By Janet Fang
Posting in Environment
Scientists have linked structural changes in the brain to the formation and retrieval of memories. This could improve the understanding of many neuropsychiatric disorders.
Scientists are homing in on the structures that help the brain avoid faulty recall. These help keep memories precise by preventing unrelated events from evoking them, Nature News reports.
A group led by Pico Caroni at the Friedrich Miescher Institute looked at synaptic structures – the structures involved in signal transmission from neuron to neuron – in the hippocampus region of mouse brains.
These mice went through a fear-conditioning procedure where their paws received electric shocks in a particular room.
- If, after a couple days, they entered that room, they would freeze in fear. In a different but similar room, they sniffed about as normal.
- However, after about 2 weeks, the mice froze in both rooms. The memory had been generalized, producing a response to a wide range of cues instead of the specific one that had been learned.
"The memory is still there, it might be there forever,” says Caroni. “But it changes.”
As the team followed changes in the hippocampus during the conditioning process, they found a large increase in the numbers of synaptic structures at the ends of neurons called granule cells. These have axons called large mossy fibres after the memories were formed. By the time the memory had been generalized, these synaptic structures had disappeared.
- Then the researchers put mice that lacked a protein necessary to form the synaptic connections through the same shock treatments. In these mice, the memory became generalized after only one day.
- But when the team introduced that protein into the mossy-fibre neurons in the hippocampus, the mice recovered the ability to maintain precise memories for weeks.
New synapses form when new skills are learned, and Caroni's experiments suggest that one of their roles is to inhibit the retrieval of the memory in response to unrelated cues.
With post-traumatic stress disorder (PTSD), traumatic memories are evoked by environments very different from those in which they originated. Signals in safe places can mistakenly evoke emotions that rightly belong to a battlefield tragedy.
The generalization of the mouse memories was a form of PTSD, but when the mice were reintroduced to the environment in which they were fear-conditioned, the synaptic structures reappeared within a couple hours and the precise memories were reinforced, saving them from anxiety in other environments.
In the same vein, one treatment for PTSD is exposure therapy, when patients are reintroduced to the place where the traumatic events happened in order to help them control their fears.
Loss of suppression of spurious associations may also be involved in other neuropsychiatric disorders, including autism spectrum disorders and schizophrenia.
Via Nature News.
The study was published on Sunday in Nature.
Image: brain cells / Ruediger et al.
May 5, 2011