Action of putative neurotransmitters in the guinea pig cochlea

Scala tympani of the guinea pig was perfused with artificial perilymph containing putative transmitter substances. Single auditory nerve fibre activity was recorded during these perfusions and the responses of these fibres were studied using computer-controlled routine. Acetylcholine significantly inhibited tone-induced responses of a large proportion of the fibres tested. Glutamate appeared to be a potent depolarising agent when applied in concentrations between 2 and 10 mM. Noradrenaline did not affect tone-induced activity, but inhibited the spontaneous activity of some fibres. The results support the view that acetylcholine is the transmitter at synapses of the olivocochlear bundle, and that glutamate may be involved in the afferent synapse. The data do not support the hypothesis that noradrenaline is the afferent transmitter.