Cellular & microcircuit analysis of spatial representations in the cortico-hippocampal system

Extracellular recordings have elucidated spatial neural representations without identifying underlying microcircuits. We labeled neurons juxtacellularly in medial entorhinal cortex of freely-moving rats with a novel friction-based pipette-stabilization system. In a linear maze novel to the animals, spatial firing of superficial layer neurons was reminiscent of grid cell activity. Layer 2 stellate cells showed stronger theta-modulation than layer 3 neurons and both fired during the ascending phase of field potential theta. Deep layer neurons showed little or no activity. Layer 2 stellate cells resided in hundreds of small patches. At the dorso-medial border of medial entorhinal cortex we identified larger patches, which contained polarized head-direction selective neurons firing during the descending theta-phase. Three axon systems interconnected the patches: centrifugal axons from superficial cells to single large patches; centripetal axons from large patch cells to single small patches, and circumcurrent axons interconnecting large patches. Our microcircuit analysis during behavior reveals modularity of entorhinal processing. If time permits I will complement these findings from entorhinal cortex with data from hippocampal whole-cell recordings in awake behaving animals.