20 Adult Neurogenesis in the Olfactory Bulb

Most organisms rely on an olfactory system to detect and analyze chemical cues from the external world in the context of essential behavior. From worms to vertebrates, chemicals are detected by odorant receptors expressed by olfactory sensory neurons, which send an axon to the primary processing center—the olfactory bulb, in vertebrates. Within this relay, sensory neurons form excitatory synapses with projection neurons and with inhibitory interneurons. Thus, due to complex synaptic interactions in the olfactory bulb circuit, the output of a given projection neuron is determined not only by the sensory input, but also by the activity of local inhibitory interneurons that are concerned by adult neurogenesis throughout life. Recent studies have provided clues about how these new neurons incorporate into preexisting networks, how they survive or die once integrated into proper microcircuits, and how basic network functions are maintained despite the continual renewal of a large percentage of neurons. We know that external influences modulate the process of late neurogenesis at various stages. Thus, this process is probably flexible, allowing brain performance to be optimized for its environment. But optimized how? And why? This chapter describes the adaptation of new interneuron production to experience-induced plasticity. In particular, how the survival of newly generated neurons is highly sensitive not only to the level of sensory inputs, but also to the behavioral context is discussed. Also discussed is how neurogenesis may finely tune the functioning of the neural network, optimizing the processing of sensory information. Adult neurogenesis maintains continual turnover...