Irvine, Calif., July 26, 2005 --A research team led by UC Irvineneuroscientists has identified how the brain processes and storesemotional experiences as long-term memories. The research, performed onrats, could help neuroscientists better understand why emotionallyarousing events are remembered over longer periods than emotionallyneutral events, and may ultimately find application in treatments forconditions such as post-traumatic stress disorder.
The study shows that emotionally arousing events activate thebrain's amygdala, the almond-shaped portion of the brain involved inemotional learning and memory, which then increases a protein called"Arc" in the neurons in the hippocampus, a part of the brain involvedin processing and enabling the storage of lasting memories. Theresearchers believe that Arc helps store these memories bystrengthening 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 aslong-term memories," said Christa McIntyre, the first author of thepaper and a postdoctoral researcher in the Department of Neurobiologyand Behavior in UCI's School of Biological Sciences, working with JamesL. McGaugh, research professor and a fellow at the Center for theNeurobiology of Learning and Memory. "On the other hand, emotionallyarousing events, such as those of September 11, tend to bewell-remembered after a single experience because they activate theamygdala."
In their experiments, the researchers placed a group of rats ina well-lit compartment with access to an adjacent dark compartment.Because rats are nocturnal and prefer dark environments, they tended toenter the dark compartment. Upon doing so, however, they were eachgiven a mild foot-shock -- an emotional experience that, by itself, wasnot strong enough to become a long-lasting memory. Some of the ratsthen had their amygdala chemically stimulated in order to determinewhat role it played in forming a memory of the experience.
When they placed the rats that received both the mildfoot-shock and the amygdala stimulation back in the well-litcompartment, the researchers found the rats tended to remain there,demonstrating a memory for the foot shock they had received in the darkcompartment. These rats, the researchers found, also showed an increasein the amount of the Arc protein in the hippocampus. On the other hand,rats that received only the mild foot-shock and no amygdala stimulationshowed no increase in Arc protein. When placed in the well-litcompartment, they tended to enter the dark compartment, suggesting theydidn't remember the foot shock.
"In a separate experiment, we chemically inactivated theamygdala in rats very soon after they received a strong foot-shock,"McIntyre said. "We found the increase in Arc was reduced and these ratsshowed poor memory for the foot shock despite its high intensity. Thisalso shows that the amygdala is involved in forming a long-termmemory."
The brain is extremely dynamic, McIntyre explained, with somegenes in the brain, called "immediate early genes," changing afterevery experience. "We know the level of the immediate early gene thatmakes the Arc protein increases in the brain, simply in response to anexposure to a new environment," she said. "Our findings show that thisgene makes more Arc protein in the hippocampus only if the experienceis emotionally arousing or important enough to activate the amygdalaand to be remembered days later."
The researchers were surprised to find no change in the genethat 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 createmore gene activation in the hippocampus. But we saw the same amount ofthe gene in the rats, regardless of the amygdala treatment. It was theArc protein, created by the gene, that was different. This gives us newinsight into the way lasting memories are stored."
The research was supported by several grants from the NationalInstitutes of Health. In addition to McIntyre and McGaugh, co-authorsof the study include Oswald Steward, UCI; Teiko Miyashita, KristopherD. Marjon and John F. Guzowski, the University of New Mexico HealthScience Center; and Barry Setlow, Texas A&M University.
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