New neural pathways are used to remember old fears, study finds

Chuck Bednar for redOrbit.com – Your Universe Online
The brain may use new pathways to recall old memories, including those of the traumatic events that can trigger post traumatic stress disorder (PTSD), according to a new study appearing in the latest edition of the journal Nature.
In an animal study funded by the US National Institutes of Health (NIH), researchers from the University of Puerto Rico School of Medicine found that the brains of rats used a separate brain pathway to recall older fear memories, not the same one used to recall it when it was fresh.
After the rats were conditioned to fear a tone associated with a mild shock, their overt behavior remained unchanged. But the pathway responsible for remembering the traumatic event was altered, and may have grown even stronger, the study authors explained.
As soon as the fear conditioning was complete, a circuit running from the brain’s prefrontal cortex to the part of the amygdala responsible for controlling fear was activated to retrieve the memory, the researchers explained.
Something changed
Several days later, however, they found that the task had been shifted to a different circuit, one running from the prefrontal cortex to an area in the thalamus. This part of the brain, which is known as the paraventricular region (PVT), then communicates with a different region of the amygdala – one that orchestrates fear learning and expression.
Dr. Quirk and his colleagues used a genetic/laser technique called optogenetics to detect the moving memory, and they believe that the PVT could cause fear to become integrated with other adaptive responses, such as stress, thereby strengthening the fear memory.
“While our memories feel constant across time, the neural pathways supporting them actually change with the time,” said study author, Dr. Gregory Quirk. “Uncovering new pathways for old memories could change scientists’ view of post-traumatic stress disorder, in which fearful events occur months or years prior to the onset of symptoms.”
Patients suffering from PTSD and other anxiety disorders often experience prolonged and exaggerated bouts of fear. The authors of the study suggests that this could involve the disruption of a gradual shifting of brain circuitry responsible for retrieving memories of those fear-inducing events.
“In people with anxiety disorders, any disruption of timing-dependent regulation in retrieval circuits might worsen fear responses occurring long after a traumatic event,” he said.
Examining the PVT further
In a second study, published in the same issue of Nature, researchers from Cold Spring Harbor Laboratory in New York and a team of colleagues from the US, France, and China explained how the long-term fear memory circuit translates stress detection into adaptive behaviors in mice.
This research team, which was led by associate professor Dr. Bo Li, discovered the same changes in memory retrieval circuitry over time, and used powerful genetic-chemical and optogenetic methods to switch pathways on and off.
By doing so, they conclusively demonstrated that neurons originating in the PVT regulate fear processing by acting on a class of neurons that store fear memories in the central region of the amygdala. This activity was traced back to the action of a messenger chemical, brain-derived neurotrophic factor, which had been previously linked to mood and anxiety disorders.
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