Cholinergic and noradrenergic nervous systems in the cynomolgus monkey cochlea

The cholinergic and noradrenergic nervous systems in the cochleas of cynomolgus monkeys were examined by histochemical and immunohistochemical demonstration of acetylcholinesterase (AChE) and tyrosine hydroxylase (TH) and/or dopamine B-hydroxylase (DBH), respectively. AChE activity was demonstrated in the efferent fibers making synaptic contact with the perikarya and the cell processes of type II neurons. Type II neurons also demonstrated AChE activity on the cell surface and their neurites, suggesting that the AChE-positive fibers in the simian cochlear nerve trunk, previously believed to be of efferent origin, are the central processes of type II neurons. TH-like and DBH-like immunoreactivity of beaded unmyelinated fibers demonstrated similar distribution throughout the cochlear nerve trunk, Rosenthal's canal, and osseous spiral lamina. During their course, they came in contact with unmyelinated efferent fibers and neurites of type II neurons and abutted blood vessels. However, their distribution was not restricted to following blood vessels or cochlear nerve fibers. These observations indicate that the sympathetic nerve supply and olivocochlear efferent system in the cochleas of monkeys differ from those of smaller animals.