Scientists have developed a new tool that lights up active conversations between neurons during a behavior or sensory experience. To create labels that would persistently tag active synapses, the scientists split a fluorescent molecule in half, one half for the talking neuron (pre-) and one half for the listening neuron (post-). When an exchange occurs (i.e. the pre-synaptic neuron 'fires' a message), the two halves come together across the synapse and light up. Moreover, fluorescent molecules of three different colors allow unique labeling of different synapses in the same animal. (Here the three colors are shown at the same synapse for simplicity.)


At first blush, reading the mind of the common fruit fly Drosophila melanogaster might seemingly not produce any profound insight toward philosophical and existential questions about our existence in the universe. However, many neuroscientists would look upon this view as provincial, since understanding the behavioral and sensory inputs from these insects may unlock many secrets of our own neurodevelopment.

Now, researchers at Northwestern University have developed a new method for lighting up active conversations between neurons when exposed to various external stimuli, such as smelling a banana. The scientists are hopeful that mapping these intricate patterns for individual neural connections will provide insights into the computational processes that underlie the workings of the human brain.

The investigators focused on three of the fruit fly’s sensory systems by utilizing different colored fluorescent protein chimeras to tag neurons within the brain, in order to determine which connections were active during a sensory stimuli experience that happened hours earlier.

The labeling technique that the Northwestern scientists created allows them to identify individual synapses that are active during a complex behavior, such as avoiding heat or olfactory inputs. Moreover, the fluorescent signal persists for hours after the communication event, allowing researchers to study the brain's activity after the fact, under a microscope.       

“Much of the brain's computation happens at the level of synapses, where neurons are talking to each other,” explained senior author Marco Gallio, Ph.D., assistant professor in the department of neurobiology at Northwestern University. “Our technique gives us a window of opportunity to see which synapses were engaged in communication during a particular behavior or sensory experience. It is a unique retrospective label.”

The findings from this study were published recently in Nature Communications through an article entitled “Dynamic labelling of neural connections in multiple colours by trans-synaptic fluorescence complementation.”

Reading the fluorescent signals, the researchers were able to tell if a fly had been exposed to either heat or cold for a 10 minute period, even an entire hour after the sensory event had happened. Furthermore, they could see that exposure to the scent of a banana activated neural connections in the olfactory system that were different from those activated when the fly smelled jasmine.

“Different synapses are active during different behaviors, and we can see that in the same animal with our three distinct labels,” noted Dr. Gallio.

The fluorescent green, yellow and blue signals enabled the researchers to label different synapses activated by the sensory experience in different colors in the same animal. To create the labels, the researchers genetically engineered the fluorescent protein to express in two separate halves—one-half for the talking neuron and one-half for the listening neuron. If those neurons talked to each other when a fly was exposed to the banana smell or heat, the two protein halves came together and were able to fluoresce, which only happened at the site of active synaptic transmission.

“Our results show we can detect a specific pattern of activity between neurons in the brain, recording instantaneous exchanges between them as persistent signals that can later be visualized under a microscope,” Dr. Gallio stated.

The scientists noted that this technique is first to persistently tag individual synapses active during complex behavior and that such a tool should help researchers better understand how brain circuits process information—with the ultimate goal of translating this knowledge and applying it to humans.