Acetylcholine (Ach) has been considered a major neurotransmitter in the inner ear efferent nerve endings. A bioassay analysis has shown that the electrical stimulation of the crossed olivocochlear bundle increased the Ach-like activity in the perilymph. Applying in vivo microdialysis techniques and high-performance liquid chromatography to the perilymph, the change of Ach level was thus measured before and after α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), a glutamate receptor agonist, was added to the perfusate. Ach was only detectable when the perfusate contained an acetylcholinesterase inhibitor. The level of Ach increased 2-3-fold immediately after AMPA was administered. Our data suggest that the afferent stimulation, such as the administration of AMPA, may therefore induce the release of Ach from the efferent nerve endings.
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