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Effects of melatonin on the melanophores of fishes, Labeo Rohita

In fishes labeo rohita adapted to black background intraperitoneal administration of melatonin (MEL) resulted in paling of the integument. In both innervated and denervated melanophores in vitro MEL caused aggregatiuon of the pigment, in vitro, studies revealed ineffectiveness of alpha (phenoxybenzamine and yobinbine) and beta (propranolol) adrenolytic agents to inhabit MEL, induced responses. Adenosine (ADE), theophylline (THE) and alpha-melanocyte stimulating hormone caused rapid dispersion of the reversible malanosome aggragation within melanophores of Labeo rohita and the receptors for its action are different form adrenoceptors. The action of MEL is assumed to be linked to inhibition of adenylate cyclase.
Lerner and his colleagues (1958) first discovered melatnin from mammalian pinea organ and named it after its potent concentrating action on amphibian melanophores. In many fish species melatonin has been reported to exert potent melanosome translocation while melanophores (Fujii, 1993; Visconti and Castrucci, 1993). In restricted area of the skin the differential responsiveness to MEL, exists (Fujii and Oshima, 1986; Masagaki and Fujii, 1999). In fishes, Labeo rohita, alpha adrenoceptors (Jain and Patil, 1992), beta adrenoreceptors and melanin concentrating hormone receptors (Jain and Patil 1990) exist.

For study scales of Labeo rohitawere removed from anterodorsal trunk region and immediately immersed in physiological saline for about 20 minutes for equilibrium before treatment with drug under study (Jain and Patil, 1992). The response of control and drug treated melanophores were based on Hogben and Slome (1931) melanonphore index. The macroscopic color changes were recorded utilizing a method based on a 9 point scale derived from Munsell Grey series (Sharma et al., 1996). The drug chemicals used are: Melatonin (Sigma, USA), α-melanocyte stimulating hormone (Peninsula Labs. Europe Ltd.,U.K.), adenonsine (Sigma, USA), theophylline ,propranonol  hydrochloride (Cipla, Bombay), phenoxy benzamine hydrochloride. Working solutions of these drugs were prepared by diluting the stock solutions with physiological saline before use. A black adapted fish, Labeo rohita, with general tint of the body equivalent to paled notably within 5 minute after receiving an intraperitoneal injection of melanin 10mg/kg. The maximum paling of skin was equivalent to I.N 4.4 at 10 minute that lasted for about an hour from the treatment. The influence of hormone declined but appeared to be long lasting one, as even after 5 hr of its administration the fish were found to be appreciably pale. The present study melanophores pretreated with alpha adregic blockers (Yohimbine and phenoxybenzamine) and beta adrenergic blocker (propanolol) and were treated with MEL. These adrenergic blockers failed to inhabit the responses due to MEL. The study provides evidence for the existence of cellular receptors for the MEL that is differet from adrenergic receptors as reported in our earlier studies (Patil and Jain, 1989; Jain and Patil, 1992).  Such possibility of existence of specific MEL receptors was also reported by Fujii and Miyashita (1978). Adenosine also dispersed the melanosome
s with Labeo melanophores. Fujii and Miyashita (1976) first suggest the role of adenosine and adenine nucleotides in the conrol of pigment dispersal in fish chromatophores.  Several other studies (Oshima, 1989 and Namoto, 1992) support this pigment dispersing action of adenosine. In Labeo rohita there is evidence for the existence of adenosine receptors of A2 type. These A2-receptors stimulate the activity of adenylate cyclase (Londos et., 1980 and Daly. 1983) elevating the cAMP level leading to dispersion okf the pigment. In the present study α-MSH MIMICKED THE ACTION OF ADNOSINE.   It centrifugal translocation of the pigment in melanophores aggregated by MEL. The response due to α-MSH is consistent with observation reported in other fishes (Sharma et al., 1996; Visconti et al., 1999 and Fujii, 2000).
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