All-optical interrogation of the hippocampal neural code underlying episodic memory

Our ability to form memories of specific events, known as episodic memory, is central to our identity and our interactions with the world. A key brain structure involved in storing and retrieving episodic memories is the hippocampus. Lesions of the hippocampus, or disruptions of the hippocampal circuit during neurodegenerative diseases such as Alzheimer's, can disrupt recall of existing memories and prevent formation of new episodic memories. The discovery of place cells in the hippocampus, which are active in specific regions of the environment, provided a possible cellular mechanism supporting the formation of episodic memories. However, it is not yet known whether place cell firing is causally linked to memory formation. Furthermore, the nature of the neural code that the hippocampus utilises to store and retrieve memories across space and time is unknown.

We will address these fundamental questions by harnessing a novel strategy for 'all-optical' interrogation of neural circuits in the intact brain. This approach uses light to simultaneously read out the activity of neurons while performing targeted optogenetic stimulation with cellular resolution in behaving mice. This allows us to identify neurons which exhibit certain types of activity and to stimulate them selectively in order to test the causal functional role of this activity, which we can assess by observing the behavioural performance of the animal. By changing the number, timing and pattern of activated neurons in the circuit we can identify the neural code which supports episodic memory.

Our experiments will probe the role of hippocampal neurons in the formation of both spatial and temporal aspects of episodic memory. We will use two different behavioural tasks in order to identify and manipulate the relevant neural codes. For spatial memory, mice will perform a spatial navigation task in a virtual reality environment, running down a track and learning to stop and lick in an area where they receive a reward. For temporal memory we will use an olfactory task where animals must remember the identity of an odor across a delay and then assess its relationship to a second odor. We will then use all-optical interrogation to test both memory formation and retrieval by identifying and artificially increasing the reliability of the relevant activity patterns in order to increase learning rates on both tasks. Additionally, we will test the role of place cells and sequences in memory retrieval by artificially driving these patterns and observing relevant behavioural alterations. Building upon this initial test of function we will then systematically vary our stimulation parameters, including the number of cells, level of synchrony and pattern of sequential activation. This will provide the first causal links between specific activity patterns of hippocampal neurons and behaviour, as well as revealing the fundamental properties of the neural code underlying episodic memory.

By determining the neural activity patterns enabling memory we will substantially further our understanding of both the healthy and diseased brain. Our knowledge of how disease states impact the manifestation of healthy neural activity patterns is still limited, in part because we do not yet know the nature of the neural code in the healthy brain. Once the activity patterns supporting memory in the healthy brain are understood, new treatments can be designed to preserve or restore them when the diseased brain malfunctions. Our findings will be especially important for guiding research in dementias and neurodegenerative diseases, which cause cognitive deficits due to disruptions of cellular function and coding in the hippocampal system.

The hippocampus is essential for the formation and retrieval of episodic memories. The discovery of hippocampal place cells suggested a cellular mechanism for spatial memory. However, a causal role of place cell firing has yet to be established; and the general neural code employed by hippocampal neurons to form and recall episodic memories is unknown. We will address these problems by leveraging a novel "all-optical" approach for interrogating neural circuits in the intact brain. By combining simultaneous 2-photon calcium imaging and 2-photon targeted optogenetic stimulation, the activity of thousands of neurons can be recorded and simultaneously manipulated at cellular and millisecond resolution in the behaving animal. We will use this approach to determine the hippocampal neural code underlying episodic memory across both space and time. First, we will test the functional role of place cells in mice performing a spatial memory task in virtual reality. If specific optogenetic activation of place cells representing a rewarded location biases the behaviour towards that displayed in the rewarded location, this provides a direct causal link between place cell activity and spatial memory. We will also assess the role of stereotypical patterns in spatial learning by artificially increasing place cell sequence reliability in naive mice. Second, complementary experiments will be performed during an olfactory paired-associates task with a delay to test whether hippocampal neural sequences support memory across time. We will compare the efficacy of consistent versus scrambled sequences of activity in boosting learning rates and memory retrieval. Our experiments will provide unprecedented causal links between specific patterns of activity in hippocampal neurons and memory. This information will guide diagnosis and novel treatments in dementias and neurodegenerative diseases, which cause memory deficits due to disruptions of cellular function and coding in the hippocampus.

Memory is a fundamental operation of the brain and is central to identity, decision making, and future planning. Any failure in the basic mechanisms governing memory has catastrophic consequences, leading to serious cognitive and psychiatric disorders. Memory dysfunctions are a hallmark of dementia, which imposes an enormous burden on society and on the wider economy, costing an estimated £26 billion a year in the UK alone (which is projected to rise to £55 billion/yr in 2040). Understanding the basic mechanisms of memory storage will therefore have the potential for tremendous impact on society.

Specifically, our work will lead to fundamental advances in our understanding of episodic memory formation and retrieval. The strategies developed in this application will enable a new generation of experiments with unprecedented precision for linking neural circuit activity with behaviour. This will also yield a platform for future systems and translational neuroscience research centered around memory and memory-related disorders such as neurodegenerative diseases, providing a sensitive assay for detecting circuit dysfunction and improving the accuracy and rate with which new therapeutics are developed. If the results of our work suggest novel therapeutic targets, our links with UCL Enterprise and UCL Business PLC (together with UCL's Contracts Department who serve as a liaison between UCL academics and Industry), will help us to exploit our discoveries in order to provide a pathway to commercialization. The PI is in close contact with relevant clinicians at UCL due to his membership in the UCL Neuroscience Strategy Committee. Finally, an appropriate intellectual property strategy is in place at UCL to maximize the opportunities of downstream funding, partnering, and ultimate exploitation and dissemination.

Given the fundamental importance of research on memory to human behaviour, the results of this work will also enrich culture and society. We will strive to engage the public and disseminate the outcomes of our research, building on our previous experiences with public engagement (e.g. TEDx talk; World Economic Forum talk). We will post updates on our website and via social media, present public lectures and provide press releases.

The work will involve the participation of many undergraduate and master's students interested in learning about the questions and techniques involved in basic neuroscience. Bolstered by the experience, many of these students are inspired to pursue a career in medicine or biomedical research. The PI and co-PI are also involved in educational outreach, having been invited to give lectures at prestigious schools in the London region, and having organized and taught on many local and international summer schools.