How traumatic events leave a mark on the brain

Researchers in the US have a discovered a potential mechanism to explain why people retain stronger memories of events that occur in emotionally charged situations.

The findings, by Hallan Hu and colleagues at the Cold Spring Harbor Laboratory, may have implications in understanding the causes of, and developing better treatments for, post traumatic stress disorder, in which people suffer vivid flashback memories of traumatic events. But the process also plays a role in normal brain activity - allowing people to distinguish between trivial and significant events and to store the latter in the long term memory.

The study looked at the effects of the stress hormone noradrenaline - known as norepinephrine in the US - in the brains of laboratory mice. Emotional stress is known to have a strong effect on the brain’s ability to lay down memories.

When injected into the mouse brain, the chemical caused the phosphorylation of type 1 glutamate receptors (GluR1) in brain cells connecting to the hippocampus and amygdale, two regions of the brain closely associated with emotional memory formation.

The team suggest that these modifications are important in the process of long term potentiation (LTP), which eases the passage of signals across synapses - the gaps between adjacent brain cells. Although the process is still poorly understood, LTP is thought to be central to memory formation.

The study looked at the release of natural noradrenaline in the brains of mice exposed to a stressful situation through being put in a cage containing traces of the urine of foxes - a common predator of mice. The researchers also returned the mice to the cages a few days later: mice genetically modified to have defective GluR1 receptors moved around the ’new’ cage much more indicating that they had little memory of their previous spell in that environment.

Roberto Malinow, head of the laboratory’s neurobiology group and one of the paper’s authors, said the mouse brain was essentially the same as that of humans and the same mechanism was likely to be at work in human memory. The findings are just one piece of a larger puzzle, said Malinow, but they may help to produce a treatment for those suffering from disorders like PTSD.

Jim McGaugh, of the Center for Neurobiology of Learning and Memory at the University of California, Irvine, confirmed that the study complements current understanding of the effects of noradrenaline in humans.

’The findings fit well with the extensive prior evidence that the release of epinephrine (noradrenaline) in the periphery and norepinephrine in the brain play an important role in regulating the strength of memories of emotionally arousing experiences,’ McGaugh told Chemistry World.  ’Their findings that norepinephrine phosphorylates GluR1 and facilitates the delivery of GluR receptors into synapses helps to increase understanding of possible mechanisms underlying the influence of emotion on memory.’

John Bonner