Outer layer of Vb neurons in medial entorhinal cortex project to hippocampal dentate gyrus in mice

Entorhinal cortical (EC)-hippocampal (HPC) circuits are crucial for learning and memory. Although it was traditionally believed that superficial layers (II/III) of the EC mainly project to the HPC and deep layers (V/VI) receive input from the HPC, recent studies have highlighted the significant projections from layers Va and VI of the EC into the HPC. However, it still remains unknown whether Vb neurons in the EC provide projections to the hippocampus. In this study, using a molecular marker for Vb and retrograde tracers, we identified that the outer layer of Vb in the medial EC (MEC) neurons directly project to both dorsal and ventral hippocampal DG, with a significant preference for the ventral DG. In contrast to the distribution of DG-projecting Vb cells, anterior thalamus-projecting Vb cells are distributed through the outer to the inner layer of Vb. Furthermore, dual tracer injections revealed that DG-projecting Vb cells and anterior thalamus-projecting Vb cells are distinct populations. These results suggest that the roles of MEC Vb neurons are not merely limited to the formation of EC-HPC loop circuits, but rather contribute to multiple neural processes for learning and memory. ### Competing Interest Statement The authors have declared no competing interest. * AAV : Adeno-associated virus AD : Anterodorsal thalamic nucleus AV : Anteroventral thalamic nucleus BLA : Basolateral amygdala CalB : CalbindinD-28K CTB : Cholera Toxin Subunit B Ctip2 : Chicken ovalbumin upstream promoter transcription factor (COUP-TF) interacting protein 2 DAPI-4′ : 6-diamidino-2-phenylindole dDG : dorsal Dentate gyrus DG : Dentate gyrus EC : Entorhinal cortex HPC : Hippocampus MEC : Medial entorhinal cortex NAc : Nucleus accumbens PCP4 : Purkinje cell protein 4 vDG : ventral Dentate gyrus Wfs1 : Wolfram syndrome 1