Dopaminergic neurons in the brain exist in the ventral mesencephalon, caudal diencephalic periaquaductal gray and hypothalamic regions. The neurons in each region differ markedly in function and molecular characteristics.
The dopaminergic neurons in the ventral mesencephalon originate in the floor and basal plates of the mesencephalon and caudal diencephalon and are therefore called 'mesodiencephalic dopaminergic (mdDA) neurons'. They are mainly located in two groups:
Meso-diencephalic dopaminergic precursor cells proliferate in the ventricular zone of the floor and basal plates of the mesencephalon and the caudal diencephalon (prosomers 1,2,3). It is postulated that the cells that finally reside in the SNC originate from the diencephalic basal plate, while the VTA dopaminergic neurons originate from the mesencephalic floor plate.
Following signals from the isthmus organizer (IsO, at the midbrain-hindbrain border) and the floor plate, the precursor cells differentiate into dopaminergic progenitor cells. This process occurs during their migration from the ventricular zone, along radial glia towards their final destinations in the tegmental mantle layer.
Mature embryonic dopaminergic neurons residing in the SNC and VTA send their growth cones towards the telencephalon. At the embryonic stage, VTA and SNC dopaminergic neurons project unselectively to the striatum and cortex. The refinement of fibers to form the 'nigro-striatal' and 'meso-limbic' pathways only occurs post-natally.