Transcription Factors May FrameAa-natGene Expression and Melatonin Synthesis at Night in the Syrian Hamster Pineal Gland

Abstract

Pineal melatonin synthesis is stimulated at night following an increase in arylalkylamine-N-acetyltransferase (AA-NAT) activity. Depending on the species, two mechanisms of enzyme activation have been described: a cAMP/phospho-cAMP response element-binding protein-dependent stimulation of Aa-nat gene transcription in the rat, presumed to occur in all rodents, or a posttranslational regulation of AA-NAT protein in ongulates. The present data obtained in the Syrian hamster indicate another route of AA-NAT regulation. Elevated nocturnal levels of Aa-nat mRNA were strongly suppressed following light exposure or adrenergic antagonist administration, demonstrating the involvement of norepinephrine in the stimulation of melatonin synthesis. However, administration of adrenergic agonists during the day did not increase Aa-nat mRNA unless a protein synthesis inhibitor was given during the previous night. This indicates that an inhibitory protein, synthesized at night, prevents melatonin synthesis during the day. By contrast, a protein synthesis inhibitor given at the beginning of the night markedly reduced Aa-nat mRNA, suggesting that a stimulatory protein (transcription factor?) is necessary for Aa-nat gene transcription at night. Noteworthy, hamsters raised in long photoperiod were responsive to adrenergic agonist injection only in the first hour after light onset, a response that may be important in this photoperiodic species in which the melatonin peak extends into the morning hours in a short photoperiod.

Generation of the Melatonin Endocrine Message in Mammals: A Review of the Complex Regulation of Melatonin Synthesis by Norepinephrine, Peptides, and Other Pineal Transmitters

“…In the pineal gland, therefore, the photoperiodic variation in ICER level (with higher levels in SP) may control the photoperiodic variation in the pattern (slope of the increase and amplitude) of the nocturnal expression of the MEL-synthesizing enzymes. In support of this hypothesis, we have observed that the amplitude of the nocturnal peak of pineal Aa-nat mRNA and activity is lower in SP than in LP in rodents: rat (Ribelayga et al, 1999a), Siberian hamster (Ribelayga et al, 2000), and Syrian hamster (Garidou et al, 2003a). The role of ICER in the modulation of genetic expression was demonstrated in the pineal gland of Crem-deficient transgenic mice, in which the amplitude of the Aa-nat mRNA nocturnal peak was markedly increased (Foulkes et al, 1996a).…”

“…Indeed, Aa-nat mRNA level displays a large nocturnal increase (150-fold) peaking at ZT 20 -22 . Similarly to enzyme activity, Aa-nat mRNA could not be increased by acute or chronic injections of adrenergic agonists during the day but is inhibited at night following injection of a ␤-or ␣-adrenergic antagonist or to light exposure (Garidou et al, 2003a). Various experiments show that the pineal gland needs to be stimulated for at least 6 to 8 h in late afternoon to induce an increase in Aa-nat…”

“…mRNA, AA-NAT activity, and MEL synthesis (GonzalezBrito et al, 1988;Garidou et al, 2003a). The mechanisms controlling MEL synthesis in the Syrian hamster, therefore, appear somehow different to those described in the rat AA-NAT gene transcription, enzyme activation, and MEL synthesis during the night require the neosynthesis of a stimulatory protein (possibly transcription factor) but are repressed during the day by an inhibitory protein (possibly ICER), these processes leading to a strong gating of MEL synthesis in the late night (Diaz et al, 2003;Garidou et al, 2003a).…”

Analysis of Adrenergic Regulation of Melatonin Synthesis in Siberian Hamster Pineal Emphasizes the Role of HIOMT

“…AA-NAT activity was assayed in pineal homogenates according to Garidou et al [29] with some modification. Each pineal was sonicated in 100 Ìl 0.05 M phosphate buffer (pH 6.6) containing 0.35 mM acetyl-CoA (Sigma).…”

“…Brain with pineal attached was sampled from midday and midnight hamsters and frozen in -30°C isopentane. Serial coronal sections (20 ÌM ) of the pineal gland were cut and treated for in situ hybridization as previously described [28][29]. Sense and antisense 35 S-labelled Aa-nat riboprobe were incubated on pineal sections at 54°C overnight then washed out at high stringency [28,29].…”