Not all of our daily experiences remain as long-term memories. It has been found that a group of neurons called "engram cells" retain the physical or chemical traces of all experiences as transient memories. However, the mechanism by which selective memories are re-consolidated upon multiple recalls for long-term storage has not been fully elucidated.
A research group led by Team Director Jun Nagai at the RIKEN Center for Brain Science, has developed a technology to visualize and analyze the expression of a gene called Fos specifically in astrocytes (a type of glia in the brain whose morphological and mechanistic properties differ from neurons). They examined Fos expression in mice when they received an electrostatic shock in a specific location (initial experience) and when they recalled the experience in that location several days later (repeated experience). The results showed that populations of astrocytes expressing Fos were found in large numbers only during repeated experience in regions where engram cells existed. Furthermore, it was found that after the initial experience, astrocytes strengthened their responsiveness to the neurotransmitter "noradrenaline" over several hours to days, and expressed Fos during re-experience. Disrupting Fos-expressing astrocyte signaling affected engram activity, as well as the stability and precision of memory, revealing that the "switch" of Fos expression has a function to re-consolidate memories of experiences that are repeated with strong emotions.
The link between emotion and memory is implicated in many mental disorders such as depression and post-traumatic stress disorder (PTSD). These research findings may be applicable to treatments that soften memories or selectively preserve them.
(Article: Masanori Nakajo)
