Characterization of Human Melatonin Synthesis Using Autoptic Pineal Tissue

Abstract

The mammalian pineal gland synthesizes rhythmically the hormone melatonin, which provides the body with a signal coding the duration of the night period. The ultimate enzymatic step in melatonin synthesis is achieved by the hydroxyindole O-methyltransferase (HIOMT); the rate-limiting enzyme is, however, the arylalkylamine N-acetyltransferase (AA-NAT). In contrast to the central importance of a transcriptional regulation of the Aa-nat gene for rodent melatonin synthesis, mechanisms in the human pineal gland are elusive. Therefore, pineal tissue, taken from regular autopsies (n = 69; postmortem intervals ranging from 9 to 147 h) was analyzed simultaneously for Aa-nat and Hiomt mRNA levels by PCR, AA-NAT activity using (14)C-acetyl-coenzyme A, HIOMT activity using S-adenosyl-l-[(14)C]-methionine, and melatonin content using an ELISA. Results were allocated to asserted time-of-death groups (day, 1,000 to 1,630 h; dusk, 1,630 to 2,200 h; night, 2,200 to 0730 h; dawn, 0730 to 1,000 h). RNA degradation rates of genes of interest ran in parallel, and, therefore, data normalization could be established, regardless of postmortem delay in tissue sampling. Aa-nat and Hiomt mRNA and HIOMT activity showed no diurnal rhythm. In contrast, a significant rhythm was found for the correlation between time of death and both AA-NAT activity and melatonin content, with elevated values during dusk and night. Presented data demonstrate that postmortem brain tissue can be used to detect the remnant of premortem adaptive changes in neuronal activity. In particular, our results give strong experimental support for the idea that transcriptional mechanisms are not dominant for the generation of rhythmic melatonin synthesis in the human pineal gland.

Homeobox Genes and Melatonin Synthesis: Regulatory Roles of the Cone-Rod Homeobox Transcription Factor in the Rodent Pineal Gland

“…For instance, in sheep, monkey, and human, pineal Aanat transcript levels do not oscillate on a circadian basis. However, in all mammalian species, the rhythm of pineal gland melatonin production is reliant on the nocturnal release of NE in the gland [29–31]. In the rodent, the release of NE from the sympathetic nerve endings binds and activates adrenergic receptors on the pinealocyte, which results in an increased intracellular cAMP level.…”

Rhythmic control of AANAT translation by hnRNP Q in circadian melatonin production

“…S1) and sheep ( Supplementary Fig. S2) AANAT mRNAs that show quite discrepant mRNA profiles (Coon et al 1996;Klein et al 1997;Ganguly et al 2002;Ackermann et al 2006) during the circadian cycle (see Supplemental Material for details). The newly identified nucleotide sequences of rat and sheep AANAT 5ЈUTRs have been deposited in GenBank with the accession numbers DQ075321 and DQ839412, respectively.…”

“…1E, middle panel; Klein et al 1997;Ganguly et al 2002;Kim et al 2005). In contrast, in primates and ungulates, AANAT protein levels increase exclusively nocturnally, despite steady-state, high levels of AANAT mRNAs (Stehle et al 2001;Ganguly et al 2002;Ackermann et al 2006). Thus, AANAT protein expression is out of phase with its mRNA expression profile.…”

Extrapineal Melatonin: General Conception, Localization and Biological Role in the Visual System

“…In rodents, the cAMP/protein kinase pathway induces transcriptional activation of the AA-NAT gene, the primary mechanism initiating melatonin biosynthesis. In contrast, the importance of a transcriptional regulation of the AA-NAT gene for melatonin production in humans is questionable [9] . Thus, analysis of pineal tissue, taken from regular autopsies, showed significant rhythm for the correlation between time of death and both AA-NAT activity and melatonin content, while AA-NAT and HIOMT mRNA, as well as HIOMT activity showed no diurnal rhythm [9] .…”

“…In contrast, the importance of a transcriptional regulation of the AA-NAT gene for melatonin production in humans is questionable [9] . Thus, analysis of pineal tissue, taken from regular autopsies, showed significant rhythm for the correlation between time of death and both AA-NAT activity and melatonin content, while AA-NAT and HIOMT mRNA, as well as HIOMT activity showed no diurnal rhythm [9] . Although dominant mechanisms for the generation of rhythmic melatonin synthesis in the human pineal gland remain to be elucidated, it is generally accepted that AA-NAT is switching melatonin synthesis with photoperiodic variation in duration, while the level of HIOMT activity may tune the seasonal magnitude of hormone production [10,11] .…”