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.

Oxidative enzyme activity and soma size in motoneurons innervating the rat slow-twitch and fast-twitch muscles after chronic activity

The effects of chronic activity induced by running training on the activity of the mitochondrial enzyme succinate dehydrogenase (SDH) and soma size in motoneurons innervating the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles were studied in rats using the retrograde neuronal tracer Nuclear Yellow. Rats were assigned to control and trained groups that were subjected to treadmill running for 10 weeks (2 h/day, 30 m/min, 5 days/week). After training, both SOL and EDL muscles showed clear adaptations (citrate synthase activity in the SOL muscle, and the fast-twitch oxidative-glycolytic fiber area of the EDL muscle increased significantly after training). The SDH activity of the motoneurons innervating both SOL and EDL muscles was unchanged by training. However, SOL motoneurons of trained rats had a significantly larger soma size and a significantly higher total SDH activity [SDH activity x soma size) than those of control. Total SDH activity was calculated to examine the absolute SDH protein content of the motoneurons. On the other hand, there was no difference in both soma size and total SDH activity of EDL motoneurons between the two groups. These data demonstrate that chronic activity has a considerably stronger impact on soma size and total oxidative enzyme activity of motoneurons innervating slow-twitch rather than fast-twitch muscles.