The new findings build on decades of animal research that shows that the brain stores different types of memories in different areas. A brain region called the amygdala, often dubbed the brain's fear center, plays a central role in the storage of emotional memories. Research in animals suggests that propranolol, which blocks a certain molecule in the amygdala, interferes with reconsolidation by preventing the synthesis of proteins needed to store the memory.

Previous research has shown that propranolol can help PTSD patients. But the new study takes the work a step further, by comparing propranolol treatment with exposure therapy, commonly used with PTSD. With this treatment, patients repeatedly recall their traumatic memory in a safe environment, eventually learning to disassociate the memory from fear. "Exposure therapy is the most effective treatment for anxiety disorders," says Kindt. "But it's not successful for everyone, and patients relapse 20 to 60 percent of the time."

The researchers found that after exposure therapy (also called extinction), the fearful response to the spider could be rapidly brought back. But the same was not true for those treated with propranolol, suggesting that the memory was truly weakened or erased. "It shows that blocking reconsolidation is really different than extinction, which has been a matter of controversy," says Alain Brunet, a psychologist at McGill University, in Montreal, who has tested propranolol in PTSD patients but was not involved in the current study. Previous research suggests that exposure produces a new form of learning, rather than degrading the fearful memory. The fact that the original memory remains intact may explain the high relapse rates with this treatment, says Kindt.

Kindt's team has already tested whether the propranolol effect lasts longer than three days--a key requirement for therapeutic use--but she declined to give the results because they have been submitted for publication. Other scientists are testing additional drugs in animals to search for potentially more potent compounds. "Whether it's clinically going to be useful on its own, or perhaps in a cocktail of other agents that also dampen emotional state, only time will tell," says Todd Sacktor,a neurologist and scientist at State University of New York Downstate Medical Center, in Brooklyn.

While Kindt's study and others are promising, larger tests are required to determine how useful propranolol treatment will be, as well as the most beneficial conditions for delivery. Because the drug is widely available for other purposes, some PTSD patients have reported trying it on their own, with little success. It may be that the drug must be delivered under very controlled circumstances in order to work.