Scientists at the Department of Neurology at the University of Bern and the University Hospital Bern have determined how sleep helps process emotions. The study was published in the journal Science.
The work expands the importance of sleep to mental health and opens up new avenues for therapeutic strategies. Rapid eye movement (REM or paradoxical) sleep is a unique and mysterious sleep state in which most dreams occur with intense emotional content. How and why these feelings are reactivated is unclear. The prefrontal cortex integrates many of these emotions during wakefulness but appears paradoxically calm during REM sleep.
“Our aim was to understand the basic mechanism and functions of this amazing phenomenon,” says Professor Antoine Adamantides from the Department of Biomedical Research (DBMR) at the University of Bern and the Department of Neurology at the Inselspital, University Hospital Bern. Bern. Processing emotions, and in particular distinguishing between danger and safety, is critical to the animals’ survival. In humans, excessive negative emotions, such as fear reactions and anxious states, lead to pathological conditions such as post-traumatic stress disorder (PTSD). In Europe, approximately 15 percent of the population suffers from persistent anxiety and severe mental illness. The research group led by Antoine Adamantis now offers insights into how the brain can help promote positive emotions and weaken negative or distressing emotions during REM sleep.
The researchers first conditioned mice to recognize safety-related and hazard-related (aversive) auditory stimuli. The activity of neurons in the mice’s brain was then recorded during the sleep-wake cycles. In this way, the researchers were able to map different regions of the cell and determine how emotional memories are transformed during REM sleep. Neurons consist of a cell body (soma) that integrates information from dendrites (inputs) and sends signals to other neurons through their axons (outputs). The obtained results showed that the cell soma remains silent during the activation of dendrites. “This means the two cell compartments are separated, in other words, the broad soma is dormant and the dendrites are broadly awake,” Adamantidis explains. This separation is important because the robust activity of dendrites allows encoding of both feelings of danger and safety, while soma inhibitors completely block the output of the circuit during REM sleep. In other words, the brain prefers to distinguish between safety and danger in its dendrites, but prevents an overreaction to emotion, especially danger.
Survival Advantage According to the researchers, the coexistence of the two mechanisms is beneficial for the organisms’ stability and survival: “This bidirectional mechanism is necessary to better distinguish between dangerous and safe signals,” says Mattia Aime of DBMR, first author of the study. If this distinction is missing in humans and excessive fear reactions are generated, it can lead to anxiety disorders. The findings are particularly relevant for pathological conditions such as post-traumatic stress disorder, in which trauma is excessively incorporated into the prefrontal cortex, day in and day out during sleep.
A breakthrough in sleep medicine These findings pave the way for a better understanding of emotional processing during sleep in humans and open new avenues for therapeutic targets to address maladaptive processing of traumatic memories, such as post-traumatic stress disorder (PTSD) and their early sleep dependent consolidation. Additional acute or chronic mental health issues that may cause this physical neurological detachment during sleep include acute and chronic stress, anxiety, depression, panic, or even anhedonia, and an inability to feel pleasure. Sleep research and sleep medicine have long been a research focus of the University of Bern and the Inselspital of the University of Bern. “We hope that our findings will be beneficial not only to patients, but also to the general public,” Adamantidis says. (Ani)
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