Optical method may overestimate neuronal signaling, study finds

Measuring a thought has always been difficult. Neuroscientists have some clever ways, but now University of Connecticut researchers describe in Scientific Reports the flashiest method may be less precise than previously assumed.

Our brains are composed 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 of measuring those signals. The most precise uses a tiny electrical probe in a neuron, but it is impractical 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 flashes light when waves of calcium ions pass through it. The calcium ions are the carriers of electrical charge; when they move through a brain cell, an electrical signal is moving through that cell. In theory, a camera can be used to record and count the flashes to reveal the number of neurons signaling.

In the Antic laboratory at UConn’s 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 really measured this signaling.

To find out, Milicevic and Ivanova, together with their colleagues, painstakingly inserted electrical leads into individual neurons one at a time, and recorded how often each fired a signal electrically. Simultaneously, they optically recorded the same group of neurons with a camera.

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

Milicevic, Ivanova, and their colleagues called these minor sub-threshold electrical signals “depolarization plateaus.” The researchers also found that, if the neuron combined one nerve impulse with a plateau, the resultant flash of light would be two or three times brighter than an ordinary signaling event consisting of one nerve impulse. The combination of one nerve impulse with a plateau potential made it look to the camera as if two or three nerve impulses fired in repetition, instead of only one.

“It was previously thought that neuron calcium signaling was simply (linearly) related to the generation of nerve impulses. It’s not. It can be boosted by underlying plateau potentials,” Milicevic says. “This makes measuring neuronal activity with optics much trickier.”

The researchers are now studying the same signaling phenomenon in neurons at double speed, taking snapshots of electrical and optical signals twice as fast as neuroscientists normally do. They want to make sure they’re not missing any other bursts of thought.