Being Born In Winter Affects Biological Clock, Influencing Neurological Disorder Risk

Why does a baby born during the winter months have a higher risk of developing bipolar depression, schizophrenia, SAD (seasonal affective disorder) and other neurological disorders compared to one born during the summer? Scientists from Vanderbilt University, USA, wrote in the journal Nature Neuroscience that exactly when a baby is born can have a dramatic and lifelong effect on the functioning of their biological clocks.

Professor of Biological Sciences Douglas McMahon, and team say their experiment provides the first proof of seasonal imprinting of biological clocks in mammals. In this case, the imprinting effect in baby mice may help us understand better why humans born during the winter months are more likely later on in life to develop some neurological disorders.

McMahon said:

"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."

The team raised baby mice from the day they were born until they were weaned in artificial winter or summer light cycles. After being weaned, their cycles either continued the same or were changed to an opposite cycle for a 28-day period. When they reached adulthood they were placed in continuous darkness while their behaviors and activity patterns were monitored.

The mice which had been born in the winter environment demonstrated a consistent slowing of their daily activity period compared to the summer-born mice, no matter what subsequent cycle they were in after weaning.

They used a gene which makes the clock cells glow green when they are active so they could examine their master biological clocks. Here they also found a slowing in the winter-born mice's gene clocks compared to those born in the summer environment.

Team member, Chris Ciarleglio, said:

"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."

The gene clock activity's imprinting close to their birth date had a considerable impact on the mice's biological clock reactions to subsequent changes in seasons during adulthood, the investigators revealed.

McMahon commented:

"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."

Further research is needed to determine when exactly the imprinting occurs and whether it has a permanent or temporary effect, the team said.

This study makes one wonder what impact seasonal light/darkness cycles very early in life might have in the development of our personalities.

McMahon said:

"We know that the biological clock regulates mood in humans. If an imprinting mechanism similar to the one that we found in mice operates in humans, then it could not only have an effect on a number of behavioral disorders but also have a more general effect on personality. It's important to emphasize that, even though this sounds a bit like astrology, it is not: it's seasonal biology!"

Human studies have found a link between schizophrenia risk and being born in winter (rather than a specific month). Scientists have suggested many factors which could influence neurological disorder risks, such as viral exposure. This study clearly shows a link between just light cycles and a specific brain function development.

Ciarleglio said:

"We know from previous studies that light can affect the development of other parts of the brain, for example the visual system. Our work shows that this is also true for the biological clock."

It took the team two years to develop a special strain of genetically engineered mice. They carried a gene that produces a naturally fluorescent green protein - when their biological clock neurons were active they would glow green, making it possible for the team to monitor them accurately.

The master biological clock is located in the suprachiasmatic nucleus (SCN) - an area in the middle brain behind the eyes.