Neurotransmitters and the development of neuronal circuits.

In the mature brain, neurotransmitters are used for synaptic communication between neurons. But during nervous system development, neurons often express and release transmitters before their axons establish contacts with their target cells. While much is known about the synaptic effects of neurotransmitters, their extrasynaptic effects are less understood. There is increasing evidence that neurotransmitters in the immature nervous system can act as trophic factors that influence different developmental events such as cell proliferation and differentiation. However, more recent work demonstrates that neurotransmitters can also influence the targeting of migrating neurons and growing axons during the formation of neuronal circuits. This chapter will focus on such guidance effects of neurotransmitters during the development of the nervous system. Elucidating extrasynaptic functions during the nervous system development might also provide insights in their potential roles for plasticity and regeneration in the adult nervous system.