Quantitative mapping of cytochrome oxidase activity in the central auditory system of the gerbil: a study with calibrated activity standards and metal-intensified histochemistry.

The objective was to obtain detailed topographic determinations of cytochrome oxidase activity in the gerbil central auditory system at the light microscopic level. Quantitative techniques were developed using (1) tissue standards calibrated to express histochemical measures as actual enzyme activity units, (2) densitometry and image analysis of histochemical reaction product formation, (3) spectrophotometry of cytochrome oxidase activity, and (4) a cobalt-intensified staining procedure compatible with autoradiography and other techniques requiring fresh-frozen brains without perfusion-fixation. Linear relationships between incubation time, section thickness, and activity of dissected brain regions, with their reaction product measured densitometrically were determined. Auditory structures with the high activities showed about 8 times the labeling intensity of the white matter or control sections inhibited with cyanide, glutaraldehyde, or heat. This indicated the high sensitivity of the method without loss of specificity. Specific activity for each of the 18 auditory structures measured were all above the units measured for whole brain homogenates, supporting the notion that basal levels of oxidative metabolism are greater for the auditory system. There was a progressive decrement in activity from brain stem to forebrain auditory structures. The more peripheral nuclei also showed a higher proportion of somatic as compared to neuropil reactivity. In contrast, auditory midbrain and thalamocortical regions were characterized primarily by neuropil reactivity. Comparison of intrinsic patterns of activity with morphological schemes to subdivide nuclei, showed a good correspondence with classical subdivisions derived from Golgi studies. The reported activities may provide a base of normative data in the gerbil for subsequent studies of central auditory functions. The method presented fulfilled established quantitative criteria and provided a more sensitive approach for regional mapping studies of brain cytochrome oxidase activity.