How our brains change during the day.

I am reminded what a rigid daily schedule my body keeps and expects every time I vary my routine slightly (changing a meal time, exercise, happy hour, bedtime)...my body doesn't like it, feels off kilter. Travel of the sort I've been doing over the past month is a huge disrupter. I'm also clear that any demanding and analytical thinking I might want to do should happen before noontime, because by 4 p.m. (a low blood sugar time for the body), my mind has become very lazy.

McClung and collaborators have done the fascinating experiment of looking at gene expression during the day in brain areas important in learning, memory, and emotion, in 146 young and old brains, finding differences on aging. The brains were from subjects who had died suddenly, as in a car accident. They built on the work of Akil and collaborators who earlier had shown the activity of ~1000 genes varies in a daily pattern that allows the time of death to be predicted within an hour. That pattern was disrupted in people with major depressive disorders. From McClung's group:  

Significance

Circadian rhythms are important in nearly all processes in the brain. Changes in rhythms that come with aging are associated with sleep problems, problems with cognition, and nighttime agitation in elderly people. In this manuscript, we identified transcripts genome-wide that have a circadian rhythm in expression in human prefrontal cortex. Moreover, we describe how these rhythms are changed during normal human aging. Interestingly, we also identified a set of previously unidentified transcripts that become rhythmic only in older individuals. This may represent a compensatory clock that becomes active with the loss of canonical clock function. These studies can help us to develop therapies in the future for older people who suffer from cognitive problems associated with a loss of normal rhythmicity.

Abstract

With aging, significant changes in circadian rhythms occur, including a shift in phase toward a “morning” chronotype and a loss of rhythmicity in circulating hormones. However, the effects of aging on molecular rhythms in the human brain have remained elusive. Here, we used a previously described time-of-death analysis to identify transcripts throughout the genome that have a significant circadian rhythm in expression in the human prefrontal cortex [Brodmann’s area 11 (BA11) and BA47]. Expression levels were determined by microarray analysis in 146 individuals. Rhythmicity in expression was found in ∼10% of detected transcripts (P less than 0.05). Using a metaanalysis across the two brain areas, we identified a core set of 235 genes (q less than 0.05) with significant circadian rhythms of expression. These 235 genes showed 92% concordance in the phase of expression between the two areas. In addition to the canonical core circadian genes, a number of other genes were found to exhibit rhythmic expression in the brain. Notably, we identified more than 1,000 genes (1,186 in BA11; 1,591 in BA47) that exhibited age-dependent rhythmicity or alterations in rhythmicity patterns with aging. Interestingly, a set of transcripts gained rhythmicity in older individuals, which may represent a compensatory mechanism due to a loss of canonical clock function. Thus, we confirm that rhythmic gene expression can be reliably measured in human brain and identified for the first time (to our knowledge) significant changes in molecular rhythms with aging that may contribute to altered cognition, sleep, and mood in later life.