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Study shows how the brain remembers pleasure, reward
Key details of the way nerve cells in the brain remember pleasure are revealed in a study by UAB researchers published in the journal, Nature Neuroscience. The molecular events that form such “reward memories” appear to differ from those created by drug addiction.
Brain circuits have evolved to encourage behaviors proven to help our species survive by attaching pleasure to them. Eating rich food tastes good because it delivers energy and sex is desirable because it creates offspring.
This study in rats supports the idea that the mammalian brain features several memory types, each using different circuits, with memories accessed and integrated as needed. Ancient memory types include those that remind us what to fear, what to seek out (reward), how to move (motor memory) and navigate (place memory). More recent developments enable us to remember the year Columbus sailed and our wedding day.
“We believe reward memory may serve as a good model for understanding the molecular mechanisms behind many types of learning and memory,” said David Sweatt, Ph.D., chair of the UAB Department of Neurobiology, director of the Evelyn F. McKnight Brain Institute at UAB and corresponding author for the study. “Our results provide a leap in the field’s understanding of reward-learning mechanisms and promise to guide future attempts to solve related problems such as addiction and criminal behavior.”
The study is the first to illustrate that reward memories are created by chemical changes that influence known memory-related genes in nerve cells within a brain region called the ventral tegmental area, or VTA. Experiments that blocked those chemical changes — a mix of DNA methylation and demethylation — in the VTA prevented rats from forming new reward memories.
Methylation is the attachment of a methyl group (one carbon and three hydrogens) to a DNA chain at certain spots (cytosine bases). When methylation occurs near a gene or inside a gene sequence, it generally is thought to turn the gene off and its removal is thought to turn the gene on. This back-and-forth change affects gene expression without changing the code we inherit from our parents. Operating outside the genetic machinery proper, epigenetic changes enable each cell type to do its unique job and to react to its environment.
Furthermore, a stem cell in the womb that becomes bone or liver cells must “remember” its specialized nature and pass that identity to its descendants as they divide and multiply to form organs. This process requires genetic memory, which largely is driven by methylation. Note, most nerve cells do not divide and multiply as do other cells. They can’t, according to one theory, because they put their epigenetic mechanisms to work making actual memories.