Smelly associations: flexibility in mouse odor learning through newborn neurons

You probably strongly associate your partner’s perfume with the person you love. A change in perfume can be nothing less than disconcerting at first, but over time you become accustomed to it (unless it’s a really bad choice, then you might want to give some advice in the perfume department..). Why is it that we can associate a smell with a person, but then also adapt this association to a new smell? Neuroscientists reported in the Journal of Neuroscience in April that (at least in mice) this is due to neurons born in the brain during adult life. We tend to think of the brain as something solidly build before birth, with only re-wiring and re-fining going on after birth. But neurons are constantly born from neural stem cells and migrate to the olfactory bulb, the part of the brain that is required for smelling.

Mouse olfactory bulb in coronal section.
Mouse olfactory bulb, in coronal section. In green are new neurons that migrated into the olfactory bulb, and granule cells.

In mice, newborn neurons migrate into the olfactory bulb where they become so-called granule cells. Granule cells are inhibitory interneurons, which means that they connect other neurons and send signals that dampen their excitability. In fact, the researchers show that most of the granule cells in the olfactory bulb are generated after birth, and integrate with existing neurons into neural circuits. But why do we need a constant stream of new neurons and new connections? To find out, the researchers switched off the newborn interneurons in the olfactory bulb by turning off the release of signals from these neurons. They then compared the performance of these treated mice in a habituation-dishabituation test with untreated mice. In this test, the mice were trained to associate one of two smells with a sugar reward. Both mice with switched-off newborn neurons and untreated mice respond more to the smell associated with a reward, digging longer near the reward-associated smell than the non-rewarded smell. So mice do not need the newly generated neurons to associate a smell with a reward. The researchers then tested if they could retrain the mice, so that they associate the other smell with the reward. For normal mice, this task is no problem. The treated mice are unable to make the switch, and spend the same amount of time digging near both smells. This suggests that the newly born neurons in the olfactory bulb give mice the flexibility to learn odor associations, and relearn them if the context changes.

Our brain keeps itself fresh, and allows you to deal with change when your partner decides to change things up to keep fresh too.

The original research paper appeared in the Journal of Neuroscience on April 23.