Reliability and Heterogeneity of Calcium Signaling at Single Presynaptic Boutons of Cerebellar Granule Cells

Abstract

res transients while loading boutons to a steady-state indicator concentration. These experiments revealed that, in the absence of exogenous calcium buffers, a single action potential evokes transients of Ca res that vary widely in different boutons both in amplitude (400 -900 nM) and time course (25-55 ms). Variation in calcium influx density, endogenous buffer capacity, and calcium extrusion density contribute to differences in Ca res among boutons. Heterogeneity in Ca res within different boutons suggests that plasticity can be regulated independently at different synapses arising from an individual granule cell. In a given bouton, Ca res signals were highly reproducible from trial to trial and failures of calcium influx were not observed. We find that a factor contributing to this reliability is that an action potential opens a large number of calcium channels (20 -125) in a bouton. Presynaptic calcium signals were also used to assess the ability of granule cell axons to convey somatically generated action potentials to distant synapses. In response to pairs of action potentials or trains, granule cell boutons showed a remarkable ability to respond reliably at frequencies up to 500 Hz. Thus, individual boutons appear specialized for reliable calcium signaling during bursts of high-frequency activation such as those that are observed in vivo.

The Mechanisms and Functions of Synaptic Facilitation

“…This approach suggested that at the calyx of Held vesicle fusion is triggered by a calcium increase of 25 μM lasting several hundred microseconds (Schneggenburger and Neher, 2000). Ca res is readily measured with fluorometric calcium indicators, and it is typically tens to a few hundred nanomolar (Atluri and Regehr, 1996; Brenowitz and Regehr, 2007; Scott and Rusakov, 2006). …”

“…During the action potential, local Ca 2+ rises to 25 μM briefly (FWHM = .34 ms, Sabatini and Regehr, 1998)). After the action potential, a residual Ca 2+ signal of 400 nM decays back to resting Ca 2+ (50 nM) with a 40 ms time constant (Brenowitz and Regehr, 2007). …”

Short-Term Presynaptic Plasticity

“…An alteration in the firing threshold for somatic or extracellular activation that involves the Na/K-ATPase could contribute to short-term depression at these synapses (Munoz-Cuevas et al 2004). A failure of an action potential to invade some axonal branches may contribute to depression in cultured hippocampal cells (Brody and Yue 2000;Prakriya and Mennerick 2000), but action potential invasion of axons is generally reliable in more intact tissue (Cox et al 2000;Kreitzer and Regehr 2001;Brenowitz and Regehr 2007).…”

Methods of measuring activity at individual synapses: A review of techniques and the findings they have made possible

“…In cerebellar granule cells, Brenowitz and Regehr (2007) found low variability of AP-stimulated calcium influx in boutons due to a large number of calcium channels. At giant mossy fiber boutons in hippocampal area CA3 studies of calcium transients following action potential stimulation determined that store-released calcium contributes roughly one-fifth of the Ca 2+ signal (Scott and Rusakov, 2006).…”