Neuroscientists identified a neural circuit in the brain that regulates REM sleep, the phase of sleep when dreams occur. The new study also showed how REM sleep controls the physiology of non-REM sleep, the other major sleep phase commonly believed to be important for memory consolidation.

Sleep is a complex phenomenon composed of two distinct states of consciousness: REM (rapid eye movement) sleep and non-REM sleep, Dr. Yu Hayashi of the International Institute for Integrative Sleep Medicine at University of Tsukuba explained. REM sleep is a time of dreams and desynchronized brain wave activity, while during non-REM sleep, large, slow waves of activity sweep through the brain. Experiences during our waking hours are consolidated and stored in our brains during non-REM sleep, with the slow waves of sleep contributing to that important memory process.

While much is known about sleep, too many questions remain unanswered. For example, which phase, REM or non-REM, is more significant? Why are two phases needed?

Until recently, scientists suspected neurons (brain cells) in a region called the pons control REM sleep. However, Hayashi and his co-lead researcher, Dr. Shigeyoshi Itohara of RIKEN Brain Science Institute, knew that pons cells arrived (during early embryonic development) from a distant brain area called the rhombic lip.

For the study, then, Hayashi and Itohara investigated what would happen if they marked the rhombic lip cells, tracked their migration to the pons, and then artificially reactivated them during sleep.

To do this, they used a method called DREADD — Designer Receptors Exclusively Activated by Designer Drugs.

This technique begins with unique, transgenic mice. These mice have been genetically modified to express a special DREADD receptor in their rhombic lip cells before these cells migrate to the pons. Next, the researchers apply a drug that binds to this special DREADD receptor in order to activate only those cells that have migrated from the rhombic lip to the pons. The researchers perform this activation while the mice are asleep (measured by brain activity recorded by electrodes placed on the tiny heads of the mice).

What did they discover? The data showed that activating cells could suppress REM sleep, leading to an increase in NREM sleep. This led the scientists to identify so-called REM-inhibitory cells.

After this, the researchers explored the REM phase to see how it relates to sleep physiology in general. Using the DREADD technique again, they shortened or elongated REM sleep duration. Here, they discovered, the amplitude of slow waves during the following NREM sleep became correspondingly smaller or larger. Their results demonstrate that sleep phases interact in a hierarchy, with NREM sleep under the control of REM sleep.

If the slow waves (during non-REM sleep) do indeed rely on REM sleep as this new study suggests, then both phases of sleep contribute to the replay of experience that is so necessary to memory and our lives. Going forward, the researchers plan to explore this aspect of sleep more extensively.

Source: Hayashi Y, Itohara S, et al. Science. 2015.