This 2015 German rodent study found:
“Histone modifications predominantly changed during memory acquisition and correlated surprisingly little with changes in gene expression.
Although long-lasting changes were almost exclusive to neurons, learning-related histone modification and DNA methylation changes also occurred in non-neuronal cell types, suggesting a functional role for non-neuronal cells in epigenetic learning.”
Chromatin modifications in two limbic system brain areas were studied – the hippocampus (CA1 region) for short-term memories and the anterior cingulate cortex for short-and long-term memory formation and maintenance. The memories were induced by context (C) and context shock (CS) exposure:
“Overall, the data provides very strong and robust evidence for the establishment of long-term memory upon CS exposure, whereas C exposure alone did not induce the formation of long-term memory.”
So, without long-term shock/emotional memories, there would be no positive long-term findings for the researchers to report. There would be no lasting:
- “Histone modifications
- DNA methylation changes
- Changes in gene expression”
The subjects were young adults at age 3 months. The CA1 and ACC studied brain areas are fully developed before this age.
It seemed feasible that if the study were performed with younger subjects, the results may have been different. For example:
“Context exposure alone did not induce the formation of long-term memory”
may not have been the finding for early learning situations.
The researchers qualified their results several times with the phrase “changes are limited to actively expressed genes.” A similar qualifier in A study of DNA methylation and age was a reminder that unexpressed genes may have also been important:
“The textbook case of DNA methylation regulating gene expression (the methylation of a promoter and silencing of a gene) remains undetected in many cases because in an array analysis, an unexpressed gene shows no signal that can be distinguished from background and is therefore typically omitted from the analysis.”
This general qualifier may not have necessarily applied to the current study, though, because the study’s design included an unexposed control group.
http://www.nature.com/neuro/journal/vaop/ncurrent/full/nn.4194.html “DNA methylation changes in plasticity genes accompany the formation and maintenance of memory”