| Summary: | Summary: The small size of presynaptic structures and their rapid function have obscured the mechanisms underlying neurotransmission and plasticity. To dissect the function of conventional small presynaptic boutons, we performed direct recording using axon varicosities of cerebellar granule cells (GCs), a parallel-fiber bouton, in dissociated culture, in which pre- and postsynaptic paired recordings are feasible. Identification and accessibility of EGFP-labeled GC boutons allowed us to patch-clamp record presynaptic voltage-gated Ca2+ currents and membrane capacitances, together with excitatory postsynaptic currents. We find that GC boutons have 20 readily releasable vesicles, which are loosely coupled to Ca2+ channels and rapidly replenished, and that synaptic strength and short-term plasticity are tightly regulated by intracellular Ca2+ buffering. Our functional dissection of small boutons thus reveals the sophisticated design of small synapses capable of reliable but plastic outputs with limited resources. : Kawaguchi and Sakaba use direct patch-clamp recording from small axonal varicosities to show that granule cell boutons have about 20 rapidly replenishable readily releasable vesicles, which are loosely coupled to Ca2+ channels, and that synaptic strength and short-term plasticity are tightly regulated by Ca2+ buffering. Keywords: synapse, direct recording, transmitter release, releasable vesicle, calcium buffering, synaptic plasticity, presynaptic bouton, vesicle replacement, cerebellar granule cell
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