Role of excitotoxicity and metabolic perturbation in the pathophysiology of acute spinal injury in vitro

The early pathophysiological processes affecting spinal locomotor networks following an acute injury remain poorly understood even though the consequence is usually loss of locomotion, not often reversible in man. Regardless of their etiology, spinal lesions are thought to include combinatorial effects of excitotoxicity (caused by hyperactivation of glutamate receptors) and severe metabolic perturbations (caused by hypoxia, oedema and generation of reactive oxygen species). To clarify the relative contribution by excitotoxicity and toxic metabolites to dysfunction of locomotor networks, spinal reflexes and intrinsic network rhythmicity, we used, as a model, the in vitro spinal cord of the neonatal rat treated (1 h) either with kainate (1 mM) or with a pathological medium (containing free radicals and hypoxic/aglycemic conditions), or with their combination. After washout with saline solution, electrophysiological responses from lumbar ventral roots were monitored for 24 h and cell damage analyzed histologically in comparison with sham preparations kept in vitro for the same length of time. Kainate suppressed fictive locomotion patterns irreversibly, even if motoneuron firing fully recovered. Reflex activity evoked by dorsal root stimuli was however continuously depressed. Intrinsic bursts induced by synaptic inhibition block were present with lower frequency and amplitude. These results were associated with significant neuronal loss around the central canal and modest white matter damage. Combining kainate with the pathological medium evoked extensive, irreversible damage to the spinal cord without chemical or electrically induced activity. The pathological medium alone simply retarded fictive locomotion and intrinsic bursting that persisted throughout. This phenomenon was associated with polysynaptic reflex depression and preferential damage to white matter. Our model suggests distinct roles of excitotoxicity and metabolic dysfunction in the acute damage of locomotor networks, indicating that different strategies might be necessary to combat the early mechanisms of acute spinal lesion.