Postnatal development of cytochrome oxidase activity in fiber tracts of the rat brain.

This paper describes postnatal changes in cytochrome oxidase (C.O.) activity in developing fiber tracts. Quantitative histochemistry was used to measure changes in C.O. activity in nine white matter regions at postnatal days (P) 7, 12, 17, 30, and 60 in the rat. At P7, enzyme activity was maximal in the spinal trigeminal tract, medial longitudinal fasciculus, and cerebellar white matter. At P12, maximal levels were measured in the medial lemniscus and cerebral peduncle. C.O. activity increased from low levels at P7 to maximal levels by P17 in the hippocampal commissure, posterior and anterior corpus callosum, and anterior commissure. In all nine regions, C.O. activity decreased by P60. Thus, peaks in C.O. activity shifted as a function of postnatal age in a caudo-rostral direction. The regional heterogeneity in the age of onset in C.O. fluctuations suggests that vulnerability to injury and metabolic dysfunction during the perinatal period will differentially affect white matter structures, depending on the age of onset of such disruptions.