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The role of GABAergic circuits in the orchestration of hippocampal neuronal assemblies

Periodic Reporting for period 1 - GABASSEMBLY (The role of GABAergic circuits in the orchestration of hippocampal neuronal assemblies)

Reporting period: 2019-01-01 to 2020-12-31

A 'cell assembly' is defined as a sparse group of neurons repeatedly activated during a certain mental process. Theoretical and experimental work suggests that the organised activation of cell assemblies underlies cognitive abilities such as memory formation and recall, planning and decision making. The mechanisms leading to the formation, reactivation and disbanding of cell assemblies are still poorly understood. Shedding light on these mechanisms is important for two reasons. First, it could provide crucial information on the neural underpinnings of memory and cognition. Second, it could pave the way to better understand conditions in which cell assembly activity appears altered (in particular neurodevelopmental disorders such as schizophrenia).
Inhibitory neurons releasing the neurotransmitter GABA are believed to be fundamental units controlling cell assembly formation and reactivation. Leading theories posit that inhibitory neurons help maintain sparsity (i.e. making sure that only a few neurons are active in a short time window) and help segregate cell assemblies representing different brain operations. However, direct evidence and detailed mechanisms for this theory are still lacking. The rodent hippocampus represents a valuable framework to investigate the role of inhibition in cell assembly activation for various reasons. First, the hippocampus is required for encoding episodic memories. Second, the sequential activation of hippocampal cell assemblies (which are excitatory pyramidal cells) can encode both spatial and temporal information. Third, the inhibitory circuits of the hippocampus are well understood.
The aim of the present project was to understand how inhibitory (GABAergic) circuits orchestrate the activation of hippocampal cell assemblies. In the first part of the project, we sought to study the relationship between a special inhibitory subtype ('hub neurons', which are born earliest in development) and cell assemblies. Since the physiological properties and connectivity of these cells was unknown, the aim was also to shed light on these aspects. The second part of the project aimed at studying the spatiotemporal dynamics of inhibition in CA1 region of the hippocampus, at examining the relationship of these dynamics with cell assembly activity and at perturbing inhibitory neurons' activity using optogenetics.
We began by studying the relationship of a special class of inhibitory neurons with CA1 cell assemblies. These neurons are born earliest during prenatal brain development and act as 'hub cells' during early postnatal development. That is, they show high functional connectivity (statistical relationship) with nearby neurons and are causally involved in network synchronisations (activation of a large number of hippocampal neurons). Given their strong link with co-activity of large numbers of cells during postnatal development, we asked whether hub cells maintain strong ties with network bursts and cell assembly activation in adulthood. We thus imaged the calcium signals from CA1 hub cells from mice running on a treadmill using 2-photon microscopy. We found that hub cells maintain a strong statistical relationship with network synchronisations occurring during rest (synchronous calcium events) and show high correlations with cell assembly activity.
Since the intrinsic properties and the anatomical connectivity of these cells in the adult hippocampus were mostly unknown prior to our study, we also decided to perform anatomical, electrophysiological and optogenetic mapping experiments to shed light on how these cells integrate into adult networks. We found that hub cells maintain distinct morpho-physiological and connectivity profiles in adulthood. Therefore, hub cells appear predetermined for exceptional functional and structural properties in both the developing and adult hippocampus.
These results were disseminated at various national and international conferences through posters and seminars (including a plenary seminar at the Federation of European Neuroscience Societies meeting 2020 and a seminar at the Society for Neuroscience meeting 2019). Results were also recently published in a peer-review open access journal. The peer-reviewed article was advertised on social media (mainly Twitter) and received very high visibility scores.
In the second part of the project, we examined how inhibitory neurons overall contribute to cell assembly dynamics in CA1. We imaged the spatiotemporal distribution of inhibition acting on cell assemblies using 2-photon microscopy. We imaged the same neurons over multiple sessions, allowing to quantify the stability of neural dynamics across days. We also tested the effect of sensory cues, allowing to discriminate possible difference in inhibitory dynamics between egocentric and allocentric modes of navigation. Finally, we began all-optical experiments in which single or multiple inhibitory neurons are optically excited or inhibited while monitoring the activity of cell assemblies. Preliminary analyses suggest that inhibitory activity changes in function of sensory stimuli and is involved in the segregation of pyramidal cell assemblies.
These data provide a significant advancement in our understanding of GABAergic inhibition and its relationship to pyramidal cell assemblies. Our characterisation of the properties of hub cells in the adult hippocampus expands our knowledge of the diversity of inhibitory neurons in cortical areas, their functional properties and the developmental logic of their organisation. The results prove that inhibitory hub cells have a strong link with neural synchrony at both neonatal and adult stages. Since synchronous activity in the first postnatal week is believed to be crucial for the development of neural networks, our study suggests that investigating the implication of hub cells in neurodevelopmental disorders may represent a promising avenue for future research. Additionally, the unique link of hub cells to neural synchrony suggests that these cells may represent a promising target for treating seizures, which are characterised by abnormal synchronous discharges.
We envisage that these data will generate two additional peer-review publications describing the role inhibition in neural synchrony and cell assembly dynamics. The results could have a significant impact on the field because they might expand our knowledge of the function of inhibition in regulating neural activity and in the physiological and pathological mechanisms of memory formation and recall. Thus, the results could engage a large scientific audience, namely: 1) cellular/circuit neuroscientists studying functional interactions between inhibitory cells and pyramidal cells in the hippocampus or other areas sharing similar cytoarchitecture; 2) systems neuroscientists investigating network dynamics, oscillations and cell assemblies; 3) computational neuroscientists modelling the impact of inhibition on network activity; 4) Artificial Intelligence engineers seeking to incorporate inhibition in biologically plausible Artificial Neural Networks.
Imaging hippocampal inhibitory neurons and pyramidal cell assemblies in vivo