Navigating in familiar and novel environments is crucial for survival, and the cerebral cortex contains networks of specialised nerve cells that both encode and help to recall particular features of any given route. In the hippocampus, a seahorse-shaped structure within the cortex, ‘place cells’ signal specific locations of an environment, and a sequence of place cells can represent an actual journey from A to B as well as the memory of that journey. Other parts of the cerebral cortex are also required for this ‘cognitive map’ of space. One specialised area, the dorsal presubiculum, contains ‘head direction cells’, which signal direction like a compass. Another, the entorhinal cortex, contains ‘grid cells’, which represent the scale of an environment. These cortical regions are interconnected and are among the first to degenerate in Alzheimer’s disease, leading to a loss of the ability to learn novel places as well as to recall familiar ones. Much of the brain works through electrical rhythms, and this rhythmicity facilitates the coordination of synchronisation of large populations of nerve cells dedicated to a given task both within and between brain regions. We have discovered a rhythmic subcortical inhibitory (GABAergic) nerve cell population in a part of the mouse brain called the medial septum that projects to both the dorsal presubiculum and entorhinal cortex but avoids the hippocampus. We named these nerve cells ‘orchid cells’ based on the shape of the axonal trajectories. Orchid cells inhibit cortical GABAergic neurons in these specific brain regions, and are very likely responsible for the coordination of cortical pyramidal neuronal activity that represents spatial information, such as routes of navigation.
Shared rhythmic subcortical GABAergic input to the entorhinal cortex and presubiculum
Tim James Viney, Minas Salib, Abhilasha Joshi, Gunes Unal, Naomi Berry, and Peter Somogyi, University Department of Pharmacology eLife 2018;7:e34395 https://elifesciences.org/articles/34395.