Three-dimensional distribution of nigrostriatal neurons in the rat: Relation to the topography of striatonigral projections

Functional regions of the rat striatum related to identified cortical terri tories were injected ionophoretically with wheat germ agglutinin coupled to horseradish peroxidase. Coronal serial sections were cut throughout the su bstantia nigra. The distributions of labelled striatal projections and nigr ostriatal neurons were studied. Using software developed in our laboratory, three-dimensional reconstructions were calculated which confirmed and exte nded the organizational scheme of striatonigral projections already reporte d by our group. These projections were organized as a set of longitudinal l amellae spatially organized so as to segregate the flow of information eman ating from striatal regions affiliated to sensorimotor and associative-limb ic cortical areas. In addition, the relationship between the striatonigral projections and the nigrostriatal neurons was studied by three-dimensional reconstruction. For each striatal injection site, two populations of retrog radely labelled nigral neurons could be discriminated by their position wit h respect to the striatal projection field. The first one occupied a proxim al position, in register with the labelled striatal projections, while the second was more distal. The populations of proximal neurons which innervate different functional striatal sectors were segregated both mediolaterally, dorsoventrally and rostrocaudally, while the populations of distal neurons were more scattered and showed a lesser degree of spatial segregation. The organization of these two populations with respect to the striatal proj ection fields suggests that the substantia nigra might control the how of c ortical information through the striatum via two different modalities, base d respectively on a closed nigrostriatal loop involving the proximal neuron s, and an open loop involving the distal ones. (C) 1999 IBRO. Published by Elsevier Science Ltd.