Neuron-astrocyte calcium dynamics in fear learning and memory
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Abstract
Memory is one of the most extensively-studied phenomenon in neuroscience as it provides the basis for our day-to-day human experience. A continued unmet need is understanding how memory processing can go wrong, whether in the degeneration of our ability to form or access memories (i.e. dementia) or in maladaptive memory processing (i.e. Post-Traumatic Stress Disorder). Animal models have enabled tremendous insight into the molecular and systems-level underpinnings of healthy and pathological memory states that advance our understanding of the human condition. Within the memory field, the primary focus has traditionally been on how neuronal activity within regions such as the hippocampus enable the formation, consolidation, maintenance, and extinction of episodic memories, i.e. the memories of our personally experienced events defined along what-where-when dimensions. While the majority of this work has focused on neurons despite extensive work indicating that memories recruit heterogenous cell types, recent work has supported the idea that astrocytes, a type of glial cell, are active modulators of learning and memory processes ranging from fear encoding to spatial navigation. To expand upon this growing literature, we investigated the role of astrocytic calcium dynamics using multiple imaging modalities within the amygdala and hippocampus, in an effort to understand how they may be intimately interacting with neurons to enable higher-level cognitive processing.To that end, my thesis puts forth three experiments that link the activity of neurons and astrocytes to cognition and behavior. In the first experiment, we recorded real-time astrocytic calcium dynamics in the basolateral amygdala (BLA) across contextual fear conditioning (CFC), recall and extinction using single-color fiber photometry. We showed that BLA astrocytes robustly responded to foot shock during CFC, resulting in persistent changes in calcium activity across subsequent days compared to controls. Additionally, astrocytic calcium dynamics became correlated with freezing epochs during CFC and recall only in mice that received foot shocks, and this activity became uncoupled across extinction days. Further, chemogenetic inhibition of BLA fear ensembles had no effect on astrocytic calcium activity or freezing behavior. The results of our first experiment revealed that astrocytes play a real-time role in the acquisition and maintenance of contextual fear as it relates to behavior. Our second experiment then sought to understand how artificial stimulation of fear affects intra-hippocampal neuron-astrocyte dynamics as well as their behavioral consequences. Here, we utilized a combination of activity-dependent labeling strategies in the dentate gyrus (DG) and in vivo fiber photometry of neurons and astrocytes in ventral CA1 of the hippocampus (vCA1) across fear acquisition, natural recall and artificial reactivation of a fear memory or engram. In line with our first experiment, both cell types in vCA1 displayed shock-responsiveness during CFC, with astrocytic calcium events uniquely modulated. Optogenetic activation of a DG-mediated fear engram was sufficient to induce calcium dynamics in neurons and astrocytes that were akin to those displayed during natural recall. Further, these dynamics in both cell types were coupled to fear-related behaviors, such as freezing, during both natural and artificial reactivation of fear. These findings provide the first evidence of neuron-astrocytic coupling as a shared mechanism that enables both natural and artificially-induced memory retrieval, as well as the behavioral expression of fear. The final experiments sought to investigate how astrocytes contribute to contextual fear learning and memory at the single-cell level, enabling a higher-resolution understanding of our previous findings. To accomplish this, we performed freely-moving one-photon calcium recording of dorsal CA1 (dCA1) astrocytes as mice underwent CFC and a subsequent day of contextual recall (Context A; Cxt A) or novel context exposure (Context B; Cxt B). Further, we performed the first longitudinal registration of astrocytes using one-photon imaging to enable analysis of their dynamics across learning. We showed that astrocytes respond to aversive foot shock with sequential calcium activity, akin to that displayed by hippocampal time cells. Notably, these sequences reappear during contextual recall (Cxt A) but not novel exposure (Cxt B), and seem to be driven by reactivated astrocytes across days. Our ongoing work will characterize these sequences across days and explore how they may be coupled with the behavioral expression of fear. These preliminary findings suggest that astrocytes may be involved in temporal coding and provide the first evidence that sequential activity during recall may serve as a memory-specific retrieval process. In summary, my dissertation work presents evidence for the active role of amygdalar and hippocampal astrocytic calcium in the real-time processing of fear learning and memory as it relates to behavior.
Description
2024