This 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 could identify 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 used a sophisticated algorithm to combine the images from 35 mice into a 3-D model of the olfactory bulb designed by graduate students Fernando Amat and Farshid Moussavi in Professor Mark Horowitz's electric engineering group. This allowed 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.
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 in
|Contact: Louis Bergeron|