Neurophysiological mechanisms of memory formation investigated with two-photon calcium imaging in the behaving mouse

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Understanding how memories are encoded is important for increasing our knowledge of brain function but will also inform the development of novel treatment for memory disorders. Memory is thought to be represented by sparse neuronal ensembles ('engrams') that are reengaged during memory retrieval. In Dr. Josselyn’s laboratory, we try to understand the rules that determine which neurons are chosen to encode a memory. In this vein, it was recently demonstrated that lateral amygdala (LA) neurons with high levels of intrinsic excitability out-compete neighboring neurons for recruitment into the corresponding engram. However, (1) the specific spatial-temporal patterns of electrophysiological activity that mediate memory allocation remain unknown. Moreover, fear memory strength is independent from the size of the engram in the LA, raising the questions as to (2) the neurophysiological difference in how a weak vs strong memory is represented in the LA, and (3) how LA engrams transform or evolve across time. Addressing these questions has been hindered in the past due to the technical difficulty of repeatedly recording the same neurons in deep neural regions across multiple days and weeks. However, this level of spatial and temporal precision is essential if we are to capture the dynamic cellular processes that underlie memory encoding, storage, and retrieval across time. My project will address this problem by using state-of-the-art imaging techniques I developed in my PhD. Previously, I built a two-photon microscope to study cellular and subcellular processes involved in sensory processing of mouse cortex in vivo. The cells were stained with a fluorescent calcium indicator and neuronal activity was reported by changes in fluorescence that were recorded in awake mice. During my post-doctoral work in the Josselyn lab, I will build upon this experience and chronically image single-cell activity in the mouse LA – a brain region crucially involved in the encoding and retention of fear associations. These recordings will be performed both during fear learning and fear memory retrieval. I will record spontaneous activity as a read-out for cellular excitability just before the memory encoding event, to investigate the influence of engram excitability on memory allocation. To better understand the role of local amygdala circuitry in engram formation, I will target and optogenetically manipulate specific cellular subtypes and combine this with functional two-photon imaging. Together, these experiments will further our understanding of the cellular mechanisms that mediate memory encoding, storage, and retrieval across time, and will lay the groundwork for investigating how these processes go awry in mouse models of Alzheimer’s disease (in which fear memories are weaker) and posttraumatic stress disorder (in which fear memories are stronger).