Motor nerve injury results in a sequence of events known as Wallerian degeneration,
which takes 1 - 2 days in rodents. Wallerian degeneration is significantly delayed in the
naturally occurring C57BL/Wldˢ mutant mouse. The ability to produce compound action
potentials and the degradation of axonal material are delayed for up to 3 weeks following
This thesis aims to examine the processes that occur after nerve section at
neuromuscular junctions of mutant Wldˢ and normal mice. Functional morphological
measurement of actively recycling synaptic vesicles at wild type nerve terminals using
the vital dye FM1-43 indicated that the loss of the ability to recycle synaptic vesicles
occurred within 24 hours, coinciding with disruption of neurofilament and SV2 proteins
(stained using immunocytochemical methods). In contrast, the degeneration of nerve
terminals in Wldˢ mice following nerve injury was significantly delayed, and once
initiated was further slowed, degeneration occurring between 3 and 10 days after
axotomy. The morphological appearance of Wldˢ nerve terminals after lesion differed
greatly from that of degenerating wild type terminals: Wldˢ terminals appeared to be
retracting from the endplate region rather than simply degenerating.
Electrophysiological recordings from degenerating junctions demonstrate that the delay
of degeneration/withdrawal from nerve terminals of Wldˢ mice is accompanied by a
coincident loss of synaptic function. These data indicate that, as in normal mice, the first
structural and functional changes to take place after nerve injury in mutant Wldˢ mice is
the disruption of nerve terminals at the NMJ. This thesis proposes that nerve terminal
retraction in the mutant mouse is caused by a withdrawal mechanism similar to that
observed during synapse elimination, a naturally occurring activity dependant
developmental phenomenon, rather than by classical Wallerian degeneration as
previously assumed. To test this hypothesis, botulinum toxin (which abolishes
neurotransmitter release) was administered to the nerve terminal region at the same time
as the nerves were lesioned. The results indicate that nerve terminal withdrawal still
occurs, but that it is reduced and further delayed in muscles treated with botulinum toxin
by 1.14 days, in comparison with denervated alone Wldˢ muscles.
Additional studies assessed the role of Schwann cells in nerve terminal withdrawal.
Terminal Schwann cells that overly the NMJ sprout in response to denervation, and have
been shown to guide axonal processes back to endplates during reinnervation.
Experiments were conducted to evaluate the Schwann cell reaction to the absence of
degeneration after nerve section, as observed during the first few days after axotomy in
the Wldˢ mouse. Schwann cell sprouts were present at various endplates in the mutant
mouse after nerve injury. However, when compared with denervated wild-type controls
there were fewer Schwann cell sprouts in the Wldˢ and sprouting in general appeared
less extensive. Thus, the presence of an intact nerve terminal and spontaneous activity
are sufficient to delay, but not abolish, Schwann cell sprouting in the Wldˢ mouse after
In sum, these studies provide an ideal opportunity for the further investigation of the
molecular and cellular mechanisms of synapse withdrawal.