The nigrostriatal nervous pathways in the brain of the cat. An autoradiographic study

By comparing the findings obtained in these series of cats with injections of 3H-leucine involving different areas of the SN and adjacent neurons of the VTA, some conclusions may be drawn concerning the origins, courses, and terminations of the nigrostriatal fibers. Three main bundles of nigrostriatal fibers originating from different groups of neurons of the SN may be identified. The main cluster of neurons of the compacta type, corresponding to the more abundant and larger neurons of this group in the caudal part of the SN and above (pars dorsalis of the SN), gives rise to fibers that leave the latter structure through its dorsomedial lip. They course successively in the VTA and in the area of Tsai and proceed rostralward through the lateral hypothalamus, subthalamus, and the internal capsule. At the level of the anterior limb of the internal capsule, they undergo an important arborization before ending in the putamen and caudate nucleus. Fibers from the main cluster of compacta neurons terminate more dorsally and those from the dorsal part of the SN end more ventrally in the neostriatum. These findings generally agree with those of VanderMaelen et al. (12) and Szabo (9), who applied the HRP technique, and Usunoff et al. (11), who used a selective silver impregnation method in cat brains. In the caudate nucleus, endings from the compacta type neurons are more abundant in certain areas, forming islands of dense silver grains. These areas of dense endings appear to correspond to the areas of greater cellular density of the striatum as more easily recognizable in the caudate nucleus of the newborn cat (Fig. 9).(ABSTRACT TRUNCATED AT 250 WORDS)