The study, which will be published Aug. 15 in the journal Biological Psychiatry, found that among the many forms of memory, only some may be critical for the development of post-traumatic stress disorder. The research suggests that explicit memory — which can be voluntarily recalled from prior experience and articulated — may not be a requirement for PTSD, but that other, more primitive forms of learning may be required.
At least six previous reports have found that some people who have experienced terrible life events that resulted in brain damage developed syndromes similar to PTSD even though they had no recollection of the events themselves.
The UCLA study was designed to answer a basic question: If traumatic early-life memories are lost, what persists of the experiences? The research team was led by Andrew Poulos, who was a UCLA postdoctoral scholar when the research was conducted. Poulos is currently on the faculty at the University at Albany.
In the laboratory, the researchers exposed rats that were 17 days old — the equivalent of just younger than 2 human years — to a single session of unpredictable stress (electric shocks to the feet that produced mild discomfort). At 80 days — roughly equal to young-adult age in humans — the scientists tested the animals for their memory of the event and measured their fear response.
“We found that the rodents, which failed to remember the environment in which they were traumatized, showed a persistent increase in anxiety-related behavior and increased learning of new fear situations,” Poulos said. “These heightened levels of fear and anxiety corresponded with drastic changes in the daily rhythms of the circulating hormone corticosterone.”
Rats tend to stand still when they experience fear. When they recall a frightening memory, they freeze; and the stronger the memory, the more they freeze.
“We saw no freezing in the rats when they were placed in the traumatic context again,” said Michael Fanselow, a UCLA professor of psychology and the study’s senior author. “If these memories are formed in adulthood, they never forget them.”
The rats were placed in an elevated wooden maze made up of two long rectangles that cross in the middle, a few feet off the floor. One rectangle has side walls; the other does not. Normally, rats explore both arms, but when they were anxious, they stayed in the arm protected by side walls, Fanselow said.
The rats also showed a disturbed rhythm for corticosterone, the body’s main stress hormone, as well as an increase in the number of receptors for corticosterone (the equivalent of cortisol in humans). In the amygdala, a region of the brain involved in learning fear, levels of a receptor for corticosterone were increased.
“We saw specific, long-lasting, probably permanent, changes in the amygdala,” Fanselow said.
The findings indicate that not remembering a traumatic event does not preclude a person or animal from experiencing some of the negative consequences of trauma, such as anxiety and heightened fear.
Because post-traumatic stress disorder affects the brain in multiple other ways, a successful treatment for PTSD will need to target these changes, he said. Because these changes are not part of a memory, “they are more difficult to detect, but still cause a major problem in those suffering from PTSD.”
Other study authors were Maxine Reger and Sarah Sterlace, who were UCLA graduate students when the research was conducted; Nehali Mehta, Irina Zhuravka and Camille Gannam, who were undergraduates at the time; and faculty members David Hovda and Dr. Christopher Giza.
- Stuart Wolpert