Humans are diurnal animals, meaning that we usually sleep at night
and are awake during the day, due at least in part to light or the lack
thereof. Light is known to affect sleep indirectly by
entraining—modifying the length of—our circadian rhythms and also
rapidly and directly due to a phenomenon known as masking. But while a
great deal is known about how light affects circadian rhythms, little is
known about the direct effects of light on sleep: Why do we tend to
wake up if the lights are flipped on in the middle of the night? Why
does darkness make us sleepy? Caltech researchers in the laboratory of
Professor of Biology David Prober
say they have discovered at least part of the answer: a specific
protein in the brain that responds to light and darkness to set the
correct balance between sleep and wakefulness.
(Image caption:
Overexpression
of the neuropeptide prokineticin 2 (Prok2) results in an increase in
expression of the sleep-promoting neuropeptide galanin (shown in white)
when zebrafish are kept in the light, but not when they are kept in the
dark, compared to wild type control animals. These images show galanin
expression in the anterior hypothalamus, a brain region known to promote
sleep in mammals. Credit: Courtesy of the Prober laboratory)
Their work is described in a paper appearing online in the journal Neuron on June 22.
“Researchers
had previously identified the photoreceptors in the eye that are
required for the direct effect of light on wakefulness and sleep,” says
Prober. “But we wanted to know how the brain uses this visual
information to affect sleep.”
The Prober laboratory uses zebrafish
as a model organism for studying sleep. The animals are optically
transparent, allowing for noninvasive imaging of their neurons; they
also have a diurnal sleep/wake pattern like that of humans. To
investigate how their sleep responds to light, Wendy Chen, a former
graduate student in Prober’s lab, led studies examining a particular
protein in the zebrafish brain called prokineticin 2 (Prok2).
Chen
genetically engineered zebrafish to overexpress Prok2, resulting in an
abundance of the protein. She found that in contrast to normal
zebrafish, these animals were more likely to fall asleep during the day
and to wake up at night. Surprisingly, the effects did not depend on the
engineered fish’s normal circadian sleep/wake cycle but rather depended
only on whether the lights were on or off in their environment. These
observations suggest that an excess of Prok2 suppresses both the usual
awakening effect of light and the sedating effect of darkness.
Chen
then generated zebrafish with mutated forms of Prok2 and its receptor,
and observed light-dependent sleep defects in these animals. For
example, Chen found that zebrafish with a mutated Prok2 receptor were
more active when the lights were on and less active when the lights were
off, the opposite of what she had observed in animals that
overexpressed Prok2 and had functional Prok2 receptors.
“Though
diurnal animals such as zebrafish spend most of their time asleep at
night and awake during the day, they also take naps during the day and
occasionally wake up at night, similar to many humans,” Prober says.
“Our study’s results suggest that levels of Prok2 play a critical role
in setting the correct balance between sleep and wakefulness during both
the day and the night.”
Next, the researchers wanted to know how
Prok2 was modulating light’s effects on sleep. To answer this question,
they decided to examine whether other proteins in the brain that are
known to affect sleep were required for the effects of Prok2 on sleep
behavior. They found that the sedating effect of Prok2 overexpression in
the presence of light requires galanin, a known sleep-promoting
protein. They also found that Prok2 overexpression increased the level
of galanin expression in the anterior hypothalamus, a key
sleep-promoting center in the brain. But in animals that were engineered
to lack galanin, overexpression of Prok2 did not increase sleep.
These
findings provide the first insights into how light may interact with
the brain to affect sleep and provide a basis for scientists to begin
exploring the genes and neurons that underlie the phenomenon. However,
further work is needed to fully explain how light and dark directly
affect sleeping and waking, and to determine whether Prok2 has a similar
function in humans. If it does, this work might eventually lead to new
sleep- and wake-promoting drugs.
