Researchers from Xijing Hospital of the Fourth Military Medical University and Imperial College London, UK, published a research paper titled " A specific circuit in the midbrain detects stress and induces restorative sleep" in the journal " Science".
The study shows that stress promotes sleep in mice, not only reducing stress but also anxiety levels, and the study identifies the mechanisms involved.
We all know that all mammals experience two main types of sleep: rapid eye movement (light sleep) and non-rapid eye movement (deeper, dreamless sleep). In previous clinical studies, sleep disorders have been shown to have a high co-morbidity with stress- and fear-related psychiatric disorders, such as depression and post-traumatic stress disorder, in which significant alterations in REM sleep state have been observed.
In the study, the researchers created a psychosocial stressor called "social defeat" in mice, and when the mice were exposed to extremely aggressive mice, the researchers observed elevated stress hormones in the mice's bloodstreams, indicating high levels of stress.
When the mice slept, the researchers tracked the activity of their neurons and found a specific set of neurons in the ventral tegmental area (VTA) of the midbrain that detects and responds to stress hormone levels and induces high levels of sleep in both non-rapid eye movement (REM) and rapid eye movement (REM).
Specifically, a subset of γ-GABA-promoter inhibitory hormone neurons receives stress inputs and, when activated, promotes REM and non-REM sleep over several hours, while also inhibiting the release of adrenocorticotropin-releasing factor.
The activity of these neurons remained high during five hours of sleep in mice, and in addition, they sent signals to neurons that regulate stress hormones, preventing them from releasing more.
Stress induces increased sleep
The study showed that this particular group of neurons not only detected stress and induced sleep, they also triggered a decrease in stress hormones.
When the mice woke up from their sleep, the researchers also tested the mice's anxiety levels. Anxiety levels were determined by measuring the amount of time the mice spent in light environments and in dark environments, since mice tend to seek out dark environments when they are anxious.
Mice that did not get stress-induced sleep spent much longer in dark environments compared to sleep-deprived mice or stressed mice with damage to specific neurons, suggesting that they were more anxious and that their levels of stress hormones remained high.
The results suggest that stress-induced sleep reduced stress levels and lessened stress-induced anxiety in the mice, whereas mice lacking stress-induced sleep continued to have elevated levels of anxiety and noradrenocorticotropic hormone after stress.
Stress increases sleep, while sleep reduces stress-induced anxiety
The researchers say the findings add to the idea that rapid eye movement sleep helps cope with stress.
Overall, the study identifies a mechanism by which stress induces sleep, suggesting that stress-induced sleep reduces stress levels and lessens stress-induced anxiety. The researchers hope to find ways to selectively target this group of neurons to enhance their positive effects through sleep.
