Postsynaptic dorsal column pathway of the rat. I. Anatomical studies

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Abstract
The projection to the dorsal column nuclei that originates in the spinal cord dorsal horn was examined using the retrograde transport of horseradish peroxidase (HRP). This projection in other animals has been called the postsynaptic dorsal column (PSDC) pathway and might be involved in nociception in the rat. Small iontophoretic injections of HRP into the cuneate nucleus (CN) labeled > 350 neurons in alternate sections within the ipsilateral gray matter of segments [cervical] C6-8. Fewer than 25 neurons were labeled in [lumbar] L4-6 by injections into CN. Injections of HRP confined to the gracile nucleus (GN) labeled > 200 neurons within a narrow band extending across the ipsilateral dorsal horn subjacent to substantia gelatinosa of L4-6. Fewer than 10 cells were labeled in C6-8 by such injections. Labeling in lumbar neurons following injections into GN was prevented by transection of the dorsal columns at [thoracic] T10, T8 or C2. Neurons labeled by such injections ascend entirely within the dorsal columns. Lesions of the dorsal columns in C2 reduced the number of labeled neurons in the cervical cord following CN injections by .apprx. 90%. Combined lesions of the dorsal columns and ipsilateral dorsal lateral funiculus (DLF) reduced the number of cells labeled in C6-8 by .apprx. 98%. The majority of labeled neurons in the cervical enlargement project to CN via the dorsal columns; a small secondary component of the cervical projection to CN appears to ascend within the DLF. The relative sizes of the projections to the dorsal column nuclei from PSDC neurons and dorsal root ganglion cells (DRG) were compared by counting labeled neurons in the gray matter of the cervical and lumbar enlargements and the corresponding DRG. In the 4 animals so examined, PSDC neurons constituted > 38% of the neurons that projected to CN and .apprx. 30% of the cells that projected to GN. The PSDC projection of the rat is capable of providing a large somatotopically organized input to the dorsal column nuclei.