Changes in the synaptology of spinal motoneurons in zebrafish following spinal cord transection

Effects of spinal cord transection on the synaptology of zebrafish spinal motoneurons were studied. The transection was made at the level of the 14th vertebra and the synaptology of motoneuron somata and dendrites was analysed at the level of the 21st to the 23rd vertebrae at one month and three months after transection. Horseradish peroxidase, applied to the myotomal muscle, was used to label motoneuron somata and dendritic branches in central and in lateral areas of the neuropil (referred to as central and lateral dendritic profiles). Boutons impinging on motoneurons were classified according to the morphology of the vesicles. We discerned R-boutons with spherical vesicles, F-boutons with flat vesicles and DC-boutons with at least one dense core vesicle. The apposition lengths of R-, F- and DC-boutons and the circumference of labelled profiles were determined to assess the proportional covering of boutons on somata and dendrites. Ratio's of covering with R- and F-boutons (R/F ratio) for somata, central and lateral dendritic profiles were 1.1, 2.1, and 2.1 in control fish and 0.5, 0.5 and 0.9 in lesioned fish at one month after transection, respectively. The total covering of motoneurons in lesioned fish was decreased by 20% on somata and by 30% on lateral dendritic profiles, whereas central dendritic profiles did not change significantly. At three months after transection the R/F ratio's for somata, central and lateral dendritic profiles were 0.5, 0.7 and 0.6, respectively. The total covering on somata and central and lateral dendritic profiles was at control levels. The anatomical aspects of the changes in synaptology indicate that in control fish 50 to 60% of the R-boutons on the motoneuron surface originate from descending axons. In contrast, almost all F-boutons seem to be from local origin.