Changes in synaptology of adult cat spinal alpha-motoneurons after axotomy

The aim of this electron-microscopic study was to analyze the distribution of synaptic contacts on the cell bodies and dendrites of permanently axotomized adult cat spinal alpha-motoneurons. Following transection and ligation of the medial gastrocnemius nerve, the synaptic covering of the cell bodies and three different dendritic compartments of homonymous alpha-motoneurons was analyzed quantitatively at 3, 6, and 12 weeks postoperatively. The synaptic boutons were classified according to their size and the shape of their synaptic vesicles. On the soma, a transient increase in the number of boutons was noted at 3 weeks and 6 weeks postoperatively, while after 12 weeks the bouton number had decreased to half of its normal value. The transient increase was mainly due to an increase in the number of F-type boutons. At 12 weeks postoperatively, the synaptic covering was reduced by 83% on the soma and by 57% on the proximal dendrites. In the distal dendritic regions, the values for synaptic covering remained largely unchanged. In summary, axotomized motoneurons exhibit a reduction in synaptic covering which is maximal on the cell body and becomes less pronounced centrifugally along the dendrites. However, if also taking into account the loss of distal dendritic branches that occurs in axotomized motoneurons, the total loss of boutons is several times larger in the dendrites than on the soma.