Parvalbumin is a mobile presynaptic Ca2+ buffer in the calyx of held that accelerates the decay of Ca2+ and short-term facilitation

Presynaptic Ca2+ signaling plays a crucial role in short-term plasticity of synaptic transmission. Here, we studied the role of mobile endogenous presynaptic Ca2+ buffer(s) in modulating paired-pulse facilitation at a large excitatory nerve terminal in the auditory brainstem, the calyx of Held. To do so, we assessed the effect of presynaptic whole-cell recording, which should lead to the diffusional loss of endogenous mobile Ca2+ buffers, on paired-pulse facilitation and on intracellular Ca2+ concentration ([Ca2+](i)) transients evoked by action potentials. In unperturbed calyces briefly preloaded with the Ca2+ indicator fura-6F, the [Ca2+](i) transient decayed surprisingly fast (tau(fast), similar to 30 ms). Presynaptic whole-cell recordings made without additional Ca2+ buffers slowed the decay kinetics of [Ca2+](i) and paired-pulse facilitation ( twofold to threefold), but the amplitude of the [Ca2+ ](i) transient was changed only marginally. The fast [Ca2+](i) decay was restored by adding the slow Ca2+ buffer EGTA (50-100 mu M) or parvalbumin (100 mu M), a Ca2+-binding protein with slow Ca2+-binding kinetics, to the presynaptic pipette solution. In contrast, the fast Ca2+ buffer fura-2 strongly reduced the amplitude of the [Ca2+](i) transient and slowed its decay, suggesting that the mobile endogenous buffer in calyces of Held has slow, rather than fast, binding kinetics. In parvalbumin knock-out mice, the decay of [Ca2+](i) and facilitation was slowed approximately twofold compared with wildtype mice, similar to what is observed during whole-cell recordings in rat calyces of Held. Thus, in young calyces of Held, a mobile Ca2+ buffer with slow binding kinetics, primarily represented by parvalbumin, accelerates the decay of spatially averaged [Ca2+](i) and paired-pulse facilitation.