Cell survival or cell death: Differential vulnerability of long descending and thoracic propriospinal neurons to low thoracic axotomy in the adult rat

Previous studies show that most short thoracic propriospinal (TPS; T5-T7) and long descending propriospinal tract (LDPT; C4-C6) neurons are lost following low-thoracic spinal cord contusion injury (cSCI), as assessed by retrograde labeling with fluorogold (FG). Gene microarray and terminal deoxynucleotidyl transferase dUTP nick end (TUNEL)/caspase-3 immunolabeling indicate that post-axotomy cell death may be responsible for the observed decrease in number of labeled TPS neurons post cSCI. Yet, no indications of post-axotomy cell death are evident within LDPT neurons following the same injury. The present experiments were devised to understand this difference. We assessed the number and size of LDPT and TPS neurons at different time points, retrogradely labeling these neurons with FG prior to delivering a moderate low-thoracic cSCI or after they were axotomized by a complete low-thoracic spinal transection. Counts of FG-filled TPS and LDPT cells indicate a large loss of both neuronal populations 2 weeks post cSCI. Propriospinal neurons in other animals were retrogradely labeled with dextran tetramethyl rhodamine prior to cSCI and tissue was processed for detection of TUNEL- or caspase-3-positive profiles at chronic times post injury. Our overall findings confirm that cell death post injury is the major factor responsible for the loss of TPS neurons during the acute period post cSCI, and that little post-axotomy cell death occurs in LDPT neurons during the first 2 months after the same injury. After chronic axotomy retrograde transport is impaired in LDPT neurons, but can be reinitiated by re-axotomy. Our results also indicate that FG is metabolized/lost from retrogradely labeled neurons at increasing survival times, and that this process appears to be accelerated by injury.