The overlap of spinothalamic and dorsal column nuclei projections in the ventrobasal complex of the rat thalamus: a double anterograde labeling study using light microscopy analysis

Projections from the spinal cord and the dorsal column nuclei (DCN) to the ventrobasal complex of the thalamus (VB) were studied in the rat by using double anterograde labeling strategy. This strategy was based on the injection of 3H-leucine into the DCN and of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the spinal cord and their subsequent transport. Adjacent 30-micron-thick sections were then processed differentially for autoradiography or for HRP by using tetramethyl benzidine (TMB) as a chromogen. Similar areas of the ventrobasal complex were labeled, in adjacent sections, after a large injection of 3H-leucine into the DCN and when wheat germ agglutinin-HRP had been injected in any part of the spinal cord. If, however, a small injection of the radioactive tracer was centered in the gracile nucleus and compared with an injection of WGA-HRP placed in the lumbar enlargement of the cord, the rostral and dorsal portions of the lateral VB were labeled from both sources. On the other hand, if tritiated leucine was injected into the cuneate nucleus, and WGA-HRP placed in the cervical enlargement, then the caudal and ventral portions of the lateral VB demonstrated overlap of both labels. The present results show that, in the rat, areas of termination of both the spinothalamic tract and the lemniscal pathway originating from the DCN overlap in the lateral VB. This overlap is somatotopically organized, thus indicating that the same area of the VB receives somatic inputs from one particular part of the body through both pathways. These results are discussed in comparison to those of comparable studies performed in the cat and in the monkey and with reference to the electrophysiological data that have demonstrated that, in the rat VB, neurons responding to noxious stimulation are intermingled with neurons exclusively responding to non-noxious stimulation.