Quantitative cytochemical analysis of cytochrome oxidase and succinate dehydrogenase activity in spinal neurons.

This paper describes cytophotometric determinations of cytochrome oxidase (COX) and succinate dehydrogenase (SDH) activities in neurons in 3 areas of the spinal motor column of the teleost fish Brachydanio rerio (the Zebrafish). Purpose of this investigation was to analyse the correlation between the oxidative metabolic capacity of motoneurons with their activity patterns. The spatial organization of the spinal cord of the zebrafish allows such an analysis, because the motoneurons which innervate different muscle fiber types (slow tonic red and fast phasic white, respectively) occupy separate areas of the motor column (Van Raamsdonk et al. 1983). We analysed the COX and SDH activities on serially sectioned neurons, We found large variations in the ratio of COX/SDH activity: the ratio was high for large neurons (in the "white" area) and low for small neurons (in the "red" area). These findings were contrary our expectations, because COX as well as SDH activity have been proposed as indicators for neuronal activity if both activities are reliable indicators, then their ratio should be constant. In addition, COX and SDH activities were analysed on serially sectioned anterior horn neurons of the cat spinal cord. In contrast to the situation in fish, we observed a statistically significant proportionality between COX and SDH activities. We conclude that the histochemical reactions for COX or SDH activity have no general validity as markers for the same type of neuronal activity.