Extensive glial apoptosis develops early after hypoxic-dysmetabolic insult to the neonatal rat spinal cord in vitro.

The current etiopathogenesis of spinal cord injury comprises a growing number of nontraumatic causes, including ischemia generating hypoxic-dysmetabolic conditions. To mimic the metabolic disruption accompanying nontraumatic acute spinal cord injury and to characterize the type and dynamics of cell death in relation to locomotor network function, we used, as a model, the rat neonatal spinal cord preparation in vitro transiently (1 h) exposed to a "pathological medium" (PM), i.e. hypoxic/aglycemic solution containing toxic radicals. PM induced, in the ventrolateral spinal region, pyknosis already detectable after 2 h and stabilized 24 h later (affecting 55% of white matter cells). Glial cells were much more vulnerable than neurons. The amplitude of fictive locomotor patterns recorded from lumbar ventral roots was decreased and periodicity delayed by PM, in keeping with substantial preservation of neuronal networks. Repeated application of PM intensified such a functional impairment. White matter astrocytes and oligodendrocytes displayed nucleolytic pyknosis mainly dependent on caspase-mediated death processes as shown by active caspase-3 and terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling (TUNEL) positivity. Expression of cleaved poly(ADP-ribose) polymerase-1 (PARP-1) (the active caspase-3 executor) also grew with similar time course. The caspase-3 inhibitor II counteracted, in a dose-dependent fashion, white matter pyknosis. Our results suggest the important involvement of apoptotic pathways in early glial cell death during the first 24 h after a hypoxic-dysmetabolic insult, associated with impaired locomotor output. Residual locomotor network activity together with distinctive apoptotic damage to white matter cells suggests that early protection against glial destruction may help to prevent subsequent damage extension responsible for paraplegia.

Kainate-induced delayed onset of excitotoxicity with functional loss unrelated to the extent of neuronal damage in the in vitro spinal cord.

While excitotoxicity is a major contributor to the pathophysiology of acute spinal injury, its time course and the extent of cell damage in relation to locomotor network activity remain unclear. We used two in vitro models, that is, the rat isolated spinal cord and spinal organotypic cultures, to explore the basic characteristics of excitotoxicity caused by transient application of the glutamate analogue kainate followed by washout and analysis 24 h later. Electrophysiological records showed that fictive locomotion was slowed down by 10 microM kainate (with no histological loss) and fully abolished by 50 microM, while disinhibited bursting with unchanged periodicity persisted. Kainate concentrations (> or =50 microM) larger than those necessary to irreversible suppress fictive locomotion could still elicit dose-dependent motoneuron pool depolarization, and dose-dependent neuronal loss in the grey matter, especially evident in central and dorsal areas. Motoneuron numbers were largely decreased. A similar regional pattern was detected in organotypic slices, as extensive cell loss was dose related and affected motoneurons and premotoneurons: the number of dead neurons (already apparent 1 h after kainate) grew faster with the higher kainate concentration. The histological damage was accompanied by decreased MTT formazan production commensurate with the number of surviving cells. Our data suggest locomotor network function was very sensitive to excitotoxicity, even without observing extensive cell death. Excitotoxicity developed gradually leaving a time window in which neuroprotection might be attempted to preserve circuits still capable of expressing basic rhythmogenesis and reconfigure their function in terms of locomotor output.