Ca2.2 Channels Sustain Vesicle Recruitment at a Mature Glutamatergic Synapse.

The composition of voltage-gated Ca channel (Ca) subtypes that gate action potential (AP)-evoked release changes during the development of mammalian CNS synapses. Ca2.2 and Ca2.3 lose their function in gating-evoked release during postnatal synapse maturation. In mature boutons, Ca2.1 currents provide the almost exclusive trigger for evoked release, and Ca2.3 currents are required for the induction of presynaptic long-term potentiation. However, the functional significance of Ca2.2 remained elusive in mature boutons, although they remain present at active zones and continue contributing significantly to presynaptic Ca influx. Here, we addressed the functional significance of Ca2.2 and Ca2.3 at mature parallel-fiber (PF) to Purkinje neuron synapses of mice of either sex. These synapses are known to exhibit the corresponding developmental Ca subtype changes in gating release. We addressed two hypotheses, namely that Ca2.2 and Ca2.3 are involved in triggering spontaneous glutamate release and that they are engaged in vesicle recruitment during repetitive evoked release. We found that spontaneous miniature release is Ca dependent. However, experiments with Ca subtype-specific blockers excluded the spontaneous opening of Cas as the Ca source for spontaneous glutamate release. Thus, neither Ca2.2 nor Ca2.3 controls spontaneous release from PF boutons. Furthermore, vesicle recruitment during brief bursts of APs was also independent of Ca influx through Ca2.2 and Ca2.3. However, Ca2.2, but not Ca2.3, currents significantly boosted vesicle recruitment during sustained high-frequency synaptic transmission. Thus, in mature PF boutons Ca2.2 channels are specifically required to sustain synaptic transmission during prolonged neuronal activity. At young CNS synapses, action potential-evoked release is gated via three subtypes of voltage-gated Ca channels: Ca2.1, Ca2.2, and Ca2.3. During postnatal maturation, Ca2.2 and Ca2.3 lose their function in gating evoked release, such that at mature synapses Ca2.1 provides the almost exclusive source for triggering evoked release. Ca2.3 currents are required for the induction of presynaptic long-term potentiation. However, the function of the still abundant Ca2.2 in mature boutons remained largely elusive. Here, we studied mature cerebellar parallel-fiber synapses and found that Ca2.2 does not control spontaneous release. However, Ca influx through Ca2.2 significantly boosted vesicle recruitment during trains of action potentials. Thus, Ca2.2 in mature parallel-fiber boutons participate in sustaining synaptic transmission during prolonged activity.