Pineal circadian clocks gate arylalkylamine N‐acetyltransferase gene expression in the mouse pineal gland

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Abstract
In mammals it has been thought that the circadian clock localizes only in the suprachiasmatic nucleus of the hypothalamus. Recent studies have revealed that certain brain regions and peripheral tissues may also have intrinsic circadian clocks. However, the roles played by ‘peripheral circadian clocks’ have not been fully elucidated. In this study, we investigated their function using mouse pineal glands, and found that expression of the arylalkylamine N‐acetyltransferase (Aa‐Nat, EC 2.3.1.87, the rate‐limiting enzyme of melatonin synthesis) gene after adrenergic receptor stimulation depended on the time of day even in vitro (gating). Phase‐dependent Aa‐Nat responses were observed in both melatonin‐proficient and melatonin‐deficient mouse pineal glands. Phosphodiesterases are unlikely to suppress Aa‐Nat induction because a phosphodiesterase inhibitor itself had no effect on the mRNA levels. Puromycin was ineffective in inducing Aa‐Nat mRNA levels in either the presence or absence of isoproterenol, suggesting that newly synthesized proteins may not be necessary to gate the Aa‐Nat responses. We also discovered circadian dependence of the expression of Period1‐luminescence in Period1–luciferase transgenic mouse pineal glands: circadian clocks may be functional in culture. Aa‐Nat mRNA levels showed no significant circadian rhythms in the absence of isoproterenol, thus suggesting that Aa‐Nat mRNA levels are induced by adrenergic mechanisms, not by a pineal circadian clock. Our results suggest that the pineal circadian clock may determine timing when Aa‐Nat gene expression can respond to inputs from the master circadian clock in the suprachiasmatic nucleus, e.g. adrenergic stimulation.

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