How Secure Is In Vivo Synaptic Transmission at the Calyx of Held?

The medial nucleus of the trapezoid body (MNTB) receives excitatory input from giant presynaptic terminals, the calyces of Held. The MNTB functions as a sign inverter giving inhibitory input to the lateral and medial superior olive, where its input is important in the generation of binaural sensitivity to cues for sound localization. Extracellular recordings from MNTB neurons show complex spikes consisting of a prepotential, thought to reflect synaptic activation, followed by a postsynaptic action potential. This makes the synapse ideal to study synaptic transmission in vivo because presynaptic and postsynaptic activity can be monitored with a single electrode. Recent in vivo and in vitro studies have observed isolated prepotentials in the MNTB suggesting that, under certain stimulus conditions, synaptic transmission fails. We investigated synaptic transmission at the calyx of Held in the MNTB of the adult cat and concluded that synaptic transmission was completely secure in terms of rate of transmitted spikes. However, synaptic transmission was found to be less secure in terms of timing. With increasing spike rate, the synaptic delay showed an increase of up to 100 μs, as well as a decrease in amplitude of the action potential. This variability in delay is of a surprisingly high magnitude given the hypothesized role of these binaural circuits in sound localization and given the fact that this is one of the largest synapses in the mammalian brain.