Nerve growth factor in glia and inflammatory cells of the injured rat spinal cord

Nerve growth factor (NGF) is crucial for the development of sympathetic and small-diameter sensory neurons and for maintenance of their mature phenotype. Its role in generating neuronal pathophysiology is less well understood. After spinal cord injury, central processes of primary afferent fibers sprout into the dorsal horn, contributing to the development of autonomic dysfunctions and pain. NGF may promote these states as it stimulates sprouting of small-diameter afferent fibers and its concentration in the spinal cord increases after cord injury. The cells responsible for this increase must be identified to develop a strategy to prevent the afferent sprouting. Using immunocytochemistry, we identified cells containing NGF in spinal cord sections from intact rats and from rats 1 and 2 weeks after high thoracic cord transection. In intact rats, this neurotrophin was present in a few ramified microglia and in putative Schwann cells in the dorsal root. Within and close to the lesion of cord-injured rats, NGF was in many activated, ramified microglia, in a subset of astrocytes, and in small, round cells that were neither glia nor macrophages. NGF-immunoreactive putative Schwann cells were prevalent throughout the thoracolumbar cord in the dorsal roots and the dorsal root entry zones. Oligodendrocytes were never immunoreactive for this protein. Therapeutic strategies targeting spinal cord cells that produce NGF may prevent primary afferent sprouting and resulting clinical disorders after cord injury.