Functional Dissection of Local Medial Entorhinal Cortex Subcircuit

Abstract

The superficial layers of the medial entorhinal cortex(MEC) contain serval functionally specialized spatial cell types. suck a grid cells, head direction cells, border cells and cells with conjunctive properties. It is currently not know how the firing patterns of these vell populations map onto the architecture og the MEC circuit. Results from recent work suggest that there are two largely non-overlapping neuronal populations within superficial layers of MEC with different prosjecting targets. One of them target the hippocampus while the other prosjects extrahippocampally. It has been shown that all funtional MEC cell types prosject to the hippocampus, and a large part of these cells were grid cells. Based on these observations we wanted to investigate if there is a firrerence in fruntional cell distribution of MEC cells projecting to the contralateral MEC and cells prosjecting to hippocampus.

Retrogradely transportable recombinant adeno-associated virus expressing Flag-tagged channelrhodopsin-2(ChR2), was injected in left MEC of 6 rats. This introduced optogenetic control over MEC neurons with direct årosjection to the contralateral MEC. Combining optogenetic and electrophysiological in vivo recordings, allowed identification of functional cell types with direct prosjection to the contralateral MEC, as these cells showed minimal response latencies to laser stimulations in the medial entorhinal cortex.

We found border cells, head direction cells, non-spatial cells and interneurons with direct projection to the MEC, but no grid cells. This distrubution is in contrasts with the one found to project to the hippocampus, where grid cells are the predominant spatial cell type. More data are requred to determine if the sparsity of respnsive grid cells reflects limited sampling, or if the contralaterally--projecting cell population has distinct functional properties.