Distinct localization of peripheral and central types of choline acetyltransferase in the rat cochlea

Abstract

We previously discovered a splice variant of choline acetyltransferase (ChAT) mRNA, and designated the variant protein pChAT because of its preferential expression in peripheral neuronal structures. In this study, we examined the immunohistochemical localization of pChAT in rat cochlea and compared the distribution pattern to those of common ChAT (cChAT) and acetylcholinesterase. Some neuronal cell bodies and fibers in the spiral ganglia showed immunoreactivity for pChAT, predominantly the small spiral ganglion cells, indicating outer hair cell type II neurons. In contrast, cChAT- and acetylcholinesterase-positive structures were localized to fibers and not apparent in ganglion cells. After ablation of the cochlear nuclei, many pChAT-positive cochlear nerve fibers became clearly visible, whereas fibers immunopositive for cChAT and acetylcholine esterase disappeared. These results suggested that pChAT and cChAT are localized in different systems of the rat cochlea; pChAT in the afferent and cChAT in the efferent structures.

Keywords: acetylcholine; choline acetyltransferase; cholinergic neurons; cochlear nervous system; spiral ganglion.

Fig. 1

Immunohistochemical localization of choline acetyltransferase of a peripheral type (pChAT)-positive structures in the rat cochlea. (A) Low power magnification of the rat cochlea. Arrowheads indicate spiral ganglia. sv, scala vestibule; st, scala tympani. (B) High power magnification of a spiral ganglion, showing intense staining of some smaller cells. (C) High power magnification of the organ of Corti. A nerve fiber weakly stained for pChAT extends to the outer cell layer (arrows). No pChAT-positive cells are seen. (D) High power magnification of pChAT-positive nerve fibers in the modiolus of the cochlea. Bars=500 µm (A), 50 µm (B–D).

Fig. 2

Immunohistochemical localization of choline acetyltransferase of a common type (cChAT)-positive structures in the rat cochlea. (A) In the modiolus, cChAT-positive fibers (arrows) are gathered near the spiral ganglion (arrowheads). (B) In the organ of Corti, cChAT-positive fibers are observed extending to the hair cells (arrows). (C) High power magnification of cChAT-positive fibers in the modiolus (arrows). (D) High power magnification of the spiral ganglion, showing no cChAT-positive cells. Bar=50 µm (A, B), 25 µm (C, D).

Fig. 3

Scattergram showing the correlation between the concentration of choline acetyltransferase of a peripheral type (pChAT) staining and cell area, indicating that pChAT-positive staining in the spiral ganglion seems more specific for the smaller cells.

Fig. 4

Acetylcholinesterase (AChE)-positive (A) and choline acetyltransferase of a common type (cChAT)-positive structures (B) in the rat cochlea. The distribution pattern of AChE-positive nerve fibers is similar to that of cChAT-positive nerve fibers. Neither AChE nor cChAT-positive cells are shown. Bar=50 µm.

Fig. 5

Immunohistochemical staining of choline acetyltransferase of a peripheral type (pChAT) in the spiral ganglion with no treatment (A) or after removal of the cochlear nucleus (B). After removal of the cochlear nucleus, many pChAT-positive fibers are visible (arrows). Bar=50 µm.

Fig. 6

Immunohistochemical staining of choline acetyltransferase of a common type (cChAT) in the organ of Corti with no treatment (A) or after removal of the cochlear nucleus (B). After removal of the cochlear nucleus, cChAT-positive nerve fibers disappeared. Bar=100 µm.