Sleep and especially dreaming has always fascinated scientists, clinicians, philosophers, and artists alike. But researching it is extremely difficult. Dreaming happen in so called REM sleep, which is a phase of night-time mammalian sleep occurring in their brain.
Even though numerous research teams have tried to look into it, the identity of the neurons that control REM sleep, and its function in sleep have been controversial, because there are no precise genetic methods to study the sleeping brain. However, now with help of modern research technology scientists are able to look into sleeping brain and reveal at least part of its secrets.
Researchers at the RIKEN Brain Science Institute have conducted a study and identified a neural circuit in the brain that regulates REM sleep. Furthermore, they figured out the function of REM sleep – it turns out it controls the physiology of the other major sleep phase, called non-REM (NREM) sleep.
Scientists began the study when they noticed something rather interesting in the brain. The brain area, where REM sleep control was suspected, is called the pons. Team members noticed that here many cells are just visitors from another brain area called the rhombic lip and only got to pons during early embryonic development. Scientists were speculating that if they could mark rhombic lip cells, they could track their migration to the pons and artificially reactivate them during sleep, but there are no effective methods to do that. Or at least there were not until now.
Now there is a method called DREADD, standing for “Designer Receptors Exclusively Activated by Designer Drugs”. This method involves transgenic mice that express a DREADD receptor in rhombic lip cells that express Atoh1 during the developmental stage, and which, as scientists got to discover almost accidentally, then migrate to the pons.
Scientists used a special drug to bind to the receptor called CNO to activate the cells in the pons during sleep. Entire experiment was observed using brain activity measuring with electrodes placed on the head. The data collected during the experiment demonstrated that activating Atoh1 cells could suppress REM sleep, which automatically leads to increased NREM sleep. This means that scientists were able to identify cells that inhibit REM sleep, but they also wanted to figure out the function of this sleep phase in general sleep physiology.
Researchers used recordings from the brains of mice during NREM sleep. This sleep phase is characterized by large slow waves of activity that sweep through the brain, while REM sleep phase is recognized from the relatively quiet, desynchronized brain activity.
Scientists used their DREADD system to manipulate REM sleep, shortening and lengthening it, and found that the amplitude of slow waves during the following NREM sleep became corresponded by becoming smaller or larger. This means that for the first time ever science can prove that sleep phases are related in hierarchical order with NREM being under the control of REM sleep.
We may not realize, but such scientific discoveries actually have rather significant implications. It helps in bettering the understanding why sleep in mammals evolved to its current two-phase structure. Even before this study science knew that our experiences during wakefulness are stored in the brain during subsequent NREM sleep – that is why slow waves are very important.
Now, however, scientists also know that REM sleep is also important in this process, because these slow waves largely depend on REM phase. Scientists already have next steps of the research planned. Currently they will continue using their DREADD and other brain research technologies being developed by neuroscientists in order to determine the actual evolutionary role of REM sleep in mammals.
Mice are good models for this research, as usual, even though their dreams usually consist of replaying the prior activities. The future area of interest for these scientists in fact is the relationship between REM sleep and the replay of experience in NREM.
This research also is a good illustration how little we know about ourselves. Some people have an opinion that science these days is so advanced that new discoveries are just splitting the needle and are rather small. However, researches like this show that we do not even know how our own sleeping works and what kind of functions each phase has.