In real time in a living animal, scientists have observed regions of the brain as they respond to odors. The Rockefeller University study with mice, reported as the cover story in the April 8 issue of the journal Neuron, promises to advance research on how animals, as well as humans, sense odors.
"The anatomical definition is superb," says Rockefeller University's Peter Mombaerts, M.D., Ph.D., who led the study. "We can identify the structures in the brain that respond to odorants beautifully with this technique."
The odorant receptors belong to a family of molecules called G protein-coupled receptors (GPCRs), and GPCRs are targeted by more than a third of the drugs on the market today. Among drugs that target GPCRs are Zyprexa (schizophrenia) and Claritin (allergy).
"As we find more ligands and better understand the structure and function of GPCRs, in the long run that's going to be useful for drug development," says Mombaerts, professor and head of the Laboratory of Developmental Biology and Neurogenetics.
Mombaerts says that the technique of visualizing the brain as it senses odors also may help clarify the role of "orphan receptors" in the field of drug discovery.
Mombaerts and first author Thomas Bozza, Ph.D., who developed the mouse strain, collaborated with Boston University researchers John P. McGann, Ph.D., and Matt Wachowiak, Ph.D., to image the areas of in the mouse olfactory bulb that are activated when the animal is exposed to a particular odor.
In this study, a mouse is genetically modified to produce in its olfactory neurons a molecule called synapto-pHluorin. This is a fusion of a pH-sensitive version of the green fluorescent protein (GFP) used by many researchers for tracking gene activity in cells, and a nerve cell protein called VAMP2.
The researchers imaged brain activity of the synapto-pHluorin mouse through the top of the animal's skull, which had been surgically thinned while the animal was anaesthetized. Synaptic firing can be imaged in 10 to 20 percent of the 2,000 glomeruli, structures in the brain's olfactory bulb that receive "smell" information from the receptors of the olfactory neurons that line the nose. The other glomeruli reside too deep within the mouse brain.
The olfactory system is very suitable for this type of imaging because the olfactory system has an unusual synaptic organization, Mombaerts explains.
"Several thousand axons, all of the same specificity, terminate in the same glomeruli, so the density of synapses is extraordinarily high, which is probably why we can see the signals so clearly," says Mombaerts.
A major advantage of the new technique is that researchers can return to the same synapto-pHluorin mouse repeatedly to track changes in brain activity, an option that is not available with other imaging techniques because the animals typically need to be sacrificed for analysis of their brains.
According to Mombaerts, to understand the sense of smell, scientists need to relate molecules, called ligands, to odorant receptors, which are either stimulated or blocked by the ligands. Currently, scientists have identified only a handful of ligands that bind to odorant receptors in mice, rats and people.
"Ideally, we would like to have an enormous data set with 100,000 chemicals and 1000 odorant receptors - a hundred million combinations - and figure out exactly at a given concentration what receptor is stimulated by what odorant," says Mombaerts.
"At that point, we will be able to understand the sense of smell because we will be able to predict the quality of an odor, which no chemist can do now, apart from a few exceptions."
The problem of identifying ligands that bind to receptor molecules in the body extends to the search for therapeutic drugs. Ligand-less receptors are known as "orphans" in the pharmaceutical industry, and Mombaerts says that the synapto-pHluorin mouse could help this effort as well.
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