A topographic re-evaluation of the nigrostriatal projections to the caudate nucleus in the cat with multiple retrograde tracers.

The anatomical organization of the cat nigrostriatal projections to the caudate nucleus was studied by retrograde tracer techniques. Horseradish peroxidase conjugated with wheat germ agglutinin and fluorescent retrograde tracers such as Fast Blue and Diamidino Yellow were injected concomitantly in different regions of the caudate nucleus. The distribution of single and double retrogradely labeled neurons was analysed in the substantia nigra pars compacta, substantia nigra pars lateralis, retrorubral area and adjacent ventral tegmental area. Adjacent sections processed for acetylcholinesterase were used as histochemical markers for the densocellular zone of the substantia nigra. The main findings of this study are: (1) The rostral caudate nucleus receives projections mainly from the caudal substantia nigra while the caudal caudate nucleus receives projections from all rostrocaudal levels of the substantia nigra. (2) The substantia nigra pars lateralis projects very specifically to the caudal caudate nucleus. (3) The ventral retrorubral area close to the medial lemniscus projects to all rostrocaudal levels of the caudate nucleus. (4) The rostral caudate nucleus receives projections mainly from the medial substantia nigra while more caudal sectors of the caudate nucleus receive projections from the medial and lateral substantia nigra. (5) A dorsoventral inversion of nigrostriatal projections from the medial substantia nigra pars compacta and the adjacent ventral tegmental area to the caudate nucleus was established. In contrast, we found zones within the retrorubral area projecting both to the dorsal and ventral caudate nucleus. (6) Distant injections of two different fluorescent tracers regarding both the dorsoventral and the rostrocaudal coordinates, yielded double-labeled neurons that were mainly located in the medial and caudal portions of the substantia nigra and in the ventral retrorubral area. However, the number of double-labeled neurons was higher after separated injections in the dorsoventral axis, suggesting that the collateralization to the caudate nucleus occurs mainly in the dorsoventral plane. (7) A clustering organization of nigrostriatal cells projecting to the caudate nucleus was detected mainly in the intermediate rostrocaudal part of the substantia nigra pars compacta and in the retrorubral area. The results of this comprehensive study on the cat nigrostriatal pathway to the caudate nucleus show novel findings on the anatomical organization of the nigrostriatal projections which might help the understanding of the complex architecture of nigral neurons projecting to the caudate nucleus in carnivores.