Optical detection of dendritic spike initiation in hippocampal CA1 pyramidal neurons

Previous studies have shown that spikes can be generated in the dendrites of CA1 pyramidal neurons. Some have suggested that, in response to synaptic inputs, spikes are initiated near the soma and propagate back into the dendrites, but some recent studies have shown that intense synaptic inputs initiate spikes in the dendrite. Here, we report the optical detection of spike propagation along the apical dendrites of hippocampal pyramidal neurons. Rat hippocampal slices were stained with the fluorescent voltage-sensitive dye, JPW1114, and optical signals monitored using a 16×16 photodiode array system at a frame rate of 4 kHz. A stimulating electrode was placed at the boundary between the stratum (str.) lacnosum-moleculare and the str. radiatum to stimulate the Schaffer collateral, and fast and slow signal components were detected in the dendritic and somatic regions. By comparing the optical signals with whole-cell recordings, we confirmed that the fast component was due to a population of dendritic spikes in pyramidal neurons. The fast component appeared in dendritic locations near the input sites in response to synaptic activation, and signal onset at the soma was delayed by a few milliseconds compared with that at the input sites. Local perfusion of a Na⁺ channel blocker near the soma eliminated the fast component at the soma, but had no effect on the fast component at the input sites. Our results indicate that dendritic spikes can be initiated in dendrites near the input site and propagate orthodromically toward the proximal dendrites and the soma.