Differential Reductions in Dopaminergic Innervation of the Motor-Related Areas of the Frontal Lobe in MPTP-Treated Monkeys
Unexpectedly widespread dopaminergic innervation has been recognized in the primate cerebral cortex. The cortical dopaminergic fibers are distributed throughout the frontal-tooccipital extent, with the highest density in the prefrontal and motor-related areas of the frontal lobe.1-10Previous anatomical studies have shown that multiple motor-related areas of the frontal lobe, such as the primary motor cortex (MI), the premotor cortex (PM), and the supplementary motor area (SMA), contain dopaminergic fibers.1–3,7,8,10 The dopaminergic innervation of the motor-related areas is diffuse, involving all cortical layers.3,7,8,10 On the other hand, the cortical dopaminergic fibers as a whole arise from the three catecholaminergic cell groups—A8, A9, and A 10—that are located in the ventral mesencephalon.3 It has been reported that the cells of origin of the mesocortical dopamine projections to the dorsal frontal cortex, including the motor-related areas, are distributed predominantly in the dorsal aspects of the A8–A10 complex.11,12 In primates, the dorsal components of these catecholaminergic cell groups largely correspond to the dorsal part of the retrorubral area, pars gamma (i e, the dorsal tier) of the substantia nigra pars compacta, and the parabrachial pigmented nucleus of the ventral tegmental area, respectively. Given that subpopulations of dopaminergic neurons in such dorsal components may issue axon collaterals to both the frontal cortex and the striatum (for data in the rat, see refs.13,15), it is most likely that at least part of the dopaminergic fibers in the motor-related areas are affected in Parkinson’s disease which is well known to be caused by the extensive loss of dopaminergic nigrostriatal neurons.
KeywordsVentral Tegmental Area Supplementary Motor Area Primary Motor Cortex Ventral Mesencephalon Immunoreactive Fiber
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