Analysis of Spatial Memory Cell Types Projections in Entorhino-Entorhinal Circuitry Using Electronphysiological Neural Recordings Combined with Optogenetics Application
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Recent studies have revealed that all functional cell types in superficial layers of medial entorhinal cortex have projections to the hippocampus. Furthermore, hippocampal functions in connection with neural circuits in medial entorhinal cortex offer a good systems approach to understanding the basic functions of episodic memory formation. Then, we draw our attentions to the connections complexity between both medial entorhinal cortex (MEC) hemispheres to investigate the pattern of connectivity of MEC-MEC functional cell type neurons, furthermore, providing further data associated with spatial memory formation for future research. There are several functional spatial cell types in superficial layer 2 of MEC that code various spatially information in navigational system. These cells including; grid cells, head-direction cells, border cells, conjunctive grid x head-direction cells, conjunctive border x head-direction cells, unknown principal cells and interneurons. By injecting retrogradely transportable recombinant adeno-associated virus carrying channelrhodopsin-2 transgene in the left MEC of two rats and also right MEC of three rats, optogenetic control over entorhinal neurons with direct projections to the contralateral MEC has been presented. Optogenetics application together with electrophysiological recordings in vivo, enabled us to determine functionally specialized spatial cell types in MEC projecting to the contralateral MEC, as these cells contained minimal response latencies to laser stimulations in MEC. We here show that, unknown principal cells and interneurons, among all functional cell types presented in MEC, had a strong projection from left MEC to the contralateral right MEC and vise versa. Grid cells that are recently found to have a massive projections to the hippocampus, here in our study, only a small number, showed responses to light that might suggest the probability of their contralateral MEC connections. However, these responses were too much weak to convince us that these cells had projections to contralateral MEC. No more data were discovered to support projections of other detected spatial cell types to the contralateral MEC at this study.