The brain's olfactory bulb lets you savor the wafting odors of the cappuccino in the classroom next door. Researchers suspect that the bulb may also send a mental alert when your labmate brings back the newest caffeinated concoction from the coffee shop. That might be because the olfactory bulb is one of the few parts of the brain that receives a lifelong infusion of fresh neurons, a restocking that could help mammals learn how to recognize new smells. Now a study shows that mice with fewer new neurons can't distinguish a familiar and a new odor--leading some to conclude that these neurons are important for learning and memory.
In people, monkeys, and rodents, the olfactory bulb gets a continuous supply of fresh nerve cells from the forebrain. It's been unclear whether the new neurons simply replace dying cells or allow the nose to learn new tricks. To find out, a team led by neurobiologist Pierre-Marie Lledo of the Institut Alfred Fessard in Gif-sur-Yvette, France, decided to see what would happen if they sabotaged this replenishment in mice.
The researchers mutated a gene that's necessary for new neurons to migrate into the olfactory bulb. Although these mice had olfactory bulbs that were 35% smaller than normal, that had no effect on their ability to smell: The knock-out mice were just as sensitive to diluted scents as normal mice. But then the team tested whether mice accustomed to one scent--paprika--noticed when the scent was replaced with a second scent--cinnamon. All normal mice perked up their noses at a paper strip soaked with a new scent, but the experimental mice didn't seem to notice the change, the researchers report in the 15 February Proceedings of the National Academy of Sciences.
Lledo suggests that faulty memory is to blame: He claims the mice fail to learn the first scent and thus don't notice when a new scent is introduced. Most other researchers remain unconvinced, although neurobiologist Arturo Alvarez-Buylla of Rockefeller University in New York says the research is a good starting point for understanding how mice sniff out new odors.