Developmental changes in potassium currents at the rat calyx of Held presynaptic terminal

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Abstract
During early postnatal development, the calyx of Held synapse in the auditory brainstem of rodents undergoes a variety of morphological and functional changes. Among ionic channels expressed in the calyx, voltage-dependent K+ channels regulate transmitter release by repolarizing the nerve terminal. Here we asked whether voltage-dependent K+ channels in calyceal terminals undergo developmental changes, and whether they contribute to functional maturation of this auditory synapse. From postnatal day (P) 7 to P14, K+ currents became larger and faster in activation kinetics, but did not change any further to P21. Likewise, presynaptic action potentials became shorter in duration from P7 to P14 and remained stable thereafter. The density of presynaptic K+ currents, assessed from excised patch recording and whole-cell recordings with reduced [K+]i, increased by 2-3-fold during the second postnatal week. Pharmacological isolation of K+ current subtypes using tetraethylammonium (1 mM) and margatoxin (10 nM) revealed that the density of Kv3 and Kv1 currents underwent a parallel increase, and their activation kinetics became accelerated by 2-3-fold. In contrast, BK currents, isolated using iberiotoxin (100 nM), showed no significant change during the second postnatal week. Pharmacological block of Kv3 or Kv1 channels at P7 and P14 calyceal terminals indicated that the developmental changes of Kv3 channels contribute to the establishment of reliable action potential generation at high frequency, whereas those of Kv1 channels contribute to stabilizing the nerve terminal. We conclude that developmental changes in K+ currents in the nerve terminal contribute to maturation of high-fidelity fast synaptic transmission at this auditory relay synapse.