Biology subjects

Suthard, R. L.

Publications and source records attributed to Suthard, R. L..

5 recordsLinked to original sources

bioRxiv2023

Engram Reactivation Mimics Cellular Signatures of Fear

Engrams, or the physical substrate of memory in the brain, recruit heterogeneous cell-types. Targeted reactivation of neurons processing discrete memories drives the behavioral expression of memory, though the underlying landscape of recruited cells and their real-time responses remain elusive. To understand how artificial stimulation of fear affects intra-hippocampal neuronal and astrocytic dynamics as well as their behavioral consequences, we expressed channelrhodopsin-2 in an activity-dependent manner in dentate gyrus neurons while performing fiber photometry of both cell types in ventral CA1 across learning and memory. Neurons and astrocytes were shock-responsive, while astrocytic calcium events were uniquely modulated by fear conditioning. Notably, optogenetic stimulation of a hippocampus-mediated engram recapitulated coordinated calcium signatures time-locked to freezing that were also observed during natural fear memory recall, suggesting that engram activation alters activity across different cell types within hippocampal circuits during the behavioral expression of fear. Together, our data reveals cell-type specific hippocampal dynamics during freezing behavior and points to neuronal-astrocytic coupling as a shared mechanism enabling the natural and artificial recall of a memory. HighlightsO_LIVentral hippocampal neurons and astrocytes are active during foot shock C_LIO_LICalcium activity is time-locked to freezing during fear conditioning and recall C_LIO_LIOptogenetic reactivation of fear recapitulates cellular signatures seen during recall C_LIO_LIReactivation of a fear memory allows prediction of freezing behavior C_LI

neuroscience↗

bioRxiv2023

Chronic activation of a negative engram induces behavioral and cellular abnormalities

Negative memories engage a brain and body-wide stress response in humans that can alter cognition and behavior. Prolonged stress responses induce maladaptive cellular, circuit, and systems-level changes that can lead to pathological brain states and corresponding disorders in which mood and memory are affected. However, its unclear if repeated activation of cells processing negative memories induces similar phenotypes in mice. In this study, we used an activity-dependent tagging method to access neuronal ensembles and assess their molecular characteristics. Sequencing memory engrams in mice revealed that positive (male-to-female exposure) and negative (foot shock) cells upregulated genes linked to anti- and pro-inflammatory responses, respectively. To investigate the impact of persistent activation of negative engrams, we chemogenetically activated them in the ventral hippocampus over three months and conducted anxiety and memory-related tests. Negative engram activation increased anxiety behaviors in both 6- and 14-month-old mice, reduced spatial working memory in older mice, impaired fear extinction in younger mice, and heightened fear generalization in both age groups. Immunohistochemistry revealed changes in microglial and astrocytic structure and number in the hippocampus. In summary, repeated activation of negative memories induces lasting cellular and behavioral abnormalities in mice, offering insights into the negative effects of chronic negative thinking-like behaviors on human health.

neuroscience↗

bioRxiv2023

Hippocampal engrams generate flexible behavioral responses and brain-wide network states

Memory engrams are both necessary and sufficient to mediate behavioral outputs. Defensive behaviors such as freezing and avoidance are commonly examined during hippocampal-mediated fear engram reactivation, yet how reactivation of these cellular populations across different contexts engages the brain to produce a variety of defensive behaviors is relatively unclear. To address this, we first optogenetically reactivated a tagged fear engram in the dentate gyrus (DG) subregion of the hippocampus across three distinct contexts. We found that there were differential amounts of light-induced freezing depending on the size of the context in which reactivation occurred: mice demonstrated robust light-induced freezing in the most spatially restricted of the three contexts but not in the largest. We then utilized graph theoretical analyses to identify brain-wide alterations in cFos co-activation during engram reactivation across the smallest and largest contexts. Our manipulations conferred greater positive cFos correlations and recruited regions spanning putative fear and defense systems as hubs in the respective networks. Moreover, reactivating DG-mediated engrams generated network topologies across experimental conditions, emphasizing both shared and distinct features. By identifying and manipulating the circuits supporting memory function, as well as their corresponding brain-wide activity patterns, it is thereby possible to resolve systems-level biological mechanisms mediating memorys capacity to modulate behavioral states. SIGNIFICANCE STATEMENTImplementing appropriate defensive behaviors across disparate environments is essential for survival. Memories can be used to select these responses. Recent work identified and artificially manipulated cellular ensembles within the hippocampus that mediate fear memory recall, yet how these populations engage brain-wide pathways that mediate defensive behaviors under environmental contingencies is unclear. We demonstrated here that reactivation across environments of various sizes elicits different behavioral responses and corresponding brain-wide network dynamics. These findings establish the flexibility of memory-bearing ensembles in generating brain and behavior states.

