Distinct types of dendritic Ca ²⁺ spikes in CA3 pyramidal neurons associated with neuronal activity changes during navigation

Active dendrites enrich single-neuron computations by performing nonlinear input integration, including generation of dendritic spikes. Diverse dendritic spike types have been found in various cortical neuron classes in vitro; however, their in vivo prevalence and roles remained elusive. We measured calcium activity in apical dendrites and soma of pyramidal cells in the hippocampal area CA3 (CA3PCs) during virtual navigation in mice. Although dendritic activity was generally synchronous with the soma, their correlation decreased with dendritic distance. We identified two types of regenerative dendritic activities: slow, large-amplitude global Ca ²⁺ events representing putative Ca ²⁺ plateaus and fast Ca ²⁺ events with large dendrite-to-soma attenuation, representing putative dendritically initiated Ca ²⁺ spikes. Ca ²⁺ plateaus contributed to spatially tuned activity but only occasionally induced new place fields, suggesting conditional induction of synaptic plasticity. In contrast, fast Ca ²⁺ spikes were often followed by transient increase in somatic activity without spatial tuning, suggesting increased excitatory inputs or excitability. Our results unveil previously unidentified mechanisms whereby dendritic activity shapes output of CA3PCs in vivo.