Development of presynaptic calcium dynamics and short-term plasticity in the SC-CA1 synapse

Following a presynaptic action potential, there is a rapid rise of [Ca 2+]i in the immediate vicinity of Ca2+ channels that triggers membrane fusion and release of transmitter from vesicles within this microdomain. This presynaptic Ca2+ signal ([Ca 2+]pre) then disperses to produce a residual Ca 2+ ([Ca2+]res) that decays over the course of tens to hundreds of milliseconds. The [Ca2+]res has important implications in synaptic plasticity and is the basis for working memory storage. Ultimately [Ca2+]res is removed from the cytoplasm either into intracellular organelles or across the plasma membrane into the extracellular environment.;Calcium influx pathways, cytoplasmic Ca2+ buffering proteins, and Ca2+ extrusion processes in rodent neurons undergo considerable change during the first postnatal month. These changes have important functional significance in short-term plasticity---in particular paired-pulse facilitation (PPF)---at presynaptic terminals where neurotransmitter release is directly dependent on the dynamics of free cytoplasmic Ca2+. To examine developmental changes in [Ca2+]res dynamics in the Schaffer collateral synapses onto CA1 pyramidal neurons in in vitro hippocampal slices, we measured the timecourse of decay of [Ca 2+]res in presynaptic terminals following single and paired orthodromic stimuli in the stratum radiatum. The contribution of the slow component compared to the total decay of [Ca2+] res was reduced from >80% in newborn mice to ∼50% in the more mature animals (>P24) and [Ca2+]res had a distinct slow rising component in newborn mice (< P4), which was not apparent in older mice. This transition from a slow decay in early neonatal periods to the rapid decay in adults occurred gradually over the first 4 weeks of postnatal life, and appeared to be coincident with the major period of maturation of these synapses. The first goal of this study was to investigate the role of internal stores in regulating presynaptic [Ca2+] and in synaptic plasticity during this important period of synaptic development.;During this same developmental period, SNAP-25, a presynaptic vesicular release protein, undergoes changes in isoform expression. Alterations in SNAP-25 isoform expression have been linked to diseases with developmental onset such as schizophrenia, attention deficit hyperactivity disorder, and epilepsy (Corradini et al., 2009). Snap25tm2Mcw mice (Tkneo), in which this developmental change of isoform expression is modified (Bark et al., 2004), appear to retain an immature state of paired-pulse facilitation. The second goal of this study was to use these Tkneo mice as a tool to better understand the mechanisms of synaptic plasticity and to better understand how SNAP-25 regulation could underlie neurological disorders. We applied a model for paired-pulse facilitation (Schiess et al., 2006), which describes facilitation as a function of two [Ca2+]res-dependent pools of vesicles with different release probabilities. This model predicted the observed external [Ca 2+]-dependent changes in paired-pulse ratio based on changes in the effectiveness of [Ca2+]res and the resultant increased probability of release during the second pulse. We compared the presynaptic Ca2+ regulation in Tkneo mice to that found at different developmental stages and observed a striking difference between both Tkneo and Snap25tm1Mcw (SNAP-25 heterozygote null, HET) mice and similar-aged wild type (WT) mice in the degree of buffer saturation.;The first portion of this study suggests that maturation of cytoplasmic Ca2+ stores plays an important role in the [Ca2+] pre regulation and the consequent synaptic plasticity that occurs during development. Interestingly, the second portion of the study indicates that [Ca2+]res regulation and synaptic plasticity in SNAP-25 Tkneo mice are not dependent on contributions from cytoplasmic Ca 2+ stores, but rather depend on contributions of cytoplasmic [Ca 2+]res saturation sensitivity and resultant effects on plasticity. Unfortunately, the contributions of SNAP-25 isoform to these mechanisms are still unknown.