BACKGROUND: Numerous studies have shown that tumor necrosis factor α (TNF-α) is closely correlated with spinal cord injury (SCI), but the mechanisms of TNF-α and therapeutic treatments for SCI are still poorly understood. OBJECTIVE: To determine the role of TNF-α in the pathogenesis of SCI. DESIGN, TIME AND SETTING: An in vivo experiment based on genetically engineered animals was performed at the Medical University of South Carolina, Charleston, South Carolina, USA, between June 2007 and October 2008.MATERIALS: TNF-α transgenic rats (Xenogen Biosciences in Cranbury, New Jersey, USA) were utilized in this study. METHODS: TNF-α transgenic (tg) and wild-type (WT) rats underwent a complete single-level laminectomy at the 10th thoracic vertebra (T10).MAIN OUTCOME MEASURES: Motor function of rat hindlimb was assessed using the Basso, Beattie, and Bresnahan hindlimb locomotor rating scale. Histological evaluation of spinal cord tissue loss was conducted. Immunohistochemistry for astrocytes, microglia/macrophages, and TNF receptors (TNFRs) was performed on spinal cord tissue sections. TNF-α mRNA expression was detected by real-time polymerase chain reaction. The concentrations of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) in the supatant were determined using an enzyme-linked immunosorbent assay kit for rat NGF or BDNF, respectively. The rats were injected subcutaneously with etanercept to verify that TNF-α was the direct effect of the modulation of behavioral and neurodegenerative outcomes in the TNF-α tg rats. RESULTS: TNF-α tg rats showed higher expression of TNF-α mRNA in the spinal cord prior to SCI. TNF-α tg rats showed worse motor deficits than WT rats in the acute period (< 3 days) after SCI (P < 0.01), while in the chronic period, TNF-α tg rats exhibited persistent elevated baseline levels of TNF-α mRNA and improved recovery in motor function and tissue healing compared to WT rats (P < 0.01). Following SCI, the number of microglia/macrophages in TNF-α tg rat was always greater than in WT rat (P < 0.01). There were no significant differences in NGF and BDNF levels in the supatant of spinal cord homogenates. TNFR1 expression was significantly greater in the TNF-α tg rats compared to the WT rats (P < 0.01). However, TNFR2 expression did not reveal a significant increase in the TNF-α tg rats compared to the WT rats. Finally, treatment with etanercept reduced injury acutely, but exacerbated the injury chronically.CONCLUSION: Overexpression of TNF-α is deleterious in the acute phase, but beneficial in the chronic phase in the response to SCI. The role of TNF-α post-injury may depend on TNF-α expression in the spinal cord and its differential binding to TNFR1. Our observations may have clinical relevance that antagonists or inhibitors of TNF-α could be administered within the early time window post-injury, and appropriate amounts of TNF-α could be administered during the chronic stage, in order to improve the final neurological recovery in patients with SCI.