A research team of Dr. Michael Ehlers at Duke University Medical Center came up with a clever way to study brain circuitry in action by incorporating a light-sensitive cation channel from the green algae Chlamydomonas reinhardtii into mice neurons, and then shining a light to trigger neural activity.
From a statement by Howard Hughes Medical Institute about these specially designed transgenic mice:
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.