Babies' biological clocks dramatically affected by birth light cycle

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 new study published online on Dec. 5 by the journal Nature Neuroscience. The experiment provides the first evidence for seasonal imprinting of biological clocks in mammals and was conducted by Professor of Biological Sciences Douglas McMahon, graduate student Chris Ciarleglio, post-doctoral fellow Karen Gamble and two undergraduate students at Vanderbilt University.

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 a number of neurological disorders including seasonal affective disorder (winter depression), bipolar depression and schizophrenia.

"Our biological clocks measure the day length and change our behavior according to the seasons. We were curious to see if light signals could shape the development of the biological clock," said McMahon.

In the experiment, 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.

The winter-born mice showed a consistent slowing of their daily activity period, regardless of whether they had been maintained on a winter light cycle, or had been shifted to summer cycle after weaning. When the scientists examined the master biological clocks in the mouse brains, using a gene that makes the clock cells glow green when active, they found a similar pattern: slowing of the gene clocks in winter-born mice compared to those born on a summer light cycle.

"What is particularly striking about our results is the fact that the imprinting affects both the animal's behavior and the cycling of the neurons in the master biological clock in their brains," said Ciarleglio.

In addition, their experiments found that the 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 commented.

Exactly when the imprinting occurs during the three-week period leading up to weaning and whether the effect is temporary or permanent are questions the scientists intend to address in future experiments.

Seasonality and Personality