Falling asleep while driving or while operating the nuclear reactor might be a piece of entertainment when we see Homer Simpson (cartoon character). But in reality these disorders portray serious life challenges in the form of sleep-related illnesses - narcolepsy, cataplexy, and rapid eye movement (REM) sleep behavior disorder.
Neurons in the brain that link all three disorders were located in an area of the brain called the ventral medial medulla and received input from another area (sublaterodorsal tegmental nucleus, or SLD) too. These neurons can thus provide a target for treatments as per researchers at the University of Tsukuba.
Sleep Disorders and the Neurons
During REM sleep, our eyes move back and forth, but our bodies remain still and we dream. This near-paralysis of muscles while dreaming is called REM-atonia, and is lacking in people with REM sleep behavior disorder.
People with REM sleep behavior disorder have their muscles move around, often going as far as to stand up and jump, yell, or punch. The team explored the neurons in the brain that normally prevent this type of behavior during REM sleep.
"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", says Professor Takeshi Sakurai, who led the study at the University of Tsukuba.
Blocking the input to these neurons in the mice models allowed them to move during their sleep, similar to someone with REM sleep behavior disorder. And thus, silencing the SLD-to-ventral medial medulla resulted in reduced number of cataplexic bouts.
Hence the study shows the role of the circuitry in controlling muscle atonia in both REM sleep and cataplexy.
"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", says Sakurai.