Inhibiting an enzyme in the brain that controls the formation of fears may lead to the first drug for post-traumatic stress disorder, say U.S. scientists.

Researchers at the Massachusetts Institute of Technology say their work could lead to a treatment for the millions of adults who suffer each year from persistent, debilitating fears — including hundreds of soldiers posted in Iraq and Afghanistan.

In a study published in the July 15 issue of Nature Neuroscience, professor Li-Huei Tsai and colleagues at the Brain and Cognitive Sciences Department of MIT showed that blocking an enzyme called Cdk5 helped mice get over fear learned in a particular context.

Conversely, the learned fear persisted when the enzyme's activity was increased in the hippocampus, the brain's centre for storing memories.

The mice received mild foot shocks in a certain environment and were re-exposed to the same environment without the foot shock. Mice with increased levels of Cdk5 activity had more trouble "letting go" of the memory of the foot shock and continued to freeze in fear.

On the other hand, in mice whose Cdk5 activity was inhibited, the bad memory of the shocks seemed to disappear when the mice learned they no longer needed to fear the environment where the foot shocks had once occurred.

"Remarkably, inhibiting Cdk5 facilitated extinction of learned fear in mice," Tsai said in a release. "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."

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.

Current treatment for PTSD symptoms such as flashbacks and depression include antidepressants and sleeping pills.