Chapter I: The organization and circuits of mesencephalic dopaminergic neurons and the distribution of dopamine receptors in the brain

The organization of the main dopaminergic cell groups in the brain, located in the ventral mesencephalic tegmentum, and the circuits in which they are inserted are reviewed here, with emphasis on rodents. Subdivisions based on cytoarchitecture (substantia nigra, ventral tegmental area and related nuclei, retrorubral field), dopaminergic phenotype (A8, A9 and A10 cell groups) and organization in dorsal and ventral tiers are discussed and compared. Dendritic release and gap junctional protein expression, interactions with glial cells, molecular and cellular features of the chemical repertoire of midbrain dopaminergic neurons and their main inputs are also reviewed. An account is given on basal ganglia circuits, including the organization of the direct, indirect and hyperdirect pathways of information processing and dopamine modulation of these pathways. Data on the dopaminergic innervation of limbic structures, including the extended amygdala, and the distribution and laminar organization of dopaminergic fibers in the cerebral cortex are summarized. The last part of the chapter focuses on the distribution of dopamine receptor subtypes and their relative densities in different brain structures. For each of the D1, D2, D3, D4 and D1B/5 receptors, an overview and distributional maps are provided, followed by data on their localization in the rat basal ganglia, cerebral cortex and limbic system, and a comparison with findings obtained in the human and nonhuman primate brain. This chapter thus presents an overview, at the molecular, cellular and systems levels, of central dopaminergic circuits involved in state-setting modulatory systems, generation and integration of motor behavior, cognitive functions and reward mechanisms.