Forget the panic button; two research teams reporting in Nature this week have discovered specific cellular mechanisms in the brain that control fear.
Andreas Lüthi and colleagues from the Friedrich Miescher Institute for Biomedical Research in Switzerland have discovered that two distinct populations of neurons in a part of the brain called the amygdala are involved in different aspects of fear memory: one in the elimination of established fear responses, and the other in the ability to refresh those memories. Working with mice, they found that selectively activating one of the two populations triggered large changes in the behavioral state.
According to the researchers, the populations are integrated into two discrete neuronal circuits that are differentially connected with parts of the brain previously implicated in conditioned fear responses. They suggest that feelings of fear for stimuli previously linked to unpleasant consequences are triggered by rapid switching in the balance of activity between these two circuits.
In a related study, Denis Paré and colleagues from the State University of New Jersey, USA, have revealed another cellular mechanism underlying the ability to unlearn established fear memories. They propose that within another part of the amygdala a different population of neurons, called intercalated amygdala neurons, also help to eliminate fear of stimuli that previously caused fear. By destroying these neurons in rats, the researchers observed a decrease in the extinction of fear memories such that the rats remained afraid.
Both studies provide insight into the various cells and circuits in the brain that maintain learned fear, and could provide therapeutic clues for fear and anxiety disorders in humans, such as post-traumatic stress disorder.
- Switching on and off fear by distinct neuronal circuits (Article p600, doi: 10.1038/nature07166)
- Amygdala intercalated neurons are required for expression of fear extinction (Letter p642, doi: 10.1038/nature07167)
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