[MURG] [>Htech] sciencedaily: not much restructuring in rats olfactory bulb (fwd from alito@organicrobot.com)

[Eugen Leitl]

I much doubt similiar holds for the neocortex.

----- Forwarded message from Alejandro Dubrovsky <alito at organicrobot.com> -----

From: Alejandro Dubrovsky <alito at organicrobot.com>
Date: Mon, 27 Oct 2003 22:29:27 +1000
To: transhumantech <transhumantech at yahoogroups.com>
Subject: [>Htech] sciencedaily: not much restructuring in rats olfactory bulb
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(
http://www.sciencedaily.com/releases/2003/10/031023071202.htm
)

Evidence That Neurons Prune Only 'Twigs' To Rewire Themselves
DURHAM, N.C. -- By using a laser microscope to spy on individual nerve
cells in living mice, researchers have discovered that neurons' wiring
remain largely stable, providing a solid scaffold to accommodate the
challenges in their environment. Specifically, the scientists found that
the neuronal branches called "dendrites" remain largely unchanged in the
highly active olfactory processing region of the mouse brain. Such
evidence suggest that dendrites in the adult brain form a stable
background even in the face of ongoing changes that form part of
everyday experience.

Besides providing a better basic understanding of the dynamic processes
of brain rewiring, the researchers believe their findings might yield
insights into such disorders as epilepsy and Alzheimer's disease, which
are marked by aberrant neural circuitry. 

Dendrites are the branches of neurons that support the multitude of
interconnections by which one neuron triggers a nerve impulse in its
neighbors in the intricate neural pathways of the brain. 

The research was reported in the November 2003 issue of the journal
Nature Neuroscience by Howard Hughes Medical Institute investigator
Lawrence Katz, Ph.D., and colleague Adi Mizrahi, both at the Duke
University Medical Center. 

"The brain faces two challenges in maintaining its functionality in a
changing environment," said Katz. "One is to remain stable enough so
that the basic things we need to do to interpret the world remain
consistent. And the other is to continually adapt to the changing
environment, which places a high premium on the ability to alter neural
circuitry." 

The brain is known to undergo large-scale wiring during embryonic
development after such drastic events as a stroke or loss of a limb.
However, said Katz, a central question in neurobiology is whether such
dendritic alterations take place during the formation of long-term
memories. 

To explore the nature of such rewiring, Mizrahi and Katz studied neurons
in the neural structure called the olfactory bulb -- the collection of
neurons that represent the initial processing stage for information from
odor sensing receptors in the nose. 

"The olfactory bulb is one of only two areas of the brain where new
neurons are being generated throughout life," said Katz. "Neurons in the
olfactory bulb are constantly losing synapses linked to sensory cells
that are dying and gaining new ones connected to new sensory cells."
Thus, he said, detailed observation of those neurons should yield a
clear look at neurons in the process of rewiring during ordinary
experience. 

The scientists used a laser microscopy technique that enabled them to
watch changes in specific neurons genetically tagged with a fluorescent
protein, as the mice were presented with changes in their environment.
The transgenic mice were developed by Duke neurobiologist Guoping Feng
and his colleagues. 

"Importantly, this technique enabled us to look in real time at the
changes in a single neuron in the same animal; not at populations of
neurons and not at different animals," said Katz. "We could follow over
time how dendrites responded to ongoing change." In initial studies, the
researchers found only subtle changes in the neurons. 

"The changes bordered on the imperceptible -- like a tree that lost or
gained only a few twigs over time," said Katz. "It wasn't what we
initially thought, that the neurons would be like rose bushes in spring,
in which a tremendous amount of dendritic structure would be gained."
Even when the scientists placed the mice in an enriched "Disneyland" of
structures and smells to explore, they saw few changes in dendritic
structure. This, despite the fact that other researchers had found that
manipulating the odor environment drastically increased turnover of
neurons in the olfactory bulb. Nor did the scientists see significant
changes when they taught the animals to seek out a particular odor to
gain a reward. 

The only way they could induce major changes, they found was to use the
molecular "sledgehammer" of a drug known to make neurons hyperactive,
"so we knew they had the capacity to undergo change," said Katz. 

"We've concluded from these findings that the overall theme of this area
of the brain is stability, and that these dendrites are not undergoing
large-scale changes under natural conditions, even in response to
changes in their environment," said Katz. "My own view is that there is
a large backbone of stability in these areas and relatively low levels
of plasticity, despite the fact that new neurons are being constantly
generated," said Katz. 

According to Katz ongoing studies are using the combination of laser
microscopy and cell tagging to study plasticity in other regions of the
brain, particularly the central site of learning, the hippocampus. 

Such studies could yield significant insights into disorders that
involve brain rewiring, he said. "Dendritic degeneration is a hallmark
of Alzheimer's disease, and dendritic changes are known to occur in
epilepsy," said Katz. "So, understanding what is normal and what is
pathological -- and the mechanisms that produce such changes -- could
offer insights into these diseases." For example, he said, by crossing
mouse strains that show epilepsy with the fluorescently tagged strain,
it would be possible to study in detail alterations in dendritic wiring
that might contribute to the disorder.


________________________________________________________________________

This story has been adapted from a news release issued by Duke
University Medical Center.



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