The impact of target size on the level of the energy metabolism of spinal motoneurons. An enzyme histochemical and morphological study.

We investigated the relationship between the enzyme histochemical properties of motoneurons and the size of their peripheral target. The study was carried out on zebrafish (Brachydanio rerio) of 18 to 53 mm body length. Motoneurons of the white, fast glycolytic muscle (WMN) and of the slower red and intermediate oxidative muscle (RIMN) were investigated. We determined the average soma size of the WMNs and the RMNs and the following histochemical characteristics: Glucose 6-phosphate dehydrogenase (G6PDH), a marker for reductive biosynthesis; Phosphofructokinase (PFK), a marker for glycolytic activity; Succinate dehydrogenase (SDH) enzyme of the citric acid cycle and NADH tetrazolium reductase (NADH-TR) a marker for oxidative activity. The product of enzyme activity and the soma volume was denoted the enzyme capacity of a soma. As a measure for the average target of the motoneurons, we took the number of endplate regions within the white and the red-intermediate muscle. The age dependent increase in G6PDH capacity of motoneuron somata correlates significantly with the increase in the number of endplate regions. On the basis of changes in the soma size, enzyme capacity and target size, we estimate that in 18 mm fish about 45% and in 53 mm fish about 60% of the oxidative capacity is used to maintain connections with the target. This holds for WMNs as well as for RIMNs. Similar calculations, on basis of data known from the literature, indicate that in motoneurons of the cat, less than 10% of the oxidative capacity is used for connections with the target. The PFK capacity increased unproportionally with age in WMNs but not in RIMNs. This indicates that the energy metabolism in WMNs of large fish tends to be anaerobic glycolytic. The study shows that enzyme histochemical characteristics of motoneurons should be interpreted in a morphological context, regarding motoneuron soma size and target size.