Enzyme histochemical profiles of fish spinal motoneurons after cordotomy and axotomy of motor nerves.

Histochemical profiles were made of identified spinal motoneurons from normal adult zebrafish and from animals subjected to cordotomy or unilateral axotomy of the motor nerves. The lesions caused an increase of the myotomal area with oxidative muscle fibers. We studied the question: do changes in the myotomal muscle configuration concur with changes in the enzyme histochemical profiles of innervating motoneurons? Based on the location and size of cell somata, two categories of motoneurons are distinguished: large white (W) motoneurons that innervate the deep fast, glycolytic muscle fibers, and smaller red and intermediate (RI) motoneurons that innervate the superficial slow oxidative and intermediate muscle fibers. In normal animals, glucose-6-phosphate dehydrogenase activity is high in the large W motoneurons and relatively low in the small RI motoneurons. The reverse holds for succinate dehydrogenase activity is high in the large W motoneurons and relatively low in the small RI motoneurons. The reverse holds for succinate dehydrogenase activity. W and RI motoneurons show similar nicotinamide adenine dinucleotide diaphorase activity. Short- (2 weeks) and long- (8 weeks) term effects of lesions were studied. The results show that: (1) the 3 types of lesions lead to prolonged changes in the enzyme histochemical profiles of spinal motoneurons. The type of change depends on the type of lesion and on the type of motoneuron; (2) unilateral axotomy of the motor nerves affects the histochemical characteristics of spinal motoneurons and the myotomal muscle fiber type configuration on the ipsi- and contralateral side. The contralateral effects are conceived as adaptations to maintain a left-right symmetry in the motor output.