Availability of data and material: All reconstructions will be uploaded to Neuromorpho.org. Physiology data will be shared upon request.Code availability: Standard software packages, Neurolucida 360, pClamp10 and R were used in analysis. Custom IGOR-Pro code for IPSC analysis will be made available upon request.

2Semilunar granule cells (SGCs) have been proposed as a morpho-functionally distinct class of 3 hippocampal dentate projection neurons contributing to feedback inhibition and memory 4 processing in juvenile rats. However, whether SGCs retain their unique structural and inhibitory 5 characteristics through postnatal development remains unresolved. Focusing on postnatal days 6 11-13, 28-42, and >120, corresponding with human infancy, adolescence, and adulthood, we 7 examined whether SGCs differ from granule cells (GCs) in somatodendritic morphology and 8 inhibitory regulation. Unsupervised cluster analysis confirmed that morphological features 9 distinguish SGCs from GCs irrespective of animal age. SGCs maintain higher spontaneous 10 inhibitory postsynaptic current (sIPSC) frequency than GCs from infancy through adulthood. 11While sIPSC frequency peaked during adolescence, and amplitude declined progressively with 12 age in both cell types, sIPSC frequency in SGCs was particularly enhanced during adolescence. 13Like GABAergic synaptic inputs, extrasynaptic GABA current amplitude in SGCs peaked in 14 adolescence and was greater than in GCs. Consistent with the developmental profile of SGC 15 synaptic and extrasynaptic GABA currents, perforant-path evoked dentate population responses 16 in vivo showed greater paired-pulse depression during adolescence. These findings highlight the 17 distinct morphology and inhibitory regulation of SGCs through development and suggest that the 18 particularly heightened inhibition of SGCs may shape dentate output during adolescence. 19

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