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发帖数: 768 | 1 Published online 3 May 2011 | Nature | doi:10.1038/news.2011.264
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Keeping memories sharp
Researchers home in on structures that help the brain avoid faulty
recall.
David Cyranoski
The specificity of memories relies on structures in the brain stopping
them from being triggered by unrelated events.EQUINOX GRAPHICS/SPL
Memories fade, events get conflated, names get attached to the wrong
faces, or, in the case of post-traumatic stress disorder, signals in
safe environments can mistakenly evoke emotions that rightly belong to a
battlefield tragedy. Understanding the proper functioning and
malfunctioning of memory retrieval holds clues for how to deal with
memory disorders and learning disabilities.
In research published in Nature1, a group led by neuroscientist Pico
Caroni at the Friedrich Miescher Institute in Basel, Switzerland
reveal structural 'traces' that help to keep memories precise by
preventing unrelated events from evoking them.
The team looked at changes in the synaptic structures — the
structures involved in signal transmission from neuron to neuron — in
the hippocampus of the brain as mice went through a fear-conditioning
procedure in which their paws received electric shocks in a particular
room. If, after a couple of days, the mice entered that room, they froze
in fear. In a different but similar room, they sniffed about as normal.
But after about two weeks the mice froze in both rooms. The memory
had been 'generalized', producing a functional 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. But it changes,"
says Caroni.
When the team followed the structural changes in the hippocampus
during the conditioning process, they found a large increase in the
numbers of synaptic structures at the terminals of neurons called
granule cells, which have axons called large mossy fibres after the
memories were formed. By the time the memory had been generalized, these
synaptic structures had disappeared.
New synapses
To confirm that the formation of the synaptic structures was involved in
memory precision, Caroni's group 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 researchers introduced that protein into the
mossy-fibre neurons in the hippocampus — even if into only 20% of the
cells — the mice recovered the ability to maintain precise memories for
weeks. "That really nails it down," says Caroni.
New synapses form when new skills are learned — the property known as
'plasticity' — and Caroni's experiments suggest that one of their roles
is to inhibit the retrieval of the memory in response to unrelated
cues, says Sam Cooke, a postdoctoral fellow at the Massachusetts
Institute of Technology in Cambridge who works on the neurobiological
basis of memory. "The fact that so many mossy-fibre terminals seem to
undergo plasticity as a single association is formed suggests that it is
not storing information about the actual context in which the shock was
received itself, but rather encoding the much larger number of contexts
in which it would be inappropriate to produce a freezing response,"
says Cooke. "It is very interesting that suppression of spurious
associations may be a major form of memory."
According to Mark Hübener, a neuroscientist at the Max Planck Institute
of Neurobiology in Martinsried, Germany, the most exciting finding is
the high spatial and temporal correlation between the formation of
specific memories and the structural changes. "The data go further
than any previous paper in linking structural changes in the mammalian
brain to the formation of very specific memories," he says.
Bad memories
Caroni says that the phenomenon could improve the understanding of
post-traumatic stress disorder (PTSD), in which traumatic memories are
evoked by environments very different from those in which they
originated. 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' in which patients are reintroduced to the
place where the traumatic events happened in order to help them control
their fears.
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Cooke thinks that a wider range of disorders might be related. "Loss
of suppression of spurious associations may also be involved in other
neuropsychiatric disorders, including autism spectrum disorders and
schizophrenia," he says.
Such links will take time to establish. But the immediate future
presents many opportunities. Caroni says there is "big time" room for
follow up. He plans to start by analysing different forms of memory in
the hippocampus, comparing, for example, emotional memories versus cold,
rational cognitive memories. "It's a way to study how systems learn. We
will look at it system by system,' he says.
References
1.Ruediger, S. et al. Nature advance online publication doi:10.
1038/nature09946 (2011).
经常把朋友的名字张冠李戴，或是把事情弄混？这也许是因为你大脑中神经元之间
的突触结构正在经历生生灭灭的变化。一项最新研究显示，这种变化会使得记忆准
确度随之改变，相关成果或许可用于治疗一些与记忆有关的疾病。
英国《自然》杂志网站刊登报告说，瑞士弗里德里希－米舍研究所等机构研究人员
观察实验鼠大脑结构变化时发现，如果实验鼠进入某个房间后遭到电击，它就会记
住这个遭遇，再进入这个房间时就会表现出恐惧，而在其他相似但不同的房间中却
没有这种表现。研究发现，在这个过程中，实验鼠大脑中相关神经元周围多出了许
多突触结构。
不过，实验鼠的记忆准确度只能维持较短的时间，在遭电击两个星期后，即使是进
入相似的房间，它也会表现出恐惧，这说明被电击的记忆还在，只是大脑开始把相
关环境混淆了。研究发现，这时其大脑中相关神经元周围的突触结构逐渐消失。
但如果再让实验鼠回到最初遭电击的房间，其相关突触结构会重新建立，记忆再次
变得准确，再进入其他房间也不再表现出恐惧。研究人员据此认为，突触结构在大
脑记忆中起着确定事件背景、保证记忆准确度的作用。
《科学时报》 (2011-05-05 A4 国际) |
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