How Light Alters Circadian Rhythms: An Animal Study

Steven Dubovsky, MD

Long-term changes in light exposure are associated with altered methylation of genes involved in circadian rhythm.

Light has long-lasting effects on functions ranging from response to threats (NEJM JW Psychiatry Mar 14 2014) to learning. These effects appear to be related to changes in circadian rhythms driven by the suprachiasmatic nucleus (SCN) of the hypothalamus. To determine how circadian rhythms are reprogrammed, European investigators compared methylation of SCN genes in young mice entrained to 22- or 24-hour days.

In animals entrained to the shorter day, compared with those entrained to the normal day, 1294 SCN genes involved in synaptic function were hyper- or hypo-methylated in the promoter region. Periods of activity in the dark (free running) were shortened in the 22-hour animals, and this alteration was associated with changes in methylation and gene expression (both hyper- and hypomethylation and suppressed and increased expression were seen). Although changes in gene expression and circadian behavior were persistent, they could be reversed by re-entrainment to a 24-hour day or blocked during entrainment to the shorter day by injection of zebularine, which inhibits DNA methylation.

Citation(s):

Azzi A et al. Circadian behavior is light-reprogrammed by plastic DNA methylation. Nat Neurosci 2014 Mar; 17:377.