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During
the most dream-filled phase of sleep, our muscles become paralyzed, preventing
the body from acting out what's going on in the brain. Now, researchers have
discovered the brain chemicals that keep the body still in sleep.
The
findings could be helpful for treating sleep disorders, the scientists report
Wednesday (July 18) in The Journal of Neuroscience.
The
brain chemicals kick into action during rapid eye movement (REM) sleep, a phase
that usually begins about 90 minutes into a night's rest. During REM, the brain
is very active, and dreams are at their most intense. But the voluntary muscles
of the body — arms, legs, fingers, anything that is under conscious control —
are paralyzed.
This
paralysis keeps people still even as their brains are acting out fantastical
scenarios; it's also the reason people sometimes experience sleep paralysis, or
the experience of waking up while the muscles are still frozen. This sensation
has been the basis for myths such as the succubus and the incubus.
The
chemistry of sleep
Exactly
how the muscles are paralyzed has been a mystery, however. Early studies pegged
a neurotransmitter called glycine as the culprit, but paralysis still occurred
even when the receptors that read glycine's presence were blocked, disproving
that notion.
So
University of Toronto researchers Patricia Brooks and John Peever cast a wider
net. They focused on two different nerve receptors in the voluntary muscles,
one called metabotropic GABAB and one called inotropic GABAA/glycine. The
latter receptor responds to both glycine and a different communication chemical
called gamma-amino butyric acid, or GABA, while the first responds to GABA and
not glycine.
The
researchers used drugs to "switch off" these receptors in rats and
discovered that the only way to prevent sleep paralysis during REM was to shut
both types off at the same time. What that means is that glycine alone isn't
enough to paralyze the muscles. You need GABA, too.
Treating
sleep disorders
Understanding
this alphabet soup of neurotransmitters is important for people who have sleep
disorders, especially an odd condition called REM behaviour disorder. In this
disorder, people don't become paralyzed during REM sleep. That means they act
out their dreams, talking, thrashing and even punching or hitting in their
sleep.
Currently,
Clonazepam, an antipsychotic drug, is used to treat REM behaviour disorder. The
new study could point to new treatments for the problem, sleep researcher
Dennis McGinty of the University of California, Los Angeles, who was not
involved in the study, said in a statement. The researchers hope that the
results could help explain the link between REM behaviour disorder and more
deadly conditions.
"Understanding
the precise mechanism behind these chemicals’ role in REM sleep disorder is
particularly important because about 80 percent of people who have it
eventually develop a neurodegenerative disease, such as Parkinson’s
disease," Peever said. "REM sleep behaviour disorder could be an
early marker of these diseases, and curing it may help prevent or even stop
their development."
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