Co-localization of glutamate, choline acetyltransferase and glycine in the mammalian vestibular ganglion and periphery

Glutamate (Glu) is considered to be the main transmitter at the central synapses of primary vestibular afferents (PVA) and glycine (Gly) is assumed to play a modulatory role. In the vestibular periphery a transmitter role for acetylcholine (ACh) has been attributed chiefly to vestibular efferents (VE), however only a subset of VE neurons displays immunoreactivity (ir) for choline acetyltransferase (ChAT) and acetylcholine esterase (AChE). Controversial results exist on the presence of these two enzymes in PVA. In this study the presence of Glu, ChAT, Gly and their co-localization in the vestibular ganglia (VG) and end organs of mouse, rat, guinea pig and squirrel monkey were investigated. In the VG all bipolar neurons display strong Glu-ir and the majority of cells show a graded ChAT-ir and Gly-ir in all species examined. ChAT and Gly are present in highly overlapping neuronal populations and with a similar gradation. In the end organs ChAT and Gly are again co-localized in the same sets of fibers and endings. In conclusion, in the vestibular ganglion and end organs ChAT appears also to be present in primary afferents rather than being restricted to efferent processes. ChAT in primary afferents might indicate a modulatory or co-transmitter function of acetylcholine.