CHICK PINEAL CLOCK REGULATES SEROTONIN N-ACETYLTRANSFERASE MESSENGER-RNA RHYTHM IN CULTURE

Melatonin production in the chick pineal gland is high at night and lo w during the day. This rhythm reflects circadian changes in the activi ty of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferas e, 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 i n the pineal cells themselves. Here we present evidence that the chick pineal clock generates a rhythm in the abundance of AA-NAT mRNA in cu ltured cells that parallels the rhythm in AA-NAT activity. In contrast , elevating cAMP hy forskolin treatment markedly increases AA-NAT acti vity without producing strong changes in AA-NAT mRNA levels, and lower ing 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 mRN A level, whereas cAMP-controlled changes occur primarily through chang es at the protein level. Related studies indicate that the clock-depen dent nocturnal increase in AA-NAT mRNA requires gene expression but no t de novo protein synthesis, and that AA-NAT mRNA levels are suppresse d at all times of the day by a rapidly turning over protein. Further a nalysis of the regulation of chick pineal AA-NAT mRNA is likely to enh ance our understanding of the molecular basis of vertebrate circadian rhythms.