Chances in the Brain Cells, From Epigenetic To the Future
Epigenetics refers to external modifications to DNA that turn genes "on" or "off." These modifications do not change the DNA sequence, but instead, they affect how cells "read" genes.
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Examples of epigenetics
Epigenetic changes alter the physical structure of DNA. One example of an epigenetic change is DNA methylation — the addition of a methyl group, or a "chemical cap," to part of the DNA molecule, which prevents certain genes from being expressed.
Another example is histone modification. Histones are proteins that DNA wraps around. (Without histones, DNA would be too long to fit inside cells.) If histones squeeze DNA tightly, the DNA cannot be "read" by the cell. Modifications that relax the histones can make the DNA accessible to proteins that "read" genes.
Depression
Epigenetic inheritance of depression-related phenotypes has also been reported in a preclinical study. Inheritance of paternal stress-induced traits across generations involved small non-coding RNA signals transmitted via the paternal germline.
Fear Conditioning
Rats subjected to one instance of contextual fear conditioning create an especially strong long-term memory. At 24 hours after training, 9.17% of the genes in the genomes of rat hippocampus neurons were found to be differentially methylated. This included more than 2,000 differentially methylated genes at 24 hours after training, with over 500 genes being demethylated. Where genes were hypermethylated, 87% of these genes were shown to be down-regulated in gene expression at the messenger RNA level. Similar results to that in the rat hippocampus were also obtained in mice with contextual fear conditioning.
The hippocampus region of the brain is where contextual fear memories are first stored (see figure of the brain, this section), but this storage is transient and does not remain in the hippocampus. In rats, contextual fear conditioning is abolished when the hippocampus is subjected to hippocampectomy just one day after conditioning, but rats retain a considerable amount of contextual fear when hippocampectomy is delayed by four weeks.
In mice, examined at 4 weeks after conditioning, the hippocampus methylations and demethylations were reversed (the hippocampus is needed to form memories but memories are not stored there) while substantial differential CpG methylation and demethylation occurred in cortical neurons during memory maintenance. There were 1,223 differentially methylated genes in the anterior cingulate cortex of mice four weeks after contextual fear conditioning.
Transgenerational fear conditioning
Studies on mice have shown that certain conditional fears can be inherited from either parent. In one example, mice were conditioned to fear a strong scent, acetophenone, by accompanying the smell with an electric shock. Consequently, the mice learned to fear the scent of acetophenone alone. It was discovered that this fear could be passed down to the mice offspring.
Despite the offspring never experiencing the electric shock themselves the mice still displayed a fear of the acetophenone scent, because they inherited the fear epigenetically by site-specific DNA methylation. These epigenetic changes lasted up to two generations without reintroducing the shock.
source: wiki & livescience