Semilunar Granule Cells Maintain Distinct Dendritic Morphology and Inhibition through Postnatal Development and Receive Heightened Inhibition in Adolescence

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