Synaptic Vesicles in the Cochlea

Abstract

The cochlea is innervated by afferent, efferent and sympathetic neurons. The afferent synapses and the efferent and sympathetic nerve terminals contain transmitter substances within their synaptic vesicles. These transmitters are likely to be chemically different in each of the three cochlear nerve populations. The character of intra-cochlear synaptic vesicles was investigated in order to throw some light on transmitter identity. Their staining properties were examined using different fixatives and after pretreatment with false transmitter substances and a monoamine depleting drug. The efferent and sympathetic cochlear terminals were identified and differentiated by surgical ablation of either the efferent olivo-cochlear bundle or by cervical sympathectomy. The nature of these terminals could also be ascertained in unoperated animals, but only after pretreatment with false transmitters or after special fixation techniques. In this way a catecholamine appeared to be the transmitter in the sympathetic nerve-terminals. The staining properties of cochlear afferent synaptic vesicles and synaptic bars made it unlikely that a catecholamine was the transmitter. Vesicles in efferent terminals reacted similarly to vesicles in cholinergic neurons.