REESTABLISHMENT OF DIRECT SYNAPTIC CONNECTIONS BETWEEN SENSORY AXONS AND MOTONEURONS AFTER LESIONS OF NEONATAL OPOSSUM CNS (MONODELPHIS-DOMESTICA) IN CULTURE

For functional recovery after spinal cord injury, regenerating fibres need to grow and to reform appropriate connections with their targets. The isolated central nervous system of neonatal opossums aged 1-9 day s has been used to analyse the precision with which neurons become rec onnected during regeneration. In culture these preparations maintain t heir electrical activity and show rapid outgrowth through spinal cord crushes or cuts. By recording electrically and by staining with horser adish peroxidase, we first demonstrated that direct reflex connections were already present at birth between sensory fibres in one segment a nd motoneurons in the same segment and in adjacent segments. As in pre vious experiments, 5 days after the spinal cord had been crushed, labe lled sensory fibres grew across the lesion to reach the next segment [ Woodward et al. (1993) J. Exp. Biol., 176, 77-88; Varga et al. (1995a) Eur. J.Neurosci., 7, 2119-2129, Varga et al. (1995b) Proc. Natl. Acad . Sci. USA, 92, 10959-10963]. Beyond the lesion the labelled axons abr uptly changed direction, traversed the spinal cord and terminated on l abelled motoneurons in the ventral horn. In preparations that had rege nerated dorsal root stimulation once again initiated ventral root refl exes. Electron micrographs revealed synapses made by labelled sensory axons on motoneurons. Double staining of growing sensory axons and rad ial glial fibres showed close association, suggesting guidance. These results indicate that the original pathway is reestablished during rep air and that appropriate connections are reformed after injury.