Reorganization of synchronous and asynchronous release in developing hippocampal autaptic neurons of mice

Presynaptic vesicle pools, particularly the readily releasable pool (RRP), are essential for synaptic transmission. The RRP comprises fast-release pool (FRP) and slow-release pool (SRP) subpopulations that mediate synchronous and asynchronous neurotransmitter release, respectively, yet their developmental regulation remains unclear. To investigate this question, we utilized a hippocampal autaptic culture system to analyze synaptic function in individual excitatory neurons at 1, 2, and 3 weeks in vitro. We observed significant enhancements in both synaptic transmission efficiency and the size of RRP with neuronal maturation. Despite the increase in vesicle availability, the probability of vesicular release (Pvr) decreased during development, suggesting a shift in RRP composition or priming status. Although both synchronous and asynchronous release increased in absolute magnitude during development, the relative contribution of asynchronous release to total release declined with maturation, indicating a shift toward synchronous release in mature neurons. Furthermore, while the overall RRP refilling rate remained constant across development, early recovery of EPSCs after depletion was modestly faster in mature neurons. This finding may suggest preferential recruitment of vesicles into the FRP in mature neurons. These findings demonstrate a developmental reorganization of presynaptic vesicle pools that optimizes synchronous release and enhances temporal precision in mature synapses.