The distance between Ca2+ channels and release sensors determines the speed and efficacy of synaptic transmission. Tight "nanodomain" channel-sensor coupling initiates transmitter release at synapses in the mature brain, whereas loose "microdomain" coupling appears restricted to early developmental stages. To probe the coupling configuration at a plastic synapse in the mature central nervous system, we performed paired recordings between mossy fiber terminals and CA3 pyramidal neurons in rat hippocampus. Millimolar concentrations of both the fast Ca2+ chelator BAPTA [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid] and the slow chelator EGTA efficiently suppressed transmitter release, indicating loose coupling between Ca2+ channels and release sensors. Loose coupling enabled the control of initial release probability by fast endogenous Ca2+ buffers and the generation of facilitation by buffer saturation. Thus, loose coupling provides the molecular framework for presynaptic plasticity.