Crossed connections of the substantia nigra in the rat

The existence of crossed multisynaptic pathways that allow for the interdependent control of activity in one substantia nigra and its contralateral counterpart has been inferred from a number of recent biochemical and neurophysiological investigations. This prompted a reexamination of the connections of the substantia nigra with an emphasis on crossed inputs to and crossed projections from that nucleus. Male albino rats received 20-50-nl pressure injections of a 1% wheat germ agglutinin-conjugated horse-radish peroxidase (WGA-HRP) solution into the substantia nigra or into surrounding areas as controls. Following a 24-hour survival period the animals were processed according to the tetramethylbenzidine protocol for the visualization of HRP. The pattern of anterograde transport of WGA-HRP after substantia nigra injections, confirming for the most part previous reports, demonstrated ipsilateral nigral efferent projections to the striatum; globus pallidus; subthalamic nucleus; the lateral dorsal, paralamellar mediodorsal, ventromedial, and parafascicular thalamic nuclei; central gray, midbrain reticular formation; superior colliculus; and peribrachial area, including the pedunculopontine nucleus. Additionally, the nigral projections to the paralamellar mediodorsal and ventromedial thalamic nuclei and to the superior colliculus were demonstrated to be bilateral. Most of these connections were confirmed by the complementary retrograde experiment. In accordance with previous reports, intranigral WGA-HRP injections retrogradely labeled neurons located in the ipsilateral prefontal cortex, motor cortex, striatum, globus pallidus, central nucleus of the amygdala, anterior hypothalamic area, subthalamic nucleus, and dorsal raphe. Additionally, labeled perikarya were observed in the ipsilateral parafascicular thalamic nucleus, in the contralateral posterior lateral hypothalamic area, and in the ipsilateral and contralateral peribrachial-pedunculopontine area. These latter nigral afferents were confirmed with complementary WGA-HRP injections into each of the regions of origin. While bilateral peribrachial-pedunculopontine innervation of the substantia nigra has been reported in the cat there has been no previous demonstration of a crossed nigral afferent system from the contralateral posterior lateral hypothalamic area. The results are discussed with reference to the pathways that may mediate the interdependent control of the activity of neurons in the left and right substantia nigra. Additionally, the association of the substantia nigra with a variety of neuronal circuits, including the cerebellofugal, tectothalamic, thalamocortical, thalamostriatal, and basal ganglia pathways, are discussed.