Activity-Dependent Bulk Endocytosis and Clathrin-Dependent Endocytosis Replenish Specific Synaptic Vesicle Pools in Central Nerve Terminals

Multiple synaptic vesicle (SV) retrieval modes exist in central nerve terminals to maintain a continual supply of SVs for neurotransmission. Two such modes are clathrin-mediated endocytosis (CME), which is dominant during mild neuronal activity, and activity-dependent bulk endocytosis (ADBE), which is dominant during intense neuronal activity. However, little is known about how activation of these SV retrieval modes impact the replenishment of the total SV recycling pool and the pools that reside within it, the readily releasable pool (RRP) and reserve pool. To address this question, we examined the replenishment of all three SV pools by triggering these SV retrieval modes during both high-and low-intensity stimulation of primary rat neuronal cultures. SVs generated by CME and ADBE were differentially labeled using the dyes FM1-43 and FM2-10, and their replenishment of specific SV pools was quantified using stimulation protocols that selectively depleted each pool. Our studies indicate that while the RRP was replenished by CME-generated SVs, ADBE provided additional SVs to increase the capacity of the reserve pool. Morphological analysis of the uptake of the fluid phase marker horseradish peroxidase corroborated these findings. The differential replenishment of specific SV pools by independent SV retrieval modes illustrates how previously experienced neuronal activity impacts the capability of central nerve terminals to respond to future stimuli.