Long ascending projections to the midbrain from cells of lamina I and nucleus of the dorsolateral funiculus of the rat spinal cord.

Small volumes (5-40 nl) of an aqueous solution of wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA-HRP) were injected unilaterally into midbrain structures of 18 adult, albino rats. In 17 of these preparations cells of many types were found to be retrogradely labeled in cervical and lumbar spinal cord segments. The data reported here concern the number and location of labeled cells from injection sites in the midbrain that affected two distinct cell populations: neurons within the marginal layer (lamina I cells) and neurons of the nucleus of the dorsolateral funiculus (NDLF cells). In ten of the preparations, only nine of which are reported in detail here, a total of 1,831 labeled lamina I cells were identified. In the lumbar enlargement they reached a density of more than 60 cells/mm. Of these, 85% projected to medial portions of the caudal, contralateral midbrain. Injection sites that were centered in the caudal periaqueductal gray (PAG) and/or in the immediately adjacent region of nucleus cuneiformis labeled the largest numbers of lamina I cells. Cells of the NDLF were retrogradely labeled in all preparations in which lamina I cells were labeled but they were also observed in five cases in which lamina I cells were not labeled. A total of 1,914 NDLF cells were labeled from all injection sites. These cells were found to have essentially a bilateral distribution with 57% of the cells located in the contralateral DLF. Although there is substantial overlap between the terminal fields of lamina I and NDLF cells within the midbrain, NDLF cells had a more diffuse target area encompassing the reticular core of the midbrain and PAG, bilaterally, while the target area for lamina I cells was comparatively discrete, being largely restricted to the more medially situated midbrain structures, contralaterally. Whether the terminations of lamina I cells in and near the PAG are from collaterals of spinothalamic neurons originating in lamina I, or a subclass of lamina I neurons that project exclusively to the midbrain, is not known. It is significant, however, that lamina I cells, known to be activated by noxious stimuli to the skin, should project to a region of the brain stem from which analgesia can be produced by electrical stimulation or by local application of opiates.