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Technique gives first real-time view of developing neurons

BY AMY ADAMS

Scientists have believed that neurons need a long period of fine-tuning and training with other neurons before they take on their adult role. But after using new technology for the first time to watch these cells develop, a team of medical school researchers found that neurons come into this world with a good idea about what they'll become as adults.

The work, which was published in Thursday's issue of Neuron, took place in the brain of a small see-through fish called a zebra fish.

Stephen Smith, PhD, professor of molecular and cellular physiology, and graduate student Christopher Niell immobilized a young fish at an age when the nerves first grow from the eye to reach the brain. Then, with the aid of a 6-foot-long laser and some fancy microscopy, the researchers were able to watch individual neurons as they matured in real time.

The pair specifically monitored hundreds of neurons in the region of the brain that respond to images. Niell set up a tiny LCD screen showing squares the size of the fish's favorite planktonic food moving up and down or left and right.

They expected to find that young neurons fire in response to a variety of different images, then refine their role over time so that in the adult fish the neurons only respond to images moving in a certain direction or near the left or right side of the visual field.

What they found was a surprise. As soon as the neurons were old enough to respond to the LCD screen, they specifically fired when they sensed only one type of movement. When the tiny square moved left to right, a distinct population of neurons turned fluorescent colors to indicate their activity.

Moving the square the reverse direction triggered a different population of neurons to light up.

"At first we felt like we let some air out of our own tires with this finding," said Smith. His previous work had supported the prevailing idea that neurons need a period of fine-tuning before establishing their final identity.

Still, the experiments mark the first time researchers have been able to watch neurons in an entire region of the brain as they fire one by one in real time.

The technical savvy involved in monitoring neurons will allow researchers to conduct experiments that were previously not possible.

Because the group could see the full branching structure of a neuron each time it fired, they could watch the branches grow and recede like a tree waving in the wind. Over time, the network of branches stabilized into the mature form.

"We're looking at a dynamic process that nobody has ever seen before," Smith said.

Understanding how neurons mature into their adult role goes beyond zebra fish and their ability to see their eventual planktonic prey. "Probably these same processes are happening in our own brains all the time," Niell said.

When people learn new skills or add memories, neurons continue the dynamic process started in the earliest stages of brain development. Seeing how neurons mature in zebra fish may explain the mysteries of how our own brains change throughout our lives.