ADULT OPOSSUMS (DIDELPHIS-VIRGINIANA) DEMONSTRATE NEAR NORMAL LOCOMOTION AFTER SPINAL-CORD TRANSECTION AS NEONATES

When the thoracic spinal cord of the North American opossum (Didelphis virginiana) is transected on postnatal day (PD) 5, the site of injury becomes bridged by histologically recognizable spinal cord and axons which form major long tracts grow through the lesion. In the present s tudy we asked whether opossums lesioned on PD5 have normal use of the hindlimbs as adults and, if so, whether that use is dependent upon axo ns which grow through the lesion site. The thoracic spinal cord was tr ansected on PD5 and 6 months later, hindlimb function was evaluated us ing the Basso, Beattie, and Bresnahan (BBB) locomotor scale. All anima ls supported their weight with the hindlimbs and used their hindlimbs normally during overground locomotion. In some cases, the spinal cord was retransected at the original lesion site or just caudal to it 6 mo nths after the original transection and paralysis of the hindlimbs ens ued. Surprisingly, however, these animals gradually recovered some abi lity to support their weight and to step with the hindlimbs. Similar r ecovery was not seen in animals transected only as adults. In order to verify that descending axons which grew through the lesion during dev elopment were still present in the adult animal, opossums subjected to transection of the thoracic cord on PD5 were reoperated and Fast blue was injected several segments caudal to the lesion, In all cases, neu rons were labeled rostral to the lesion in each of the spinal and supr aspinal nuclei labeled by comparable injections in unlesioned, age-mat ched controls. The results of orthograde tracing studies indicated tha t axons which grew through the lesion innervated areas that were appro priate for them. (C) 1998 Academic Press.