We constantly alternate between light and deep sleep throughout the night. Until recently, scientists were unsure how these transitions happened. Our normal sleep cycle consists of rapid eye movement (REM) and non-REM (NREM) sleep stages (1) that are marked by different patterns of neuronal activity. Rapid neuronal firing usually accompanies REM sleep, and a slower and more occasional firing occurs during NREM sleep. Interruptions in these sleep stages can result in sleep disorders such as narcolepsy.

In a new study published in Science, Takeshi Sakurai, an expert in sleep medicine at the University of Tsukuba, and his team reported that a sharp increase in dopamine signaling triggers the transition from NREM to REM sleep (1). This finding could provide clues for understanding sleep disorders. 

This study “really lends a new perspective on the very complex neural circuitry that is underlying our sleep-wake arousal states,” said William Giardino, a neuroscientist at Stanford University who was not involved in the study.

Sakurai and his team focused on the role of dopamine in the midbrain region called the ventral tegmental area (VTA). Dopamine is a neurotransmitter that affects sleep patterns. Based on previous studies that reported a dynamic dopamine neuronal firing pattern in the VTA during REM and NREM sleep, Sakurai and his team suspected that there were multiple dopamine neurons in the VTA with distinct functions in regulating sleep.

To monitor dopamine signaling at various locations within the VTA, the scientists injected viral vectors containing dopamine sensors, called G protein-coupled receptor activation based sensors (GRABDA), into the basolateral amygdala (BLA) and other regions of mouse brains (2). 

“Sleep cycles are different between species, but the mechanisms that control the different stages are very similar,” Sakurai said. For example, mice have fragmented sleep and need more frequent sleep throughout the day than humans, but the neural pathways that govern this sleep are similar to those of humans. 

Unlike prior technologies such as microdialysis that can only provide time point measurements of the dopamine levels in the brain, these GRAB_DA sensors allow for real-time monitoring. The implanted optical fibers in these VTA regions enable dopamine neuron stimulation or inhibition using light and genetic engineering. 

“These new classes of sensors use fluorescence to make the neurons glow proportional to the level of dopamine and the activity of the dopamine receptor signaling,” Giardino said. According to Giardino, this new sensor, which scientists at Peking University recently developed, is the centerpiece that enabled the breakthroughs in this study (3). 

Sakurai and his team found that the increase in dopamine levels in the BLA region — but not in any other region of the VTA — terminated NREM and initiated REM sleep. By stimulating the GRAB_DA sensors in the BLA region with light, Sakurai and his team accelerated this sleep transition from about 8 minutes to just 2 minutes in mice. They also found that inhibiting the dopamine D2 receptors (DRD2) in the BLA increased REM sleep duration.

These findings have implications for treating narcolepsy, which is currently incurable. “There are so many challenges to treat narcolepsy with medications,” Giardino said. “It’s really imperative that we identify the underlying neurobiology to enhance effective treatment for narcolepsy, and understanding the circuitry regulating REM sleep is an important part of that.” 

Narcoleptic patients tend to experience a sudden loss of muscle strength, known as cataplexy, before they fall asleep uncontrollably. To investigate whether dopamine signaling was involved in this condition, Sakurai and his team monitored the behavior of narcoleptic mice after feeding them chocolate to stimulate dopamine release. The narcoleptic mice showed a large increase in BLA dopamine levels, whereas wild-type mice only showed a slight increase. This increase in BLA dopamine level triggered cataplexy in the narcoleptic mice and induced sleep, suggesting that the dopamine signaling in BLA plays a distinct role in cataplexy.

“Sleep and wakefulness is very complex,” said Seiji Nishino, a neuroscientist at Stanford University who was not involved in the study. According to Nishino, targeting one specific neural receptor, such as the dopamine receptor in the BLA region, may lead to an effective cataplexy treatment. However, dopamine signaling is not the only pathway that regulates sleep. Other neurotransmitters such as acetylcholine and neurons in the brainstem are also important in sleep regulation. 

Giardino explained that targeting only DRD2 as a treatment strategy could be problematic. Blocking DRD2 in the BLA of humans would require either gene editing or an incredibly invasive, risky, and involved procedure to infuse a drug into a very select region of the brain. This process is not something that he thinks will be done on humans on a large scale anytime soon.

Sakurai and his team are continuing their efforts to understand the dopamine signaling pathway. They are particularly interested in investigating the target sites of DRD2 neurons to better understand REM sleep regulation.

“REM sleep is very important for health," Sakurai said. “The appropriate method to control the timing and amount of REM sleep might be beneficial for treating various kinds of sleep disorders.” 

References

1. Hasegawa, E. et al. Rapid eye movement sleep is initiated by basolateral amygdala dopamine signaling in mice. Science  375, 994-1000 (2022).
2. Sun, F. et al. A Genetically Encoded Fluorescent Sensor Enables Rapid and Specific Detection of Dopamine in Flies, Fish, and Mice. Cell  174, 481-496 (2018)
3. Sun, F. et al. Next-generation GRAB sensors for monitoring dopaminergic activity in vivo. Nat Methods  17, 1156-1166 (2020).