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New Understanding of Basic Units of Memory

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Duke Health News 919-660-1306

DURHAM, N.C. – A molecular "recycling plant" permits nerve
cells in the brain to carry out two seemingly contradictory
functions – changeable enough to record new experiences, yet
permanent enough to maintain these memories over time.

The discovery of this molecular recycling plant, detailed in
a study appearing early online Sept. 19 in the journal Neuron,
provides new insights into how the basic units of learning and
memory function. Individual memories are "burned onto" hundreds
of receptors that are constantly in motion around nerve
synapses – gaps between individual nerve cells crucial for
signals to travel throughout the brain.

According to the study's leader, Duke University Medical
Center neurobiologist Michael Ehlers,
M.D., Ph.D., these receptors are constantly moving around the
synapse and often times they disappear or escape. Ehlers
discovered that a specific set of molecules catch these elusive
receptors, take them to the recycling plant where they are
reprocessed and returned to the synapse intact.

"These receptors constantly escape the synapse and are in a
perpetual state of recycling," said Ehlers, who is also a
Howard Hughes Medical Institute investigator. "This process
occurs on a time scale of minutes or hours, so the acquisition
of new neurotransmitter receptors and their recycling is an
on-going process. Memory loss may result from receptors
escaping from the synapse."

All this activity takes place on millions of tiny "nubs," or
protrusions in the synapses known as dendritic spines. The
recycling plants are located within the body of these dendritic
spines.

"We believe that the existence of this recycling ability
explains in part how individual dendritic spines retain their
unique identity amidst this constant molecular turnover,"
Ehlers said. "The system is simultaneously dynamic and
stable."

While these findings should be able to help neurobiologists
as they attempt to understand the molecular foundations of
learning and memory, Ehlers believes that this knowledge could
also be helpful in explaining what happens in certain
neurological disorders, such as Alzheimer's disease,
schizophrenia, or learning disorders like autism.

For example, it appears that in animal models of the early
phases of Alzheimer's disease, often before any symptoms become
apparent, the dendritic spines gradually lose their ability to
transport and recycle the receptors.

"If the receptors don't get recycled, you see a gradual loss
of synaptic function that is associated with reduced cognitive
ability," Ehlers said. "These dendritic spines are where
learning and memories reside. These are the basic units of
memory."

Other Duke members of the team were Jiuyi Lu, Thomas Helton,
Thomas Blanpied, Bence Racz and Thomas Newpher. Richard
Weinberg of the University of North Carolina – Chapel Hill was
also a member of the team. The research was supported by the
National Institutes of Health.

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