Neurotransmission in the inner ear.

The present view on cochlear neurotransmission can be summarized as follows: There are two main types of synapses on cochlear hair cells, afferent and efferent ones. Afferent synaptic structures are abundant on inner hair cells whereas similar structures on the outer hair cells are less frequent and appear to be rudimentary. Presynaptic vesicles seem to be rare in outer hair cells. For the inner hair cell--afferent terminal--the presence of a chemical transmission mechanism is generally accepted. The transmitter substance has not yet been unequivocally demonstrated. Glycine, catecholamines, GABA and 5-HT can be eliminated as candidates as these compounds do not activate afferent fibres. There are good reasons, however, to consider amino acids. Most of the experimental results support glutamate as the transmitter (e.g. effectiveness of glutamate, kainic acid, glutamate diethylester). Aspartate is less likely. It is not yet well understood, however, why glutamate has to be applied in concentrations of up to 10(-3) M intracochlearly in order to activate afferent fibres and why elevated glutamate levels could not be demonstrated in perilymph collected during acoustical stimulation, whereas this same perilymph was able to activate afferent nerve terminals when applied intracochlearly. Efferent endings use acetylcholine as a transmitter. Enzymes for synthesis and breakdown of acetylcholine are present; acetylcholine is effective at the synaptic junction, as are cholinergic compounds and specific blockers. However, there may be different types of efferent endings in both the cochlear and vestibular organs.(ABSTRACT TRUNCATED AT 250 WORDS)