The histochemical localization of acetylcholinesterase in the rainbow trout saccular macula by electron microscopy.

The hair cells of the teleost saccular macula are morphologically similar to type II vestibular hair cells of higher vertebrates. The two types of nerve endings at the base of the saccular hair cells are the non-vesiculated endings, thought to be afferent, and the vesiculated terminals, presumed to be efferent. The vesicles in the presumptive efferent endings are predominantly round and clear, and a few are dense cored. The morphological characteristics of the vesiculated endings resemble those of presumptive efferent cholinergic endings present in the inner ear of higher vertebrates. In the present study, vesiculated nerve endings were examined histochemically for acetylcholinesterase. The reaction product was observed along the plasma membranes of the vesiculated nerve endings synapsing both with the hair cells and afferent endings, but was not seen at non-vesiculated endings on hair cells. No staining was observed in control specimens incubated in the presence of eserine sulfate, an inhibitor of acetylcholinesterase. These results suggest that the vesiculated nerve endings in the trout saccular macula contain cholinergic elements.