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The season in which babies are born can have a dramatic and persistent effect on how their biological clocks function. That is the conclusion of a study offering the first evidence for seasonal imprinting of biological clocks in mammals. The research was conducted by Professor of Biological Sciences Douglas McMahon, graduate student Chris Ciarleglio, PhD’09, postdoctoral fellow Karen Gamble and two Vanderbilt undergraduates.
The imprinting effect, which was found in baby mice, may help explain the fact that people born in winter months have a higher risk of several neurological disorders, including seasonal affective disorder (winter depression), bipolar depression and schizophrenia.
In the study, groups of mouse pups were raised from birth to weaning in artificial winter or summer light cycles. After they were weaned, they were maintained in either the same cycle or the opposite cycle for 28 days. Once they were mature, the mice were placed in constant darkness and their activity patterns were observed.
Winter-born mice showed a consistent slowing of daily activity, regardless of whether they had been maintained on a winter light cycle or had been shifted to summer cycle after weaning. When scientists examined the master biological clocks in the mice’s brains, using a gene that makes the clock cells glow green when active, they found a similar pattern: a slowing of the gene clocks in winter-born mice as compared to those born on a summer light cycle.
“What is particularly striking about our results,” says Ciarleglio, “is the fact that the imprinting affects both the animal’s behavior and the cycling of the neurons in the master biological clock in its brain.”
In addition, their experiments found that imprinting of clock-gene activity near birth had dramatic effects on the reaction of the biological clock to changes in season later in life. The biological clocks and behavior of summer-born mice remain stable and aligned with the time of dusk while that of the winter-born mice varied widely when they were placed in a summer light cycle.
“The mice raised in the winter cycle show an exaggerated response to a change in season that is strikingly similar to that of human patients suffering from seasonal affective disorder,” McMahon says.
The new study raises an intriguing but highly speculative possibility: Seasonal variations in the day/night cycle that individuals experience as their brains are developing may affect their personality.
Results of the study were published online Dec. 5 by the journal Nature Neuroscience.
Ciarleglio completed his graduate studies and is now assistant director of the Vanderbilt Brain Institute. The undergraduate contributors to the study were John Axley, BA’10, and Benjamin Strauss, BS’08, who have graduated and gone on to graduate school and medical school. Karen Gamble, the contributing postdoctoral fellow, is now a faculty member at the University of Alabama-Birmingham.