Recovery of synapses in axotomized adult cat spinal motoneurons after reinnervation into muscle

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.