The study shows that emotionally arousing events activate the brain's amygdala, the almond-shaped portion of the brain involved in emotional learning and memory, which then increases a protein called "Arc" in the neurons in the hippocampus, a part of the brain involved in processing and enabling the storage of lasting memories. The researchers believe that Arc helps store these memories by strengthening the synapses, the connections between neurons.
The study will appear in today's issue of the Proceedings of the National Academy of Sciences.
"Emotionally neutral events generally are not stored as long-term memories," said Christa McIntyre, the first author of the paper and a postdoctoral researcher in the Department of Neurobiology and Behavior in UCI's School of Biological Sciences, working with James L. McGaugh, research professor and a fellow at the Center for the Neurobiology of Learning and Memory. "On the other hand, emotionally arousing events, such as those of September 11, tend to be well-remembered after a single experience because they activate the amygdala."
In their experiments, the researchers placed a group of rats in a well-lit compartment with access to an adjacent dark compartment. Because rats are nocturnal and prefer dark environments, they tended to enter the dark compartment. Upon doing so, however, they were each given a mild foot-shock – an emotional experience that, by itself, was not strong enough to become a long-lasting memory. Some of the rats then had their amygdala chemically stimulated in order to determine what role it played in forming a memory of the experience.
When they placed the rats that received both the mild foot-shock and the amygdala stimulation back in the well-lit compartment, the researchers found the rats tended to remain there, demonstrating a memory for the foot shock they had received in the dark compartment. These rats, the researchers found, also showed an increase in the amount of the Arc protein in the hippocampus. On the other hand, rats that received only the mild foot-shock and no amygdala stimulation showed no increase in Arc protein. When placed in the well-lit compartment, they tended to enter the dark compartment, suggesting they didn't remember the foot shock.
"In a separate experiment, we chemically inactivated the amygdala in rats very soon after they received a strong foot-shock," McIntyre said. "We found the increase in Arc was reduced and these rats showed poor memory for the foot shock despite its high intensity. This also shows that the amygdala is involved in forming a long-term memory."
The brain is extremely dynamic, McIntyre explained, with some genes in the brain, called "immediate early genes," changing after every experience. "We know the level of the immediate early gene that makes the Arc protein increases in the brain, simply in response to an exposure to a new environment," she said. "Our findings show that this gene makes more Arc protein in the hippocampus only if the experience is emotionally arousing or important enough to activate the amygdala and to be remembered days later."
The researchers were surprised to find no change in the gene that produced the Arc protein when the rat's amygdala was stimulated. "We weren't expecting the gene to be uncoupled from the Arc protein," McIntyre said. "We thought an activation of the amygdala would create more gene activation in the hippocampus. But we saw the same amount of the gene in the rats, regardless of the amygdala treatment. It was the Arc protein, created by the gene, that was different. This gives us new insight into the way lasting memories are stored."
The research was supported by several grants from the National Institutes of Health. In addition to McIntyre and McGaugh, co-authors of the study include Oswald Steward, UCI; Teiko Miyashita, Kristopher D. Marjon and John F. Guzowski, the University of New Mexico Health Science Center; and Barry Setlow, Texas A&M University.
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Proceedings of the National Academy of Sciences