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New experiments in light stimulation are
helping scientists learn more about neural systems. Optical
excitation using fiber optics can be used to stimulate specific
areas of the brain and is an alternative to electrode stimulation.
Electrode stimulation is unable to target single types of neurons
and instead activates the firing of all neurons in one neural area.
This new technology may allow us to uncover what roles specific
neurons play.
Light stimulation every 200 milliseconds generates electrical
activity in an area of the brain associated with depression.
The New York Times published an article
yesterday about this new research.
The Beam of Light That Flips a Switch That Turns on the
Brain
By INGFEI CHEN
Published: August 14, 2007
It sounds like a science-fiction version of stupid pet tricks: by
toggling a light switch, neuroscientists can set fruit flies
a-leaping and mice a-twirling and stop worms in their squiggling
tracks.
But such feats, unveiled in the past two years, are proof that a
new generation of genetic and optical technology can give
researchers unprecedented power to turn on and off targeted sets of
cells in the brain, and to do so by remote control.
These novel techniques will bring an
“exponential change” in the way scientists learn about
neural systems, said Dr. Helen Mayberg, a clinical neuroscientist
at Emory University, who is not involved in the research but has
seen videos of the worm experiments.
“A picture is worth a thousand
words,” Dr. Mayberg said.
Some day, the remote-control technology
might even serve as a treatment for neurological and psychiatric
disorders.
These clever techniques involve
genetically tinkering with nerve cells to make them respond to
light.
One of the newest, fastest strategies
co-opts a photosensitive protein called channelrhodopsin-2 from
pond scum to allow precise laser control of the altered cells on a
millisecond timescale. That speed mimics the natural electrical
chatterings of the brain, said Dr. Karl Deisseroth, an assistant
professor of bioengineering at Stanford.
“We can start to sort of speak the
language of the brain using optical excitation,” Dr.
Deisseroth said. The brain’s functions “arise from the
orchestrated participation of all the different cell types, like in
a symphony,” he said.
Laser stimulation can serve as a musical
conductor, manipulating the various kinds of neurons in the brain
to reveal which important roles they play.
This light-switch technology promises to
accelerate scientists’ efforts in mapping which clusters of
the brain’s 100 billion neurons warble to each other when a
person, for example, recalls a memory or learns a skill. That quest
is one of the greatest challenges facing neuroscience.
The channelrhodopsin switch is
“really going to blow the lid off the whole analysis of brain
function,” said George Augustine, a neurobiologist at Duke
University in Durham, N.C.
Dr. Deisseroth, who is also a psychiatrist
who treats patients with autism or severe depression, has ambitious
goals. Brain cells in those disorders show no damage, yet something
is wrong with how they talk to one another, he said.
“The high-speed dynamics of the
system are probably off,” Dr. Deisseroth said. He wants to
learn whether, in these neuropsychiatric diseases, certain neurons
falter or go haywire, and then to find a way to tune
patients’ faulty circuits.
A first step is establishing that it is
possible to tweak a brain circuit by remote control and observe the
corresponding behavioral changes in freely moving lab animals. On a
recent Sunday at Stanford, Dr. Deisseroth and Feng Zhang, a
graduate student, hovered over a dark brown mouse placed inside a
white plastic tub. Through standard gene-manipulating tricks, the
rodent had been engineered to produce channelrhodopsin only in one
particular kind of neuron found throughout the brain, to no
apparent ill effect.
Mr. Zhang had implanted a tiny metal tube
into the right side of the mouse’s partly shaved head.
Now he carefully threaded a translucent
fiber-optic cable not much wider than a thick human hair into that
tube, positioned over the area of the cerebral cortex that controls
movement.
“Turn it on,” Dr. Deisseroth
said.
Mr. Zhang adjusted a key on a nearby laser
controller box, and the fiber-optic cable glowed with blue light.
The mouse started skittering in a left-hand spin, like a dog
chasing its tail.
“Turn it off, and then you can see
him stand up,” Dr. Deisseroth continued. “And now turn
it back on, and you can see it’s circling.”
Because the brain lacks pain receptors,
the mouse felt no discomfort from the fiber optic, the scientists
said, although it looked a tad confused. Scientists have long known
that using electrodes to gently zap one side of a mouse’s
motor cortex will make it turn the opposite way. What is new here
is that for the first time, researchers can perturb specific neuron
types using light, Dr. Deisseroth said.
Electrode stimulation is the standard tool
for rapidly driving nerve cells to fire. But in brain tissue, it is
unable to target single types of neurons, instead rousing the
entire neural neighborhood.
“You activate millions of cells, or
thousands at the very least,” said Ehud Isacoff, a professor
of neurobiology at the University of California, Berkeley. All
variety of neurons are intermixed in the cortex, he said.
Neuroscientists have long sought a better
alternative than electrode stimulation. In the past few years, some
have jury-rigged ways to excite brain cells by using light; one
technique used at Yale made headless fruit flies flap away. But
these methods had limitations. They worked slowly, they could not
target specific neurons or they required adding a chemical agent.
More recently, Dr. Isacoff, with Dirk Trauner, a chemistry
professor at the University of California, Berkeley, and other
colleagues engineered a high-speed neural switch by refurbishing a
channel protein that anchors in the cell membrane of most human
brain cells. The scientists tethered to the protein a
light-sensitive synthetic molecular string that has glutamate, a
neurotransmitter, dangling off the end.
