Synaptic organization of the olfactory bulb based on chemical coding of neurons

Olfaction is one of the chemical senses in both vertebrate and invertebrate animals essential for a variety of social behaviors. Recent molecular biological and physiological studies using optical recording have indicated elaborate mechanisms in the main olfactory bulb for processing input from olfactory receptor neurons and control of output to higher centers in the brain. The current challenge is to identify a structural basis for understanding such elaborate molecular and functional organization. Immunocytochemistry and other advanced technologies have enabled us to label bulbar neurons selectively, and they have shown that the olfactory bulb has much greater heterogeneity in chemical and structural neuronal organization and in synaptic connectivity than previously believed. This review describes the structural aspects of the main olfactory bulb of rats and summarizes the findings for its synaptic organization based on chemical coding of neurons. Current uncertainties and issues that need to be clarified in the future are also discussed.