Crossed Entorhino-Dentate Projections Form and Terminate With Correct Layer-Specificity in Organotypic Slice Cultures of the Mouse Hippocampus.

Lars Hildebrandt-Einfeldt,Thomas Deller,Andreas Vlachos,Maximilian Lenz,Mandy H Paul,Carolin Stoffer,Alexander Drakew,Kenrick Yap,Nadine Zahn

doi:10.3389/fnana.2021.637036

Lars Hildebrandt-Einfeldt, Thomas Deller + Show 7 more

Open Access

https://doi.org/10.3389/fnana.2021.637036

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Abstract

The entorhino-dentate projection, i.e., the perforant pathway, terminates in a highly ordered and laminated fashion in the rodent dentate gyrus (DG): fibers arising from the medial entorhinal cortex (MEC) terminate in the middle molecular layer, whereas fibers arising from the lateral entorhinal cortex (LEC) terminate in the outer molecular layer of the DG. In rats and rabbits, a crossed entorhino-dentate projection exists, which originates from the entorhinal cortex (EC) and terminates in the contralateral DG. In contrast, in mice, such a crossed projection is reportedly absent. Using single and double mouse organotypic entorhino-hippocampal slice cultures, we studied the ipsi- and crossed entorhino-dentate projections. Viral tracing revealed that entorhino-dentate projections terminate with a high degree of lamina-specificity in single as well as in double cultures. Furthermore, in double cultures, entorhinal axons arising from one slice freely intermingled with entorhinal axons originating from the other slice. In single as well as in double cultures, entorhinal axons exhibited a correct topographical projection to the DG: medial entorhinal axons terminated in the middle and lateral entorhinal axons terminated in the outer molecular layer. Finally, entorhinal neurons were virally transduced with Channelrhodopsin2-YFP and stimulated with light, revealing functional connections between the EC and dentate granule cells. We conclude from our findings that entorhino-dentate projections form bilaterally in the mouse hippocampus in vitro and that the mouse DG provides a permissive environment for crossed entorhinal fibers.

Highlights

Entorhino-dentate axons relay cortical information from the entorhinal cortex (EC) to the dentate gyrus (DG; e.g., Van Hoesen et al, 1972; Wilson and Steward, 1978; Jones, 1993; Förster et al, 2006; Nilssen et al, 2019; Lee et al, 2020)
The entorhino-dentate connections are subdivided into two major projections: one projection arises from layer II neurons in the medial entorhinal cortex (MEC) and its axons terminate in the middle molecular layer of the DG
Entorhinal Axons Terminate With Correct Laminar-Specificity in the DG of Single Organotypic EC-DG Slice Cultures

Summary

INTRODUCTION

Entorhino-dentate axons relay cortical information from the entorhinal cortex (EC) to the dentate gyrus (DG; e.g., Van Hoesen et al, 1972; Wilson and Steward, 1978; Jones, 1993; Förster et al, 2006; Nilssen et al, 2019; Lee et al, 2020). The entorhino-dentate connections are subdivided into two major projections: one projection arises from layer II neurons in the medial entorhinal cortex (MEC) and its axons terminate in the middle molecular layer of the DG. The other projection arises from layer II neurons in the lateral entorhinal cortex (LEC) and its axons terminate more superficially in the outer molecular layer of the dentate. In rats (Steward et al, 1974; Zimmer and Hjorth-Simonsen, 1975; Steward, 1976; Deller et al, 1996) and rabbits (HjorthSimonsen and Zimmer, 1975) a crossed projection from the EC to the DG exists. Following ipsilateral entorhinal denervation, the remaining crossed entorhino-dentate axons branch and sprout new collaterals (Deller et al, 1996). In the absence of such signals, crossed EC fibers should enter the DG and interact with axons from the ipsilateral EC, raising the question, whether these crossing fibers terminate with the same layer specificity and intermingle with EC axons from the ipsilateral side

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