NeuroscienceMay 5, 2009

Gene Activity in the Brain's Internal Clock and Its Role in Measuring Day Length

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 28, 2026

Abstract

The core of the suprachiasmatic nuclei (SCN) integrates a new photoperiod quickly, influencing the expression of clock-controlled genes.

Daily changes in gene expression reflect integration of a new photoperiod rather than a passive light effect.
The transition from long to short photoperiod in Syrian hamsters leads to rapid adjustments in clock gene expressions.
Clock-controlled genes exhibit slower adaptation following changes in photoperiod compared to core clock genes.
The pineal gland shows differential daily functioning as a result of changes in SCN activity due to photoperiod shifts.
In hamsters exposed to a short photoperiod for 26 weeks, all analyzed genes show similar daily expression except for Clock.
The distinct expression of Clock in the photorefractory phase suggests its potential role in this physiological state.

Simplified

In mammals, the 24 h-rhythmicity of many physiological events is driven by the circadian clock contained in the suprachiasmatic nuclei (SCN). In the SCN, clock gene expressions produce the rhythmicity and control the expression of clock-controlled genes which play a role in the distribution of daily messages. The daily expression of all these genes is modulated by the duration of the light phase (i.e. photoperiod). The aim of this study was first to determine if these daily changes of expression reflect a real integration of a new photoperiod by the circadian clock or reflect only a passive effect of the light. In this way, we performed a time course of the modifications of gene expression after a transfer of Syrian hamsters from long to short photoperiod (LP and SP). Our results demonstrate that the core of the SCN (clock genes) integrates quickly a new photoperiod which entrains a slow adaptation of the clock-controlled gene expressions and induces a differential daily functioning of an SCN-target tissue, the pineal gland. We next asked the question whether SCN are involved in the photorefractory phase observed in Syrian hamsters exposed to SP for 26 weeks. All genes analyzed present a similar daily expression in SP-refractory and in SP with the exception of Clock. Its particular expression in SP-refractory is different than ones observed in SP or in LP. Thus, Clock seems to play a role in the development of the photorefractory phase, or this physiological state may modify the expression of Clock in the SCN. As a conclusion, it appears that the photoperiodic time measurement involves daily modifications of the molecular functioning of the SCN and that SCN also play a role in the measurement of the duration of the time passed in a short photoperiod.

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Gene Activity in the Brain's Internal Clock and Its Role in Measuring Day Length

Abstract

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