Localization of choline acetyltransferase and calcitonin gene-related peptide immunoreactivities in the vestibular end-organs of the guinea pig.

The localization of choline acetyltransferase (ChAT) and calcitonin gene-related peptide (CGRP) immunoreactivities in the vestibular end-organs of the guinea pig was examined using immunohistochemistry. A large number of ChAT immunoreactive (ChAT-IR) terminals were found adjacent to the base of vestibular sensory cells. ChAT-IR terminals frequently formed synapses with the chalices of afferent nerves receiving synaptic inputs from type I cells and also formed direct contacts with type II cells. ChAT-IR terminals were filled with small clear vesicles, indicating that they are associated with the vestibular efferent system. All the vestibular efferent terminals were identified to be ChAT immunoreactive by electron microscopy. On the other hand, vestibular efferent terminals did not always show CGRP immunoreactivity. CGRP-immunoreactive (CGRP-IR) efferent terminals frequently made synaptic contacts with the chalices and type II cells. These results suggest that acetylcholine (ACh) and CGRP frequently coexist in the vestibular efferent system and may participate in the regulation of the vestibular function.