From the University of Bern
May 13, 2022 -- Researchers
at the Department of Neurology of the University of Bern and University
Hospital Bern identified how the brain triages emotions during dream sleep to
consolidate the storage of positive emotions while dampening the consolidation
of negative ones. The work expands the importance of sleep in mental health and
opens new ways of therapeutic strategies.
Rapid eye movement (REM
or paradoxical) sleep is a unique and mysterious sleep state during which most
of the dreams occur together with intense emotional contents. How and why these
emotions are reactivated is unclear. The prefrontal cortex integrates many of
these emotions during wakefulness but appears paradoxically quiescent during
REM sleep. "Our goal was to understand the underlying mechanism and the
functions of such a surprising phenomenon", says Prof. Antoine Adamantidis
from the Department of Biomedical Research (DBMR) at the University of Bern and
the Department of Neurology at the Inselspital, University Hospital of Bern.
Processing emotions,
particularly distinguishing between danger and safety, is
critical for the survival of animals. In humans, excessively negative emotions,
such as fear reactions and states of anxiety, lead to pathological states like
Post-Traumatic Stress Disorders (PTSD). In Europe, roughly 15% of the
population is affected by persistent anxiety and severe mental illness. The
research group headed by Antoine Adamantidis is now providing insights into how
the brain helps to reinforce positive emotions and weaken strongly negative or
traumatic emotions during REM sleep. This study was published in the journal
Science.
A Dual mechanism
The researchers first
conditioned mice to recognize auditory stimuli associated with safety and
others associated with danger (aversive stimuli). The activity of neurons in
the brain of mice was then recorded during sleep-wake cycles. In this way, the
researchers were able to map different areas of a cell and determine how
emotional memories are transformed during REM sleep.
Neurons are composed of
a cell body (soma) that integrates information coming from the dendrites
(inputs) and send signals to other neurons via their axons (outputs). The
results obtained showed that cell somas are kept silent while their dendrites
are activated. "This means a decoupling of the two cellular compartments,
in other words soma wide asleep and dendrites wide awake", explains
Adamantidis. This decoupling is important because the strong activity of the
dendrites allows the encoding of both danger and safety emotions, while the
inhibitions of the soma completely blocks the output of the circuit during REM
sleep. In other words, the brain favours the discrimination of safety versus
danger in the dendrites, but block the over-reaction to emotion, in particular
danger.
A survival advantage
According to the
researchers, the coexistence of both mechanisms is beneficial to the stability
and survival of the organisms: "This bi-directional mechanism is essential
to optimize the discrimination between dangerous and safe signals", says
Mattia Aime from the DBMR, first author of the study. If this
discrimination is missing in humans and excessive fear reactions are generated,
this can lead to anxiety disorders. The findings are particularly relevant to
pathological conditions such as post-traumatic stress disorders, in which
trauma is over-consolidated in the prefrontal cortex, day after day during
sleep.
Breakthrough for sleep
medicine
These findings pave the
way to a better understanding of the processing of emotions during sleep in
humans and open new perspectives for therapeutic targets to treat maladaptive
processing of traumatic memories, such as Post Traumatic Stress Disorders
(PTSD) and their early sleep-dependent consolidation. Additional acute or
chronic mental health issues that may implicate this somatodendritic decoupling
during sleep include acute and chronic stress, anxiety, depression, panic, or
even anhedonia, the inability to feel pleasure. Sleep research and sleep
medicine have long been a research focus of the University of Bern and the
Inselspital, Bern University Hospital. "We hope that our findings will not
only be of interest to the patients, but also to the broad public", says
Adamantidis.
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