The role of dendritic action potentials and Ca2+ influx in the induction of homosynaptic long-term depression in hippocampal CA1 pyramidal neurons

Long-term depression (LTD) of synaptic efficacy at CA1 synapses is believed to be a Ca2+-dependent process. We used high-speed fluorescence imaging and patch-clamp techniques to quantify the spatial distribution of changes in intracellular Ca2+ accompanying the induction of LTD at Schaffer collateral synapses in CA1 pyramidal neurons. Low-frequency stimulation (3 Hz), which was subthreshold for action potentials, produced small changes in [Ca2+](i) and failed to elicit LTD. Increasing the stimulus strength so that action potentials were generated produced both robust LTD and increases in [Ca2+](i). Back-propagating action potentials at 3 Hz in the absence of synaptic stimulation also produced increases in [Ca2+](i), but failed to induce LTD. When subthreshold synaptic stimulation was paired with back-propagating action potentials, however, large increases in [Ca2+](i) were observed and robust LTD was induced. The LTD was blocked by the N-methyl-D-aspartate receptor (NMDAr) antagonist APV, and stimulus-induced increases in [Ca2+](i) were reduced throughout the neuron under these conditions. The LTD was also dependent on Ca2+ influx via voltage-gated Ca2+ channels (VGCCs), because LTD was severely attenuated or blocked by both nimodipine and Ni2+. These findings suggest that back-propagating action potentials can exert a powerful control over the induction of LTD and that both VGCCs and NMDArs are involved in the induction of this form of plasticity.