Evidence for the existence of homolateral and contralateral projections from the substantia nigra to the subthalamic nucleus in the rat

Hemichorea/ballism is a rare neurological disorder but the crucial involvement of the subthalamic nucleus (STN) in its pathophysiology is appreciated since decades. The idiopathic Parkinson’s disease is a common neurodegenerative disorder but the key role of the STN in the pathophysiological origin of the parkinsonian state became only recently evident. The STN was believed to exert an inhibitory, probably – GABA-mediated, effect on its projection nuclei, and this belief is one of the major reasons to overlook the involvement of the STN in the parkinsonian pathophysiology. It is now firmly established that the STN projection neurons are glutamatergic, excitatory, and heavily innervate by widely branching axons of the substantia nigra (SN), the internal pallidal segment (GPI), followed by the external pallidal segment (GPE) and the pedunculopontine tegmental nucleus (PPN).