Fast Ca2+ Buffer-Dependent Reliable but Plastic Transmission at Small CNS Synapses Revealed by Direct Bouton Recording

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 preand 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.