How does sleep help to process emotions?

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). Researchers at the University of Bern and University Hospital of Bern, headed by Professor Antoine Adamantidis, are now providing insights into how the brain helps to reinforce positive emotions and weaken strongly negative or traumatic emotions during REM sleep — the sleep state during which most of our dreams occur — with their results published in the journal Science.

The researchers first conditioned mice to recognise 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,” Adamantidis said.

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 block the output of the circuit during REM sleep. In other words, the brain favours the discrimination of safety versus danger in the dendrites but blocks the over-reaction to emotion, in particular danger. According to the researchers, the coexistence of both mechanisms is beneficial to the stability and survival of the organisms.

“This bidirectional mechanism is essential to optimise the discrimination between dangerous and safe signals,” said Mattia Aime, 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 PTSD, in which trauma is over-consolidated in the prefrontal cortex, day after day during sleep.

The team’s 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 PTSD. Additional acute or chronic mental health issues that may implicate this somatodendritic decoupling during sleep include acute and chronic stress, anxiety, depression, panic and even anhedonia, or the inability to feel pleasure.

Image credit: ©stock.adobe.com/au/Kuz Production

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