Unlike the circuitry of the visual system, that of the olfactory system was thought to be hardwired: Once the neurons had formed, no amount of sensory input could change their arrangement.
Now researchers at Rockefeller University and their collaborators have upturned this scientific dogma by showing that there is a sensitive period during which the external environment can alter a circuit in the fly brain that detects carbon dioxide, a gas that alerts flies to food and mates. This research, to be published in the December 6 issue of Neuron, may suggest that this brain plasticity isn't limited to the carbon dioxide detection circuit. Rather, it may be a general feature of the olfactory system itself.
"The circuit has a genetic plan, but that genetic plan can adjust to real world conditions," says Leslie Vosshall, head of the Laboratory of Neurogenetics and Behavior. "This paper is the first compelling case that the olfactory system is plastic."
Using several imaging techniques, Vosshall and her colleagues traced the carbon dioxide circuit, a well-described pathway that consists of three different types of neurons, the axons and dendrites of which form an entangled ball called a glomerulus. The researchers exposed flies to elevated levels of carbon dioxide to see whether it would alter the shape of this circuit or how it functioned. The glomerulus's volume was already increased after two days of exposure (from birth) and kept on increasing for five days, at which point it stopped. The increase in this specific glomerulus could only be induced by elevated levels of carbon dioxide and was also reversible.
After those initial few days, however, the researchers saw a different story unfold. If they didn't expose the flies to carbon dioxide within the first five days, genetics locked in the glomerulus's size such that no matter how long the flies were exposed to the gas, the glomerulus's volume didn't increase. These findings suggest that the fly's external environment can rewire the carbon dioxide detection circuit only during a five-day window of development.