The researchers embedded these light-sensitive ion channels into the subthalamic nucleus in rats and flashed pulses of light at the same rate used in deep brain stimulation. The treatment, however, failed to alleviate any of the rats' physical symptoms, leading the researchers to conclude that stimulating the subthalamic nucleus on its own is an inadequate treatment approach.
"Neurons being stimulated with optogenetics don't generally respond very quickly, and it seemed to me that the researchers were flashing their lights faster than the neurons could keep up with," said Grill. "The data bore this out, as the neurons appeared to be responding randomly rather than in sync with the flashes. And previous research that we conducted showed that random patterns of deep brain stimulation are not effective at relieving symptoms."
It took more than a decade for Grill to be able to test his theory, but two recent developments allowed him to follow his hunch. Researchers developed a faster form of optogenetics called Chronos that could keep up with the speeds traditionally used in deep brain stimulation. And Chunxiu Yu, a research scientist with expertise in optogenetics, joined Grill's laboratory. Also contributing to the work in Grill's laboratory were Isaac Cassar, a biomedical engineering doctoral student, and Jaydeep Sambangi, a biomedical engineering undergraduate.
In the new paper, Yu embedded the Chronos optogenetics machinery into the subthalamic nucleus neurons of rats that have been given Parkinson's disease-like conditions in one-half of their brains. This model helps researchers determine when a treatment is successful because the resulting physical movement symptoms only occur on one side of the rat's body. They then delivered deep brain stimulation using light flashes at the standard 130 flashes per second.
According to Grill, their result has several important implications. One is that researchers need to consider the kinetic properties of how rapidly optogenetic approaches can act when designing their experiments and pay close attention to performance in their studies. Another insight was the way that other neurons outside of the subthalamic nucleus responded to the treatment. While there was not a large difference in their average activity levels, there was a dramatic shift in the pattern in which those neurons fired, which offers clues as to how deep brain stimulation works.
But perhaps the most important result is simply that the technique worked at all. Besides offering a much clearer look at neural activity by removing electrical artifacts, the ability to deliver deep brain stimulation to precise subsets of neurons should allow researchers to begin probing exactly which parts of the brain need to be stimulated and how therapies might be tailored to treat different motor control symptoms on a case-by-case basis.
MEDICA-tradefair.com; Source: Duke University