December 7, 2010

Light Cycles At Birth Affect Biological Clocks

(Ivanhoe Newswire) -- We're all familiar with the term biological clock.  Some of us are perhaps familiar with the sound it makes when it starts to tick.  What we may not be familiar with, however, is that the season in which babies are born can have a remarkable and unrelenting effect on how the gears of their biological clocks function.

The conclusion of a recent experiment provides the foremost evidence for seasonal imprinting of biological clocks in mammals.  The imprinting effect "“ evident in baby mice "“ could help explain the fact that people born in winter months have an increased risk neurological disorders, including but not limited to seasonal affective disorder (winter depression), bipolar depression, as well as 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," which Douglas McMahon, Professor of Biological Sciences at Vanderbilt University and lead author of the study, was quoted as saying.

McMahon and colleagues, in an effort to turn forward the clocks of modern science, raised groups of mouse pups from birth to weaning in artificial or summer light cycles for the experiment.  Subsequent to the pup's weaning, they were maintained in either the identical cycle or an opposite cycle for 28 days.  Once they were mature, the mice were placed in continuous darkness where their activity patterns were carefully observed.

"When they are born, the brains of mice are less developed than those of a human baby.  As a result, their brains are still being wired during this period," McMahon said.
The winter-born mice displayed an incessant slowing of their daily activity period, in spite of whether they had been maintained on a winter light cycle, or moreover shifted to summer cycle following weaning.  The scientists then examined the master biological clocks in the mouse brains "“ using a gene that makes the clock cells glow green when active "“ and discovered a parallel pattern: slowing of the gene clocks in winter-born mice in comparison to mice born on 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 added.

When they examined what was happening in the brains of the different groups, they discovered a prominently similar pattern.

In the summer-born mice, the activity of the neurons in the SCN peaked at the time of dusk and was unceasing for approximately 10 hours.  When the winter-born mice were matured in the winter cycle, their neuronal activity peaked one hour after the time of dusk and continued for 10 hours.  Nevertheless, in the winter-born mice switched to a summer cycle, the master bioclock's activity peaked two hours prior to the time of dusk and prolonged for 12 hours.
Their analysis showed that these variations are caused by alterations in the activity patterns of the individual neurons, rather than by network-level effects.

"It is quite striking how closely the neuronal wave form and period line up with their behavior," McMahon said.

The new-fangled study raises an intriguing yet exceedingly provisional likelihood: seasonal variations in the day/night cycle that individuals experience as their brains are developing may ultimately affect their personality.

"We know that the biological clock regulates mood in humans," said McMahon.  "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!"

There are countless potential seasonal signals that may affect brain development, one of which being exposure to flu virus.  This study demonstrates that seasonal light cycles can affect the development of a specific brain function.

"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," which Chris Ciarleglio, graduate student at Vanderbilt University and co-author, was quoted as saying.

Precisely when the imprinting occurs throughout the three-week period leading up to weaning, and whether the effect is provisional or permanent are questions the scientists intend to address and further delve into via forthcoming experiments.

SOURCE: Natural Neuroscience, 5 December 2010