A model of the prespike at the calyx of Held synapse

The calyx of Held synapse is a giant axosomatic synapse in the brainstem that functions as a fast and reliable auditory relay. Because of its large size, the presynaptic action potential can be picked up in a recording of the postsynaptic cell. This so-called prespike is caused by the ephaptic coupling of presynaptic membrane currents via the synaptic cleft. We constructed an electric model of the prespike and estimated values for the leak conductance of the synaptic cleft, the capacitance of the release face and the presynaptic calcium current density based on simultaneous pre- and postsynaptic voltage clamp recordings. These values allowed us to estimate an impact of presynaptic capacitive and ionic currents on the cleft potential, and suggested that fenestration of the calyx of Held during development might be important to further reduce the cleft leak conductance of the cleft to minimize its impact on the presynaptic calcium channels.

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