Paving the way from nose to noggin

STANFORD (US) — A new technique that traces neural pathways across the brain is offering insight into how the brain and nose work together to sniff out fear.

It’s a complex process that starts with the scent being picked up by specific receptors in the nose. But until now it wasn’t clear exactly how these scent signals proceeded from the nose to the brain for neural processing.

Using a newly developed technique, scientists for the first time are able to map the path that the scent signals take from the olfactory bulb, the part of the brain that first receives signals from odor receptors in the nose, to higher centers of the mouse brain where the processing is done.

The research is reported in the journal Nature.

“No one could trace signals across neural connections to a specific type of neuron at a specific location before,” says Liqun Luo, professor of biology at Stanford University.

This is Luo’s first study of the mouse olfactory system, but his lab has spent 10 years studying olfactory pathways in the fruit fly. Because mouse brains are so much larger and more complex that those of flies, Luo and postdoctoral researcher Kazunari Miyamichi had to develop an entirely new experimental technique.

These techniques can be used to do more than just study how mice smell. “The tools we’ve developed can be applied to trace neural connections of any part of the nervous system,” Luo says, including to understand how mouse brains process information from their other senses, or how the brain controls movement.

The tools can also be adapted for use in rats and other mammalian species.

To trace the neural pathways, mouse brains were injected with two viruses, one after the other.

The researchers first injected a low-grade virus into the higher centers of a mouse brain, where it infected nearby neurons. The first virus left the neurons susceptible to infection by the second virus, which was injected two weeks later. The second virus—fluorescent red in color—was designed by collaborator Edward Callaway at the Salk Institute.

Genes introduced by the first virus allowed the next virus to infect its way from the higher brain to the olfactory bulb, going in the opposite direction of scent signals. By following the backward progress of the second virus, the scientists identified the neurons in the olfactory bulb where the virus ended up, thanks to the red fluorescence.

The scientists then sliced each mouse brain into about 60 thin sections, and took photos of all of them through a microscope. They then combined the images from 35 mice into a 3-D model of the olfactory bulb, allowing them to look for patterns between where the virus started in the higher brain centers and where in the olfactory bulb it finished its journey.

Professor Liqun Luo, left, in his lab with postdoctoral fellow Kazunari Miyamichi. (Credit: L.A. Cicero)

Trigger for innate fear

They found that most of the nerve pathways heading to the higher processing centers that direct the mice’s innate like or dislike of certain odors, and trigger a response to them, originated from one region—the top part of the olfactory bulb. This could explain how the mouse brain directs the animal’s innate fear response to cat or fox urine.

This is in contrast to the neurons heading to the brain areas which process learned responses to odor. The neurons associated with learned responses are scattered all over the olfactory bulb, and their relative lack of organization could reflect their flexibility in allowing the mice to learn to avoid or be attracted to new smells.

The group also found that each neuron in the brain’s higher centers receives signals from at least four neurons in the olfactory bulb, each of which receives input from a large number of like odor receptors. This progressive funneling and processing helps explain how the brain integrates the information from many different odors, Luo says.

“There might be similar organizational principles in flies and mice, despite the evolutionary distance between them.”

Luo will use the techniques in this study to take a more detailed look at other parts of the mouse olfactory bulb and brain, with the eventual goal of understanding how the brain processes specific odors.

The study was funded by the Howard Hughes Medical Institute and the National Institutes of Health.

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