Novel tool that can be used to visualize single neurons in living brain tissue

Carnegie Mellon University neuroscientist Alison Barth has developed the first tool to identify and study individual neurons activated in a living animal.

This advance, described in the July 21 issue of The Journal of Neuroscience, ultimately could lead to the development of targeted drugs that directly affect specific neurons involved in neurological diseases that alter behavior, learning and perception.

While neuroscientists have made great strides in identifying the general areas of the brain that perform certain tasks, these methods have worked at the gross level and with poor resolution, according to Barth, an assistant professor of biological sciences at the university's Mellon College of Science. To overcome these limitations, Barth created a transgenic mouse that couples the green fluorescent protein (GFP) with the gene c-fos, which turns on when nerve cells are activated. Using this method, researchers can see specific neurons glow as they are activated by external stimuli such as sensory experience or drug treatment.

"Our transgenic mouse is a novel tool that can be used to visualize, in living brain tissue, a single neuron that has been activated in response to an animal's experience," Barth said.

Barth used the fosGFP mice to identify neurons that are activated during a specific rearing condition – experiencing the world through one whisker. By locating a cluster of glowing neurons, she was able to precisely identify the area of the brain involved in processing sensory input from the single whisker. Once the neurons of interest had been located, Barth then examined each neuron to determine how its electrophysiological and synaptic properties changed in response to sensory input. Her results are the first to show alterations in the rate at which neurons transmit electrical signals after increased sensory input in vivo.

Barth's technology is based on the decades-long understanding that a neuron must turn on new genes to firmly encode memories in the brain. Each time c-fos is activated in Barth's transgenic mouse, so is GFP. The result is an animal whose neurons literally glow when they are activated by stimuli.

"The fosGFP mice offer better access than ever before to the specific neurons that have been activated by an animal's experience," Barth said.

Although scientists can detect c-fos expression using another technique, it requires disrupting membranes and disturbing connections between nerve cells. Barth's method circumvents these drawbacks, allowing scientists to study living neurons at the cellular level.

Using the fosGFP mouse to identify a discrete area of the brain involved in inputting sensory information from a single whisker, Barth found that the electrical properties of neurons in the area stimulated by sensation were different than those of neurons deprived of sensation. Specifically, she discovered that neurons in the sensory-stimulated area underwent changes that made them less likely to send a signal to surrounding neurons.

"These changes are hypothesized to be part of a dynamic interplay between forces that maintain neural firing within an optimal range and those that strengthen particular connections between cells, thought to underlie learning," Barth said.

The fosGFP mouse is a broadly applicable tool for many neuroscientists, according to Barth, who has patented the mouse and licensed it commercially.

The fosGFP mouse should help scientists see which neurons are active in different neurological diseases and has broad implications for rational drug design in the treatment of schizophrenia as well as many other psychiatric diseases, according to Barth. For instance, the drug Clozapine, which is used to treat schizophrenia, is effective at relieving symptoms associated with the disease, but it isn't clear which part of the brain or which specific neurotransmitter receptors are being affected by the drug. Using the fosGFP mouse to study Clozapine's mechanism of action may provide a better understanding not only of which neurons are activated by the drug, but also how they change on continued exposure to the drug.