Key points• The dorsal (DHC) and ventral (VHC) regions of the rodent hippocampus are anatomically, behaviourally, and biochemically distinct.• The intrinsic electrophysiological properties of CA1 pyramidal neurones from these regions however, have not been fully characterized.• In this study, we found that VHC neurones were intrinsically more excitable than DHC neurones.• The difference in intrinsic excitability stems from a higher input resistance (R in ) and more depolarized resting membrane potential observed in the soma and apical dendrite of VHC neurones.• Morphological analysis of reconstructed neurones revealed significantly more dendritic surface area for DHC than VHC neurones.• Simulations using morphologically realistic passive models indicate that morphological differences could, in principle, underlie somatic but not dendritic differences in R in .Abstract The hippocampus has a central role in learning and memory. Although once considered a relatively homogenous structure along the longitudinal axis, it has become clear that the rodent hippocampus can be anatomically and functionally divided into a dorsal component generally associated with spatial navigation, and a ventral component primarily associated with non-spatial functions that involve an emotional component. The ventral hippocampus (VHC) is also more sensitive to epileptogenic stimuli than the dorsal hippocampus (DHC), and seizures tend to originate in the VHC before spreading to other brain regions. Although synaptic and biochemical differences in DHC and VHC have been investigated, the intrinsic excitability of individual neurones from the DHC and VHC has received surprisingly little attention. In this study, we have characterized the intrinsic electrophysiological properties of CA1 pyramidal neurones from the DHC and the VHC using the whole-cell current-clamp method. Our results demonstrate that somatic current injections of equal magnitude elicit significantly more action potentials in VHC neurones than DHC neurones, and that this difference stems from the more depolarized resting membrane potential (RMP; 7 mV) and higher input resistance (R in ; 46 M measured from RMP) observed in VHC neurones. These differences in RMP and R in were also observed in dendritic whole-cell current-clamp recordings. Furthermore, morphological reconstructions of individual neurones revealed significant differences in the dendritic branching pattern between DHC and VHC neurones that could, in principle, contribute to the lower somatic R in of DHC neurones. Together, our results highlight significant differences in the intrinsic electrophysiological properties of CA1 pyramidal neurones across the longitudinal hippocampal axis, and suggest that VHC neurones are intrinsically more excitable than DHC neurones. This difference is likely to predispose the VHC to hyperexcitability.

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