The living microscope slide
The larval zebrafish (Danio rerio) is a neuroscientist's dream. For the first few days of its life, its body is almost completely transparent. This optical clarity allows researchers to look directly into its brain and watch neurons fire in real time. Its entire central nervous system is fully functional by 72 hours after fertilization. The brain of a larva is only about 0.8 by 0.6 by 0.2 millimeters and contains roughly 100,000 neurons, a complex but manageable number compared to the billions in a human brain.
This transparency makes the zebrafish an subject for optogenetics. The technique involves genetically modifying specific neurons to produce light-sensitive proteins called opsins, which are often derived from algae. These proteins act like switches. When a researcher shines a laser of a specific color on a modified neuron, the opsin activates or deactivates the cell. This provides control, allowing scientists to turn individual brain cells on or off at will while observing the effects on the entire neural network. The fish are living, behaving subjects, giving immediate feedback on how manipulating a single cell can alter behavior like swimming.
Mapping behavior neuron by neuron
At the Janelia Research Campus, scientists combine optogenetics with advanced light-sheet microscopy to create 3D activity maps of the entire zebrafish brain. This method allows them to image nearly every neuron in the brain once every 1.3 seconds. Researchers can observe the cascade of neural activity that follows a sensory input, like the sight of a potential predator, or they can trigger a motor output, like an escape maneuver, just by activating a single sensory neuron.
A team led by group leader Misha Ahrens has developed systems where a zebrafish larva is held stationary but can "swim" through a virtual reality environment. As the fish behaves, scientists record the activity of almost all of its neurons. Then, using targeted lasers, they can silence a group of active neurons and instantly see how the fish's behavior changes. This all-optical approach allows researchers to identify the complete set of neurons involved in a specific action. The technique has revealed that neurons scattered across widely separated brain regions often work together to control a single behavior, a discovery difficult to make with conventional methods that study isolated brain areas.