The superior olivary complex of the hamster has multiple periods of cholinergic neuron development

Cholinergic neurons of the superior olivary complex share a common embryological and phylogenetic origin with brainstem motor neurons and serve as the major descending efferent pathway either to the cochlea as part of the olivocochlear system or to the cochlear nucleus. In this study, we investigated the developmental expression patterns of choline acetyltransferase (ChAT) and its co-localization with calcitonin gene-related peptide within the superior olivary complex and neighboring brainstem motor nuclei. At embryonic day 12, neurons in the ventral nucleus of the trapezoid body were first to express ChAT. The temporal expression pattern of both ChAT mRNA and immunoreactivity in this periolivary region mimicked motor neurons in the facial and trigeminal motor nuclei. Just before birth, shell neurons surrounding the lateral superior olive expressed ChAT. Neither ChAT-positive periolivary neurons nor shell neurons co-expressed calcitonin gene-related peptide during development or in the adult. Immediately following birth, intrinsic neurons within the lateral superior olive expressed ChAT but not calcitonin gene-related peptide. However, a transient increase in the number of ChAT-positive neurons in the lateral superior olive coincided with the onset of the calcitonin gene-related peptide co-expression within these neurons. We conclude that ChAT expression appears first in periolivary regions containing medial olivocochlear neurons, precedes the expression of calcitonin gene-related peptide in the superior olivary complex, and is co-expressed with calcitonin gene-related peptide within the lateral superior olive containing lateral olivocochlear neurons. These data suggest that the lateral olivocochlear system co-expresses ChAT and calcitonin gene-related peptide, whereas the medial olivocochlear system does not.