The making of the master clock

Abstract

A genetic basis for the anatomic master circadian clock in mammals has been found.

Keywords: Lhx1; Ror-alpha; VIP; circadian rhythms; suprachiasmatic nucleus.

Figure 1.. Circadian oscillations at the molecular, tissue and behavioral level.

At the molecular level, the transcription factor Lhx1 controls the expression of peptides within the neurons of the SCN; when Lhx1 is not expressed, neither are the peptides. At the tissue level this peptide expression allows the coupling of the SCN neurons that produces the circadian behavior of the SCN. When Lhx1 is active, the circadian rhythms are strong; without Lhx1, the rhythms are weaker. At the behavioral level the more robust clock that results from Lhx1 activity (middle row) is slower to adapt to shifted light schedules. The white and grey boxes indicate times when lights are on (white) or off (grey). Black marks are the hypothetical daily activity of a mouse in a running wheel. The mice are normally active when it is dark. When Lhx1 is expressed, the mice find it more difficult to adapt to a sudden change in the light cycle, which results in a lot of activity when it is light. When Lhx1 is absent (bottom), the mice are better able to adjust to restrict their activity to the hours of darkness.