DISTINCT TYPES OF DENDRITIC CA<SUP >2+</SUP> SPIKES IN CA3 PYRAMIDAL NEURONS ASSOCIATED WITH NEURONAL ACTIVITY AND TUNING DURING NAVIGATION

Active dendrites, especially regenerative dendritic spikes (d-spikes) are thought to enable neurons to perform diverse computations supporting adaptive behavior. In hippocampal CA1 pyramidal cells (PCs), slow dendritic Ca²⁺ plateaus have been shown to instruct new place field formation by inducing robust and rapid synaptic plasticity (behavioral timescale synaptic plasticity, BTSP). However, little is known about dendritic activity and its impact on somatic tuning in PCs of the CA3 area. Using in vivo two-photon Ca²⁺ imaging we studied the dynamics of somatic and dendritic activity in CA3PCs during goal-oriented virtual navigation in mice and sought to determine the types of dendritic Ca²⁺ spikes and assess how they affect somatic activity and spatial tuning. We observed that dendritic activity was generally synchronous with the soma showing distance dependent decrease in correlation. We identified two types of somato-dendritic Ca²⁺ activities that could be explained by regenerative dendritic spikes: slow, large-amplitude global Ca²⁺ events representing putative Ca²⁺ plateaus, and fast-decaying Ca²⁺ events with high dendrite/soma amplitude ratio and limited propagation representing putative dendritically initiated fast Ca²⁺ spikes. These two types of dendritic spikes had different impact on somatic activity. Global Ca²⁺ plateaus contributed to spatially tuned activity of CA3PCs and occasionally could induce new PFs via a mechanism consistent with CA3 BTSP rules. In contrast, fast Ca²⁺ spikes were often followed by a temporary increase in somatic activity without specific spatial tuning. Our results indicate that different types of regenerative dendritic activities have distinct impact on the output of CA3PCs in vivo.