Catecholaminergic Innervation of the Spinal Cord in the North American Opossum, Didelphis virginiana

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Abstract
Axons presumed to contain catecholamines were visualized in the spinal cord of the North American opossum using antibodies to tyrosine hydroxylase (TH) by an indirect antibody peroxidase-antiperoxidase (PAP) technique. Axons showing TH-like immunoreactivity (TH-IR) coursed primarily in the dorsal part of the lateral funiculus, although they were also present more ventrally. An occasional TH-IR axon was seen in the dorsal funiculus. Within the gray matter, TH-IR axons were distributed most densely within the intermediolateral cell column (ILC). Such axons were more numerous in the presumptive sacral parasympathetic nucleus than within the adjacent gray matter, but their density was less than that within the ILC. Many TH-IR axons were also found within laminae I-V and X, but the density of innervation in the ventral horn was relatively low. The brainstem origin of TH-IR axons in the spinal cord was studied using a combination of the retrograde transport of fluorescent dyes and immunofluorescence as well as the retrograde transport of horseradish peroxidase and PAP immunohistochemistry. The injections of retrograde markers were made at either cervical, thoracic or lumbar spinal levels. With both techniques, spinally projecting TH-IR neurons were located within the nucleus periventricularis hypothalami, the nucleus paraventricularis hypothalami dorsalis, the area hypothalamica posterior, the ventral part of the nucleus coeruleus, the nucleus coeruleus pars alpha, the lateral part of the nucleus reticularis pontis, and several nuclei of the ventrolateral medulla. Few or no cells within the parabrachial area, the region of the Kolliker-Fuse nucleus or the area adjacent to the superior olivary complex (the location of the A5 group of rats) provided TH-IR projections to the spinal cord. Our results suggest that catecholaminergic projections to the spinal cord of the marsupial opossum are similar in termination and origin to those described for rats and other placental mammals, but differences do exist.

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