INHIBITORS OF MESSENGER-RNA AND PROTEIN-SYNTHESIS AFFECT DIFFERENTLY SEROTONIN ARYLALKYLAMINE N-ACETYLTRANSFERASE ACTIVITY IN CLOCK-CONTROLLED AND NON CLOCK-CONTROLLED FISH PINEAL

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 other s (e.g., trout) the clock is lacking. The main rhythmic output of the pineal photoreceptor is melatonin, an internal 'zeitgeber' of the orga nisms. The nocturnal rise in melatonin secretion results from an incre ase in the activity of the arylalkylamine-N-acetyltransferase (AA-NAT) which converts serotonin to N-acetylserotonin. In the present study w e investigated the effects of transcription and translation inhibitors on AA-NAT activity in pike and trout pineal organs in culture. Cycloh eximide, anisomycin, and puromycin inhibited the rise in AA-NAT activi ty observed during the first 2, 4 or 6 h of the dark phase, in both sp ecies. Actinomycin D was active only in the pike. Six hours of treatme nt during the first half of the night induced inhibition of AA-NAT act ivity, providing that forskolin tan adenylyl cyclase stimulator) was p resent in the culture medium. When the treatment was run for 3, 6 or 1 2 h, starting at midday of a 12L/12D cycle, basal and forskolin-stimul ated AA-NAT activity (measured at midnight) were dramatically reduced. Such a treatment had no effect on trout AA-NAT activity. It is conclu ded that: (1) the dark-induced rise in AA-NAT activity and melatonin s ecretion are dependent on newly synthesized protein in both pike and t rout pineal; (2) AA-NAT regulation takes place at the translational an d post-translational levels in both species; (3) AA-NAT regulation occ urs also at the transcriptional level in the pike, but not in the trou t; and (4) the cAMP-dependent activation of AA-NAT requires transcript ion in the pike, not in the trout. The presence of a cell population a cting as a circadian clock in the pike pineal, but not in the trout pi neal, can explain the difference between these two species. Thus, we s uggest that the clock mechanism operates at the genetic level in these cells. Further comparative studies between clock-controlled and non-c lock-controlled pineals might prove interesting to demonstrate the dif ference between these two regulatory pathways. (C) 1998 Elsevier Scien ce B.V. All rights reserved.