Upon absorbing violet light, the string plugs the glutamate into
the protein’s receptor and sparks a neuron’s natural
activation process: the channel opens, positive ions flood inside,
and the cell unleashes an electrical impulse.
In experiments published in May in the
journal Neuron, the Berkeley team bred zebrafish that carried the
artificial glutamate switch within neurons that help sense
touch.
“If I were a fish, and somebody
poked me in the side,” (in this case, with a fine glass tip),
Dr. Isacoff said, “I would escape.” But when the
translucent fish were strobed with violet light, the overstimulated
creatures no longer detected being prodded. Blue-green light
reversed the effect.
One advantage of the Berkeley approach,
Dr. Isacoff said, is that it can be adapted for many types of
proteins so they could be activated by light. But for the method to
work, scientists must periodically douse cells with the glutamate
string.
In contrast, Dr. Deisseroth’s
laboratory at Stanford has followed nature’s simpler design,
borrowing a light-sensitive protein instead of making a synthetic
one.
In 2003, Georg Nagel, a biophysicist then
at the Max Planck Institute of Biophysics in Frankfurt, and
colleagues characterized channelrhodopsin-2 from green algae. This
channel protein lets positive ions stream into cells when exposed
to blue light. It functioned even when inserted into human kidney
cells, the researchers showed.
Neuroscientists realized that this pond
scum protein might be used to hot-wire a neuron with light. In
2005, Edward Boyden, then a graduate student at Stanford, Mr. Zhang
and Dr. Deisseroth, joining with the German researchers,
demonstrated that the idea worked. And in separate research
published last spring, Mr. Zhang and Dr. Boyden, now at the
Massachusetts Institute of Technology, each found a way to also
silence neurons: a bacterial protein called halorhodopsin, when
placed in a brain cell, can cause the cell to shut down in response
to yellow light.
The Stanford-Germany team put both the
“on” and “off” toggles into the motor
neurons or muscle cells of transgenic roundworms. Blue light made
the creatures contract their muscles and pull back; yellow let them
relax their muscles and inch forward.
Dr. Augustine and associates at Duke next
collaborated with Dr. Deisseroth to create transgenic mice with
channelrhodopsin in different brain cell populations. By quickly
scanning with a blue laser across brain tissue, they stimulated
cells containing the switch. They simultaneously monitored for
responses in connecting neurons, by recording from an electrode or
using sensor molecules that light up.
“That way, you can build up a
two-dimensional or, in principle, even a three-dimensional
map” of the neural circuitry as it functions, Dr. Augustine
said.
Meanwhile, other researchers are exploring
light-switch technology for medical purposes. Jerry Silver, a
neuroscientist at Case Western Reserve University in Cleveland, and
colleagues are testing whether they can restore the ability to
breathe independently in rats with spinal cord injuries, by
inserting channelrhodopsin into specific motor neurons and pulsing
the neurons with light.
And in Detroit, investigators at Wayne
State University used blind mice lacking photoreceptors in their
eyes and injected a virus carrying the channelrhodopsin gene into
surviving retinal cells. Later, shining a light into the
animals’ eyes, the scientists detected electrical signals
registering in the visual cortex. But they are still investigating
whether the treatment actually brings back vision, said Zhuo-Hua
Pan, a neuroscientist.
At Stanford, Dr. Deisseroth’s group
has identified part of a brain circuit, in the hippocampus, that is
underactive in rats, with some symptoms resembling depression. The
neural circuit’s activity — and the animals’
— perked up after antidepressant treatment, in findings
reported last week in the journal Science. Now the team is
examining whether they can lift the rats’ low-energy behavior
by using channelrhodopsin to rev up the sluggish neural zone.
But human depression is complex, probably
involving several brain areas; an easy fix is not expected. The
light-switch technologies are not likely to be used for depression
or other disorders in people any time soon. One concern is making
sure that frequent light exposure does not harm neurons.
Another challenge — except in eye
treatments — is how to pipe light into neural tissue. Dr.
Deisseroth’s spinning mouse demonstration suggests that fiber
optics could solve that issue. Such wiring would be no more
invasive, he said, than deep brain stimulation using implanted
electrodes, currently a treatment for Parkinson’s
disease.
An even bigger obstacle, however, is that
gene therapy, a technology that is still unproven, would be needed
to slip light-switch genes into a patient’s nerve cells.
Clinical trials are now testing other gene therapies against
blindness and Parkinson’s in human patients.
But even if those succeed, introducing a
protein like channelrhodopsin from a nonmammal species could set
off a dangerous immune reaction in humans, warned Dr. Howard
Federoff, a neuroscientist at Georgetown University and chairman of
the National Institutes of Health committee that reviews all
gene-therapy clinical trial protocols in the United States.
In the near term, Dr. Deisseroth predicts
that the remote-control technology will lead to new insights from
animal studies about how diseases arise, and help generate other
treatment ideas.
Such research benefits could extend beyond
the realm of neuroscience: The Stanford group has sent DNA copies
of the “on” and “off” light-switch genes to
more than 175 researchers eager to try them in all stripes of
electrically excitable cells, from insulin-releasing pancreas cells
to heart cells.
Orignal article is here
In an optical switch in a mammalian neuron, red marks synapses and
green shows photosensitive protein on the cell membrane.
STOPPING ON YELLOW A genetically modified C. elegans worm stopped
in response to yellow light that inhibits its neural activity.
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Neuroscience 
