Developmental changes in presynaptic Ca2+ clearance kinetics and synaptic plasticity in mouse Schaffer collateral terminals

Presynaptic Ca2+ influx pathways, cytoplasmic Ca2+ buffering proteins and Ca2+ extrusion processes undergo considerable change during the first postnatal month in rodent neurons. These changes may be critical in establishing short-term plasticity at maturing presynaptic terminals where neurotransmitter release is directly dependent on the dynamics of cytoplasmic residual Ca2+ ([Ca2+]res). In particular, the robust paired-pulse facilitation characteristic of adult neurons is almost entirely lacking in newborns. To examine developmental changes in processes controlling [Ca2+]res, we measured the timecourse of [Ca2+]res decay in presynaptic terminals of Schaffer collateral to CA1 synapses in acute hippocampal slices following single and paired orthodromic stimuli in the stratum radiatum. Developmental changes were observed in both the rise time and slow exponential decay components of the response to single stimuli such that this decay was larger and faster in the adult. Furthermore, we observed a greater caffeine-sensitive basal Ca2+ store, which was differentially affected when active uptake into the endoplasmic reticulum was blocked, in the presynaptic fields of the Schaffer collateral to CA1 terminals of P6 and younger mice when compared to adults. These transitions in [Ca2+]res dynamics occurred gradually over the first weeks of postnatal life and correlated with changes in short-term plasticity.