Optogenetics Allows Control of Spinal Circuits

Optogenetics has been shown to control neurons — and subsequently muscle movement — when applied to the spinal cords of awake, alert animals.

Neuroscientists have long studied spinal circuits in the brain in efforts to better treat spinal cord injuries. In this study, conducted by researchers at MIT’s McGovern Institute for Brain Research, optogenetics allowed exploration of the function of inhibitory interneurons. These interneurons form circuits with many other neurons in the spinal cord, and the circuits execute commands from the brain, with additional input from sensory information from the limbs.

“With optogenetics, you are attacking a system of cells that have certain characteristics similar to each other,” said professor Dr. Emilio Bizzi, the lead researcher. “It’s a big shift in terms of our ability to understand how the system works.”

In the study, inhibitory spinal neurons were engineered to express the protein channelrhodopsin-2, which stimulates neural activity when exposed to blue light. Such light was shone at different points along the thoracic spine, allowing the researchers to observe the effects of neuron activation in greater detail.

When the inhibitory neurons were activated in the freely moving mice, their hind legs were completely, but reversibly, immobilized.

The researchers also found that activating inhibitory neurons had no effect on the transmission of sensory information from the limbs to the brain, or on normal reflexes.

“The spinal location where we found this complete suppression was completely new,” said Dr. Vittorio Caggiano, a postdoctoral researcher. He added that this “front-to-back suppression,” which affects only motor behavior without affecting sensory behavior, is also a new concept.

The researchers plan to explore other types of spinal cord neurons, as well as how input from the brain influences spinal circuits.

“Further studies will highlight the contribution of single populations of neurons in the spinal cord for the control of limbs and control of movement,” Caggiano said.

The work was funded by the Human Frontier Science Program and the National Science Foundation. The research was published in PLoS One (doi: 10.1371/journal.pone.0100865).

For more information, visit www.mit.edu.