TRANSMITTER RELEASE IN INNER HAIR CELL SYNAPSES - A MODEL ANALYSIS OFSPONTANEOUS AND DRIVEN RATE PROPERTIES OF COCHLEAR NERVE-FIBERS

The inner hair cell (IHC) synapse is one of the stages of cochlear pro cessing that determine the relation between sound pressure level and s pike rate in auditory nerve fibres. Transmitter released in the non-st imulated condition is held responsible for the wide range of spontaneo us spike rates (SR) observed in these fibres. Properties of stimulated spike activity in auditory nerve fibres, including rate threshold and operating range of a fibre, are known to systematically vary with SR. This paper presents a model analysis of the relation between IHC tran smembrane potential and transmitter release rate as becoming manifest in these spontaneous and driven rate properties. A previously develope d computational model is used to identify those transfer properties of its synapse section which lead to reproduction of the variation of ra te thresholds, shapes of rate-intensity functions and maximal driven r ate with SR known from the literature. First a simple additive release model, in which driven transmitter release depends linearly on IHC po tential, is elaborated. Its results lead to the hypothesis that the tr ue release function is non-linear and variable across synapses generat ing different SR. An exponential release function is then introduced, with parameters varying across SR in a physiologically dictated way. T his approach leads to adequate reproduction of the variation in rate t hresholds and rate-intensity functions with SR. Finally, the model is applied in an inverse way to directly estimate the release function fr om given rate-intensity functions. The conclusion of both forward and inverse model analyses is that transmitter release is a non-linear fun ction of IHC potential which, by the systematic variation of its param eters across SR, effectively leads to the physiological variation in d ynamic range across fibres of different SR. Possible relations of thes e results with ultrastructural morphology and basic physiology of IHC synapses are discussed.