Neurobiologists have discovered a mechanism by which the constantly changing brain retains memoriesfrom that dog bite to that first kiss. They have found that the brain co-opts the same machinery by which cells stably alter their genes to specialize during embryonic development.
Courtney Miller and David Sweatt reported their findings in the March 15, 2007 issue of the journal Neuron, published by Cell Press.
Their studies aimed at exploring whether a process called DNA methylation plays a role in forming memories. In this process, molecules called methyl groups are attached to genes, which switches them off. Conversely, lack of methyl groups enables the genes to remain activated.
Cells use methylation during embryonic development to selectively deactivate genes to enable the cells to specialize into different types as the embryo develops. Such regulation is dubbed "epigenetic," since it constitutes a layer of genetic control beyond the regulation inherent in the structure of genes themselves.
Methylation causes a permanent change in gene activity during development. So, while previous studies by Sweatt and others had hinted that the methylation mechanism remains active in adult brains, researchers had generally believed that methylation would not constitute a mechanism for long-term establishment of memories. However, as misregulation of DNA methylation occurs in some brain disorders like schizophrenia and forms of mental retardation, Miller and Sweatt designed experiments to test whether methylation specifically regulates memory formation.
In their experiments, the researchers conditioned fearful memories in rats by giving the animals mild shocks when they were in a specific training chamber. The researchers could then test whether the rats remembered the conditioning by observing whether they froze when placed in the chamber.