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A window on the brain
New technique affords a view inside the brains of mice as they sense odors
“The anatomical definition is superb,” says Rockefeller University’s Peter Mombaerts, watching a computer monitor as tiny blobs of green and red appear, then intensify over a black-and-white image of a mouse brain. “We can identify each of the structures beautifully.”
Mombaerts and research associate Thomas Bozza are reviewing a movie of the brain of a living mouse as it is exposed to odors. Each time the mouse gets a whiff of a chemical called hexanal, which smells something like freshly cut grass, specific sections of the mouse’s brain light up. Mombaerts and Bozza are watching as mice, quite literally, process scent.
Making this possible is a technique, developed by Mombaerts, Bozza, and colleagues at Boston University, to monitor the flow of information from one brain cell to the next. The scientists can essentially see the tiny bursts of light as messages pass from one neuron to the next. The technique allows researchers to see 10 to 20 percent of the 2,000 structures in the brain’s olfactory bulb, which receives smell information from the nose (the other structures are too deep within the mouse brain to be seen).
The system promises to advance research on how animals, as well as humans, sense odors, and it may also help scientists develop new drugs that target a family of molecules in the brain known as G protein-coupled receptors (GPCRs). About a third of drugs on the market today, from Zyprexa (for schizophrenia) to Claritin (for allergy relief), act on GPCRs.
“Look,” Bozza says, pointing to his monitor, “it’s easy to see which specific parts of the olfactory bulb are being used as the mouse responds to the odors.”
Mombaerts, who is head of the Laboratory of Developmental Biology and Neurogenetics, and Bozza collaborated with Boston University researchers John P. McGann and Matt Wachowiak. The mouse strain used is genetically modified to produce a molecule called synapto-pHluorin in its olfactory sensory neurons. The molecule is a fusion of a pH-sensitive green fluorescent protein — the same type used by many scientists to track gene activity in cells — and a nerve cell protein called VAMP2. Bozza developed the mouse strain to study olfactory physiology in a live animal.
The researchers anaesthetized the mice, then surgically thinned their skulls until they were virtually transparent. They then placed the mice under a microscope and shined blue light — the wavelength that causes the green fluorescent protein to glow — onto them. The microscope snapped a series of digital images through each mouse’s skull as a nozzle sprayed short bursts of hexanal nearby.
The initial imaging was done by Wachowiak at Boston University. Bozza is reproducing the system in the Mombaerts lab in the Bronk building.
The technique allows the researchers to return to the same mouse repeatedly to track changes in brain activity. This option is not available with other imaging techniques because the animals must be euthanized before their brains can be analyzed.
(Anticipating a wide demand, the genetically engineered mice have been sent to The Jackson Laboratory in Bar Harbor, Maine, which will distribute them to researchers later this year.)
The olfactory system is very suitable for this type of imaging because of its unusual organization, Mombaerts explains. Each neuron releases information into the synapse through the cell’s structure called an axon. “Several thousand axons, all of the same specificity, terminate in the same area of the brain, so the density of synapses is extraordinarily high. That’s probably why we can see the signals so clearly,” says Bozza.
To understand the sense of smell, scientists 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 1,000 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, apart from a few exceptions, no chemist can do now.”
The next step is to transfer that knowledge to drug developers who need better data about the locations of specific receptors in order to design drugs that target them. Ligand-less receptors are known as orphans in the pharmaceutical industry. The technique of visualizing the brain as it senses odors also may help clarify the roles these orphan receptors play in many body processes.
“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.

May 14, 2004



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