Neurons that see the light

Are you having trouble staying awake? Did you get enough sleep? Is it a warm spring day with the sun shining bright? Are you performing a task that should be engaging and rewarding? Do you have narcolepsy?   Narcolepsy is a neurodegenerative disease and chronic sleep disorder that is characterized by extreme sleepiness at inappropriate times and places. This daytime drowsiness seems unaffected by the amount of nighttime sleep [1]. So, if you're dancing in the middle of a wild flower field on a sunny spring day and are still drowsy, you might have narcolepsy.  

 

Recently, researchers at University of California, Los Angeles (UCLA) in the Semel Institute for Neuroscience and Human Behavior, have determined that dysfunction within the hypocretin system produces narcolepsy in humans and mice [2]. The hypocretin system is responsible for arousal, wakefulness, and food intake. It is located within the hypothalamus of the brain, which appears responsible for body temperature, circadian cycle, and other automatic and metabolic bodily functions.  

 

Now, the same research group has published an article in the “Journal of Neuroscience” on October 26, 2011 that reports the discovery of a group of neurons within the hypocretin system that are responsible for light-induced arousal [3]. Basically, these neurons wake people when light is present. Without these neurons, people would be unable to stay awake no matter the amount of sun. This is typical of narcolepsy.

 

In the paper, the researchers looked at mice with and without the light-induced arousal neurons in a series of tasks in both light and dark scenarios. Interestingly, the researchers discovered that the mice without the light-induced arousal neurons performed worse in only the light scenarios that used rewards. Moreover, in the mice with the neurons, the light-induced arousal neurons were only activated in the light scenarios [4].

 

The researchers believe that the inability to produce arousal for reward tasks during the light scenarios might be the reason narcoleptics have trouble focusing and staying awake during the day. Narcoleptics would not receive the same stimulus from performing tasks during daylight hours. Also, the researchers believe that the disruption of these light-induced arousal neurons might create further disruptions within the entire arousal system. These disruptions might lead to some of the symptoms of depression, since it has been reported that narcoleptics have a higher rate of depression than people with normal hypocretin systems.

 

Hopefully, this will lead to treatments for both narcolepsy and depression. Currently, narcoleptics rely on behavioral changes (naps throughout the day) and stimulants (Amphetamine) to combat the symptoms of narcolepsy. The behavioral changes are hard to implement within a work day. Bosses rarely allow people to nap at their desk and almost never have beds available. The stimulants often have nasty side effects, such as addiction, hypertension and psychosis, and the stimulants are often unavailable to people with certain medical conditions and women who are pregnant.

 

Of course, some treatments might be available soon. Researchers have already injected the light-induced arousal neurons from rat pups into adult rats. The neurons were able to survive the grafting process [5]. Furthermore, if the light-induced arousal neurons are still present in the brain, stimulation of the neurons with Transcranial magnetic stimulation (TMS) might reactivate the neurons. TMS has been shown to help with depression [6], but TMS as a therapy or treatment is very new and extremely controversial.

 

Either way, the research done at UCLA and other facilities will hopefully continue to add insights into the brain’s arousal systems that could lead to new treatments for people with many different neurological diseases.