Abstract
Subthreshold depolarization enhances neurotransmitter release evoked by action potentials and plays a key role in modulating synaptic transmission by combining analog and digital signals. This process is known to be Ca2+ dependent. However, the underlying mechanism of how small changes in basal Ca2+ caused by subthreshold depolarization can regulate transmitter release triggered by a large increase in local Ca2+ is not well understood. This study aimed to investigate the source and signaling mechanisms of Ca2+ that couple subthreshold depolarization with the enhancement of glutamate release in hippocampal cultures and CA3 pyramidal neurons. Subthreshold depolarization increased presynaptic Ca2+ levels, the frequency of spontaneous release, and the amplitude of evoked release, all of which were abolished by blocking L-type Ca2+ channels. A high concentration of intracellular Ca2+ buffer or blockade of calmodulin abolished depolarization-induced increases in transmitter release. Estimation of the readily releasable pool size using hypertonic sucrose showed depolarization-induced increases in readily releasable pool size, and this increase was abolished by the blockade of calmodulin. Our results provide mechanistic insights into the modulation of transmitter release by subthreshold potential change and highlight the role of L-type Ca2+ channels in coupling subthreshold depolarization to the activation of Ca2+-dependent signaling molecules that regulate transmitter release.