Main Determinants of Presynaptic Ca2+Dynamics at Individual Mossy Fiber–CA3 Pyramidal Cell Synapses

Abstract

Synaptic transmission between hippocampal mossy fibers (MFs) and CA3 pyramidal cells exhibits remarkable use-dependent plasticity. The underlying presynaptic mechanisms, however, remain poorly understood. Here, we have used fluorescent Ca²⁺indicators Fluo-4, Fluo-5F, and Oregon Green BAPTA-1 to investigate Ca²⁺dynamics in individual giant MF boutons (MFBs) in area CA3 traced from the somata of granule cells held in whole-cell mode. In an individual MFB, a single action potential induces a brief peak of free Ca²⁺(estimated in the range of 8–9 μm) followed by an elevation to ∼320 nm, which slowly decays to its resting level of ∼110 nm. Changes in the somatic membrane potential influence presynaptic Ca²⁺entry at proximal MFBs in the hilus. This influence decays with distance along the axon, with a length constant of ∼200 μm. In giant MFBs in CA3, progressive saturation of endogenous Ca²⁺buffers during repetitive spiking amplifies rapid Ca²⁺peaks and the residual Ca²⁺severalfold, suggesting a causal link to synaptic facilitation. We find that internal Ca²⁺stores contribute to maintaining the low resting Ca²⁺providing ∼22% of the buffering/extrusion capacity of giant MFBs. Rapid Ca²⁺release from stores represents up to 20% of the presynaptic Ca²⁺transient evoked by a brief train of action potentials. The results identify the main components of presynaptic Ca²⁺dynamics at this important cortical synapse.