Abstract
The daily profiles of melatonin and arylalkylamine N-acetyltransferase (AANAT) protein, the key enzyme of the melatoninergic pathway, in three anatomical segments of gut were studied in carp which were held under natural (NP) or long (LP; LD 16:08) or short (SP; LD 08:16) photoperiods or continuous light (LL; LD 24:00) or continuous darkness (DD; LD 00:24) for 30 days. The levels of melatonin and the density of a ~23 kDa AANAT protein, in each gut segment, exhibited a daily rhythm with a peak at midday, irrespective of LD regimens to which the carp were held. None of the photo schedules had any significant effects on mesor values of gut melatonin and AANAT. However, compared to the rhythm features in NP fish, a significant reduction in the amplitude and a significant phase delay in the midday peak, as depicted by the value of acrophase (Ø), were noted in the gut of SP and DD carp. Collectively, it appears reasonable to argue that environmental lighting conditions may not be the synchronizer of daily periodicity in gut melatoninergic system, though the secondary effects of available light in determining its rhythm features in carp may not be ignored.
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