UCLA life scientists have now found a way to reduce traumatic memories.
Veterans of war, rape victims and other people who have seen horrific crimes that haunt them may someday have such memories weakened in their brains.
"I think we will be able to alter memories someday to reduce the trauma from our brains," the study's senior author, David Glanzman, a UCLA professor of integrative biology and physiology and of neurobiology, said.
They discovered that the long-term memory for sensitisation in the marine snail can be erased by inhibiting the activity of a specific protein kinase - a class of molecules that modifies proteins by chemically adding to them a phosphate (an inorganic chemical), which changes the proteins' structure and activity.
The protein kinase is called PKM (protein kinase M), a member of the class known as protein kinase C (PKC), which is associated with memory.
The research has important potential implications for the treatment of post-traumatic stress disorder, as well as drug addiction, in which memory plays an important role, and perhaps Alzheimer's disease and other long-term memory disorders.
"Almost all the processes that are involved in memory in the snail also have been shown to be involved in memory in the brains of mammals," Glanzman, who added that the human brain is far too complicated to study directly, said.
PKM is rare in that while most protein kinases have both a catalytic domain, which is the part of the molecule that does its work, and a regulatory domain, akin to an on-off switch that can be used by other signalling pathways to shut off the activity of the kinase, PKM has only the catalytic domain - not the regulatory domain.
"This means that once PKM is formed, there is no way to shut it off," Glanzman, who is a member of UCLA's Brain Research Institute, said.
"Once it is activated, PKM's continual activity maintains a memory until PKM degrades," he explained.
Glanzman and his colleagues, researchers Diancai Cai, lead author of the study, Kaycey Pearce, and Shanping Chen, all of whom work in his laboratory, studied a simple kind of memory called sensitisation.
Glanzman said if a predator attacks marine snails, the attack heightens their sensitivity to environmental stimuli, a "fundamental form of learning that is necessary for survival and is very robust in the marine snail".
"The advantage of Aplysia is that we know the neurons that produce this reflex; we know where they are in the nervous system," he said.
The scientists removed the key neurons from the snail's nervous system and put them in a Petri dish, thereby recreating in the dish the two-neuron "circuit", a sensory neuron and a motor neuron, that produces the reflex.
"The point is to reduce the problem so we can study on a fundamental biological level how PKM is maintaining long-term memory," Glanzman said.
They succeeded in erasing a long-term memory, both in the snail itself and in the circuit in the dish. They are the first scientists to show that long-term memory can be erased at a connection between just two neurons.
"We found that if we inhibit PKM in the marine snail, we will erase the memory for long-term sensitisation," Glanzman said.
"In addition, we can erase the long-term change at a single synapse that underlies long-term memory in the snail," he stated.
The study appears in the April 27 issue of the Journal of Neuroscience, a premier neuroscience journal.