Alpha1 and alpha2 adrenoceptor mediated melanosome aggregatory responses in vitro in Oreochromis mossambica (Peters) melanophores.

Effects of specific and non-specific adrenoceptor agonists and antagonists were examined on the isolated scale melanophores of O. mossambica in physiological Ringer solution. The responses were recorded as melanophore size index. It was observed that adrenaline, nor-adrenaline, phenylpropanolamine, clonidine and phenylepherine induced melanosome aggregation in a dose-dependent manner. Denervation of the fish melanophores increased the sensitivity of the melanophores to adrenaline but not to nor-adrenaline. Phentolamine (3.55 x 10(-5) M), prazosin (2.38 x 10(-5) M) and yohimbine (2.821 x 10(-5) M) significantly inhibited the aggregatory responses of the fish melanophores to adrenaline, nor-adrenaline, clonidine and phenylepherine. The blocking effect of yohimbine was significantly higher than that of prazosin. It is concluded that the effect of adrenaline is directly mediated through the receptors and alpha2 adrenoceptors are predominantly involved in the aggregatory responses of this fish melanophores, while alpha1 adrenoceptors presence has been indicated.

Responses of isolated scale melanophores of a fresh water carp, Cirrhinus mrigala (Ham.) to zidovudine.

Zidovudine (AZT) induced concentration related aggregation in C. mrigala melanophores. Denervated melanophores failed to respond to AZT. Specific and nonspecific alpha adrenoceptor antagonists completely blocked the responses of fish melanophores to AZT. Histamine and prostaglandin antagonists also inhibited aggregation of the melanophores induced by AZT. The results suggest that AZT may release a mixture of neurotransmitter like substances, which cause the aggregation of this fish melanophores.

Mechanism of histamine induced dispersal response in the isolated web melanophores of a frog, Rana tigerina (Daud.).

Effects of histamine and specific H1, H2 receptor agonists have been investigated on the isolated web skin melanophores of the frog, R. tigerina. Histamine, 2-methyl-histamine and 4-methyl-histamine all induced dose dependent dispersion in the frog melanophores. Reserpine treatment of the frog skin, either in vivo, or in vitro potentiated the dispersal response of the frog skin melanophores elicited by histamine. Histamine induced dispersal responses in the frog integumentary melanophores were partially blocked by mepyramine and ranitidine. Atropine failed to inhibit the dispersal response in any degree. Propranolol partially inhibited the dispersion of frog melanophores induced by histamine and 2-methyl-histamine. However, propranolol completely blocked the dispersal response of the mealnophores to 4-methyl-histamine. It is suggested that histamine induced dispersion in the frog skin melanophores in vitro is mediated partially through specific H1, H2 and beta adrenergic receptors.

Control of melanosome movements in isolated skin melanophores of a catfish Clarias batrachus (Linn.).

Adrenergic and cholinergic receptors have been studied in isolated skin melanophores of a catfish Clarias batrachus. Catecholamines induced a strong aggregatory effect on the melanophores. Melanosome aggregation induced by adrenaline and noradrenaline was partially blocked by alpha adrenergic receptor blockers and a beta receptor blocker. Cholinomimetic drugs aroused a significant dispersion of melanophroes. Atropine effectively blocked the dispersal, responses of melanophores to acetylcholine and carbachol, while, hexamethonium blocked the nocotine induced dispersal responses of the melanophores.

Effect of histaminergic drugs on tail melanophores of tadpole, Bufo melanostictus.

The H1 and H2 receptor agonist histamine caused a powerful aggregation of B. melanostictus tail melanophores, which was completely blocked by metiamide, a specific H2 receptor antagonist, while mepyramine an H1 receptor blocker partially blocked the aggregating response. The strong melanin aggregating effect of 4-methyl histamine a specific H2 receptor agonist and its complete blockade by metiamide further supports the conclusion that there exists a dominant population of H2 type of histamine receptors along with sparse population of H1 receptor on the tail melanophores of the toad, which mediate centripetal movement of melanin granules within the pigment cells leading to blanching of the animal.

Acetylcholine content of the brain and visceral organs of the teleostean fish; Heteropneustes fossilis and Clarias batrachus.

The acetylcholine content of various organs of two teleostean fish, Heteropneustes fossilis and Clarias batrachus has been estimated biologically on the eserinized frog rectus muscle. The acetylcholine content in the stomach of both the fish was 0.341 and 0.22 mug/g, respectively. Values were lower than those of other parts of the alimentary tract. The ilia contained the most acetylcholine, being 0.644 and 0.551 mug/g respectively, for the two fish. In the brain, heart, kidney and liver of both the fish, a considerable amount of acetylcholine was detected. Acetylcholine was not found in the spleen of Clarias batrachus, but it was present in the spleen of Heteropneustes fossilis.

Histamine content of the digestive tract of a catfish Clarias batrachus (Linn.) in relation to spawning, sex and seasonal variation.

Histamine content of the digestive tract of a catfish Clarias batrachus has been estimated biologically on the atropinized guineapig ileum. A significant decrease of histamine content has been observed with the decrease of gonosomatic index (G.S.I.) during breeding season in female fishes, while no such type of change in histamine content of male fishes during breeding season has been noted. In the summer histamine content of both the sexes was signivicantly lowered in comparison with the histamine content during breeding season in both the sexes. It is concluded that higher histamine content in prespawing female fishes during breeding season may be due to the inhibition of the enzyme histaminase by gonadotrophines. It is also suggested that the route of inactivation of histamine during breeding season may be different in both the sexes, while in summer a common factor may be involved in depletion of histamine content of both the sexes.