Presynaptic diversity revealed by Ca²⁺-permeable AMPA receptors at the calyx of held synapse

GluA2-lacking Ca²⁺-permeable AMPARs (CP-AMPARs) play integral roles in synaptic plasticity and can mediate excitotoxic cellular signaling at glutamatergic synapses. However, the developmental profile of functional CP-AMPARs at the auditory brainstem remains poorly understood. Through a combination of electrophysiological and live-cell Ca²⁺ imaging from mice of either sex, we show that the synaptic release of glutamate from the calyx of Held nerve terminal activatesCP-AMPARsin the principal cells of the medial nucleus of the trapezoid body in the brainstem. This leads to significant Ca²⁺influx through these receptors before the onset of hearing at postnatal day 12 (P12). Using a selective open channel blocker of CP-AMPARs, IEM-1460, we estimate that 80% of the AMPAR population are permeable to Ca²⁺ at immature P4-P5 synapses. However, after the onset of hearing, Ca²⁺ influx through these receptors was greatly reduced. We estimate that CP-AMPARs comprise approximately 40% and 33% of the AMPAR population at P18 -P22 and P30 -P34, respectively. By quantifying the rate of EPSC block by IEM-1460, we found an increased heterogeneity in glutamate release probability for adult-like calyces (P30 -P34). Using tetraethylammonium (TEA), a presynaptic potassium channel blocker, we show that the apparent reduction of CP-AMPARs in more mature synapses is not a consequence of presynaptic action potential (AP) speeding. Finally, through postsynaptic AP recordings, we show that inhibition of CP-AMPARs reduces spike fidelity in juvenile synapses, but not in more mature synapses. We conclude that the expression of functional CP-AMPARs declines over early postnatal development in the calyx of Held synapse.