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Here is another article on using focused
beams of light to stimulate neurons from Howard Hughes Medical
Institute.
The illustration, which comes from a painting by
Duke University student Yifan Xu, conceptually illustrates a beam
of light shining into the olfactory bulb activating a mitral cell.
HHMI investigator Michael Ehlers and colleagues report they have
developed mice that express channelrhodopsin-2, a light-gated
cation channel from the green algae Chlamydomonas reinhardtii, in
neurons of the central nervous system. This enables researchers to
trigger neural activity with high spatial and temporal
precision—a powerful tool for those striving to map
functional circuits in the brain.
Researchers have devised a clever way to
activate neurons in a living mouse by shining light on the surface
of the animal's brain. The “light switch” that turns
neurons on is actually a light-sensitive protein that is produced
by algae. When this protein is genetically engineered into the
neurons of living mice, researchers can precisely trigger those
neurons with light, causing them to generate electrical impulses.
The scientists who developed the new
method believe it will change how researchers map the function of
brain circuits in living animals. “We believe that this
light-induced activation technique is a major technical
breakthrough in the functional analysis of neural circuitry,”
said the leader of the research team, Michael Ehlers, a Howard
Hughes Medical Institute researcher at Duke University Medical
Center. “This technique will soon become the standard method
for these types of experiments.”
The researchers published a research article describing the new
technique in the April 19, 2007, issue of the journal Neuron. The
research team included Ehlers and Duke colleagues Benjamin
Arenkiel, Guoping Feng and George Augustine. Other co-authors were
from the University of Coimbra and the Gulbenkian Science Institute
in Portugal, and from Stanford University.
In developing their technique, the
researchers drew on work by other scientists studying
channelrhodopsin-2, a protein found in green algae. One of the
unique features of the protein is that it enables algae to migrate
toward light. The researchers found that when they introduced the
gene for channelrhodopsin-2 into neurons in culture, the protein
rendered the neurons light-sensitive. When the scientists exposed
those neurons to light, they found that the light stimulated neural
activity in the neurons in culture. Co-author Karl Deisseroth at
Stanford was among those who demonstrated that the
channelrhodopsin-2 could render neurons light-sensitive in culture.
“A major question was whether this
algal protein could be expressed in animals throughout development
and still remain functional and not cause any problems,” said
Ehlers. “When Guoping Feng produced the transgenic mice in
his laboratory, he found that they developed normally and showed no
obvious neurological or behavioral problems,” he said.
“In our laboratory, we then studied
the effect of using a fiber optic light source to illuminate the
brains of these animals with light pulses,” said Ehlers.
“We found that the light-evoked activity response was very
rapid, and it corresponded precisely to the pulse location of the
light,” he said. By repeating light pulses at
one-thousandth-of-a-second intervals, the researchers showed that
they could trigger repeated trains of electrical signals in the
neurons. The light beams they used were as fine as 100 microns in
diameter, said Ehlers. By comparison, a human hair is about 200
microns in diameter.
The new light-activation technique has
advantages over other methods that are being used in functional
mapping of neural circuitry, said Ehlers. One widely used method
involves presenting a sensory stimulus such as an odorant to an
animal and recording the electrical activity the stimulus triggers
in the sensory neural circuitry. This method is slower and less
specific than the light-activation method developed by Ehlers and
his colleagues. Another approach researchers have used involves
introducing chemical receptors into neurons genetically and then
using them to trigger specific neurons. This technique is very
useful for some applications but is slower and can be
experimentally difficult, Ehlers said.
In a related study published April 15,
2007, in an advance online publication of Nature Neuroscience,
Karel Svoboda and colleagues also took advantage of
channelrhodopsin-2, using the protein in brain slices to map
functional brain wiring across great distances. Svoboda is a group
leader at HHMI’s Janelia Farm Research Campus.
To demonstrate the utility of their new
technique, Ehlers and his colleagues used it to map how nerve
signals from the olfactory bulb travel to a brain structure called
the olfactory cortex. The olfactory bulb is the neural relay
station that receives impulses from odor receptors, and the
olfactory cortex in the brain initially processes odor information
into odor perception.
The researchers used different light
patterns to activate various neurons in the olfactory bulb
structures called glomeruli in the mouse brain. During these
experiments, they used recording electrodes to measure the
resulting electrical activity of neurons in the olfactory cortex.
Their goal was to see if the new technique would help them
understand how signals from the structures in the olfactory bulb
converge on the olfactory cortex. After analyzing the pattern of
electrical responses they observed in their experiments, they
concluded that cortical neurons require input from a distributed
array of neurons in the olfactory bulb in order to fire. “We
could never find a single glomerulus or clump of glomeruli that
would strongly activate a neuron in the cortex,” said Ehlers.
“Distributed input from many glomeruli was required to cause
a cortical neuron to fire.”
To improve their light-driven mapping
technique, the researchers plan to apply additional genetic
techniques that will enable them to insert the channelrhodopsin-2
gene into specific neuronal subtypes. While their current
transgenic method is specific for neurons, it introduces the
channelrhodopsin-2 gene into many subtypes of neurons. Also, the
researchers will explore methods for light stimulation of parallel
brain regions using computer-driven patterns of illumination, which
could be a precursor to new technologies for neural activation in
the brain and spinal cord.
Howard Hughes
Medical Institute
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Neuroscience 
