How traumatic memories hide in the brain, and how to retrieve them: Special brain mechanism discovered to store stress-related, unconscious memories —ScienceDaily – August 17, 2015 – Northwestern University
At first, hidden memories that can’t be consciously accessed may protect the individual from the emotional pain of recalling the event. But eventually those suppressed memories can cause debilitating psychological problems, such as anxiety, depression, post-traumatic stress disorder or dissociative disorders.
A process known as state-dependent learning is believed to contribute to the formation of memories that are inaccessible to normal consciousness. Thus, memories formed in a particular mood, arousal or drug-induced state can best be retrieved when the brain is back in that state.
The findings show there are multiple pathways to storage of fear-inducing memories, and we identified an important one for fear-related memories,”
“This could eventually lead to new treatments for patients with psychiatric disorders for whom conscious access to their traumatic memories is needed if they are to recover.”
The best way to access the memories in this system is to return the brain to the same state of consciousness as when the memory was encoded, the study showed.
Two amino acids, glutamate and GABA, are the yin and yang of the brain, directing its emotional tides and controlling whether nerve cells are excited or inhibited (calm).
Under normal conditions the system is balanced. But when we are hyper-aroused and vigilant, glutamate surges.
Glutamate is also the primary chemical that helps store memories in our neuronal networks in a way that they are easy to remember.
GABA, on the other hand, calms us and helps us sleep, blocking the action of the excitable glutamate. The most commonly used tranquilizing drug, benzodiazepine, activates GABA receptors in our brains.
There are two kinds of GABA receptors.
- One kind, synaptic GABA receptors, works in tandem with glutamate receptors to balance the excitation of the brain in response to external events such as stress.
- The other population, extra-synaptic GABA receptors, are independent agents. They ignore the peppy glutamate. Instead, their job is internally focused, adjusting brain waves and mental states according to the levels of internal chemicals, such as GABA, sex hormones and micro RNAs
Extra-synaptic GABA receptors change the brain’s state to make us aroused, sleepy, alert, sedated, inebriated or even psychotic.
However, Northwestern scientists discovered another critical role; these receptors also help encode memories of a fear-inducing event and then store them away, hidden from consciousnes
“It’s an entirely different system even at the genetic and molecular level than the one that encodes normal memories,” said lead study author Vladimir Jovasevic,
This different system is regulated by a small microRNA, miR-33, and may be the brain’s protective mechanism when an experience is overwhelmingly stressful.
The findings imply that in response to traumatic stress, some individuals, instead of activating the glutamate system to store memories, activate the extra-synaptic GABA system and form inaccessible traumatic memories.
Traumatic Memories Rerouted and Hidden Away
Memories are usually stored in distributed brain networks including the cortex, and can thus be readily accessed to consciously remember an event.
But when the mice were in a different brain state induced by gaboxadol, the stressful event primarily activated subcortical memory regions of the brain. The drug rerouted the processing of stress-related memories within the brain circuits so that they couldn’t be consciously accessed.
Vladimir Jovasevic, Kevin A Corcoran, Katherine Leaderbrand, Naoki Yamawaki, Anita L Guedea, Helen J Chen, Gordon M G Shepherd, Jelena Radulovic. GABAergic mechanisms regulated by miR-33 encode state-dependent fear. Nature Neuroscience, 2015; DOI: 10.1038/nn.4084