TRANSCRIPTS ENCODING 2 MELATONIN SYNTHESIS ENZYMES IN THE TELEOST PINEAL ORGAN - CIRCADIAN REGULATION IN PIKE AND ZEBRAFISH, BUT NOT IN TROUT

In this report the photosensitive teleost pineal organ was studied in three teleosts, in which melatonin production is known to exhibit a da ily rhythm with higher levels at night; in pike and zebrafish this inc rease is driven by a pineal clock, whereas in trout it occurs exclusiv ely in response to darkness. Here we investigated the regulation of me ssenger RNA (mRNA) encoding serotonin N-acetyltransferase (AA-NAT), th e penultimate enzyme in melatonin synthesis, which is thought to be pr imarily responsible for changes in melatonin production. AA-NAT mRNA w as found in the pineal organ of all three species and in the zebrafish retina. A rhythm in AA-NAT mRNA occurs in vivo in the pike pineal org an in a light/dark (L/D) lighting environment, in constant lighting (L /L), or in constant darkness (DID) and in vitro in the zebrafish pinea l organ in L/D and L/L, indicating that these transcripts are regulate d by a circadian clock. In contrast, trout pineal AA-NAT mRNA levels a re stable in. vivo and in vitro in LID, L/L, and D/D. Analysis of mRNA encoding the first enzyme in melatonin synthesis, tryptophan hydroxyl ase, reveals that the in vivo abundance of this transcript changes on a circadian basis in pike, but not in trout. A parsimonious hypothesis to explain the absence of circadian rhythms in both AA-NAT and trypto phan hydroxylase mRNAs in the trout pineal is that one circadian Syste m regulates the expression of both genes and that this system has been disrupted by a single mutation in this species.