Researchers found that activating GABAergic neurons in the medulla of the brain immediately induced REM sleep - the dream phase - in mice.
Yang Dan, a professor of molecular and cell biology at UC-Berkeley, and colleagues found that activating certain neurons in a brain region called the medulla induced rapid eye movement (REM) sleep in mice in a matter of seconds, while deactivating the neurons impaired their ability to enter REM sleep.
The team recently published their findings in the journal Nature.
REM is a stage of mammalian sleep that makes up around 25% of our sleep cycle, with the first phase normally occurring around 70-90 minutes after falling asleep.
As well as being characterized by fast, random eye movements and temporary muscle paralysis, REM is involved in the dreaming process. Researchers hypothesize that the lack of muscle control that occurs during REM sleep is to stop us from acting out our dreams.
Activating GABAergic neurons triggered REM sleep in mice
For their study, Dan and colleagues set out to gain a better understanding of whether GABAergic neurons in the medulla - which are active during REM sleep - play a role in REM sleep generation.
- 90% of our dreams are lost within 1 minute of waking up
- We spend an average of 6 years of our lives dreaming
- Babies do not dream of themselves until the reach the age of 3 years.
GABAergic neurons protrude from the ventral part of the medulla, situated at the top of the spinal cord and certain regions of the brainstem and the hypothalamus - a brain region involved in hormone release and many other bodily functions.
The team used a technique called optogenetics on mice that were genetically engineered to express a marker protein in GABAergic neurons only.
The optogenetics method involved placing a light-sensitive ion channel into the GABAergic neurons using a virus. Once inserted, the researchers activated the GABAergic neurons in mice by stimulating them with a laser light via an optical fiber placed in the brain. The team was able to deactivate the GABAergic neurons with laser light by inserting an inhibitory ion pump into the neurons.
The researchers tracked the activity of GABAergic neurons in the medulla of the mice during both activation and deactivation with optogenetics. They also assessed how switching these neurons on or off for short periods affected the sleep and wake behavior of the mice.
The team found that activating the GABAergic neurons in the medulla of sleeping mice induced REM sleep within seconds, while deactivating these neurons during sleep either reduced their ability to enter REM or stopped that sleep phase completely.
Commenting on the findings, Dan says:
"Because of the strong induction of REM sleep - in 94% of the recorded trials our mice entered REM sleep within seconds of activating the neurons - we think this might be a critical node of a relatively small network that makes the decision whether you go into dream sleep or not."
Findings could shed light on psychiatric, neurological disorders
While activating the GABAergic neurons in mice that were awake did not impact their wakefulness, it did affect their appetite, causing them to eat more. The researchers note that in normal mice, activity of GABAergic neurons is highest during waking periods and during eating and grooming - which the team notes are two enjoyable activities for mice.
As such, Dan believes GABAergic neurons in the medulla produce the opposite effect to nerve cells involved in stress, such as noradrenergic neurons in a region of the brain called the pons.
"Other people have found that noradrenergic neurons, which are active when you are running, shut down when eating or grooming," says Dan. "So it seems like when you are relaxed and enjoying yourself, the noradrenergic neurons switch off and these GABAergic neurons in the medulla turn on."
As well as helping to gain a better understanding of the brain mechanisms behind sleep and dreaming, the researchers say their findings mean they are now able to switch dreaming on and off in mice at will, which could have clinical implications.
"Many psychiatric disorders, especially mood disorders, are correlated with changes in REM sleep, and some widely used drugs affect REM sleep, so it seems to be a sensitive indicator of mental and emotional health," says first author Franz Weber, a postdoctoral fellow at UC-Berkeley.
Not only could studying the sleep circuit offer insight into psychiatric disorders, Weber says it could aid understanding of neurological disorders that impact sleep, such as Parkinson's disease and Alzheimer's disease.
Last year, Medical News Today reported on a study in which researchers shed light on why some people often remember their dreams while others do not.