The hippocampal CA3 region is thought to play crucial roles in episodic memory functions
because of the extensive recurrent connections between CA3 pyramidal cells (CA3PCs).
However, different methods provided contradicting observations about the synaptic
connectivity between CA3PCs. Therefore, we estimated the connectivity rate between
individual CA3PCs using a new approach that is not affected by the confounds of conventional
methods. Specifically, we used voltage imaging with the Voltron sensor in acute slices
from rats of either sex to test CA3PC connections by detecting spontaneous spiking
and subthreshold responses in anatomically identified neurons. We detected 164 monosynaptic
excitatory connections in 3078 tested CA3PC-CA3PC pairs. This 5.3% connectivity rate
was much higher than that we observed with the theoretically more sensitive patch
clamp method in similar experimental conditions, but it remained below the anatomically
observed number of CA3PC-CA3PC contacts. We verified that the imaged excitatory connections
were mediated by AMPA receptors. Our results also showed that the recurrent connections
did not enrich into preferred connectivity motifs and followed a distribution that
was consistent with random connectivity in general. Moreover, voltage imaging revealed
CA3PCs with distinct firing properties and somatic locations corresponding to previously
established heterogeneity and showed that specific connectivity rules create preferred
information routes among these subpopulations. Finally, we showed that there is at
least one condition, influencing patch clamp recordings but not voltage imaging, that
affects the observable functional connections between CA3PCs. Altogether, our results
obtained with a new voltage imaging approach argue for high local connectivity rates
between CA3PCs.