neuroscience↗

bioRxiv2022

Chronic Gq activation of ventral hippocampal neurons and astrocytes differentially affects memory and behavior

Network dysfunction is implicated in numerous diseases and psychiatric disorders, and the hippocampus serves as a common origin for these abnormalities. To test the hypothesis that chronic modulation of neurons and astrocytes induces impairments in cognition, we activated the hM3D(Gq) pathway in CaMKII+ neurons or GFAP+ astrocytes within the ventral hippocampus across 3, 6 and 9 months. CaMKII-hM3Dq activation impaired fear extinction at 3 months and acquisition at 9 months. Both CaMKII-hM3Dq manipulation and aging had differential effects on anxiety and social interaction. GFAP-hM3Dq activation impacted fear memory at 6 and 9 months. GFAP-hM3Dq activation impacted anxiety in the open field only at the earliest time point. CaMKII-hM3Dq activation modified the number of microglia, while GFAP-hM3Dq activation impacted microglial morphological characteristics, but neither affected these measures in astrocytes. Overall, our study elucidates how distinct cell types can modify behavior through network dysfunction, while adding a more direct role for glia in modulating behavior. HighlightsO_LICaMKII- and GFAP-Gq activation impacted memory, anxiety, and social behaviors. C_LIO_LINovel environment exploration was affected by CaMKII- and GFAP-Gq activation. C_LIO_LICaMKII-Gq modified microglial number, while GFAP-Gq affected microglial morphology. C_LIO_LINeither cell manipulation affected astrocytic number or morphology. C_LI

animal behavior and cognition↗

bioRxiv2022

Basolateral amygdala astrocytes are engaged by the acquisition and expression of a contextual fear memory

Astrocytes are key cellular regulators within the brain. The basolateral amygdala (BLA) is implicated in fear memory processing, yet most research has entirely focused on neuronal mechanisms, despite a significant body of work implicating astrocytes in learning and memory. In the present study, we used in vivo fiber photometry to record from amygdalar astrocytes across fear learning, recall, and three separate periods of extinction. We found that BLA astrocytes robustly responded to foot shock during acquisition, that their activity remained remarkably elevated across days in comparison to unshocked control animals, and that their increased activity persisted throughout extinction. Further, we found that astrocytes responded to the initiation and termination of freezing bouts during contextual fear conditioning and recall, and this behavior-locked pattern of activity did not persist throughout the extinction sessions. Importantly, astrocytes do not display these changes while exploring a novel context, suggesting that these observations are context or memory-dependent. Chemogenetic inhibition of fear ensembles in the BLA did not affect freezing behavior or astrocytic calcium dynamics. Overall, our work presents a real-time role for amygdalar astrocytes in fear processing and provides new insight into the emerging role of these cells in cognition and behavior. Significance StatementWe show that basolateral amygdala astrocytes are robustly responsive to footshock, exhibit unique calcium event characteristics following contextual fear acquisition, and ramp up activity at the initiation and termination of freezing bouts during fear conditioning and recall. This astrocytic calcium response to freezing behavior is not observed during extinction sessions, despite unique calcium events continuing through three days of training compared to no-shock controls. We find that astrocytes display context specific changes in calcium signaling, but chemogenetic inhibition of BLA fear ensembles does not impact freezing behavior or calcium dynamics. These findings show that astrocytes play a key, real-time role in fear learning and memory.

neuroscience↗