L-type Ca2+ channels mediate regulation of glutamate release by subthreshold potential changes.

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.

Modulating and monitoring the functionality of corticostriatal circuits using an electrostimulable microfluidic device.

The central nervous system is organized into different neural circuits, each with particular functions and properties. Studying neural circuits is essential to understanding brain function and neuronal diseases. Microfluidic systems are widely used for reconstructing and studying neural circuits but still need improvement to allow modulation and monitoring of the physiological properties of circuits. In this study, we constructed an improved microfluidic device that supports the electrical modulation of neural circuits and proper reassembly. We demonstrated that our microfluidic device provides a platform for electrically modulating and monitoring the physiological function of neural circuits with genetic indicators for synaptic functionality in corticostriatal (CStr) circuits. In particular, our microfluidic device measures activity-driven Ca2+ dynamics using Ca2+ indicators (synaptophysin-GCaMP6f and Fluo5F-AM), as well as activity-driven synaptic transmission and retrieval using vGlut-pHluorin. Overall, our findings indicate that the improved microfluidic platform described here is an invaluable tool for studying the physiological properties of specific neural circuits.

Distinct synaptic vesicle recycling in inhibitory nerve terminals is coordinated by SV2A.

Proper brain function requires a balance between excitatory and inhibitory neuronal activity. This balance, which is disrupted in various neural disorders, ultimately depends on the functional properties of both excitatory and inhibitory neurons; however, how the physiological properties of presynaptic terminals are controlled in these neurons is largely unknown. In this study, we generated pHluorin-conjugated, synaptic vesicle-specific tracers that are preferentially expressed in excitatory or inhibitory nerve terminals. We found that synaptic vesicle recycling is ∼1.8-fold slower in inhibitory nerve terminals than excitatory nerve terminals, resulting in reduced efficacy of synaptic transmission in inhibitory presynaptic terminals during repetitive activities. Interestingly, this relative difference in trafficking efficiency is mediated by synaptic vesicle protein 2A (SV2A), which is more highly expressed in inhibitory synapses and differentially controls sorting of synaptic protein, synaptotagmin I. These findings indicate that SV2A coordinates distinct properties of synaptic vesicle recycling between excitatory and inhibitory synapses.