Parvalbumin expression identifies subicular principal cells with high projection specificity

The calcium-binding protein parvalbumin is an established marker for a subset of cortical inhibitory interneurons with similar biophysical features and connectivity. However, parvalbumin is also expressed in a small population of excitatory cells in layer 5 of the neocortex with specific sub-cortical projection targets. Parvalbumin may thus also in principal cells identify particular subclasses with distinct connectivity and function. Here we investigated whether parvalbumin is expressed in excitatory neurons of the hippocampal formation and if so, whether it delineated neurons with specific features. We report parvalbumin-expressing glutamatergic cells in the distal subiculum, which -based on location, connectivity and gene expression - separated into two subclasses: neurons in deep layers, which specifically project to the antero-ventral thalamus and neurons in superficial layers, which project to the mamillary bodies. Contrary to most adjacent pyramidal cells parvalbumin-positive neurons were non-bursting and displayed straight apical dendrites devoid of oblique dendrites. Functionally, the projections diverged from classical driver/modulator subdivisions. Parvalbumin expression thus marks two sub-types of subicular projection neurons with high target specificity and unique functional features.