A “misfit” fluorescent dye, developed to round out a spectrum of dyes, is becoming a cornerstone of a new brain imaging tool developed at Howard Hughes Medical Institute’s Janelia Research Campus. The tool, which is named Voltron, lets researchers track neuron activity in living animals more precisely and for longer time periods than was once possible, the researchers said. Voltron is a modular system made from a dye molecule linked by a tag to an engineered, voltage-sensitive protein that is situated on the neurons’ membranes. Scientists can “form Voltron” by applying the special dye to animals engineered to have the dye-grabbing protein in neurons of interest. The fluorescent dyes used in Voltron are both bright and long-lasting. Courtesy of Jonathan Grimm. The ultrabright, synthetic dye emits 10 times more light than fluorescent proteins. Voltron’s specially engineered protein causes the intensity to change when specific neurons switch on, allowing researchers to detect neural signals. The team has engineered mice, fly, and zebrafish brains to contain this special protein. Voltage changes from a signaling neuron alter the protein’s behavior, making the dye molecules brighten and dim with millisecond precision. When the researchers put the animals under a microscope and shine light on them, the dye emits colorful light that can be captured in videos. In a mouse cortex, Voltron allowed single-trial recording of spikes and subthreshold voltage signals from dozens of neurons simultaneously, over 15 minutes of continuous imaging. In larval zebrafish, Voltron enabled the precise correlation of spike timing with behavior. Voltron makes neurons fluoresce under the microscope. When the cells’ voltage changes, so does the fluorescence. That lets scientists know when neurons are firing. Courtesy of Ahmed Abdelfattah. The Janelia team has already sent Voltron components to more than 100 other labs that have requested the tool after a preprint was published on bioRxiv. “Our philosophy is to make the tools we develop as broadly available as possible, as soon as possible,” group leader Eric Schreiter said. For scientists in the field, Voltron brings within reach the ability to directly watch voltage changes in living animals’ neurons. Janelia group leader Glenn Turner, for example, has inserted electrodes into individual fruit fly brain cells for years to measure voltage changes in neurons. Voltron lets him monitor multiple cells at once for 10 to 15 minutes at a time without clunky electrodes. With Voltron, he can even distinguish signals from neighboring neurons. Mouse neurons (yellow dots) are labeled using Voltron. The overlay shows voltage signals measured using the tool, and spikes indicate that a cell has sent a message. Courtesy of Ondrej Novak. Scientists can currently use Voltron with light-sheet microscopy and other kinds of light microscopes. Schreiter and neuroscientist Ahmed Abdelfattah would like to develop a Voltron variation that works with two-photon imaging, a higher-resolution microscopy technique. The research was published in Science (https://doi.org/10.1126/science.aav6416).