Contribution of the mouse calyx of Held synapse to tone adaptation

The calyx of Held synapse is a giant synapse in the medial nucleus of the trapezoid body (MNTB) of the ventral brainstem, which is involved in sound localization. Although it has many release sites, it can show transmission failures and display an increase in synaptic delay during high-frequency signalling. Its apparent lack of reliability and precision raises the question whether this synapse makes a sizeable contribution to tone adaptation, the decline in response to sustained or repetitive auditory stimuli. We observed evidence for the presence of both ipsilateral and contralateral inhibition, but these effects were already present in the inputs to the MNTB, suggesting that synaptic inhibition within the MNTB does not contribute to tone adaptation. During trains of brief tones at variable intervals, there were no clear changes in reliability or precision at tone intervals of 20 ms or longer. A progressive decrease in the number of spikes measured in the MNTB was observed at shorter tone intervals, but this decrease largely originated upstream from the MNTB. In addition, for tones with short intervals, during the train a progressive increase in first-spike latencies was observed, but much smaller changes were observed in the delay between excitatory postsynaptic potentials and postsynaptic action potentials within the MNTB. We conclude that despite the failures and variability in synaptic delay that are present at the calyx of Held synapse, their contribution to tone adaptation is relatively small compared with upstream factors.