The distribution of choline acetyltransferase in the rat amygdaloid complex and adjacent cortical areas, as determined by quantitative micro‐assay and immunohistochemistry

The distribution of choline acetyltransferase (ChAT) within the amygdaloid complex has been studied to evaluate what should primarily represent the terminal field of the cholinergic projection from the basal forebrain. Two currently available methods have been combined for the comparison: immunohistochemistry with a monoclonal antibody against ChAT, by a double peroxidase‐antiperoxidase procedure, and quantitative histochemistry involving micro‐assay of the ChAT activity of contiguous microdissected samples. Both methods indicate prominent ChAT activity in the basolateral amygdaloid nucleus (especially rostrally), the nucleus of the lateral olfactory tract (especially layer II), and the amygdalohippocampal area. Regions of lower ChAT activity were not accurately represented, by the immunohistochemistry, but could be discriminated by the quantitative assays. Lowest activity was found in the medial nucleus of the amygdala. Most other regions had activities at least as high as average brain or neocortex. Gradients of enzyme activity were found within several regions, including the basolateral and lateral amygdaloid nuclei and the nearby posterior piriform cortex. In the piriform cortex, a region of particularly high ChAT activity was found at its medial edge near the nucleus of the lateral olfactory tract. The immunohistochemical method shows a few intensely reactive somata in layer III within this zone. Comparison of the results seen with immunohistochemistry and quantitative histochemistry suggests an advantage in using them together, since their respective strengths and weaknesses tend to complement each other.