Distinct Electrophysiological Signatures Define Neuronal Subtypes in the Fasciola Cinereum

The fasciola cinereum (FC) is a small, conserved hippocampal subregion whose function has remained largely unexplored. Anatomically situated between dorsal CA1 and the third ventricle in rodents, the FC receives diverse cortical and subcortical inputs yet is often omitted from hippocampal circuit models. There remains a fundamental knowledge gap regarding the cell types and intrinsic properties of neurons in FC and whether they are distinct from neighboring hippocampal subregions. Here, we performed ex vivo whole-cell patch-clamp recordings in mouse hippocampal slices to characterize FC neurons. We found that FC cells are functionally distinct from neighboring CA1 pyramidal cells, exhibiting significantly reduced excitability, delayed spike initiation, and enhanced afterhyperpolarization (AHP) currents, consistent with strong potassium conductance. Notably, we identified two electrophysiologically distinguishable FC neuron excitatory cell subtypes, differing in excitability and potassium channel activity. Pharmacological analyses demonstrated that Kv2.1 and Kv7 potassium channels play a key role in shaping the intrinsic properties of FC neurons, underlying their reduced excitability. These findings suggest that the FC is a heterogeneous structure, molecularly and functionally specialized for gating excitability within the hippocampal circuit. Significance statement The fasciola cinereum (FC) is a small and often overlooked hippocampal subregion whose physiological properties remain largely unexplored. Here, we show that FC neurons display distinct intrinsic electrophysiological signatures that set them apart from the neighboring hippocampal area CA1. We find that FC excitatory neurons have low excitability and can be segregated into two subgroups with differing potassium channel activity. These findings establish a physiological baseline for FC function and provide a fundamental framework for future studies on its role in normal and pathological brain states.