Summary
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 a/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<0.05) and HK activity correlated well with G6PDH activity (r=0.94;p<0.05), butnot with PFK (r=0.64;p>0.05). In the W-MNs, there was no correlation between SDH and NADH-TR (r=−0.59;p>0.05) or between HK and G6PDH (r=0.50;p>0.05) and the correlation coefficient between HK and PFK activity was close to zero (r=0.04;p>0.05).
It was concluded that in the R/I-MNs gwhich are continuously ctive, firing activity is fuelled by oxidative metabolsm. 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.
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De Graaf, F., Van Raamsdonk, W., Van Asselt, E. et al. Histochemical profiles of motoneurons innervating muscle fibres with different activity patterns in the zebrafish,Brachydanio rerio . Histochem J 23, 273–280 (1991). https://doi.org/10.1007/BF01045046
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DOI: https://doi.org/10.1007/BF01045046