Interneurons in the Olfactory Bulb: Roles in the Plasticity of Olfactory Information Processing

Abstract

Odor information is substantially modulated in the first relay of the central nervous system, the olfactory bulb (OB). This chapter focuses on how the large number of local inhibitory interneurons in the OB contribute to the structural and functional plasticity of the OB neuronal circuits, and odor information processing within them. The major OB interneurons, granule cells (GCs) and periglomerular cells (PGCs), form the characteristic synaptic structures, called dendrodendritic reciprocal synapses with excitatory projection neurons, the mitral and tufted cells. Dendrodendritic synaptic inhibition of OB interneurons induces feedback inhibition, lateral inhibition, and synchronization to mitral/tufted cells. Dendrodendritic synaptic inhibition is regulated by various signals, including olfactory sensory inputs, top-down inputs from the olfactory cortex (OC), and neuromodulatory signals. Outputs from mitral/tufted cells are in turn substantially influenced by these signals via the plasticity of dendrodendritic inhibition of OB interneurons. A second intriguing property of OB interneurons is that they are continually generated throughout life, conferring high plasticity on the OB neuronal circuits. The life-and-death decision of adult-born GCs is regulated by sensory experience and the behavioral state of the animal, indicating that the fate decision of new OB interneurons is also under the control of multiple signals. Further, behavioral analysis of mice with suppression of adult neurogenesis revealed abnormalities in many kinds of odor-guided behaviors. These observations collectively indicate that the structural and functional plasticity of OB interneurons plays crucial roles in odor information processing, and that this plasticity contributes to the expression of proper olfactory behaviors.