EXPRESSION OF MELATONIN SYNTHESIS GENES IS CONTROLLED BY A CIRCADIAN CLOCK IN THE PIKE PINEAL ORGAN BUT NOT IN THE TROUT

The photosensitive teleost pineal organ exhibits a daily rhythm in mel atonin production. In most teleosts, including the pike, this is drive n by an endogenous pineal clock. An exception is the trout, in which t he pineal melatonin rhythm. is a direct response to darkness. This fun damental difference in the regulation of melatonin production in two c losely related species provides investigators a novel opportunity to s tudy the molecular mechanisms of vertebrate clock function. We have st udied the circadian regulation of mRNA encoding two melatonin synthesi s enzymes by Northern blot analysis. These hive enzymes are serotonin N-acetyltransferase (AA-NAT), the penultimate enzyme in melatonin synt hesis, and tryptophan hydroxylase (TPH), the first enzyme in melatonin synthesis. A clock controls expression of both AA-NAT and TPH mRNAs i n the pineal organ of pike, but not that of trout, in which the levels of these mRNAs are tonically elevated. A parsimoneous explanation of this is that a single circadian system regulates the expression of bot h AA-NAT and TPH genes in most teleosts, and that in trout this system has been disrupted, perhaps by a single mutation. ((C) Elsevier, Pari s).