Differences in the Neurons That Project from the Dorsal Column Nuclei to the Diencephalon,Pretectum, and Tectum in the Cat

The dorsal column nuclei (DCN) project to a number of targets in the nervous system besides the ventroposterolateral nucleus (VPL) of the thalamus. Recent evidence obtained using double-labeling techniques indicates that DCN's diencephalic-projecting neurons differ in their location and morphology from those that project to some of its other targets, such as the cerebellum and tectum. The purpose of the present study was to characterize anatomically the DCN neurons that project another of DCN's targets, the pretectum, and to determine if any of these neurons have collateral projections to the tectum or diencephalon.

The projections were studied using two double-labeling methods. One method made use of either tritiated inactivated horseradish peroxidase (³H]apoHRP) or tritiated N-acetyl wheatgerm agglutinin (³H]WGA) as a marker and HRP or WGA conjugated to HRP. The other method made use of the dyes Fast Blue and Nuclear Yellow. In each cat, one marker was injected into the DCN-recipient portions of the pretectum, tectum, or diencephalon, and the other marker was injected into another of these three targets.

Neurons labeled by pretectal or tectal injections were of all sizes, fusiform and multipolar in shape, and similarly located. They were scattered through the rostral zone of DCN, but were distributed at the periphery of and at the junction between the gracile and cuneate nuclei in DCN's middle and caudal zones.

In contrast to the pretectal-and tectal-labeled neurons, neurons labeled by diencephalic injections were round and large. They were found throughout the DCN complex, but were concentrated in DCN's middle and caudal zones. When both the pretectum and diencephalon were injected in the same cat, the two groups of neurons occupied similar locations in the rostral zone, but were distinct in the middle and caudal zones, with the pretectal-projecting neurons surrounding the clusters of diencephalic-projecting neurons. Very few neurons were double-labeled.

These results demonstrate that the projections to the pretectum, tectum, and diencephalon originate from different populations of neurons within specific domains in DCN. When these results are compared with the results of electrophysiological and other anatomical studies, it appears that the pretectal- and tectal-projecting neurons may be part of a previously unrecognized system originating in DCN. In contrast with the well-known lemniscal system, recognized for its function in tactile discrimination, and composed of DCN's VPL-projecting neurons together with VPL's projections to the cerebral cortex, this other system may serve some role in the regulation of posture or the coordination of movement.