Afferent input to nucleus submedius in rats: retrograde labeling of neurons in the spinal cord and caudal medulla.

In cats, spinal and medullary input to the thalamic nucleus submedius (Sm) arises almost exclusively from neurons in the marginal zone. As a result, it has been proposed that Sm may be specifically involved in nociception. In the present study, we determined the locations of neurons in the spinal cord and caudal medulla that project to Sm in rats. Iontophoretic injections of Fluoro-Gold or pressure injections of Fast blue were made into Sm. In each of the 6 rats that received small injections of Fluoro-Gold into Sm, only a small number (mean = 90) of retrogradely labeled neurons were found throughout the 18 segments of the spinal cord examined. Surprisingly, almost no labeled neurons (less than 1%) were counted in the marginal zone of the spinal cord. The majority were located in the deep dorsal horn and intermediate zone/ventral horn. In contrast, many neurons were labeled in the marginal zone of nucleus caudalis. Injections of Fluoro-Gold into any of a number of nuclei near Sm also labeled only a small number of neurons in the spinal cord and almost no neurons in the marginal zone. Using identical injection parameters, we injected Fluoro-Gold into the ventrobasal complex or posterior thalamic group. Hundreds of neurons in the spinal cord, including many in the marginal zone, were labeled following these injections. These results indicate that the techniques used to inject Fluoro-Gold into Sm were capable of labeling many projection neurons, including those in the marginal zone. Larger pressure injections of Fast blue were also made into Sm of 3 rats. The distribution of labeled neurons in nucleus caudalis and the spinal cord was similar to that following iontophoretic injections of Fluoro-Gold. Again, few marginal zone neurons were labeled in the spinal cord in any of these rats. Therefore, our results indicate that few spinothalamic tract neurons appear to project to Sm or any of several adjacent nuclei, and virtually no marginal zone neurons in the spinal cord project to these areas.