Narcolepsy (excessive daytime sleepiness), cataplexy (sudden loss of muscle tone), and rapid eye movement (REM) sleep behavior disorder are serious sleep-related disorders. The researchers have found neurons in the brain that link all three disorders and could provide a target for treatments.
The research led by Professor Takeshi Sakurai at the University of Tsukuba is published in the Journal of Neuroscience.
Most of the dreams occur during REM sleep in which the eyes move back and forth, but our bodies remain still. This near-paralysis of muscles while dreaming is called REM-atonia. In patients with REM sleep behavior disorder, REM-atonia is absent due to which the muscles move around often leading to stand up and jump, yell, or punch. Researchers have found neurons in the brain that normally prevent this type of behavior during REM sleep.
Sakurai said, "The anatomy of the neurons we found matched what we know. They were connected to neurons that control voluntary movements, but not those that control muscles in the eyes or internal organs. Importantly, they were inhibitory, meaning that they can prevent muscle movement when active."
On blocking the input to these neurons, the mice began to move during their sleep similar to someone with REM sleep behavior disorder.
Narcolepsy is characterized by suddenly falling asleep at any time during the day, even in mid-sentence. Cataplexy is characterized by sudden loss of muscle tone and collapse; even if they are awake their muscles act as if they are in REM sleep. It was found that the special neurons were related to these two disorders.
By triggering cataplexic attacks in mouse model of narcolepsy by chocolate, the researchers tested their hypothesis.
Sakurai said, "We found that silencing the SLD-to-ventral medial medulla reduced the number of cataplexic bouts. The glycinergic neurons we have identified in the ventral medial medulla could be a good target for drug therapies for people with narcolepsy, cataplexy, or REM sleep behavior disorder. Future studies will have to examine how emotions, which are known to trigger cataplexy, can affect these neurons."
The glycinergic neurons that link all the three disorders can serve as a possible target for the treatment of sleep disorders.