The effects of remaining axons on motoneuron survival and NOS expression following axotomy in the adult rat

It is well known that target removal or distal axotomy in adult animals results in no detectable loss of motoneurons in the spinal cord. By performing axotomy in the seventh cervical (C7) spinal nerve at different distances from the spinal cord (0, 2, 4, 8 mm respectively), this study examines the effects of the remaining axons on motoneuron survival as well as NOS expression. Results of the present study show that axotomy in adult peripheral nerve can induce significant spinal motoneuron death if axotomy is performed close enough to the spinal cord. The closer the axotomy to the spinal cord, the higher the rate of motoneuron loss was observed. The most significant motoneuron loss was found in animals with axotomy at 0 mm to the cord, which was coincident with the highest percent of NOS-positive motoneurons. The rate of survival of motoneurons increases and the percent of NOS-positive motoneurons decreases when the distance of the axotomy to the cord increases from 0 to 4 mm. No significant motoneuron loss nor NOS-positive motoneurons were observed when axotomy was performed at 4 mm and distally. These results indicate that the survival of spinal motoneurons in adult rat following axotomy is largely dependent on the length of the remaining axons. The longer the remaining axon: the better for motoneuron survival. The minimal length of axon for motoneuron survival in adult rat seems to be at least 4 mm.