Two distinct cell types of the medial mammillary body forming segregated subcircuits

The medial mammillary body (MM) is an integral component of the Papez circuit and the extended hippocampal system essential for cognitive and emotional functions. However, whether MM contains morpho-electrophysiologically distinguishable, genetically identifiable neuron types, and how they interact to process information underlying diverse MM functions, has remained largely unexplored. Here we employed a multidisciplinary approach in mice, combing genetic labeling, electrophysiological recording, morphological reconstruction, viral tracing, activity monitoring and manipulation, and behavioral testing to perform an integrative analysis of MM. We identified two major neuron types in MM, distinguished by the expression of calbindin (CB) and parvalbumin (PV). These neuron types occupy complementary MM territories and exhibit discernable anatomical and physiological characteristics. Further, they display segregated outputs and differential inputs, with scarce local connectivity, forming independent subcircuits for parallel information processing. Using optogenetic activation and calcium fiber photometry, we demonstrated that CB-expressing MM neurons, but not PV-expressing ones, drive place aversion and hyperlocomotion and exhibit elevated activity during locomotion. In summary, our findings reveal the neuronal composition of MM, delineate its local and long-range circuit organization, and uncover functionally divergent, cell-type-specific subcircuits, establishing a robust framework for future investigations in both healthy and diseased states.