Tool to image cellular brain activity developed
individual neurons activated in a living animal, ultimately leading
to the development of targeted drugs that directly affect neurons
involved in neurological diseases that alter behaviour and
perception.
The development of this tool could 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.
Dr. Alison Barth, an assistant professor of biological sciences at the university's Mellon College of Science, said neuroscientists had made great strides in identifying the general areas of the brain that perform certain tasks. However, these methods have worked at the gross level with poor resolution.
Barth told DrugResearcher.com: "To be able to identify specific neural circuits that underlie behaviour or pathology is an essential step in being able to design new drugs that specifically target neural subsets that are activated in disease processes."
"This technique can help us understand whether a particular drug has a direct or indirect effect on neurons in a part of the brain that is suspected to underlie a particular disease."
In the study, Barth created a transgenic mouse that couples a 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 visualise, in living brain tissue, a single neuron that has been activated in response to an animal's experience," said Barth.
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 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.
Barth said: "The fosGFP mice offer better access than ever before to the specific neurons that have been activated by an animal's experience."
Although scientists can detect c-fos expression using another technique, it requires disrupting membranes and disturbing connections between nerve cells. Barth's method overcomes 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.
Barth said: "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."
According to Barth, the fosGFP mouse is a broadly applicable tool for many neuroscientists, who has patented the mouse and licensed it commercially.
She added: "We are currently collaborating with investigators who are interested in the neural basis for some of these disorders, and it is reasonable to expect that the outcome of that research may suggest better cellular targets for new pharmaceuticals."
This advance, described in the July 21 issue of The Journal of Neuroscience, could have implications for drugs such as Clozaril(clozapine). Clozaril is used to treat schizophrenia, which is effective at relieving symptoms associated with the disease. Currently it is not clear which part of the brain or which specific neurotransmitter receptors the drug is affecting.
Using the fosGFP mouse to study Clozaril mechanism of action may provide a better understanding not only of which neurons the drug activates, but also how they change on continued exposure to the drug.
Commenting on the future Barth said: "If we can understand the pharmacological profile of neurons in the amygdale that are overactive during anxiety, for example, there is the opportunity to design specific compounds to affect their activity without affecting the activity of other surrounding cells."
