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Cortical maps for space

Final Report Summary - GRIDCODE (Cortical maps for space)

The overall objective of GRIDCODE was to decipher how function is coded, divided and integrated among components of the grid-cell circuit of the medial entorhinal cortex (MEC) and the brain regions associated with it. Using a combination of transgenic interventions, in utero labelling, multisite multichannel tetrode recording and optical imaging, we have sought to determine the number and distribution of grid modules, the organization and mechanisms of the grid-cell network, and the relationship of grid cells to place cells in the hippocampus. We have obtained answers to most of these questions as well as others that were not predicted at the time when the projected started. The project results suggest that grid cells are likely caused by interactions between recurrently connected cells in multiple distinct modularly organized continuous attractor networks. The work was organized into 6 workpackages (WP):
In WP1, we demonstrated independent grid modules in MEC and described their quantitative relationship. We found a minimum of 5 independent modules. In follow-up experiments we are testing rats in a 4 x 4 m2 recording arena to capture cells with grid spacing so wide that standard environments did not capture them originally. We expect these recordings to provide the final number of modules.
In WP2, we determined the scale relationship between grid modules. We showed that when the increase in grid spacing is plotted across animals as a function of module number, increments are almost linear, with a scale factor of ~1.42 (sqrt 2).
In WP3, we addressed the development of the MEC circuit. We have shown that grid modules with short spatial frequencies are born earlier than the ones with longer frequencies. This has allowed us to selectively tag MEC cells with a specific birth date, which eventually will make it possible to test, at adult age, whether cells born on different embryonic days have different functional properties (e.g. belong to different grid modules) and if there is a dorso-ventral order in the sequence of early vs late-born MEC neurons.
In WP4, we have determined how grid cells in layers of MEC depend on each other. We have shown that grid cells in layer II are both stellate cells and pyramidal cells, we have quantified their distributions, and we have estimated properties of cells in the deep layers of MEC, using viral retrograde labelling techniques.
In WP5, we are establishing the mechanism for transformation of spatial activity in MEC to place-cell activity in the hippocampus. We are using monotranssynaptic tracing with rabies virus to determine the functional identity of MEC cells with direct connections to an identified hippocampal cell. The procedure has been settled and we are seeing labelling in entorhinal input neurons as a result of infection of very low numbers of cells in the hippocampus. Labelled neurons are made to express a fluorescent calcium indicator or a microbial opsin, so that we can identify these input cells functionally in freely-moving mice. Labelled cells include grid cells as well as other functional cell types, pointing to converge from multiple classes of cells onto individual place cells. We expect to complete this work during the next year.
In WP6, we are determining whether patterns of input from modules of grid cells contribute to the formation of distinct place cell maps in principal cells of the hippocampus. A new silicon-probe-based technology has been introduced to enable simultaneous data collection from hippocampus and MEC and we are now in the process of collecting hundreds of cells in each area to finally determine patterns of interaction between the two brain structures. The project has also pointed to the importance of interneuron subpopulations in controlling remapping in hippocampal place cells.
GRIDCODE has reached all major subgoals and workpackage results have been published repeatedly in high-impact journals such as Nature and Science. The key questions have been retained on all projects but modifications in subgoals and approaches have been made along the way.