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