Transcriptomic continuity and an age-dependent onset of hemoglobin expression characterize morphological PV types in the hippocampus

The diversity reflected by >100 different neural cell types fundamentally contributes to brain function and a central idea is that neuronal identity can be inferred from genetic information. Recent large-scale transcriptomic assays seem to confirm this hypothesis, but a lack of morphological information has limited the identification of several known cell types. For example, parvalbumin interneurons (PV-INs) comprise of a main transcriptomic cluster within all inhibitory cells. However, transcriptomics alone has not resolved the different morphological PV types that exist. To close this gap, we used single-cell RNA-seq in morphologically identified PV-INs, sampled from 10 days to 3 months-old mice and studied their transcriptomic states in the morphological, physiological, and developmental domains. Our findings reveal a small number of genes whose expression is different between morphologically different types, but overall our analysis indicates high transcriptomic similarity among PV-INs. Furthermore, morphological PV types display uniform cell adhesion molecule (CAM) profiles, despite the fact that these types have very different wiring patterns, suggesting that CAM expression in mature PV cells do not reflect wiring specificity after development. Finally, our results reveal a pronounced change of transcriptomic states between postnatal days 20 and 25, during which PV-INs display a rapid onset of hemoglobin gene expression which remains stable in later development. Together, our results suggest that while morphological PV types in the CA1 are distinguished by their anatomy and in vivo activity, their largely continuous transcriptomic and homogenous biophysical landscapes are not predictive of these distinct identities. Pvalb positive cells were collected from mice between ages P10 and P77. There were 62 vertical basket cells (vBCs), 16 horizontal basket cells (hBCs), 9 vertical bistratified cells (vBICs), 14 horizontal bistratified cells (hBICs), 7 vertical axo-axonic cells (vAACs), 6 cortical cells (Corticals), and 14 unclassified cells (UCs) Please note that the readme_Series_design.tsv list the samples included in each processed data file.