To demonstrate a dependence of spinal cord motoneurons on the communication with their targets, the expression of immediate early gene c-fos and neurotrophin genes in the lumbar (L3-L6) spinal cord neurons was examined in Sprague-Dawley rats (male > or = 9-weeks-old) with unilateral sciatic nerve transection. Using in situ hybridization, we detected the expression of c-fos mRNA in the motoneurons of the spinal cord segments within 45 min to 3 h of peripheral nerve transection (n = 4 in each time point). The expression of c-fos mRNA was also correlated positively with the expression of Fos antigen using immunohistochemistry, while no change in calbindin and parvalbumin antigens were noted. The expression of BDNF mRNA increased at 90 min after sciatic nerve transection. However, no detectable enhancement in the expression of NGF mRNA was observed. DNA fragmentation in neurons was observed using the incorporation of digoxigenin-dUTP by terminal transferase into 3'-OH terminals of DNA fragments in the ipsilateral section of the spinal cords 48h after nerve injury. Nuclei that exhibited DNA fragmentation were not observed in the spinal cord of the control animals. Lastly, we observed that the majority of astrocytes did not have DNA fragmentation. Because the detection of DNA fragmentation using this assay is one of early detections of apoptosis or programmed cell death, the result suggested we could detect early cell death in spinal cord, and indicated a target dependence of the neurons in the spinal cord after transection of sciatic nerve.