Projections of calbindin expressing neurons in layer II of the entorhinal cortex
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The hippocampal formation and the parahippocampal region have been implicated in memory and spatial processes. In the parahippocampal region, the entorhinal cortex (EC) is considered to mediate input to and output from the hippocampal formation, with neurons in layers II and III as the primary input source. Layer II of the EC comprises at least two distinct populations of principal cells. One expresses reelin and projects to the dentate gyrus (DG). Another population expresses calbindin (CB), and reports on the postsynaptic targets of these neurons in rodents have provided strongly conflicting data, including that these neurons would selectively project to the ipsilateral CA1 or commissurally to the contralateral EC. The aim of this study was therefore to investigate the projection targets of the CB-positive population in layer II of the EC in rodents. The EC can be divided on the basis of functional and cytoarchitectonic differences into two subregions, the medial (MEC) and the lateral (LEC) entorhinal cortices. I therefore analysed the projections of CB-positive neurons in both subdivisions with a focus on the potential projections to CA1 or the contralateral EC. Fluorescent retrograde tracers were injected dorsally in CA1 of rats, whereas in mice dorsal CA1 and the contralateral MEC were targeted with different tracers. Immunohistochemistry for CB was performed on horizontal sections in order to verify the presence of colocalisation with retrogradely labelled cells. All sections were analysed using standard and confocal fluorescence microscopy. In both MEC and LEC of the rat and the mouse, only a minority of CA1 projecting cells expressed CB. Similarly, in the mouse MEC and LEC, only a minority of neurons projecting to the contralateral MEC expressed CB. Comparable to the rat, the majority of CB-positive neurons in layer II of the EC were not retrogradely labelled in any of the experimental conditions. The results indicate that CB-positive neurons in EC layer II have heterogeneous projection targets, where the projection target of the majority of cells is still unknown.