Ultrastructural characteristics of synaptic endings in the cochlear nucleus having acetylcholinesterase activity

Abstract Karnovsky's method was used to examine the ultrastructural distribution of acetylcholinesterase (AChE) activity in the anteroventral cochlear nucleus (AVCN) of the cat and chinchilla. The density of synaptic endings with reaction product is greatest in the superficial layer of granule cells covering the AVCN. In addition to primary cochlear nerve afferents, an abundance of feedback fibers terminate in the AVCN. Some feedback fibers are from neurons intrinsic to the AVCN, while others arise from neurons with long axons located in nuclei at higher levels. By destroying the cochlea, we demonstrated that endings of cochlear nerve afferents do not have demonstrable AChE activity. However, at least 3 types of terminals not belonging to cochlear nerve axons do have AChE activity. These 3 groups of endings differ in external configuration and in the shape of their synaptic vesicles. None of these groups has characteristics which permit identification of AChE-type endings without the histochemical reaction, because many endings without detectable AChE activity are morphologically similar to those with activity. In the AVCN proper most endings with AChE activity are boutons containing flattened vesicles. Some of these probably represent terminals of feedback fibers from the superior olive. The superficial granular layer has two different types of endings with AChE activity: boutons and mossy endings. Both of these contain round synaptic vesicles about 49 nm in diameter (vesicles of cochlear nerve afferent endings are round but 56.5 nm in diameter). Sources of these endings are not known. Our results indicate that in the AVCN the shape of synaptic vesicles is not a parameter which is useful in predicting if nerve endings will have histochemical evidence of AChE activity.