if they have previously experienced chronic stress. Here is what they had to say:
The researchers found that animals who underwent chronic stress prior to a traumatic experience engaged a distinctive brain pathway that encodes traumatic memories more strongly than in unstressed animals.
About 8 million Americans suffer from nightmares and flashbacks to a traumatic event. This condition, known as post-traumatic stress disorder (PTSD), is particularly common among soldiers who have been in combat, though it can also be triggered by physical attack or natural disaster.
Blocking this type of memory formation may offer a new way to prevent PTSD, says Ki Goosens, the senior author of the study, which appears in the journal Biological Psychiatry.
"The idea is not to make people amnesic but to reduce the impact of the trauma in the brain by making the traumatic memory more like a 'normal,' unintrusive memory," says Goosens, an assistant professor of neuroscience and investigator in MIT's McGovern Institute for Brain Research. The paper's lead author is former MIT postdoc Michael Baratta.
Goosens' lab has sought for several years to find out why chronic stress is so strongly linked with PTSD. "It's a very potent risk factor, so it must have a profound change on the underlying biology of the brain," she says.
To investigate this, the researchers focused on the amygdala, an almond-sized brain structure whose functions include encoding fearful memories. They found that in animals that developed PTSD symptoms following chronic stress and a traumatic event, serotonin promotes the process of memory consolidation. When the researchers blocked amygdala cells' interactions with serotonin after trauma, the stressed animals did not develop PTSD symptoms. Blocking serotonin in unstressed animals after trauma had no effect.
"That was really surprising to us," Baratta says. "It seems like stress is enabling a serotonergic memory consolidation process that is not present in an unstressed animal."
For example, "flashbulb" memories, formed in response to a highly emotional experience, are usually much more vivid and easier to recall than typical memories.
Goosens and colleagues further discovered that chronic stress causes cells in the amygdala to express many more 5-HT2C receptors, which bind to serotonin. Then, when a traumatic experience occurs, this heightened sensitivity to serotonin causes the memory to be encoded more strongly, which Goosens believes contributes to the strong flashbacks that often occur in patients with PTSD.
"It's strengthening the consolidation process so the memory that's generated from a traumatic or fearful event is stronger than it would be if you don't have this serotonergic consolidation engaged," Baratta says.
This memory consolidation process can take hours to days to complete, but once a memory is consolidated, it is very difficult to erase. However, the findings suggest that it may be possible to either prevent traumatic memories from forming so strongly in the first place, or to weaken them after consolidation, using drugs that interfere with serotonin.
"The consolidation process gives us a window in which we can possibly intervene and prevent the development of PTSD. If you give a drug or intervention that can block fear memory consolidation, that's a great way to think about treating PTSD," Goosens says. "Such an intervention won't cause people to forget the experience of the trauma, but they might not have the intrusive memory that is ultimately going to cause them to have nightmares or be afraid of things that are similar to the traumatic experience."
"The consolidation of traumatic memories requires this serotonergic cascade and we want to block it, not enhance it," she adds. "This study suggests we should rethink the use of SSRIs in PTSD and also be very careful about how they are used, particularly when somebody is recently traumatized and their memories are still being consolidated, or when a patient is undergoing cognitive behavior therapy where they're recalling the memory of the trauma and the memory is going through the process of reconsolidation."