The overall goal of this project is to advance understanding of the roles and interrelationships of inflammatory responses, demyelination, and axonal conduction block in the loss of function that follows traumatic injury to the spinal cord. This goal is directly relevant to the profound medical problems of treating acute and chronic spinal cord injury. A recent clinical trial has shown limited, but significant improvement in neurological outcome from spinal trauma, using very high doses of methylprednisolone within 8 hours of injury. However, it is still uncertain how this drug achieves its benefit, and it is difficult to identify and develop other effective treatments while the mechanisms of secondary pathological damage remain poorly understood. The present experiments will concentrate on the possible role of inflammatory responses in such secondary pathology. Pairwise-randomized, blinded experiments will be performed to examine the effects of intraperitoneal injections of silica dust (a macrophage toxin) on inflammatory responses and on functional and histological outcome from focal trauma. Injuries of the lower thoracic spinal cord will be produced in anesthetized adult guinea pigs, using a technique of compression that results in injuries of moderate severity. The outcome from injury will be monitored with behavioral tests, daily during the first week and weekly thereafter for up to three months. Animals will then be fixed for histological examination of the spinal cord in plastic sections (1 micromole) stained with toluidine blue. Quantitative morphometric sampling of myelinated axons and microvasculature will be performed in sections from the center of the lesion and from surrounding areas of the cord that experience axonal degeneration but little direct mechanical damage. Measures of functional change will be related to the quantitative morphometry of myelinated axons and vasculature in the chronic lesion and compared between experimental and control groups. Effects of silica injections on the inflammatory response will be examined in separate experiments in which the animals will be fixed for histology at 1, 2, 4, 8, or 14 days after injury. Effects of silica injections on the development of the inflammatory response will be related to the timing of effects on the secondary loss of behavioral and electrophysiological function, and to changes in circulating leukocytes. Similar techniques of injury and quantitative analysis will than be used to examine whether neurological outcome in the guinea pig model is improved by methylprednisolone (30 mg/kg i.v. bolus beginning at 45 minutes post injury) and whether any improvement is related to a particular histological change or a particular time in the evolution of secondary pathological events in the spinal cord, particularly that associated with the peak of phagocytic activity.
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