Symptoms of post-traumatic stress disorder (PTSD) can develop even without memory of the trauma, according to a recent study.

Researchers from SUNY Albany and the University of California Los Angeles found that explicit memory may not be an absolute requirement for PTSD, whereas other forms of learning, such as fear conditioning, may be required. Researchers said there are case reports of people who have experienced terrible life events that resulted in brain damage, some of whom developed syndromes similar to PTSD even though they had no recollection of the event itself.

"These data highlight the importance of the many ways in which the brain processes traumatic experiences. Psychotherapy tends to focus heavily on the articulation of trauma memories. However, the current study highlights that these explicit memories may not represent all brain processes that drive distress and disability," Dr. John Krystal Editor of Biological Psychiatry said.

For the study, researchers exposed juvenile rodents to a single session of unpredictable stress. Later, as adults, they tested the animals for their memory of the event and also measured their fear response.

"We found that our 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," researcher Andrew Poulos said in a statement," These heightened levels of fear and anxiety corresponded with drastic changes in the daily rhythms of the circulating hormone corticosterone."

Corticosterone is a hormone that, in part, regulates the body's stress response. Interestingly, within the amygdala, a brain region crucial for the learning of fear, levels of a receptor for corticosterone were also increased.

"Future experiments in our laboratory will allow us to determine if this increase in glucocorticoid receptors within the amygdala and/or aberrant hormone levels sets up the organism for increased fear and anxiety," Poulos said.

Based on the findings, not remembering a traumatic event does not preclude "an organism from experiencing some of the negative consequences of trauma, such as anxiety and heightened fear."