Identification of neuronal ensembles mediating fear generalization in the infralimbic cortex

点击翻译按钮获取中文摘要

PROJECT SUMMARY Posttraumatic stress disorder (PTSD) afflicts 8% of the US population. One prominent symptom of PTSD is the over-generalization of traumatic memories. Generalization refers to the transfer of learned responding to stimuli that are similar, but not identical, to the original conditioned stimulus (CS). Despite the importance of understanding generalization, in both basic memory processing and PTSD, very little is known about the neuronal substrates. Memory is thought to be encoded in sparse, distributed groups of neurons known as “neuronal ensembles.” Over time, neuronal ensembles supporting memory expression are dynamic. Generalization is also dynamic, with a tendency to increase over time, suggesting a relationship between the two. A major question in neuroscience is how ensemble dynamics mediate changes in the expression of generalization over time. One brain region shown to contribute to generalization is the infralimbic cortex (IL). Despite a known role for the IL in generalization, the identification of IL neuronal ensembles mediating generalization processes over time is unknown. One obstacle in the way of studying neuronal ensembles is the inherent technical difficulty in ensemble identification and manipulation. New molecular tools, such as ArcCreERT2 x EYFP transgenic mice, for permanently “tagging” active cells (Arc/arg3.1 expression) during learning for comparison with cells active during memory recall, has advanced the field. In Aim 1, we use ArcCreERT2 x EYFP mice to tag cells in the IL during learning for comparison with cells activated during recall of either the CS or after a novel tone (generalization or discrimination) at recent or remote retention intervals. While “tagging” technology provides a window into neuronal ensemble dynamics, the resolution of the approach is limited. To provide greater temporal resolution (subsecond) for identifying neuronal ensembles dynamics, we incorporate in vivo Ca2+ imaging to visualize, measure, and compare coordinated IL ensemble activity associated with fear generalization over time. In Aim 2, a causal link between global IL activity and fear generalization will be established using chemogenetics to bi-directional modulate IL pyramidal neuron excitation. Activity-dependent (ArcCreERT2) chemogenetics will be used to test whether the synthetic reactivation of a small number of neurons whose activity was previously associated with generalization (i.e., a ”generalization ensemble”) is sufficient to drive generalization expression. I have outlined a research plan well- suited for undergraduate neuroscience research. Nearly all proposed techniques have already been successfully carried out in my lab. It is anticipated the data generated by these experiments will not only strengthen undergraduate research at Vassar but will lead to a greater understanding of fear generalization processes at the level of neuronal ensembles, with the hope of informing treatment strategies for PTSD.