Role of tumor necrosis factor-alpha in neuronal and glial apoptosis after spinal cord injury

We investigated the role of tumor necrosis factor (TNF)-alpha in the onset of neuronal and glial apoptosis after traumatic spinal cord crush injury in rats. A few TUNEL-positive cells were first observed within and surroundin g the lesion area 4 h after injury, with the largest number observed 24-48 h after injury. Double-labeling of cells using cell type-specific markers r evealed that TUNEL-positive cells were either neurons or oligodendrocytes. One hour after injury, an intense immunoreactivity to TNF-alpha was observe d in neurons and glial cells in the lesion area, but also seen in cells sev eral mm from the lesion site rostrally and caudally. The level of nitric ox ide (NO) also significantly increased in the spinal cord 4 h after injury. The injection of a neutralizing antibody against TNF-alpha into the lesion site several min after injury significantly reduced both the level of NO ob served 4 h thereafter as well as the number of apoptotic cells observed 24 h after spinal cord trauma. An inhibitor of nitric oxide synthase (NOS), N- G-monomethyl-L-arginine acetate (L-NMMA), also reduced the number of apopto tic cells. This reduction of apoptotic cells was associated with a decrease in DNA laddering on agarose gel electrophoresis. These results suggest tha t: (i) TNF-alpha may function as an external signal initiating apoptosis in neurons and oligodendrocytes after spinal cord injury; and (ii) TNF-alpha -initiated apoptosis may be mediated in part by NO as produced by a NOS exp ressed in response to TNF-alpha. (C) 2000 Academic Press.