Choline acetyltransferase-immunoreactive cochlear efferent neurons in the chick auditory brainstem.

Cholinergic neurons in the chick auditory brainstem were studied with the aid of an antiserum to choline acetyltransferase (ChAT), the biosynthetic enzyme for acetylcholine. ChAT-immunoreactive (ChAT-I) neurons were found in a ventrolateral and a dorsomedial cell group. The ventrolateral group is a rostrocaudally directed column of cells that surround the superior olive (SO), are ventromedial to the ventral facial nucleus (VIIv), and are lateral to the nucleus pontis lateralis (PL) as far rostrally as the nucleus subceruleus ventralis. Cells in the dorsomedial group were found in the pontine reticular formation medial to the dorsal facial nucleus and lateral to the abducens nerve root. Occasionally, small ChAT-I cells were found in the crossed dorsal cochlear tract and in the medial vestibular nucleus near the dorsal border of the caudal nucleus magnocellularis (NM). No ChAT-I neurons or fibers were observed in NM, nucleus angularis, nucleus laminaris, in the nuclei of the lateral lemniscus, or in the nucleus mesencephalicus lateralis pars dorsalis. To determine which cholinergic neurons project to the cochlea, a double-labeling technique was used combining ChAT-I and the retrograde transport of biotinylated dextran amine (BDA) from the inner ear. Double-labeled cells were found bilaterally in both the ventrolateral and dorsomedial cell groups, with the exception of large ChAT-I cells dorsal to the SO, which do not appear to project to the cochlea. Cholinergic cells that project to the cochlea were classified into three morphological groups: multipolar, elongate, and round-to-oval. Both the ventrolateral and the dorsomedial cell groups appear to have a mixture of these different cell types. The average somal area of cholinergic cochlear efferents was 246 microns 2. Only about 70% of the cochlear efferent neurons, however, are cholinergic.