Cytochrome oxidase activity in the auditory system of the mouse: a qualitative and quantitative histochemical study

Detailed qualitative and quantitative determinations of cytochrome oxidase activity in the central auditory system of BALB/cJ mice were obtained at the light microscopic level. Cytochrome oxidase activity was determined using quantitative densitometry calibrated with standards of spectrophotometrically assayed enzymatic activity. This was done together with a cobalt-intensified histochemical procedure using fresh-frozen brains without perfusion-fixation. The resulting method showed improved sensitivity and allowed quantification of histochemical labeling as actual enzyme activity units. Adjacent sections were processed for either Nissl, fiber or Golgi stains to correlate the histochemical labeling with tissue morphology. The more peripheral auditory nuclei showed primarily somatic labeling with specific cell types showing predominant reactivity. However, higher auditory structures, including the inferior colliculus, medial geniculate and auditory cortex, showed predominantly neuropil reactivity. Comparison of mean cytochrome oxidase activities for the 27 auditory regions quantified revealed a trend for decreasing activity from the brainstem to the forebrain in central lemniscal structures. The extra-lemniscal auditory regions at each level showed lower activity than the corresponding lemniscal regions. The regions with the higher activity values showed around 10 times the labeling density of the white matter, indicating the high sensitivity of the method. The darkly labeling auditory structures were clearly delineated from surrounding neural regions, supporting the concept that basal levels of oxidative metabolic capacity are larger for the auditory system. It was concluded that the quantitative approach to cytochrome oxidase histochemistry may be applied successfully to the mouse brain. The normative data presented may be used as a starting point for other investigations of the effects of experimental manipulations on the metabolic activity of the auditory system.