A comparison of the intranigral distribution of nigrotectal neurons labeled with horseradish peroxidase in the monkey, cat, and rat.

The location of neurons in the substantia nigra's pars reticulata (SNR) that send their axons to the superior colliculus was compared in the monkey, cat, and rat using the horseradish peroxidase (HRP) retrograde cell-labeling method. Although several cases of large, unilateral HRP deposits in the superior colliculus show that in all three species, the nigrotectal cells are confined, for the most part, to the rostral one-half of SNR, the following differences were noted in the precise location of the nigrotectal neurons and in the degree of bilaterality of the nigrotectal projection. In the monkey, labeled nigrotectal cells were particularly numerous in the extreme rostrolateral portion of SNR. From this region of high concentration, a progressively decreasing number of cells spreads medially in a ventral stratum immediately dorsal to the pes pedunculi. No labeled cells were found in the extreme medial part of SNR. A substantial number of HRP-positive cells were present in the contralateral SNR in a similar distribution. In the cat, labeled cells were less selectively localized in SNR's mediolateral expanse, being distributed more or less randomly in its middle portion with a scattering of cells in both medial and lateral parts of SNR. Although some cell labeling occurred in the contralateral SNR, it was less substantial than in the monkey. In the rat, the HRP-positive cells were especially concentrated throughout the mediolateral extent of a ventral stratum of SNR immediately dorsal to the pes pedunculi. Although some cells were located more dorsally, they were far fewer in number and consistently less heavily labeled. Only one or two labeled cells could be detected in the contralateral SNR of the rat. These anatomical differences suggest that the influence of the corpus striatum on the tectal control of orienting responses may vary considerably from one mammalian species to the next.