Introduction

The CDC says that the average human adult requires 7 plus hours of sleep every night for everyday function, but what happens during those hours? Your brain will go through different stages of sleep known as REM sleep. There is not too much known about REM sleep and it continues to be researched all the time. One thing that is still researched about REM sleep is how it is activated and deactivated. Your brain will go through many stages of activating and deactivating REM sleep in just one night of slumber, causing your brain to switch between REM and non-REM sleep constantly. In a recent study, a couple of scientists proposed an idea about how REM sleep is activated based on an experiment done with mice. This study proposes that there are GABAergic neurons which reside in the dorsomedial hypothalamus which function as a switch for REM and non-REM sleep. Because of this research, other researchers may now move forward on examining these neurons in humans and their function they may play for regulating our natural sleep cycle. 

Background

When your body falls asleep the first stage of sleep is non-REM sleep, and this lasts typically an hour or two. Then your body will transition into what is known as REM sleep, and this is where things get wild. REM sleep is when your brain is most active while sleeping and this is usually when you start to have vivid dreams (Sleep Basics, 2020). For the rest of the night, your brain will switch between cycles of REM and non-REM sleep. Understanding REM sleep and how it is activated can give us greater insight on the role that REM sleep plays for sleeping and why it is necessary for the brain to conduct during sleeping. Having a better understanding of REM sleep may let us know why we dream as well, or what role dreams play for helping our brain function in everyday life.

 The hypothalamus plays key roles for not only our brain to function, but the whole body. It is responsible for maintaining homeostasis inside of us; it regulates all our autonomic functions inside us such as maintaining an appropriate body temperature and energy levels. It also plays a major role in the endocrine system as it has many connections with the pituitary gland (Pop Miana Gabriela et al., 2018). Growing our understanding of the hypothalamus is crucial for modern science as we have only recently learned of its enormous role it plays in so many basic bodily functions. 

Methods

To measure the activity of the mouses brain the scientists hooked up the mouse to an electrode which is connected to an EEG. The EEG will be able to read brain activity based on the neuronal impulses sent by neurons inside the brain. The scientists wanted to see the neurons inside the dorsomedial hypothalamus, so they used a virus expressing a calcium indicator which can be seen under a fluorescent microscope. A normal microscope uses visible light to illuminate an image of a sample and then magnifies that image so it can be seen. A fluorescence microscope however uses a higher intensity light source which excites a specific fluorescent species so it can be seen under the microscope (Fluorescent Microscopy, 2021). This virus contains the fluorescence needed to be seen under the microscope so that is why it was injected into the dorsomedial hypothalamus of the rat brain. This is when the scientists saw the two distinct subgroups of neurons which were activated at different times during the rat’s slumber. While one subgroup was active, the other was being suppressed and the scientists found this interesting. 

Results

The scientists questioned if the subgroupings of neurons could be distinguished based on their axonal projections since the neurons were all very similar being both GABAergic and galaninergic. The scientists would use retrograde tracing targeting the preoptic area and raphe pallidus due to the axons projecting to mostly these areas. The preoptic area of the hypothalamus is primarily responsible for managing body temperature but is involved in numerous physiological and behavioral processes due to its many interconnections inside the brain (Yu Sangho etal., 2017). The raphe pallidus on the other hand seems to play a role in our immune system. It is typically activated in order to induce a fever to battle a present infection (Hornung JP, 2012). The subgroups of neurons found from the previous step would be injected with a different tracer and an interesting finding came forward. The neuron subgroup which was activated during non-REM sleep would mostly project to the preoptic area while the other subgroup which was activated during REM sleep would mostly project to the raphe pallidus.

Discussion

Overall, these findings were very insightful. The scientists managed to find two subgroups of neurons which serve as an activation/deactivation switch for REM sleep. During REM sleep, one subgroup would be actively suppressed while the other being constantly activated. This tells me that there is a clear argument for these neurons playing a major role in the mouses sleep cycle. What leaves me puzzled is why these neurons target the areas they do. The preoptic area and raphe pallidus are not really known for their contributions to sleep functioning, but these findings show them obviously playing a role in the mouses sleep patterns. I am really curious to what would happen if this experiment were conducted on a human instead. Would the raphe pallidus and preoptic area still play major roles in REM and non-REM sleep patterns? I am also curious to if more and/or different structures inside the hypothalamus would be activated during REM sleep inside humans during sleep. Regardless, it is safe to say that the hypothalamus does affect the sleep of our rodent friends. I hope this study inspires more scientists to continue research of the relationship between the hypothalamus and sleep.

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