Quantitative histochemical mapping of enzymes of the cholinergic system in cat cochlear nucleus.

Activities of choline acetyltransferase and acetyicholinesterase were measured in samples dissected from freeze-dried sections of cat cochlear nuclei and the immediate surroundings. Refined methods of sampling and record-keeping were systematically applied at the histologic level. Selected sections were studied for nine cochlear nuclei, and a more complete threedimensional map of enzyme distributions was obtained for a tenth nucleus. Activities of both enzymes were highest in granular regions and lowest among the entering auditory-nerve fibers; the range was more than 100-fold in both cases Although the choline acetyltransferase activity for the cochlear nucleus as a whole was only about one sixth that of cat whole brain, activities within granular regions of the nucleus were comparable to that of whole brain. Acetyicholinesterase activities within these granular regions were more than five times that of whole brain. In most regions, the activities of the two enzymes were distributed similarly. A notable exception was in the superficial layers of the dorsal cochlear nucleus, where acetyicholinesterase activities were 2-3 times those of average cochlear nucleus, while choline acetyltransferase activities were about average. Very high choline acetyltransferase activities, in some cases approaching those measured for the facial nerve, were found within the vestibular nerve root where centrifugal fibers travel to cochlear and vestibular receptors. The results imply that acetyicholine may have an important role in certain parts of the cat cochlear nucleus.