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 melatonin production. In most teleosts, including the pike, this is driven by an endogenous pineal clock. An exception is the trout, in which the pineal melatonin rhythm is a direct response to darkness. This fundamental difference in the regulation of melatonin production in two closely related species provides investigators a novel opportunity to study the molecular mechanisms of vertebrate clock function. We have studied the circadian regulation of mRNA encoding two melatonin synthesis enzymes by Northern blot analysis. These two enzymes are serotonin N-acetyltransferase (AA-NAT), the penultimate enzyme in melatonin synthesis, and tryptophan hydroxylase (TPH), the first enzyme in melatonin synthesis. A clock controls expression of both AA-NAT and TPH mRNAs in 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 both AA-NAT and TPH genes in most teleosts, and that in trout this system has been disrupted, perhaps by a single mutation.