TY - JOUR T1 - A toolbox of criteria for distinguishing Cajal-Retzius cells from other neuronal types in the postnatal mouse hippocampus JF - eneuro JO - eNeuro DO - 10.1523/ENEURO.0516-19.2019 SP - ENEURO.0516-19.2019 AU - Max Anstötz AU - Gianmaria Maccaferri Y1 - 2020/01/06 UR - http://www.eneuro.org/content/early/2020/01/06/ENEURO.0516-19.2019.abstract N2 - The study of brain circuits depends on a clear understanding of the role played by different neuronal populations. Therefore, the unambiguous identification of different cell types is essential for the correct interpretation of experimental data. Here, we emphasize to the broader neuroscience community the importance of recognizing the persistent presence of Cajal-Retzius cells in the molecular layers of the postnatal hippocampus, and then we suggest a variety of criteria for distinguishing Cajal-Retzius cells from other neurons of the hippocampal molecular layers, such as GABAergic interneurons and semilunar granule cells. The toolbox of criteria that we have investigated (in male and female mice) can be useful both for anatomical and functional experiments, and relies on the quantitative study of neuronal somatic/nuclear morphology, location and developmental profile, expression of specific molecular markers (GAD67, reelin, COUP-TFII, calretinin and p73), single cell anatomy, and electrophysiological properties.We conclude that Cajal-Retzius cells are small, non-GABAergic neurons that are tightly associated with the hippocampal fissure, and that, within this area of interest, selectively express the proteins p73 and calretinin. We highlight the dangers of using markers such as reelin or COUP-TFII to identify Cajal-Retzius cells or GABAergic interneurons because of their poor specificity. Lastly, we examine neurons of the postnatal hippocampal molecular layers and show cell type-specific differences in their dendritic/axonal morphologies and density distributions, as well as in their membrane properties and spontaneous synaptic inputs. These parameters can be used to distinguish biocytin-filled and/or electrophysiologically recorded neurons and should be considered to avoid interpretational mistakes.SIGNIFICANCE STATEMENT The unambiguous identification of specific neuronal types in brain circuits is critical for a correct interpretation of experimental data. Cajal-Retzius cells of the molecular layers of the postnatal hippocampus are intermingled with other neurons, such as GABAergic interneurons and semilunar granule cells. Although the presence of Cajal-Retzius cells in the hippocampus is commonly acknowledged at early developmental stages, it is often overlooked at later points of maturation. This lack of attention may increase the risk of misclassification of Cajal-Retzius cells for GABAergic interneurons or other cell types and may lead to interpretative mistakes. Here, we provide a set of simple cell type-specific criteria for their unambiguous identification. Lastly, we also warn against equivocal criteria that may lead to erroneous classifications. ER -