Distinct electrophysiological signatures reveal neuronal heterogeneity in the mouse fasciola cinereum

The fasciola cinereum (FC) is a small, conserved hippocampal subregion whose cellular properties remain poorly understood. Anatomically positioned between dorsal CA1 and the third ventricle, the FC receives diverse cortical and subcortical inputs but is often excluded from hippocampal circuit models. Here, we performed ex vivo whole-cell patch-clamp recordings in mouse hippocampal slices to define the intrinsic and synaptic properties of FC neurons. We find that FC neurons are electrophysiologically and morphologically distinct from neighboring CA1 and CA2 pyramidal cells, exhibiting reduced intrinsic excitability, delayed spike initiation, and enhanced afterhyperpolarizations. These properties are strongly shaped by potassium conductances, with Kv2 (KCNB) and Kv7 (KCNQ) channels contributing to the suppression of firing. We find that FC neurons span a continuum of intrinsic excitability which is structured by variability in potassium channel-mediated conductances and is inversely coordinated with excitatory synaptic input, such that less excitable neurons receive stronger synaptic drive. Together, these findings establish a mechanistic framework in which intrinsic membrane properties and synaptic input are co-organized across the FC population, providing new insight into how this understudied hippocampal region may contribute to the regulation of neural excitability. Key PointsO_LIThe fasciola cinereum (FC) is a small and poorly understood hippocampal subregion that has recently been implicated in hippocampal related function and seizure activity. C_LIO_LIHere we show that neurons in the FC are morphologically and functionally distinct from cells in neighboring CA1 and CA2 hippocampal subregions, and find substantial physiological diversity of FC neurons, including differences in firing behavior, membrane properties, and intrinsic excitability. C_LIO_LIPharmacological analyses show that Kv2 and Kv7 potassium channels play a key role in shaping the intrinsic properties of FC neurons. C_LIO_LILess excitable FC neurons receive stronger excitatory synaptic input, revealing an inverse relationship between intrinsic membrane excitability and synaptic drive. C_LIO_LIThese findings identify the FC as a physiologically specialized hippocampal subregion and provide a foundation for understanding its role in hippocampal function and neurological disease. C_LI