T. Brannstrom et Jo. Kellerth, Recovery of synapses in axotomized adult cat spinal motoneurons after reinnervation into muscle, EXP BRAIN R, 125(1), 1999, pp. 19-27
Peripheral axotomy of adult cat spinal motoneurons induces a marked loss of
synaptic boutons from the cell bodies and dendritic trees. The aim of the
present study was to analyze the recovery of synaptic contacts in axotomize
d motoneurons following reinnervation into muscle. Adult cat spinal motoneu
rons were first deprived of their muscular contacts for 12 weeks and, then,
allowed to reinnervate their target muscle. Two years later, regenerated m
otoneurons were labeled with horseradish peroxidase to allow quantitative u
ltrastructural analyses of the synaptic covering of the cell bodies and den
drites. Presynaptic boutons were classified according to their size and the
shape of their synaptic vesicles. Results show that a recovery of synaptic
covering occurs in the axotomized neurons after muscle reinnervation, but
it affects various bouton types to different degrees. The number of S-type
boutons synapsing with the soma was 70% higher after reinnervation than at
12 weeks after axotomy, while the number of F-type boutons had increased by
only 13%. Compared with the normal situation, the number of S-type boutons
synapsing with the proximal dendrites increased from 82% at 12 weeks after
axotomy to 180% in the reinnervated state. In conclusion, in adult cat spi
nal motoneurons, the reestablishment of muscular contact is followed by a n
ormalization of some of the synaptological changes induced by a prolonged s
tate of axotomy. In certain respects restitution is incomplete, but in othe
rs it results in overcompensation.