Photoreceptor proteins and melatonin rhythm generating AANAT in the carp pineal: Temporal organization and correlation with natural photo-thermal cues

Gut melatonin response to microbial infection in carp Catla catla

The purpose of present study was to demonstrate the response of gut melatoninergic system to Aeromonas hydrophila infection for 3 or 6 days and search for its correlation with the activity of different antioxidative and digestive enzymes to focus their interplay under pathophysiological conditions in carp (Catla catla). Microscopic study of gut in infected fish revealed degenerative changes in the tunica mucosa and lamina propria layers with sloughed off epithelial cells in the lumen. The activity of each digestive enzyme was reduced, but the levels of melatonin, arylalkylamine-N-acetyl transferase protein, the key regulator of melatonin biosynthesis, and different enzymatic antioxidants in gut were gradually and significantly increased with the progress of infection. Gut melatonin concentrations in A. hydrophila challenged carp by showing a positive correlation with the activity of each antioxidative enzyme, and a negative correlation with different digestive enzymes argued in favor of their functional relation, at least, during pathological stress. Moreover, parallel changes in the gut and serum melatonin titers indicated possible contribution of gut to circulating melatonin. Collectively, present carp study provided the first data to suggest that endogenous gut melatonin may be implicated to the mechanism of response to microbial infections in any fish species.

Temporal and histological evaluation of melatonin patterns in a 'basal' metazoan

While recent advances suggest functional pleiotropy of melatonin in higher organisms, an understanding of the biological significance of this ancient molecule in early evolutionary groups is lacking. Here, endogenous melatonin production was identified for the first time in the sea anemone Actinia equina, a nonsymbiotic hexacorallian cnidarian. Day/night activity profiles of melatonin in this anemone indicated that melatonin levels oscillate with significant nocturnal peaks. However, dynamic changes in melatonin concentration did not persist under constant dark conditions and therefore were not circadian in nature. Thus, the oscillating pattern of melatonin in A. equina is presumed to be the result of alternative, simpler melatonin control mechanism that likely involves direct regulation by the daily photocycle. As nocturnal melatonin signals still potentially provide 'time-of-day' information and can illustrate the seasonally changing length of the biological night, we hypothesize that melatonin may be relevant to temporal coordination of timed processes also in anthozoans. Spatial patterns of melatonin distribution found in this study indicate abundant melatonin distribution in the endodermal filaments wrapped around gametes. This finding supports the possibility that one of the melatonin-responsive processes in this basal metazoan species may involve reproductive functions.

Neural regulation of dark-induced abundance of arylalkylamine N-acetyltransferase (AANAT) and melatonin in the carp (Catla catla) pineal: an in vitro study

In all the vertebrates, synthesis of melatonin and its rhythm-generating enzyme arylalkylamine N-acetyltransferase (AANAT) reaches its peak in the pineal during the night in a daily light-dark cycle, but the role of different neuronal signals in their regulation were unknown for any fish. Hence, the authors used specific agonist and antagonists of receptors for different neuronal signals and regulators of intracellular calcium (Ca(2+)) and adenosine 3',5'-cyclic monophosphate (cAMP) in vitro to study their effects on the abundance of AANAT and titer of melatonin in the carp (Catla catla) pineal. Western blot analysis followed by quantitative analysis of respective immunoblot data for AANAT protein, radioimmunoassay of melatonin, and spectrophotometric analysis of Ca(2+) in the pineal revealed stimulatory effects of both adrenergic (α(1) and β(1)) and dopaminergic (D(1)) agonists and cholinergic (both nicotinic and muscarinic) antagonists, inhibition by both adrenergic and dopaminergic antagonists and cholinergic agonists, but independent of the influence of any agonists or antagonists of α(2)-adrenergic receptors. Band intensity of AANAT and concentration of melatonin in the pineal were also enhanced by the intracellular calcium-releasing agent, activators of both calcium channel and adenylate cyclase, and phophodiesterase inhibitor, but suppressed by inhibitor of calcium channel and adenylate cyclase as well as activator of phophodiesterase. Moreover, an inhibitory effect of light on the pineal AANAT and melatonin was blocked by both cAMP and proteasomal proteolysis inhibitor MG132. Collectively, these data suggest that dark-induced abundance of AANAT and melatonin synthesis in the carp pineal are a multineuronal function, in which both adrenergic (α(1) and β(1), but not α(2)) and dopaminergic signals are stimulatory, whereas cholinergic signals are inhibitory. This study also provides indications, though arguably not conclusive evidence, that in either case the neuronal mechanisms follow a signal-transduction pathway in which Ca(2+) and cAMP may act as the intracellular messengers. It also appears that proteasomal proteolysis is a conserved event in the regulation of AANAT activity in vertebrates.

Neuronal regulation of photo-induced pineal photoreceptor proteins in carp Catla catla

In the present in vitro study on the pineal in carp Catla catla, specific agonist and antagonists of receptors for different neuronal signals and regulators of intra-cellular Ca(++) and cAMP were used to gather basic information on the neuronal signal transduction cascade mechanisms in the photo-induced expression of rod-like opsin and alpha-transducin-like proteins in any fish pineal. Western-blot analysis followed by quantitative analysis of respective immunoblot data for both the proteins revealed that photo-induced expression of each protein was stimulated by cholinergic (both nicotinic and muscarinic) agonists and a dopaminergic antagonist, inhibited by both cholinergic antagonists and a dopaminergic agonist, but not affected by any agonists or antagonists of adrenergic (alpha(1), alpha(2) and beta(1)) receptors. Moreover, expression of each protein was stimulated by voltage gated L type calcium channel blocker, adenylate cyclase inhibitor and phosphodiesterase activator; but suppressed by the activators of both calcium channel and adenylate cyclase, and by phosphodiesterase inhibitor. Collectively, we report for the first time that both cholinergic and dopaminergic signals play an important, though antagonistic, role in the photo-induced expression of photoreceptor proteins in the fish pineal through activation of a signal transduction pathway in which both calcium and cAMP may act as the intracellular messengers.