Response of sensorimotor pathways of the spinal cord to injury and experimental treatments.
PhD thesis, University of Glasgow.
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Many spinal cord injuries (SCI) are incomplete, variable numbers of spared fibres passing the lesion level and supporting some residual function below the injury. One approach to improving function following injury is to develop therapies that maximise the potential of these spared fibres. The aim of the work in this thesis was to investigate the spontaneous plasticity that occurs in spinal cord pathways following injury and then determine whether olfactory ensheathing cell (OEC) transplants or treatment with antibodies blocking the function of the myelin inhibitors Nogo and MAG could enhance this plasticity.
An electrophysiological approach was used to investigate these questions in rats which were subjected to a lesion of the dorsal columns at the C4/5 segmental level. This lesion interrupts the main component of the corticospinal tract which descends in the ventromedial dorsal column and also interrupts the ascending collaterals of sensory fibres from forelimb nerves. In this study, changes in the function of both of these pathways were assessed by recording cord dorsum potentials (CDPs) after stimulation in the pyramids (corticospinal activation) or of the radial nerve (sensory fibre activation). To enable plasticity in the corticospinal system to be investigated a method was developed for maximally activating the corticospinal projection on one side of the pyramids, whilst avoiding activation of the opposite pyramid and structures surrounding the pyramids. It was found that this could be achieved by careful positioning of bipolar stimulating electrodes.
Before investigating the effect of potential plasticity inducing agents, the degree to which plasticity occurs in the absence of treatments was first assessed in F344 rats. The function of corticospinal and sensory pathways was compared in normal animals, acutely lesioned animals, and at 1 week and 3 months after a dorsal column lesion. Corticospinally-evoked CDPs above the lesion were not altered following an acute lesion but were larger in 1 week and 3 month dorsal column lesioned animals than in normal animals. The increase in amplitude was similar in both lesioned animal groups. This suggests that plasticity occurs at the intact connections formed by corticospinal fibres axotomised more distally, that it occurs within a week of the lesion and persisits for at least 3 months. Corticospinally-evoked CDPs were almost abolished below an acute dorsal column lesion and remained of minimal amplitude 1 week after lesioning. However, there was some recovery of CDPs between 1 week and 3 months. This suggests plasticity either at the connections formed by spared fibres of the minor non-dorsal column components of the corticospinal tract or in propriospinal pathways originating above the lesion. This plasticity has a longer time-course than that at the connections of axotomised fibres above the lesion. Plasticity of the connections formed by larger diameter sensory fibres in the radial nerve was also seen below the level of the dorsal column lesion. This had a similar time course to the plasticity of corticospinal connections above the lesion CDPs being larger both 1 week and 3 months after injury compared to normal animals. A modest enhancement of transmission in both corticospinal and sensory systems therefore occurred following a dorsal column lesion.
To investigate whether OEC transplants enhance plasticity after spinal cord injury, OECs were transplanted such that they became distributed within the spinal cord for several mm either above or below the lesion. Electrophysiological methods were then used, as above, to investigate whether transmission in the corticospinal and sensory fibre systems following a dorsal column lesion was improved in transplanted animals compared to 3 month survival animals. However, corticospinal actions rostral to the lesion were not enhanced by OEC transplants above the lesion and sensory transmission caudal to the lesion was not enhanced by cells below the lesion. OEC transplants are therefore unlikely to support recovery by promoting plasticity in the spinal cord after injury.
To investigate whether antibodies designed to block the function of the myelin inhibitors Nogo and MAG would enhance plasticity following spinal cord injury, antibodies were delivered intrathecally via implanted osmotic minipumps over a period of six weeks following a dorsal column lesion. Vehicle treated and normal animals were investigated for comparison. Placement of the cannula and/or delivery of vehicle alone was found to have a detrimental effect on corticospinal actions above the lesion when compared to normal animals. Treatment with an anti-Nogo antibody (GSK577548) raised against a human Nogo-A fragment and targeting the amino-Nogo terminal was found to enhance transmission of corticospinal actions both above and below the dorsal column lesion. Corticospinal actions above the lesion were significantly greater than in the vehicle treated controls but did not exceed those in normal animals because of the detrimental effect of the intrathecal cannulae/vehicle treatment. Transmission at the terminals of sensory afferent fibres below the level of the lesion was also enhanced by anti-Nogo treatment. In this case the actions of sensory pathways were significantly greater than those in both vehicle treated and normal animals. The fact that enhanced transmission occurs on the ‘wrong side’ of the lesion to be explained by axonal regeneration and the sensory transmission is enhanced over normal, strongly suggests that anti-Nogo induces plasticity in spinal pathways. In contrast, treatment with the anti-MAG antibody (GSK249320A) had no effect on either corticospinal or sensory-evoked activity in the spinal cord above or below the lesion, CDPs evoked by these pathways being comparable to that in vehicle treated controls. Anti-MAG does not appear to induce plasticity but may have neuroprotective actions which cannot be adequately tested in this lesion model.
The results show that both corticospinal and sensory fibre systems show modest spontaneous plasticity following a dorsal column lesion. Plasticity at the terminations of axotomised fibres occurs relatively rapidly (within one week) while plasticity in spared systems occurs more slowly. This spontaneous plasticity does not appear to be enhanced by transplants of OECs, so that improvements in spinal cord function previously demonstrated in transplanted animals are probably due to a neuroprotective mechanism. The results obtained using function blocking antibodies targeting myelin inhibitors suggest that anti-Nogo but not anti-MAG treatment may enhance plasticity in the spinal cord after injury. This observation adds to the accumulating evidence that interfering with Nogo-A signalling may be a useful approach for improving function after spinal cord injury.
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