DEVELOPMENTAL PLASTICITY OF RETICULOSPINAL AND VESTIBULOSPINAL AXONS IN THE NORTH-AMERICAN OPOSSUM, DIDELPHIS-VIRGINIANA

We have shown previously that rubral axons grow around a lesion of the ir spinal pathway in the North American opossum if it is made at early stages of development. In the present experiments, we have asked whet her reticular and vestibular axons have the same ability. The spinal c ord was hemisected at postnatal day 20, 12, or 5, well within the crit ical period for rubrospinal plasticity, and, approximately 30 days lat er, bilateral injections of fast blue were made about four segments ca udal to the lesion. The pups were killed 4 or 5 days after the injecti ons. In most of the animals lesioned on postnatal day 20, labeled neur ons were not found in the medial part of the pontine reticular nucleus or the dorsal part of the lateral vestibular nucleus ipsilateral to t he lesion. The spinal projections from both areas are exclusively ipsi lateral. When the lesions were made at postnatal day 12 or 5, however, labeled neurons were present in both areas, suggesting that they supp orted axons that had grown caudal to the lesion. As was expected from previous studies, rubral neurons were labeled contralateral to the les ion in all three groups. In the opossum, as in other species, the red nucleus projects contralaterally. We conclude that reticular and vesti bular axons, like axons from the red nucleus, grow around a lesion of their pathway during development and that the critical period for thei r plasticity ends earlier than that for rubrospinal axons. (C) 1994 Wi ley-Liss, Inc.