Blocking a molecule in the brain may "cure" post-traumatic stress disorder, according to US researchers.
Patients with PTSD can be offered counselling
They showed that inhibiting a specific enzyme removed fear in mice and report to journal Nature Neuroscience that the finding may lead to new treatments.
Around a third of people may suffer PTSD after an exceptionally traumatic event, such as a terrorist attack or a natural disaster.
Experts said it was early days but the findings were worth exploring further.
There is currently no treatment for PTSD although antidepressants and sleeping pills can help with the symptoms, which include flashbacks, anger, anxiety and depression.
Professor Li-Huei Tsai and colleagues in the Brain and Cognitive Sciences Department at MIT looked at the effects of an enzyme called Cdk5 in the brains of genetically engineered mice which had been given mild foot shocks.
When re-exposed to the same environment but without the shocks, mice in whom the researchers had increased levels of Cdk5 activity had difficulty letting go - or extinguishing - the memory of the foot shock and continued to freeze in fear.
But in mice whose Cdk5 activity was blocked, the bad memory of the shocks disappeared when the mice learned that they no longer needed to fear the environment where the foot shocks had occurred.
The enzyme activity was modified in the hippocampus - the brain's centre for storing memories.
Emotional disorders such as post-traumatic stress and panic attacks stem from the inability of the brain to stop experiencing the fear associated with a specific incident or series of incidents.
A study conducted by the US Army in 2004 found that one in eight soldiers returning from Iraq reported symptoms of PTSD.
The National Institute of Clinical and Health Excellence estimate five in 100 men and 10 in 100 women in the UK will get PTSD in their lifetime.
In guidance published in 2005 NICE said the condition was under-recognised in the NHS and better screening and treatment was needed.
Professor Tsai said: "This data points to a promising therapeutic avenue to treat emotional disorders and raises hope for patients suffering from post-traumatic stress disorder or phobia."
Dr Jonathan Bisson, senior lecturer in psychiatry at the University of Cardiff and co-chair of the NICE guideline group said the finding was "potentially a significant advance".
He added: "Translation of them into an effective treatment for PTSD is a long way off, and may not be possible.
"But the results are consistent with current theories on the development and maintenance of PTSD symptoms and it is an area very worthy of further investigation."