Experts estimate that up to 20 percent of U.S. troops returning from Iraq and Afghanistan suffer from post-traumatic stress disorder, a condition that can be stubbornly difficult to treat.

But what if PTSD could have been prevented in the first place?

Scientists have done something similar in traumatized mice. Days after a harrowing experience being restrained on wooden boards, they were given a drug that triggers a brain receptor thought to be involved in how mice — and people — respond to fear.

“We prevented PTSD-like symptoms,” said Emory University neuroscientist Raul Andero Gali, lead author of a study published last week in the journal Science Translational Medicine.

The study raises the possibility that similar drugs could one day be given to people after combat, car accidents or other types of trauma. The prospects for developing such a treatment are very good, said Sheena Josselyn, a neuroscientist at the University of Toronto who was not involved in the research.

Andero’s team set out to find genes that could help explain why some trauma victims are more vulnerable to PTSD, an anxiety disorder in which they continue to feel stressed or frightened even when they are no longer in danger.

The scientists taped mice to the wooden boards for two hours each — a technique known to cause PTSD symptoms, including learning and memory problems and anxiety.

The researchers later killed the animals, along with a control group that had not been subjected to the trauma.

That was followed by the removal of amygdala tissue, a region of the brain that plays a key role in the formation of emotional memories. Using a sophisticated screening method, the researchers combed through tens of thousands of genes and looked for differences in how the two groups of mice expressed those genes.

One stood out. Known as OPRL1, it contains instructions for making a receptor for a brain chemical called nociceptin.

The scientists traumatized more mice and then trained them to expect a mild electric shock after hearing a 30-second tone. Some of the mice received injections of the drug.

The next day, researchers repeatedly played them the tone but did not give them the shock.

Mice that had received the drug quickly learned not to fear the tone. The untreated mice, however, continued to freeze in place and act in other ways that indicated they were still traumatized.