Localization of nigrostriatal, nigrothalamic and nigrotectal neurons in ventricular coordinates in macaques

The topography of the substantia nigra and its subdivisions was first analysed in macaques by using a topographic technique based on ventricular landmarks. This study shows the stability of the contours of the substantia nigra and its subdivisions in various species of macaques. The anteroposterior sequence of four subdivisions was standardized by defining eight verticotransverse levels, regularly interspaced and systemically used for each experimental case. Neurons of the substantia nigra sending axons to the striatum, thalamus and superior colliculus were identified by the technique of retrograde transport of horseradish peroxidase. The nigrostriatal labeled neurons were essentially located in the ipsilateral pars compacta but also scattered dorsally in the pars mixta and ventrally in the pars reticulata. In addition, the existence of a crossed nigrostriatal pathway was demonstrated in monkeys. Nigrothalamic labeled neurons were found in the whole pars reticulata at rostral levels and only in the medial part at more caudal levels. Comparatively, nigrotectal labeled neurons were also found in the whole pars reticulata at rostral levels, but caudally, they were confined to the lateral part of the pars reticulata and the pars lateralis. It thus appears that these three nigral components may overlap at some levels of the substantia nigra. This is discussed in relation to the existence of branched axons already documented. However, the present results underline the strong tendency of the nigrotectal neurons to be segregated from the nigrothalamic ones and to be laterally located in monkeys. In addition, two nigrotectal components have been identified on the basis of their topography and their somata size: one with large somata located in the pars lateralis, probably specific to primates, and the other with smaller somata located in the pars reticulata. These two components may indicate the existence of two different functional systems.