Chick pineal clock regulates serotonin N-acetyltransferase mRNA rhythm in culture

Melatonin production in the chick pineal gland is high at night and low during the day. This rhythm reflects circadian changes in the activity of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AA-NAT; EC 2.3.1.87), the penultimate enzyme in melatonin synthesis. In contrast to the external regulation of pineal rhythms in mammals by the suprachiasmatic nucleus, rhythmic changes in AA-NAT activity in cultured chick pineal cells are controlled by an oscillator located in the pineal cells themselves. Here we present evidence that the chick pineal clock generates a rhythm in the abundance of AA-NAT mRNA in cultured cells that parallels the rhythm in AA-NAT activity. In contrast, elevating cAMP by forskolin treatment markedly increases AA-NAT activity without producing strong changes in AA-NAT mRNA levels, and lowering cAMP by norepinephrine treatment decreases enzyme activity without markedly decreasing mRNA. These results suggest that clock-controlled changes in AA-NAT activity occur primarily through changes at the mRNA level, whereas cAMP-controlled changes occur primarily through changes at the protein level. Related studies indicate that the clock-dependent nocturnal increase in AA-NAT mRNA requires gene expression but not de novo protein synthesis, and that AA-NAT mRNA levels are suppressed at all times of the day by a rapidly turning over protein. Further analysis of the regulation of chick pineal AA-NAT mRNA is likely to enhance our understanding of the molecular basis of vertebrate circadian rhythms.