Why Bad Memories Stick

Stressful Events May Sear Themselves Into the Memory, Lab Tests Show

Medically Reviewed by Louise Chang, MD on October 04, 2007

Oct. 4, 2007 -- Stressful events can create bad memories that are hard to forget, and scientists may have figured out why.

The theory: In stressful situations, the stress hormone norepinephrine may prime the brain to remember what happened in order to avoid the same threat in the future.

Norepinephrine tweaks a certain type of chemical receptor in the brain called GluR1. As a result, the brain lowers its threshold for learning, making it easier to make memories.

The findings come from researchers including Hailan Hu, PhD, and Roberto Manilow, MD, PhD, of New York's Cold Spring Harbor Laboratory. Their report appears in the journal Cell.

Bad Memories in the Making

The scientists studied the formation of bad memories in mice, not in people.

"We expect that the molecular mechanisms are the same" for humans, Manilow says in a news release.

The researchers injected the mice with epinephrine (which can boost norepinephrine in the central nervous system) or salt water (which doesn't affect norepinephrine).

The mice were then briefly placed in a new cage and were allowed to explore the cage.

The next day, the mice revisited the same cage. They got a mild electrical shock as soon as they were placed in the cage.

Finally, the mice made one more trip to that cage a day later. This time, they didn't get shocked. Instead, they were videotaped during their three minutes in the cage.

The researchers watched to see which group of mice stood still longer: those that had gotten the epinephrine or salt water shot on the first day of the experiment.

The epinephrine group stood still longer than the salt water group. The scientists took that as a sign of fear-based learning.

Further tests indicate that the GluR1 receptors were important in that process. Other chemical chain reactions in the brain are probably also involved, the scientists note.