Descending dopaminergic control of brainstem locomotor circuits

The locomotor role of meso-diencephalic dopaminergic (DA) neurons is traditionally associated with their ascending pathways to the basal ganglia, which project to the Mesencephalic Locomotor Region (MLR), a brainstem region controlling locomotion. However, descending DA projections to the MLR were recently reported in lamprey, where they promote locomotion through the activation of D-1 receptors in the MLR. In rodents, the DA innervation of the MLR is conserved and originates from the substantia nigra pars compacta (A9) and medial zona incerta (A13). Intriguingly, the DA innervation of the MLR degenerates in a monkey model of Parkinson's disease. Here, we review the current knowledge on these newly uncovered descending DA pathways to brainstem circuits and discuss their possible roles in locomotor control.