Histochemical profiles of motoneurons innervating muscle fibres with different activity patterns in the zebrafish, Brachydanio rerio.

Enzyme histochemical profiles of spinal motoneurons in the zebrafish were determined. Five enzymes of glucose metabolism were chosen: glucose-6-phosphate dehydrogenase (G6PDH), hexokinase (HK), phosphofructokinase (PFK), succinate dehydrogenase (SDH) and NADH tetrazolium reductase (NADH-TR). Motoneurons were traced with Fluorogold and classified as those that innervate white muscle fibres (W-MNs) and those that innervate red and intermediate muscle fibres (R/I-MNs). The average enzyme activities per volume of tissue in the somata of both populations differed at most by 25%. Both the average soma volume and the average number of muscle fibres innervated are three times larger for the W-MNs than for the R/I-MNs. This suggests that the total amount of enzyme activity within a neuron soma matches target size. In the R/I-MNs, the activities of SDH and NADH-TR were closely correlated (correlation coefficient, r = 0.99; p less than 0.05) and HK activity correlated well with G6PDH activity (r = 0.94; p less than 0.05), but not with PFK (r = 0.64; p greater than 0.05). In the W-MNs, there was no correlation between SDH and NADH-TR (r = -0.59; p greater than 0.05) or between HK and G6PDH (r = 0.50; p greater than 0.05) and the correlation coefficient between HK and PFK activity was close to zero (r = 0.04; p greater than 0.05). It was concluded that in the R/I-MNs, which are continuously active, firing activity is fuelled by oxidative metabolism. We suggest that in the W-MNs glucose is stored in the form of glycogen and that, despite high levels of NADH-TR present, the energy for intermittent firing activity is provided by glycolysis.