Multimodal Evaluation of TMS - Induced Somatosensory Plasticity and Behavioral Recovery in Rats With Contusion Spinal Cord Injury

Introduction: Spinal cord injury (SCI) causes partial or complete damage to sensory and motor pathways and induces immediate changes in cortical function. Current rehabilitative strategies do not address this early alteration, therefore impacting the degree of neuroplasticity and subsequent recovery. The following study aims to test if a non-invasive brain stimulation technique such as repetitive transcranial magnetic stimulation (rTMS) is effective in promoting plasticity and rehabilitation, and can be used as an early intervention strategy in a rat model of SCI.Methods: A contusion SCI was induced at segment T9 in adult rats. An rTMS coil was positioned over the brain to deliver high frequency stimulation. Behavior, motor and sensory functions were tested in three groups: SCI rats that received high-frequency (20 Hz) rTMS within 10 min post-injury (acute-TMS; n = 7); SCI rats that received TMS starting 2 weeks post-injury (chronic-TMS; n = 5), and SCI rats that received sham TMS (no-TMS, n = 5). Locomotion was evaluated by the Basso, Beattie, and Bresnahan (BBB) and gridwalk tests. Motor evoked potentials (MEP) were recorded from the forepaw across all groups to measure integrity of motor pathways. Functional MRI (fMRI) responses to contralateral tactile hindlimb stimulation were measured in an 11.7T horizontal bore small-animal scanner.Results: The acute-TMS group demonstrated the fastest improvements in locomotor performance in both the BBB and gridwalk tests compared to chronic and no-TMS groups. MEP responses from forepaw showed significantly greater difference in the inter-peak latency between acute-TMS and no-TMS groups, suggesting increases in motor function. Finally, the acute-TMS group showed increased fMRI-evoked responses to hindlimb stimulation over the right and left hindlimb (LHL) primary somatosensory representations (S1), respectively; the chronic-TMS group showed moderate sensory responses in comparison, and the no-TMS group exhibited the lowest sensory responses to both hindlimbs.Conclusion: The results suggest that rTMS therapy beginning in the acute phase after SCI promotes neuroplasticity and is an effective rehabilitative approach in a rat model of SCI

Axonal and neuromuscular synaptic phenotypes in Wld(S), SOD1(G93A) and ostes mutant mice identified by fiber-optic confocal microendoscopy

We used live imaging by fiber-optic confocal microendoscopy (CME) of yellow fluorescent protein (YFP) expression in motor neurons to observe and monitor axonal and neuromuscular synaptic phenotypes in mutant mice. First, we visualized slow degeneration of axons and motor nerve terminals at neuromuscular junctions following sciatic nerve injury in WldS mice with slow Wallerian degeneration. Protection of axotomized motor nerve terminals was much weaker in WldS heterozygotes than in homozygotes. We then induced covert modifiers of axonal and synaptic degeneration in heterozygous WldS mice, by N-ethyl-Nnitrosourea (ENU) mutagenesis, and used CME to identify candidate mutants that either enhanced or suppressed axonal or synaptic degeneration. From 219 of the F1 progeny of ENU-mutagenized BALB/c mice and thy1.2-YFP16/WldS mice, CME revealed six phenodeviants with suppression of synaptic degeneration. Inheritance of synaptic protection was confirmed in three of these founders, with evidence of Mendelian inheritance of a dominant mutation in one of them (designated CEMOP_S5). We next applied CME repeatedly to living WldS mice and to SOD1G93A mice, an animal model of motor neuron disease, and observed degeneration of identified neuromuscular synapses over a 1–4 day period in both of these mutant lines. Finally, we used CME to observe slow axonal regeneration in the ENU-mutant ostes mouse strain. The data show that CME can be used to monitor covert axonal and neuromuscular synaptic pathology and, when combined with mutagenesis, to identify genetic modifiers of its progression in vivo

Role of activity in neuromuscular synaptic degeneration: insights from Wlds mice

The nervous system is a dynamic structure. Both during development and in the adult, synapses display activity-dependent plasticity which can modify their structure and function. In the neonate, activity influences the stability of functional connections between the muscle and nerve. In adults, the process of neurotransmitter release and the structure of the postsynaptic muscle can also be altered by external stimuli such as exercise. It is important to understand this plasticity of the neuromuscular system, the ways in which it can be modified, and its relationship to the maintenance or degeneration of synapses. After injury, peripheral nerve undergoes Wallerian Degeneration, during which the connections between axons and muscle fibres are lost, followed by the fragmentation of the nerve itself. The primary goal of this thesis was to determine whether activity modulates this process; that is, whether enhancing or reducing neuromuscular activity creates a susceptibility to degeneration or alternatively provides any protection against it. Developing greater understanding of this process is essential in relation to neurodegenerative disorders in which the benefits of activity, in the form of exercise, are controversial. Using Wlds mice, in which synaptic degeneration occurs approximately ten times more slowly than normal after nerve injury, I investigated the influence of both decreased (tetrodotoxin induced paralysis) and increased (voluntary wheel running) activity in vivo on this process. Paralysis prior to axotomy resulted in a significant increase in the rate of synapse degeneration. Using a novel method of repeatedly visualising degenerating synapses and axons in vivo I also established that this effect was specific to the synapse, as it did not affect the degeneration of axons. In contrast, voluntary wheel running had no effect on the rate of either axonal or neuromuscular synapse degeneration, but induced a slight modification of neuromuscular transmission. To provide a more stringent test I developed a novel assay based on overnight, ex vivo incubation of nerve-muscle preparations at 32°C. I first demonstrated that this procedure separates the different degeneration time courses for neuromuscular synapse degeneration in wild-type and Wlds preparations. I then extended the study to investigate further ways of modulating synaptic degeneration. First, I tested the effects of electrical stimulation. Intermittent high frequency (100Hz) stimulation reduced the level of protection. Finally, I tested the effects of NAMPT enzymatic inhibitor FK866 on synaptic degeneration. Interestingly, the synaptic protection observed in Wlds muscles was enhanced in the presence of FK866. The results of my findings are relevant to understanding the plasticity of synapses and its relationship to degeneration. Together, these studies highlight the potential of genetic and epigenetic factors, including activity, to regulate neuromuscular synapse degeneration. My study also provides proof of concept for a novel organotypic culture system in which to identify pharmacological modulators of synaptic degeneration that could form part of a second-line screen for neuroprotective compounds or phenotypes. My findings may be viewed in the wide context of neurodegenerative disease, since synaptic use or disuse is widely thought to influence susceptibility, onset and progression in such disorders