M. Bernard et al., CHICK PINEAL CLOCK REGULATES SEROTONIN N-ACETYLTRANSFERASE MESSENGER-RNA RHYTHM IN CULTURE, Proceedings of the National Academy of Sciences of the United Statesof America, 94(1), 1997, pp. 304-309
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.