The brain is
extraordinarily good at alerting us to threats: they all send electrical
impulses buzzing down our sensory neurons, pinging our brain’s fear circuitry
and, in some cases, causing us to fight or flee. The brain is also adept at
knowing when an initially threatening or startling stimulus turns out to be
harmless or resolved. But sometimes this system fails and unpleasant
associations stick around, a malfunction thought to be at the root of post-traumatic
stress disorder (PTSD). New research has identified a neuronal circuit
responsible for the brain’s ability to purge bad memories, findings that could
have implications for treating PTSD and other anxiety disorders. Like most
emotions, fear is neurologically complicated. But previous work has
consistently implicated two specific areas of the brain as contributing to and
regulating fear responses. The amygdala, two small arcs of brain tissue deep
beneath our temples, is involved in emotional reactions, and it flares with
activity when we are scared. If a particular threat turns out to be harmless, a
brain region behind the forehead called the prefrontal cortex steps in and the
fright subsides. Our ability to extinguish painful memories is known to involve
some sort of coordinated effort between the amygdala and the prefrontal cortex.
The new study, by
researchers at the National Institutes of Health, however, confirms that a
working connection between the two brain regions is necessary to do away with
fear. Normally mice that repeatedly listen to a sound previously associated
with a mild foot shock will learn that on its own the tone is harmless, and
they will stop being afraid. Using optogenetic stimulation technology, or
controlling specific neurons and animal behavior using light, the authors found
that disrupting the amygdala–prefrontal cortex connection prevents mice from
overcoming the negative association with the benign tone. In neurobiology
speak, memory extinction fails to occur. They also found that the opposite is
true—that stimulating the circuit results in increased extinction of fearful
memories. Until now investigators were unsure whether the amygdala–prefrontal
cortex communication pathway could on its own control fear extinction; both
structures interact with many other brain regions, and so isolating their
effects of on behavior was a challenge. Optogenetics made the discovery
possible, allowing the NIH group to precisely assess only the connection
between the two brain regions in real time, providing a more accurate
correlation between neuronal activity and behavior.
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