Voltage-gated Ion Channel Expression in the Mouse Hypothalamic Mammillary Bodies

The mammillary bodies (MB) are a hypothalamic subregion and a node in the Papez circuit, a brain-wide circuit associated with encoding both spatial and episodic memories in mammals. Despite mounting evidence suggesting that this structure is a key component in these functions, very little is known about the cell type composition, gene expression and intrinsic properties of MB neurons, and how these properties contribute to their function within the Papez circuit. Recent single cell sequencing of the MB revealed the transcriptomic and spatial organization of the MB, which is composed of subpopulations of glutamatergic neurons defined by unique neurochemical markers. Furthermore, MB subpopulations differentially express transcripts encoding specific voltage-gated ion channels. We found that voltage-gated sodium (Nav) and potassium (Kv) channel family members, as well as hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, are differentially expressed among neuronal subpopulations and anatomically-distinct subnuclei. Given that mutations in several of the channels within these families are strongly associated with neurodevelopmental disorders (NDDs), characterized by deficits in learning and memory, we hypothesize that these ion channels play a critical role in the function of MB neurons, and their dysfunction in disease states. Using fluorescence in situ hybridization (FISH) and focusing on the channels most frequently mutated in neurodevelopmental disorders, SCN1A, KCNQ2, KCNQ3 and HCN1, we found that the mRNA for these channels shows a pattern of differential enrichment in specific MB subnuclei. Due to the differences in ion channel expression patterns, we also investigated the firing properties of neurons within MB subnuclei using slice electrophysiology. Focusing on Pvalb (PV) - expressing neurons, which are found in multiple MB subpopulations across subnuclei, we asked if their transcriptional differences may be reflected in their intrinsic membrane properties. Recordings from these neurons have revealed that PV+ neurons in the median nucleus (MnM) fire repetitively with a lower threshold, while PV+ neurons in the lateral nucleus (LM) have a higher threshold to fire, but burst and and quickly adapt. Taken together, these data show that MB neurons are composed of multiple distinct subpopulations, with unique patterns of voltage-gated ion channel expression, which in turn contribute to differences in their intrinsic firing properties. This project was supported by the National Institute of Mental Health (NIMH) R01MH112739 (to A.C.J) This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.