Optical method may overestimate neuronal signaling, study finds

Measuring a thought has always been difficult. Neuroscientists have some clever methods, but now researchers at the University of Connecticut describe in Scientific reports the flashier method may be less accurate than previously thought.

Our brain is made up of billions of cells. Some of them, called neurons, send electrical signals back and forth. These signals are the physical manifestation of thoughts in the mind.

Scientists have two methods for measuring these signals. The most precise uses a tiny electrical probe into a neuron, but it’s not practical to do this on more than a few cells at a time. To watch large groups of neurons signal each other, researchers use a special dye that emits light when waves of calcium ions pass through it. Calcium ions carry electrical charges; when they move into a brain cell, an electrical signal passes through that cell. In theory, a camera can be used to record and count flashes to reveal the number of neurons signaling.

In the Antic lab at the UConn School of Medicine, neuroscientists Katarina Milicevic, Violetta Ivanova and colleagues study how Alzheimer’s disease affects neuronal signaling. They wondered how well the optical method actually measured this signaling.

To find out, Milicevic and Ivanova, along with their colleagues, carefully inserted electrical wires into individual neurons, one at a time, and recorded the frequency with which each fired an electrical signal. Simultaneously, they optically recorded the same group of neurons with a camera.

Their results were unexpected. They found that the flashes of light occurred more often than the neurons actually reported. Sometimes, subtle waves of calcium ions passed through a neuron. These subtle waves were not enough to activate the cell’s main signaling potential (a nerve impulse), but enough to flash the dye for the camera.

Milicevic, Ivanova and their colleagues called these minor subthreshold electrical signals “depolarization plateaus.” The researchers also found that if the neuron combined a nerve impulse with a plateau, the resulting flash of light would be two or three times brighter than an ordinary signaling event consisting of a single nerve impulse. The combination of a nerve impulse with a potential plateau made the camera appear as if two or three nerve impulses were being fired repeatedly, instead of just one.

“It was previously thought that neuronal calcium signaling was simply (linearly) linked to the generation of the nerve impulse. This is not the case. It may be stimulated by underlying plateau potentials,” says Milicevic. “This makes measuring neuronal activity with optics much trickier.”

Researchers are now studying the same signaling phenomenon in neurons at double the speed, taking snapshots of electrical and optical signals twice as fast as neuroscientists normally do. They want to make sure that no other thought escapes them.