Journal of Neuroscience, Vol 10, 2672-2686, Copyright © 1990 by Society for Neuroscience
Afferent input to nucleus submedius in rats: retrograde labeling of neurons in the spinal cord and caudal medulla
RJ Dado and GJ Giesler Jr
Department of Cell Biology and Neuroanatomy, University of Minnesota, Minneapolis 55455.
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.
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