Inhibitors of messenger RNA and protein synthesis affect differently serotonin arylalkylamine N-acetyltransferase activity in clock-controlled and non clock-controlled fish pineal

Jun 19, 1998Brain research

Blocking RNA and protein production differently affects serotonin-related enzyme activity in fish pineal glands with and without internal clocks

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Abstract

In pike, newly synthesized proteins are essential for the rise in AA-NAT activity and melatonin secretion during the dark phase.

Photoreceptor cells in the pineal organ of fish show variations in circadian systems between species, with pike having a cellular circadian clock and trout lacking one.
The activity of the enzyme AA-NAT, which converts serotonin to melatonin, is influenced by transcription and translation inhibitors in both pike and trout.
Inhibition of AA-NAT activity occurs with cycloheximide, anisomycin, and puromycin treatment during the early dark phase in both species.
Actinomycin D specifically inhibits AA-NAT activity in pike but not in trout.
Forskolin stimulation leads to a significant reduction in AA-NAT activity when transcription is inhibited in pike, indicating a complex regulatory mechanism.
The differences in AA-NAT regulation suggest that pike possess a distinct circadian clock mechanism at the genetic level compared to trout.

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The pineal organ of fish contains photoreceptor cells. In some species (e.g., pike) each photoreceptor is a cellular circadian system which contains a photoreceptive unit, the clock and an output unit. In others (e.g., trout) the clock is lacking. The main rhythmic output of the pineal photoreceptor is melatonin, an internal 'zeitgeber' of the organisms. The nocturnal rise in melatonin secretion results from an increase in the activity of the arylalkylamine-N-acetyltransferase (AA-NAT) which converts serotonin to N-acetylserotonin. In the present study we investigated the effects of transcription and translation inhibitors on AA-NAT activity in pike and trout pineal organs in culture. Cycloheximide, anisomycin, and puromycin inhibited the rise in AA-NAT activity observed during the first 2, 4 or 6 h of the dark phase, in both species. Actinomycin D was active only in the pike. Six hours of treatment during the first half of the night induced inhibition of AA-NAT activity, providing that forskolin (an adenylyl cyclase stimulator) was present in the culture medium. When the treatment was run for 3, 6 or 12 h, starting at midday of a 12L/12D cycle, basal and forskolin-stimulated AA-NAT activity (measured at midnight) were dramatically reduced. Such a treatment had no effect on trout AA-NAT activity. It is concluded that: (1) the dark-induced rise in AA-NAT activity and melatonin secretion are dependent on newly synthesized protein in both pike and trout pineal; (2) AA-NAT regulation takes place at the translational and post-translational levels in both species; (3) AA-NAT regulation occurs also at the transcriptional level in the pike, but not in the trout; and (4) the cAMP-dependent activation of AA-NAT requires transcription in the pike, not in the trout. The presence of a cell population acting as a circadian clock in the pike pineal, but not in the trout pineal, can explain the difference between these two species. Thus, we suggest that the clock mechanism operates at the genetic level in these cells. Further comparative studies between clock-controlled and non-clock-controlled pineals might prove interesting to demonstrate the difference between these two regulatory pathways.

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Inhibitors of messenger RNA and protein synthesis affect differently serotonin arylalkylamine N-acetyltransferase activity in clock-controlled and non clock-controlled fish pineal

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