyl cyclase. The presence of luzindole (melatonin receptor antagonist) almost completely abolished the inhibition of cAMP formation induced by saccharin, D-tryptophan, and cyclo(Leu-Trp) but only slightly affected the inhibitory effect of NHD. In contrast, the presence of an alpha2-adrenergic receptor antagonist, yohimbine, almost completely abolished the inhibition of cAMP formation induced by NHD but had only a minor effect on that induced by the other tastants. Thus saccharin, D-tryptophan, and cyclo(Leu-Trp) are melatonin receptor agonists whereas NHD is an alpha2-adrenergic receptor agonist, but both pathways lead to the same transduction output and cellular response. Formation of D-myo-inositol 1,4,5-trisphosphate (IP3) in melanophores was reduced (15-58%, no concentration dependence) by saccharin, D-tryptophan, and cyclo(Leu-Trp) stimulation but increased by NHD stimulation. Tastant stimulation did not affect cGMP. Although some of the above tastants were found to be membrane permeant, their direct activation of downstream transduction components in this experimental system is questionable. MT1 and MT2 melatonin receptor mRNAs were identified in rat circumvallate papilla taste buds and nonsensory epithelium, suggesting the occurrence of MT1 and MT2 receptors in these tissues. Melatonin stimulation reduced the cellular content of cAMP in taste cells, which may or may not be related to taste sensation.

Noradrenaline-induced pigment aggregating response of melanophores in normal, denervated and reinnervated cichlid skin

The present study examines noradrenaline (NA) effects on melanophore pigment aggregation in normal, denervated and reinnervated teleost skin in vitro. Many axons were present in the melanophore-containing layer of normal skin. One week after a nerve crush lesion the skin was devoid of axons. By 1 month the skin was partly reinnervated. One day after nerve crush NA-sensitivity was markedly increased compared to controls. Sensitivity then approached normality but it remained elevated for at least one month. We conclude that melanophore supersensitivity develops very rapidly upon denervation and then gradually fades away during reinnervation.