The work, published in the April 21 issue of Nature, takes advantage of tiny, see-through zebrafish. Stephen Smith, PhD, professor of molecular and cellular physiology at the Stanford University School of Medicine, and his graduate student Jackie Hua immersed 3-day-old fish in a breathable, Jell-O-like substance that kept the fish alive but immobile. The researchers could then focus video cameras on the fish's developing brain to watch how the branches of individual neurons grew and shrank over time.
It turns out that determining which of the branches will grow follows an age-old axiom: The squeaky neuron gets the grease. "Louder neurons drown out their quieter neighbors," Smith said.
Working out this seemingly simple rule took some technical finesse. Hua created zebrafish with a few brain cells that made a protein that prevented them from firing their normal electrical signals. These cells were also engineered to produce a protein that glowed green under the appropriate light.
Hua looked for green neurons in her immobilized fish to see how their branches fared compared with neighboring neurons that fired normally. The green neurons didn't compete well.
Although the poorly-firing green neurons still formed extensive branching structures, which the researchers call the neuron's arbor, most of those branches eventually receded while neighboring neurons formed a large number of stable connections. When the fish were five days old, the green neurons had a smaller, less complex arbor than those of neighboring neurons.