Scientists have been looking for a blood test or other biological marker for post-traumatic stress disorder, which can be difficult to diagnose.
Experimenting in rats, they may have found one.
In a study published Monday in the Proceedings of the National Academy of Science, researchers stressed out dozens of laboratory rats by exposing them to used cat litter. Even though the rats had never been exposed to a cat, they recognized the scent of cat urine as a threat.
Seven days later, the researchers tested the rats for symptoms of PTSD. Animals were deemed to be affected if they cowered in a closed section of a maze -- counter to their instinct to explore -- and failed to habituate to a loud clapping-like noise, continuing to jump even after hearing it 30 times.
The researchers then analyzed gene expression in blood and two parts of the brain. The rats with what were seen as PTSD symptoms had a distinct pattern of glucocorticoid receptor signaling.
Glucocorticoids are a class of hormones. The most important is cortisol, which is released in response to stress and plays a crucial role in metabolism and the immune system.
In a second phase of the experiment, rats were injected with corticosterone -- a hormone that activates the glucocorticoid receptors -- an hour after they were placed in a cage with the cat litter.
Compared with rats that did not get the injections, they showed far fewer PTSD symptoms when they were tested a week later.
The lead researcher, Dr. Nikolaos Daskalakis, a neuroendocrinologist at
In the aftermath of trauma, some people are more vulnerable than others to develop PTSD. Researchers are still trying to figure out why.