Axon guidance in the dopamine system.

Meso-diencephalic dopamine neurons (mdDA) neurons are located in the retrorubral field (RRF), substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) and give rise to prominent ascending axon projections. These so-called mesotelencephalic projections are organized into three main pathways: the mesostriatal, mesocortical and mesolimbic pathways. Mesotelencephalic pathways in the adult nervous system have been studied in much detail as a result of their important physiological functions and their implication in psychiatric, neurological and neurodegenerative disease. In comparison, relatively little is known about the formation of these projection systems during embryonic and postnatal development. However, understanding the formation of mdDA neurons and their projections is essential for the design of effective therapies for mdDA neuron-associated neurological and neurodegenerative disorders. Here we summarize our current knowledge of the ontogeny of mdDA axon projections in subsystems of the developing rodent central nervous system (CNS) and discuss the cellular and molecular mechanisms that mediate mdDA axon guidance in these CNS regions.