The Functional Role of Olfactory Bulb Granule Cell Subtypes Derived From Embryonic and Postnatal Neurogenesis

Hiroo Takahashi, Seiichi Yoshihara, Akio Tsuboi

It has been shown in a variety of mammalian species that sensory experience can regulate the development of various structures, including the retina, cortex, hippocampus, and olfactory bulb (OB). In the mammalian OB, the development of dendrites in excitatory projection neurons, such as mitral and tufted cells, is well known to be dependent on odor experience. Odor experience is also involved in the development of another OB population, a subset of inhibitory interneurons that are generated in the ventricular-subventricular zone throughout life and differentiate into granule cells (GCs) and periglomerular cells. However, the roles that each type of interneuron plays in the control of olfactory behaviors are incompletely understood. We recently found that among the various types of OB interneurons, a subtype of GCs expressing the oncofetal trophoblast glycoprotein 5T4 gene is required for odor detection and discrimination behaviors. Our results suggest that embryonic-born OB interneurons, including 5T4-positive GCs, play a crucial role in fundamental olfactory responses such as simple odor detection and discrimination behaviors. By contrast, postnatal- and adult-born OB interneurons are important in the learning of more complicated olfactory behaviors. Here, we highlight the subtypes of OB GCs, and discuss their roles in olfactory processing and behavior, with a particular focus on the relative contributions of embryonically and postnatally generated subsets of GCs in rodents.