Rapid eye movement (REM or REM) sleep is a unique and mysterious sleep state in which most dreams occur with intense emotional content. How and why these emotions are reactivated remains a poorly understood process. The prefrontal cortex integrates most of these emotions during wakefulness but paradoxically appears at rest during REM sleep.
Better understand the processing of emotions during sleep
From Survival to Mental Illness: the main author, Prof. Antoine Adamantidis of the University of Bern and clinician in the Department of Neurology at the Inselspital (University Hospital of Bern), recalls that the processing of emotions, in particular the ability to distinguish between danger and safety, is an essential function for survival. In humans, excessively negative emotions, such as fear and anxiety reactions, lead to disease states such as post-traumatic stress disorder.
In Europe, 15% of the population suffer from anxiety and serious mental disorders.
The study: the researchers conditioned mice to recognize auditory stimuli associated with safety and others associated with danger. The activity of neurons in the mouse brain was then recorded during sleep-wake cycles. In this way, the researchers were able to map how emotional memories are transformed during REM sleep. Neurons are composed of a cell body (soma) which integrates information from dendrites (inputs) and sends signals to other neurons through their axons (outputs). The maps reveal that:
- the cellular somas are kept silent while the dendrites are activated: there is decoupling between the 2 cellular compartments, ie the soma is deeply asleep and the dendrites are wide awake;
this decoupling allows:
- the dendrites, which are very active in encoding both the emotions of danger and safety,
- the soma, inhibited, from completely blocking the output of signals during REM sleep;
- in other words, the brain favors safety over danger.
The coexistence of these 2 mechanisms is beneficial to stability and survival: “This bidirectional mechanism is essential to optimize the discrimination between dangerous signals and safety signals”says another lead author of the study, Mattia Aime.
When this mechanism malfunctions, excessive fear responses are generated, which can lead to anxiety disorders.
A “sleep medicine” to regulate emotions: these results open the way to a better understanding of the processing of emotions during sleep in humans and designate new therapeutic targets to treat traumatic memories, such as post-traumatic stress disorder (PTSD). Other acute or chronic mental health conditions that may involve this “somato-dendritic” uncoupling could also be involved, such as, acute and chronic stress, anxiety, depression, panic or even anhedonia or inability to feel pleasure.