Old fear memories are recalled via a different pathway than the original pathway used when the memory was fresh, according to a new study funded by the NIH. Researchers suspect that the new pathways may help keep an old memory fresh, as seen in post traumatic stress disorder.
Researchers using a rat model found that the pathway used to retrieve a fearful memory changed over time, which they suspected may also influence its staying power. After fear-conditioning the rats, it was evident that a circuit from the prefrontal cortex to the amygdala was engaged to retrieve the memory. However, several days later, a different circuit, from the prefrontal cortex to an area of the thalamus called the paraventricular region, was used to retrieve the memory, reported Gregory Quirk, PhD, of the University of Puerto Rico School of Medicine, in San Juan, and colleagues.
The researchers used optogenetics to study the pathways. They suspect that the paraventricular region may integrate fear with other adaptive responses like stress, which can make a fear memory even stronger.
The discovery was backed up by another study published in Nature that also confirmed the shift in memory retrieval circuitry that occurs over time. Bo Li, PhD, and Mario Penzo, PhD, of Cold Spring Harbor Laboratory in New York, concluded that neurons that originated in the paraventricular region regulate fear processing by acting on another class of neurons that store fear memories in the central amygdala.
The researchers traced the activity in the paraventricular region to a messenger chemical called brain-derived neurotrophic factor (BDNF), which has previously been implicated in mood and anxiety disorders such as post traumatic stress disorder (PTSD). In the mouse model, BDNF activated the amygdala neurons where the memory was stored, causing the mice to freeze in fear, suggesting that it may have a role in both fear memory formation and expression of fear.
- NIH News Release: http://www.nih.gov/news/health/jan2015/nimh-19.htm