Synaptic contact between vestibular afferent nerve and cholinergic efferent terminal: its putative mediation by nicotinic receptors.

The topography and fine structure of cholinergic efferent fibers were examined in the rat vestibular labyrinth using choline acetyltransferase (ChAT) immunohistochemistry. Fiber plexus of cholinergic fibers were observed beneath the sensory cell layer in all vestibular end-organs. Electron microscopic examination revealed that cholinergic fibers make synaptic contacts with chalices of vestibular nerve branches that surround type I hair cells. In situ hybridization histochemistry of nicotinic acetylcholine receptor subunit mRNAs showed that rat vestibular ganglion cells express both alpha 4 and beta 2 subunit mRNAs. These findings suggest that cholinergic efferents modulate vestibular information from type I hair cells by nicotinic receptors at the level of nerve chalices.