Local Microcircuitry of PaS Shows Distinct and Common Features of Excitatory and Inhibitory Connectivity.

The parasubiculum (PaS) is located within the parahippocampal region, where it is thought to be involved in the processing of spatial navigational information. It contains a number of functionally specialized neuron types including grid cells, head direction cells, and border cells; and provides input into layer 2 of the medial entorhinal cortex where grid cells are abundantly located. The local circuitry within the PaS remains so far undefined but may provide clues as to the emergence of spatially tuned firing properties of neurons in this region. We used simultaneous patch-clamp recordings to determine the connectivity rates between the 3 major groups of neurons found in the PaS. We find high rates of interconnectivity between the pyramidal class and interneurons, as well as features of pyramid-to-pyramid interactions indicative of a nonrandom network. The microcircuit that we uncover shares both similarities and divergences to those from other parahippocampal regions also involved in spatial navigation.