Ultrastructural evidence for a preferential elimination of glutamate-immunoreactive synaptic terminals from spinal motoneurons after intramedullary axotomy

Abstract

After axotomy in the ventral funiculus of the cat spinal cord, about half of the population of lesioned motoneurons die at 1-3 weeks postoperatively, whereas the other half survives and generates new axons through the lesion area. To identify conditions that may promote survival and regeneration of motoneurons subjected to this kind of injury, the authors examined ultrastructurally lesion-induced changes in the number and distribution of nerve terminals on the somata and proximal dendrites of alpha-motoneurons in the 7th lumbar spinal segment (L7) of the cat spinal cord. Intramedullary axotomy resulted in a profound reduction in the number of nerve terminals impinging on the somata and proximal dendrites, with the maximal effect seen at 3 weeks postlesion. At that time, only 12-25% of the normal number of terminals remained on the cell somata, and 22-33% remained on proximal dendrites. Thereafter, a gradual increase in terminal numbers occurred, reaching normal levels at 34 weeks after the lesion. Already at 2 days postoperatively and, most obviously, at 3 weeks postoperatively, type S nerve terminals were eliminated to a larger degree than type F terminals. Postembedding immunohistochemistry confirmed that the largest reduction at 3 weeks was seen for excitatory glutamate-immunopositive type S nerve terminals (90%), whereas inhibitory glycine-immunoreactive and gamma-aminobutyric acid (GABA)-immunoreactive type F terminals were affected less (70% reduction). This led to a distinct shift in the ratio between the numbers of terminals that were immunopositive for glycine and GABA and the numbers of terminals that were labeled for glutamate. For the cell body, this ratio increased from 3.7 in normal material to 14.5 in lesioned motoneurons, whereas the corresponding values for proximal dendrites were 3.8 and 7.5. The preferential elimination of glutamatergic inputs to lesioned motoneurons may reflect an active reorganization of the synaptic input to diminish the excitotoxic influence on these neurons, thereby promoting the survival of motoneurons after intramedullary axotomy.