Fluorescence imaging makes important discovery about visual connections in the brain

Scientists at the Virginia Tech Carilion Research Institute (Roanoke, VA) used fluorescence imaging to examine how retinal ganglion cells develop in a mouse model, discovering that the process is not as straightforward as previously thought.

Related: Imaging method reveals stunning details of brain connections

“Retinal neurons associated with vision generate connections in the brain, and as the brain develops it strengthens and maintains some of those connections more than others. The disused connections are eliminated,” says Michael Fox, an associate professor at the Virginia Tech Carilion Research Institute who led the study. “We found that this activity-dependent pruning might not be as simple as we’d like to believe.”

“It’s widely accepted that synaptic connections from about 20 retinal ganglion cells converge onto cells in the lateral geniculate nucleus during development, but that number reduces to just one or two by the third week of a mouse’s life,” Fox says. “It was thought that the mature retinal ganglion cells develop several synaptic terminals that cluster around information exchange points.”

Using a technique dubbed “brainbow,” the scientists tagged the terminals with proteins that fluoresce different colors. The researchers thought one color, representing the single source of the many terminals, would dominate in the clusters. Instead, several different colors appeared together, intertwined but distinct. The results showed individual terminals from more than one retinal ganglion cell in a mature mouse brain.

A fluorescence imaging technique dubbed 'brainbow' involves tagging synaptic terminals with proteins that fluoresce different colors
Using a technique dubbed “brainbow,” the Virginia Tech Carilion Research Institute scientists tagged synaptic terminals with proteins that fluoresce different colors. The researchers thought one color, representing the single source of the many terminals, would dominate in the clusters. Instead, several different colors appeared together, intertwined but distinct. (Credit: Michael Fox/Virginia Tech)

Along with the brainbow technique, Fox’s team also imaged these synaptic connections with electron microscopy. The data confirmed the results from brainbow analysis—retinal axons from numerous retinal ganglion cells remained present on adult brain cells.

“These results are not what we expected, and they will force us to reevaluate our understanding of the architecture and flow of visual information through neural pathways,” Fox says. “The dichotomy of these results also sheds important light on the benefits of combining approaches to understand complicated problems in science.”

Fox and his team are working to understand exactly how many retinal terminals converge and how they might convey information differently. Once the scientists understand the intricacies of the brain’s visual circuitry, they might be able to start developing therapeutics for when it goes wrong.

Full details of the work appear in the journal Cell Reports; for more information, please visit http://dx.doi.org/10.1016/j.celrep.2015.08.003.

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