Caltech researchers have developed a method to easily see neural connections and the flow of communications in real time within living flies. The work is a step forward toward creating a map of the entire fly brain's many connections, which could help scientists understand the neural circuits within human brains as well. The study is published in the journal eLife.

"If an electrical engineer wants to understand how a computer works, the first thing that he or she would want to figure out is how the different components are wired to each other," says Lois. "Similarly, we must know how neurons are wired together in order to understand how brains work."

When two neurons connect, they link together with a structure called a synapse, space through which one neuron can send and receive electrical and chemical signals to or from another neuron. Even if multiple neurons are very close together, they need synapses to truly communicate.

The Lois laboratory has developed a method for tracing the flow of information across synapses, called TRACT (Transneuronal Control of Transcription). Using genetically engineered Drosophila fruit flies, TRACT allows researchers to observe which neurons are "talking" and which neurons are "listening" by prompting the connected neurons to produce glowing proteins.

With TRACT, when a neuron "talks" — or transmits a chemical or electrical signal across a synapse — it will also produce and send along a fluorescent protein that lights up both the talking neuron and its synapses with a particular color.

Any neurons "listening" to the signal receive this protein, which binds to a so-called receptor molecule on the receiving neuron's surface. The binding of the signal protein activates the receptor and triggers the neuron it's attached to in order to produce its own, differently colored fluorescent protein.

In this way, communication between neurons becomes visible. Using a type of microscope that can peer through a thin window installed on the fly's head, the researchers can observe the colorful glow of neural connections in real time as the fly grows, moves, and experiences changes in its environment.

TRACT can be localized to focus in on the wiring of any particular neural circuit of interest, such as those that control movement, hunger, or vision. Lois and his group tested their method by examining neurons within the well-understood olfactory circuit, the neurons responsible for the sense of smell.

Their results confirmed existing data regarding this particular circuit's wiring diagram. In addition, they examined the circadian circuit, which is responsible for the waking and sleeping cycle, where they detected new possible synaptic connections.

TRACT, however, can do more than produce wiring diagrams. The transgenic flies can be genetically engineered so that the technique prompts receiving neurons to produce proteins that have a function, rather than colorful proteins that simply trace connections.

Because the TRACT method is completely genetically encoded, it is ideal for use in laboratory animals such as Drosophila and zebrafish; ultimately, Lois hopes to implement the technique in mice to enable the neural tracing of a mammalian brain.

"TRACT is a new tool that will allow us to create wiring diagrams of brains and determine the function of connected neurons," he says. "This information will provide important clues towards understanding the complex workings of the human brain and its diseases."