Postsynaptic calcium transients evoked by activation of individual hippocampal mossy fiber synapses

Control of Ca2+ within dendritic spines is critical for excitatory synaptic function and plasticity, but little is known about Ca2+ dynamics at thorny excrescences, the complex spines on hippocampal CA3 pyramidal cells contacted by mossy fiber terminals of dentate granule cell axons. We have monitored subthreshold stimulus-dependent postsynaptic Ca2+ transients in optically and ultrastructurally characterized complex spines and find that such spines can act as discrete units of Ca2+ response. In contrast to the more common "simple" spines, synaptically evoked Ca2+ transients at complex spines have only a small NMDA receptor-dependent component and do not involve release of calcium from internal stores. Instead, they result mainly from AMPA receptor-gated Ca2+ influx through voltage-activated calcium channels on the spine; these channels provide graded amplification of the response of thorny excrescences to individual mossy fiber synaptic events.