Brain research. Molecular brain researchNov 3, 2004

Midday darkness and its effects on daily rhythm genes in the rat’s internal clock and melatonin-making gland

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 25, 2026

Abstract

Period1 and Period2 mRNA levels in the suprachiasmatic nucleus decreased significantly after 0.5 and 2 hours of dark exposure, respectively.

Dark exposure during the day could induce phase shifts in circadian rhythms, which may involve changes in mRNA levels of circadian clock genes.
No significant changes were observed in Period1 and Aa-Nat mRNA levels in the pineal gland due to darkness, though Period2 was moderately affected.
Isoproterenol, a stimulant, significantly increased Period1 and Aa-Nat mRNA levels in the pineal gland but did not affect Period2.
The findings suggest that the pineal gland may respond to dark information during the day, challenging previous understandings of its gating mechanism.

Simplified

The suprachiasmatic nucleus (SCN) of the mammalian hypothalamus contains a central circadian pacemaker, which adjusts circadian rhythms within the body to environmental light-dark cycles. It has been shown that dark exposure in the day causes phase shifts in circadian rhythms, but it does not induce changes in the melatonin levels in the pineal gland. In this study, we examined the effect of dark exposure on two "circadian clock" genes Period1 and Period2 mRNA levels in the rat SCN, and on Period1, Period2, and arylalkylamine N-acetyltransferase (Aa-Nat, the rate-limiting enzyme in melatonin synthesis) gene expression in the pineal gland. Period1 and Period2 mRNA levels were significantly decreased in the SCN after 0.5 and 2 h, respectively, therefore suggesting that changes in those mRNA levels may be the part of the mechanisms of dark-induced phase shifts. Period1 and Aa-Nat mRNA levels in the pineal gland were not affected by darkness, but Period2 was moderately affected. Since Period1 and Aa-Nat mRNA levels in the pineal gland did not respond to dark stimulation, we further examined whether the pineal gland itself is capable of responding to adrenergic stimulation at this time of the day. Isoproterenol significantly induced Period1 and Aa-Nat mRNA levels; however, it did not affect Period2. Although previous studies have reported that during the day the SCN "gates" the dark information reaching the pineal, our data demonstrate that dark information may reach the pineal during the daytime.

Full Text

Full text is available at the source.

View Full Text ↗

Midday darkness and its effects on daily rhythm genes in the rat’s internal clock and melatonin-making gland

Abstract

Full Text

You found one interesting study. We’ll send the next 7.

what lands in your inbox each week:

Recent issues from the circadian biology brief