Contributions of SERCA pump and ryanodine-sensitive stores to presynaptic residual Ca2+

Abstract

The presynaptic Ca2+ signal, which triggers vesicle release, disperses to a broadly distributed residual [Ca2+] ([Ca2+]res) that plays an important role in synaptic plasticity. We have previously reported a slowing in the decay timecourse of [Ca2+]res during the second of paired pulses. In this study, we investigated the contributions of organelle and plasma membrane Ca2+ flux pathways to the reduction of effectiveness of [Ca2+]res clearance during short-term plasticity in Schaffer collateral terminals in the CA1 field of the hippocampus. We show that the slowed decay timecourse is mainly the result of a transport-dependent Ca2+ clearance process; that presynaptic caffeine-sensitive Ca2+ stores are not functionally loaded in the unstimulated terminal, but that these stores can effectively take up Ca2+ even during high frequency trains of stimuli; and that a rate limiting step of sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) kinetics following the first pulse is responsible for a large portion of the observed slowing of [Ca2+]res clearance during the second pulse. We were able to accurately fit our [Ca2+]res data with a kinetic model based on these observations and this model predicted a reduction in availability of unbound SERCA during paired pulses, but no saturation of Ca2+ buffer in the endoplasmic reticulum.

Presynaptic residual calcium and synaptic facilitation at hippocampal synapses of mice with altered expression of SNAP-25

“…These measurements were made from records of fEPSP slopes recorded in CA1 stratum radiatum following surgical isolation of Schaffer collateral axons from their somata in CA3. In our previous studies (Schiess et al, 2006; Scullin and Partridge, 2010; Scullin et al, 2010), we have used both fEPSP slope ratios and population spike (PS) amplitude ratios to determine PPF at the SC-CA1 synapse with results that are comparable to those observed in figures 1A and C. As discussed in Experimental Procedures, due to our interest in the presynaptic [Ca 2+ ] res we chose to use measurements of PS amplitude for the remaining analyses in this study.…”

“…We have previously demonstrated that there is significant supra-linear summation of [Ca 2+ ] res during paired-pulses in the adult rat hippocampus such that the ratio of ∫ΔF/F 0 resulting from paired-pulses is more than twice that elicited by a single pulse (Schiess et al, 2006). While our ΔF/F 0 measurement does not directly assess the Ca 2+ that is responsible for vesicular release, it is representative of the saturation of the mechanisms that determine the timecourse of presynaptic [Ca 2+ ] res (Scullin and Partridge, 2010). Furthermore, we and others have shown [Ca 2+ ] res to be a critical determinant of the facilitation of the post-synaptic response (Schiess et al, 2010; Wu and Saggau, 1994; Zucker and Regehr, 2002).…”

“…We have shown previously that uptake into and release from endoplasmic reticulum (ER) are primary mechanisms regulating [Ca 2+ ] res clearance in the SC presynaptic terminal, and that these mechanisms undergo developmental changes between P3 and P15 (Scullin and Partridge, 2010; Scullin et al, 2010). Because our ∫νΔF/F 0 data (Figure 4D) were consistent with a SNAP-25-dependent change in [Ca 2+ ] res clearance, we assessed the differences among the genotypes in the contribution of the endoplasmic reticulum to presynaptic Ca 2+ regulation.…”

“…We used two tests to demonstrate that the measured ΔF/F 0 signal was consistent with a [Ca 2+ ] response predominately from the presynaptic Schaffer collateral axons and axon terminals (Atluri and Regehr, 1996; Kamiya and Ozawa, 1999; Scullin and Partridge, 2010; Sinha et al, 1997; Wu and Saggau, 1994). First, in the presence of 10 μM CNQX, 25 μM D-AP5, and 20 μM bicuculline, the postsynaptic response was blocked, while the presynaptic fiber volley and ΔF/F 0 signal were left unchanged confirming that the ΔF/F 0 signal was not a postsynaptic response.…”

Facilitated glutamate release at Schaffer collateral to CA1 synapses has access to an exclusive population of NMDA receptors

“…Experiments were performed in coronal hippocampal slices prepared from a total of 79 P21 – 28 C57BL/6 mice ( Mus musculus ) as previously described (Scullin and Partridge, 2010). Briefly, animals were deeply anaesthetized by i.p.…”