Researchers have uncovered new evidence in mice that may explain how emotionally charged situations can leave such a powerful mark on our memories.
Surges of the stress hormone norepinephrine (also known as noradrenaline) that often accompany strong emotions spark a series of molecular events that ultimately strengthen the connections between neurons, the team reports in the October 5, 2007, issue of the journal Cell, a publication of Cell Press.
“This phenomenon is something everyone can identify with,” said Roberto Malinow of the Cold Spring Harbor Laboratory in New York. “You can probably remember where you were when you heard about 9/11, but you probably don't know where you were on 9/10. We've identified one mechanism that may underlie this effect.”
The parts of the brain where memories are stored need to distinguish between significant experiences and those that carry less importance, giving priority to the transformation of the former into long-term memory, the researchers explained. One factor that scientists believe to be critical in that process is the emotional load of an event. Indeed, studies have shown that heightened states of emotion can facilitate learning and memory. In some situations, this process can even become pathological, Malinow said, as occurs in posttraumatic stress disorder (PTSD), a condition characterized by persistent vivid memories of traumatic events.
The stress hormone norepinephrine was known to play a central role in the emotional control of memory through its effect on receptors in the brain. During emotional arousal, the stress hormone is released by neurons that project widely to many brain regions, including the hippocampus and the amygdala, which are involved in the formation of emotional memory.
Brain stimulation by norepinephrine had also been found to induce a phenomenon known as long-term potentiation (LTP). LTP involves a lasting increase in the strength of nerve connections, or synapses. That process is considered to be the cellular basis for learning and memory.
“There were all these potential ways in which excitability or transmission might be enhanced by norepinephrine,” said Manilow. Yet, exactly how the stress hormone influences the processes involved in memory formation remained mysterious.
One way to strengthen synapses is to increase the number of so-called GluR1 receptors at neurons' receiving ends, he added. Malinow's group now shows that norepinephrine can do just that.