exposure to dim light at night can lead to depressive symptoms in
rodents—but these negative effects can be reversed simply by returning
to a standard light-dark cycle, a new study suggests.
hamsters exposed to light at night for four weeks showed evidence of
depressive symptoms, those symptoms essentially disappeared after about
two weeks if they returned to normal lighting conditions.
changes in the brain that occurred after hamsters lived with chronic
light at night reversed themselves after returning to a more normal
findings add to the growing evidence that suggest chronic exposure to
artificial light at night may play some role in the rising rates of
depression in humans during the past 50 years, said Tracy Bedrosian,
lead author of the study and doctoral student in neuroscience at Ohio
“The results we found in hamsters are consistent with what we know about depression in humans,” Bedrosian said.
But the new study, published online in the journal Molecular Psychiatry, also offers some hope.
good news is that people who stay up late in front of the television
and computer may be able to undo some of the harmful effects just by
going back to a regular light-dark cycle and minimizing their exposure
to artificial light at night,” Bedrosian said. “That’s what the results
we found in hamsters would suggest.”
conducted the study with Ohio State colleagues Randy Nelson, professor
of neuroscience and psychology, and Zachary Weil, research assistant
professor in neuroscience.
study is the latest in a series out of Nelson’s lab that have linked
chronic exposure to light at night to depression and obesity in animal
new study found that one particular protein found in the brain of
hamsters—and humans—may play a key role in how light at night leads to
found that blocking effects of that protein, called tumor necrosis
factor, prevented the development of depressive-like symptoms in
hamsters even when they were exposed to light at night.
study involved two experiments using female Siberian hamsters, which
had their ovaries removed to ensure that hormones produced in the ovary
would not interfere with the results.
the first experiment, half of the hamsters spent eight weeks in a
standard light-dark cycle of 16 hours of light (150 lux) and 8 hours of
total darkness each day. The other half spent the first four weeks with
16 hours of normal daylight (150 lux) and 8 hours of dim light—5 lux, or
the equivalent of having a television on in a darkened room.
these hamsters were moved back to a standard light cycle for either one
week, two weeks or four weeks before testing began.
were then given a variety of behavior tests. Results showed that
hamsters exposed to chronic dim light at night showed less total
activity during their active period each day when compared to those in
standard lighting conditions.
hamsters exposed to dim light also showed greater depressive symptoms
than did the others– such as less interest in drinking sugar water that
they usually enjoy.
within two weeks of returning to a standard light cycle, hamsters
exposed to dim night light showed no more depressive-like symptoms than
did hamsters that always had standard lighting. In addition, they
returned to normal activity levels.
the behavioral testing, the hamsters were sacrificed and the
researchers studied a part of their brains called the hippocampus, which
plays a key role in depressive disorders. Findings showed that hamsters
exposed to dim light showed a variety of changes associated with
importantly, hamsters that lived in dim light showed increased
expression of the gene that produces tumor necrosis factor. TNF is one
of a large family of proteins called cytokines — chemical messengers
that are mobilized when the body is injured or has an infection. These
cytokines cause inflammation in their effort to repair an injured or
infected area of the body. However, this inflammation can be damaging
when it is constant, as happens in hamsters exposed to dim light at
have found a strong association in people between chronic inflammation
and depression,” said Nelson, who is a member of Ohio State’s Institute
for Behavioral Medicine Research.
“That’s why it is very significant that we found this relationship between dim light at night and increased expression of TNF.”
addition, results showed that hamsters that lived in dim light had a
significantly reduced density of dendritic spines—hairlike growths on
brain cells which are used to send chemical messages from one cell to
Changes such as this have been linked to depression, Bedrosian said.
hamsters that were returned to a standard light-dark cycle after four
weeks of dim light at night saw their TNF levels and even their density
of dendritic spines return essentially to normal.
“Changes in dendritic spines can happen very rapidly in response to environmental factors,” Bedrosian said.
a second experiment, the researchers tested just how important TNF
might be in causing the negative effects seen in hamsters exposed to
light at night. In this experiment, some hamsters were given a drug
called XPro1595, which is a TNF inhibitor—meaning that it negates the
effects of some forms of TNF in the brain.
showed that hamsters exposed to dim light at night did not show any
more depressive-like symptoms than standard-light hamsters if they were
given XPro1595. However, the drug did not seem to prevent the reduction
of dendritic spine density in hamsters exposed to dim light.
results provide further evidence of the role TNF may play in the
depressive symptoms seen in hamsters exposed to dim light. But the fact
that XPro1595 did not affect dendritic spine density means that more
needs to be learned about exactly how TNF works, Nelson said.
Source: Ohio State University