Pineal AA-NAT Research Articles

Transcripts encoding two melatonin synthesis enzymes in the teleost pineal organ: circadian regulation in pike and zebrafish, but not in trout.

In this report the photosensitive teleost pineal organ was studied in three teleosts, in which melatonin production is known to exhibit a daily rhythm with higher levels at night; in pike and zebrafish this increase is driven by a pineal clock, whereas in trout it occurs exclusively in response to darkness. Here we investigated the regulation of messenger RNA (mRNA) encoding serotonin N-acetyltransferase (AA-NAT), the penultimate enzyme in melatonin synthesis, which is thought to be primarily responsible for changes in melatonin production. AA-NAT mRNA was found in the pineal organ of all three species and in the zebrafish retina. A rhythm in AA-NAT mRNA occurs in vivo in the pike pineal organ in a light/dark (L/D) lighting environment, in constant lighting (L/L), or in constant darkness (D/D) and in vitro in the zebrafish pineal organ in L/D and L/L, indicating that these transcripts are regulated by a circadian clock. In contrast, trout pineal AA-NAT mRNA levels are stable in vivo and in vitro in L/D, L/L, and D/D. Analysis of mRNA encoding the first enzyme in melatonin synthesis, tryptophan hydroxylase, reveals that the in vivo abundance of this transcript changes on a circadian basis in pike, but not in trout. A parsimonious hypothesis to explain the absence of circadian rhythms in both AA-NAT and tryptophan hydroxylase mRNAs in the trout pineal is that one circadian system regulates the expression of both genes and that this system has been disrupted by a single mutation in this species.

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.

Avian melatonin synthesis: photic and circadian regulation of serotonin N-acetyltransferase mRNA in the chicken pineal gland and retina.

The circadian rhythms in melatonin production in the chicken pineal gland and retina reflect changes in the activity of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase; AA-NAT; EC 2.3.1.87). Here we determined that the chicken AA-NAT mRNA is detectable in follicular pineal cells and retinal photoreceptors and that it exhibits a circadian rhythm, with peak levels at night. AA-NAT mRNA was not detected in other tissues. The AA-NAT mRNA rhythm in the pineal gland and retina persists in constant darkness (DD) and constant lighting (LL). The amplitude of the pineal mRNA rhythm is not decreased in LL. Light appears to influence the phase of the clock driving the rhythm in pineal AA-NAT mRNA in two ways: The peak is delayed by approximately 6 h in LL, and it is advanced by > 4 h by a 6-h light pulse late in subjective night in DD. Nocturnal AA-NAT mRNA levels do not change during a 20-min exposure to light, whereas this treatment dramatically decreases AA-NAT activity. These observations suggest that the rhythmic changes in chicken pineal AA-NAT activity reflect, at least in part, clock-generated changes in mRNA levels. In contrast, changes in mRNA content are not involved in the rapid light-induced decrease in AA-NAT activity.

Pineal serotonin N-acetyltransferase: expression cloning and molecular analysis.

Pineal serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, or AA-NAT) generates the large circadian rhythm in melatonin, the hormone that coordinates daily and seasonal physiology in some mammals. Complementary DNA encoding ovine AA-NAT was cloned. The abundance of AA-NAT messenger RNA (mRNA) during the day was high in the ovine pineal gland and somewhat lower in retina. AA-NAT mRNA was found unexpectedly in the pituitary gland and in some brain regions. The night-to-day ratio of ovine pineal AA-NAT mRNA is less than 2. In contrast, the ratio exceeds 150 in rats. AA-NAT represents a family within a large superfamily of acetyltransferases.