Scientists have shown how it may be possible to use the body's own natural stress hormone to soothe the agonies of post-traumatic stress disorder.
Trauma can have a long-term effect
A team at the University of Texas discovered mice were less agitated by the memory of an electric shock after being given shots of corticosterone.
The researchers believe the hormone works by creating new memories to compete with those causing anxiety.
Tests are now underway to see if similar hormone shots can help humans.
The research appears online in the Journal of Neuroscience.
Days after experiencing a traumatic event - a mild electrical shock - mice in the study still showed a fearful response when re-exposed to the place where it happened, a condition that could be a model for post-traumatic stress disorder in humans.
But mice receiving the corticosterone shot at the time they "relived" the event experienced a significant drop in that fear.
Lead researcher Dr Craig Powell said: "Corticosterone appears to enhance new memories that compete with the fearful memory, thereby decreasing its negative emotional significance."
His colleague, Dr Jacqueline Blundell, said: "The natural release of stress hormones during recall of a fearful memory may be a natural mechanism to decrease the negative emotional aspects of the memory.
"Conversely, patients with post-traumatic stress disorder have blunted stress hormone responses and thus may not decrease fearful memories normally over time."
In the study, mice were placed in a plastic box and given an electrical shock to the feet.
The mice were returned to the box two days later, and their fear - gauged by how long they "froze" in place - remained high.
Shortly after being placed in the box again, the mice were injected with the stress hormone corticosterone.
A day after the injection, the mice showed significantly less fear when they were returned to the box.
But in order for the effect to work, the corticosterone had to be given after the mice were returned to the site of the initial trauma, causing the memory to be re-activated.
Giving it beforehand, or giving it without placing the mice in the box, had no effect when tested a day later.
However, when the injections were given over four days, the timing became less important. Giving the hormone either before or after secondary exposure to the box reduced fear.
The Texas team is now working with veterans suffering from post-traumatic stress disorder to see if hormone shots can help them.
Dr Jonathan Bisson, an expert in psychiatry at University Hospital of Wales, said: "This research sounds to add to existing work that suggests cortisol is involved in the development and maintenance of post-traumatic stress disorder and that corticosteroids have potential as a treatment."
Cortisol and corticosterone are both stress hormones. Cortisol is more abundant in humans, corticosterone in other animals, such as rodents.