When you experience an emotionally-charged event, it's something that is etched into your memory, and now scientists think they know why. In experiments with mice, researchers found that powerful surges of hormone norepinephrine -- surges that occur during emotional episodes -- cause a series of events that strengthen the connections between neurons, sealing these events into memory. "The question is why is it that you can remember some trivial events that occur at a time when there is high emotional arousal," said lead researcher Dr. Roberto Malinow of the Cold Spring Harbour Laboratory, New York. "For example, everyone remembers where they were when they heard about September 11, even though they may have been in some trivial place."

The researchers found that norepinephrine can modify brain cell-receptors, making them easier to go into synapses -- the tiny spaces between brain cells -- making it easier to learn and form memories, Malinow said. In studies with mice, Malinow's group found that norepinephrine, coupled with emotional stress, plays an important role in lowering the threshold for certain brain cell-receptors called GluR1. This, in turn, causes a boost in memory.

When the researchers put lab mice through behavioural tests, they found that exposure to norepinephrine made normal mice remember events more clearly. But, mice with mutations of the GluR1 receptors that were exposed to norepinephrine did not show improved memory. Norepinephrine is known to play a part in the emotional control of memory. During emotional stress, norepinephrine is released by neurons (brain cells) in many areas of the brain involved with forming emotional memories. Malinow thinks this finding could lead to new treatments for conditions like post-traumatic stress disorder. One expert sees this study as an extension of similar work, which showed that norepinephrine is involved in the memory of fearful events that can lead to post-traumatic stress disorder.