Journal of Neuroscience, Vol 1, 121-125, Copyright © 1981 by Society for Neuroscience
A comparison of the intranigral distribution of nigrotectal neurons labeled with horseradish peroxidase in the monkey, cat, and rat
RM Beckstead, SB Edwards and A Frankfurter
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.
