Abstract
Although the architecture of a dopaminergic (DA) system within the primary motor cortex (M1) was well characterized anatomically, its functional significance remained obscure for a long time. Recent studies in rats revealed that the integrity of DA fibers in M1 is a prerequisite for successful acquisition of motor skills. This essential contribution of DA for motor learning is plausible as it modulates M1 circuitry at multiple levels thereby promoting plastic changes that are required for information storage: at the network level, DA increases cortical excitability and enhances the stability of motor maps. At the cellular level, DA induces the expression of learning-related genes via the transcription factor c-Fos. At the level of synapses, DA is required for the formation of long-term potentiation, a mechanism that likely is a fingerprint of a motor memory trace within M1. DA fibers innervating M1 originate within the midbrain, precisely the ventral tegmental area (VTA) and the medial portion of substantia nigra (SN). Thus, they could be part of the meso-cortico-limbic pathway – a network that provides information about saliency and motivational value of an external stimulus and is commonly referred as “reward system.” However, the behavioral triggers of the release of dopamine in M1 are not yet identified. As alterations in DA transmission within M1 occur under various pathological conditions such as Parkinson disease or ischemic and traumatic brain injury, a deeper understanding of the interaction of VTA/SN and M1 may reveal a deeper insight into a large spectrum of neurological disorders.
Highlights
The primary motor cortex (M1) receives dopaminergic (DA) projections from mesencephalic brainstem nuclei
This essential contribution of DA for motor learning is plausible as it modulates M1 circuitry at multiple levels thereby promoting plastic changes that are required for information storage: at the network level, DA increases cortical excitability and enhances the stability of motor maps
Cortically projecting DA neurons in primates can be found beyond ventral tegmental area (VTA) and substantia nigra (SN): in owl and rhesus monkeys, 15% of DA efferents toward M1 are located within the retrorubral field [RRF, referred to as region A8; [2, 47]], a region that completely lacks mesocortically projecting neurons in rodents [48, 49]
Summary
The primary motor cortex (M1) receives dopaminergic (DA) projections from mesencephalic brainstem nuclei The integrity of this DA meso-cortical pathway has been recently shown to be a prerequisite of successful motor learning in rats. Rats were trained in a single-pellet reaching task [8], while selective destruction of DA terminals was performed by injecting 6-hydroxi-dopamine (6-OHDA) either directly in M1 or within the rostral VTA/SN, both contralateral to the preferred paw. DA depletion of M1 resulted in an impaired gain in motor performance between subsequent training sessions (inter-session learning) when compared to controls. This impairment could be resolved by substituting levodopa directly within M1 using osmotic mini-pumps.
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