Periodic Reporting for period 4 - IN-Fo-trace-DG (Role of GABAergic interneurons in the formation of new memory traces in the Dentate Gyrus ofbehaving mice)
Reporting period: 2023-04-01 to 2024-12-31
Our daily life depends on the processing and storing of a continuous stream of information, which enables us to rapidly adapt our behaviour to changes of the environment. Current theories of memory formation suggest that experience-dependent modifications in the balance between excitation and inhibition enable a selected group of neurons to form a new cell association during the learning process, which represents the newly formed memory trace, the engram. The functional and structural changes at the level of synapses, single cells and cell populations that are associated with the learning process are, however, largely unknown. This is a crucial issue because impaired memory is a leading global problem implicated in various diseases including posttraumatic stress disorder, anxiety and depression, and can have various causes such as stress, sleep deficit or medication. In order to be able to effectively treat memory disorders, we need first to understand how new memories are formed in the central nervous system. In this project, we aim to address this fundamental question on the rodent dentate gyrus (DG) as the main input region of the hippocampus, functionally vital for the acquisition of new memories. The main goal of In-Fo-trace-DG is to understand (1) how learning-associated principal cell associations emerge representing new memories and (2) delineate the nature and relevance of major functional elements supporting the formation of cell associations.
To address these important objectives, we developed techniques to record the activity of neurons within the DG at high temporal, spatial and subcellular resolution and compared the obtained data with the ones in the hippocampal areas CA1 and CA2/3. We combined chronic two-photon imaging of GCAM6s/f-mediated calcium signals with spatial navigation of head-fixed mice exposed to a virtual reality, to record activity of granule cells (GCs), the principal cells of the DG, hilar glutamatergic mossy cells and GABAergic inhibitory interneurons (INs) during spatial map formation. Using two-photon imaging of synaptic inputs mediated by the medial entorhinal cortex (MEC) and the activity of principal cells, we were further able to quantify spatial input information provided to the dentate gyrus and the hippocampal areas CA1-3 at high stability and resolution as well as the hippocampal principal cell output. Using the two approaches, we made several important discoveries. While imaging large numbers of principal cell somata in the CA1 and the CA2/3 region of mice exposed to two different virtual environments, we show that they precisely and specifically represent with their activity the environments. On subsequent days of reexposure to the same environments, however, principal cells continuously change the representations of the learned spatial sceneries, they remap. In contrast, GCs of the dentate gyrus show a stable spatial code over many days with low place- or context-specificity. Thus, our results suggest a constant reassignment of synaptic weights along the hippocampal trisynaptic loop to continuously update memories based on a stable code provided by the dentate gyrus. We further revealed that cell associations representing a novel environment, which mice have never seen before, slowly emerge in the dentate gyrus on ~3-4 subsequent days of repeated exposure to the same environment. In contrast, CA1 principal cell associations are instantly present once the animal experiences a novel scene but their day-to-day stability of spatial representations declines across subsequent days. We are currently examining the cellular and network mechanisms underlying the different temporal emergence of spatial memories in the dentate gyrus and CA1. Using two-photon imaging of synaptic boutons in the molecular layer of the dentate gyrus and the stratum lacunosmum moleculare of CA1-3 originating from neurons in the medial entorhinal cortex (MEC), we demonstrated that MEC inputs to the dentate gyrus and CA3 as well as CA1 show different spatial coding properties. During navigation through two different environments, we observed that spatially modulated MEC inputs reconfigure their spatial representations more strongly than the hippocampal principal cells. Indeed, hippocampal principal cell output activity was more reliable and stable than that of their MEC inputs. Using decoder analysis based on the experimental data we revealed that higher spatial reliability allows a low number of principal cells, particularly in the DG and CA1, to provide a more rapid and accurate information about location and the context than the MEC input. Thus, conversion of dynamic MEC inputs into stable hippocampal spatial maps may enable fast encoding and efficient recall of spatio-contextual information.
In summary, these results have substantially changed our view on the function of the dentate gyrus. In particular, they shed new light on the emergence, differentiation and reliability of spatial representations by GC associations in this brain region compared to the neighboring hippocampal areas CA1-3. Our data and accumulating evidence proposes that the dentate gyrus provides a highly stable spatial code that serves as a blueprint for the formation of specific engrams of temporally varying contents and contexts in downstream hippocampal areas. Such an encoding scheme will allow one to associate memories acquired in the same global environment but still discriminate between slightly different or temporally separate instances of these memories. The next step in this project will be to elucidate the role of key neuronal elements particularly GABAergic neurons and mossy cells in the emergence of engrams in the dentate gyrus. Almost 10-15 years ago there was only little known on the function role of the dentate gyrus in episodic memory. Based on the recent data obtained within this project we expect to the end of the project, to change our way of thinking about the relevance of the different neuron types for the emergence of new memories in the dentate gyrus circuitry.