Increased interhemispheric connectivity of a distinct type of hippocampal pyramidal cells.

Neurons typically generate action potentials at their axon initial segment based on the integration of synaptic inputs. In many neurons, the axon extends from the soma, equally weighting dendritic inputs. A notable exception is found in a subset of hippocampal pyramidal cells where the axon emerges from a basal dendrite. This structure allows these axon-carrying dendrites (AcD) a privileged input route. We found that in male mice, such cells in the CA1 region receive stronger excitatory input from the contralateral CA3, compared to those with somatic axon origins. This is supported by a higher count of putative synapses from contralateral CA3 on the AcD. These findings, combined with prior observations of their distinct role in sharp-wave ripple firing, suggest a key role of this neuron subset in coordinating bi-hemispheric hippocampal activity during memory-centric oscillations. Significance Neurons fall into multiple subtypes, many with distinct morphologies which likely reflect correlate with their distinct functions. Hippocampal CA1 pyramidal cells are mostly treated as a homogeneous group that forms transient functional ensembles related to memory formation and consolidation. A large fraction of these neurons has an unusual axon origin from a basal dendrite. This feature facilitates participation during network states with strong perisomatic inhibition around the soma, a property indicative of a prominent role in memory-forming networks. We now report that pyramidal cells with axon-carrying dendrites receive particularly strong input from the contralateral hemisphere. This distinct connectivity links neuronal morphology with network connectivity and points towards the existence of a specialized subpopulation of hippocampal pyramidal cells for inter-hemispheric communication.