Periodic Reporting for period 2 - HippAchoMod (Deciphering the cholinergic modulation of the hippocampal place code.) Reporting period: 2018-08-08 to 2019-08-07 Summary of the context and overall objectives of the project A central component of the brain’s episodic memory circuit, where complex representations of the environment emerge, is the hippocampal formation. Place cells of the hippocampus exhibit location-specific firing and they are part of an internal map of the environment that guides the behavior of the organism. Activity of place cells also carries information about non-spatial components of the environment. Our initial goal was to uncover how the medial septum, the master clock of the navigation circuit key for synchronizing information flow among its components, contribute to the formation of the place code. However, we observed the high prevalence of so-called context-selective activity of place cells. Hippocampal memory processes are key for assisting the planning of actions and decision making. Context-selective activity emerges in tasks in which the animal has to decide which way to go when alternating between two sides of a maze. Some hippocampal place cells fire only when the animal turn to one of the sides. Context-dependent activity (here, context corresponds to left / right routes), by coding the future choice of the animal, may help the decision circuit choose the correct path to follow. Alternatively, these neurons have a code for the left and another code for the right trajectory.Episodic memory traces are composed of a multiple types of information and the interconnected meshwork of memories forms the foundation of our personality. Thus, uncovering what kind of information the hippocampus codes is fundamental for understanding the brain mechanisms that determine who we are. As life expectancy is increasing, so is the neuropsychiatric diseases that lead to the deterioration of our capability of living a meaningful life. The hipppocampus-centered memory system of our brain is especially vulnerable. Therefore, understanding the components and formation of the hippocampal code is pivotal for identifying processes we can manipulate in order to prevent the break-down of our mental capacities.We have thus now an additional goal: besides revealing a key mechanism of the formation of the place code (medial septal modulation), we aim to characterize the non-spatial information possibly contributing to planning and decision making. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far The main goal of the Project was to uncover how the medial septum modulates the hippocampal spatial code. Because of unexpected findings we aimed to pursue another objective: characterization of the context-dependent activity of hippocampal place cells. The experimental work was carried out during the Outgoing Phase which was extended by 6 months. During this period the grant was suspended. The collected data was analyzed during the return phase.Regarding the experimental part of the Outgoing Phase, we implemented a location-contingent closed-loop optogenetic stimulation strategy: the medial septum was light-activated when the animal entered a pre-defined location monitored by distance sensors.We injected 36 mice, of which 19 were implanted and 11 generated data. First, we observed a reduction in the animals’ running speed while traversing the stimulated location, paralleled by the transient disruption of theta oscillation. About 50% of place cells changed their spatial firing. The most common effect was the emergence of a new place field. The disappearance of existing place fields was also observed. Notably, in control sessions without delivering any light about 30% of place cells exhibited spontaneous remapping. Sham stimulation through an optic fiber terminating outside the brain did not cause alteration of spatial activity different than that observed in control, non-manipulated sessions. Thus, our results indicate that the medial septum can indeed, change the spatial code carried by the location-coupled activity of place cells.As mentioned earlier the return phase was devoted to the analysis of the collected data. Characterization of control sessions unraveled an unexpectedly high prevalence of context-dependent activity of place cells in the stem part (central corridor) of the maze. Analysis of 182 place cells from 22 control sessions (unperturbed or stimulated without light responsive opsin) of 5 mice revealed that, 53.5%, an unexpectedly high proportion of place cells, exhibited multiple forms of context-correlated activity: i) one place field selective for either left or right trials; ii) two place fields (38.2% of all stem place cells) present in opposite trials; iii) two place fields had varying level of context-dependence; iv) non-splitter place cells with statistically non-distinguishable left and right activity. Strikingly, the proportion of splitter cells increased as the learning proceeded. Comparison of spatial activity in correct and error trials revealed, unexpectedly, a high correlation between trials with common origin (past choice i.e. where the mouse came from) but opposite upcoming choice (which side the mouse entered). This latter finding implies that splitter cells’ contextual code is retrospective rather than prospective.I presented our findings on one local (restricted to the NYU Neuroscience Institute) and three international conferences and on a seminar organized in my home institute. I also gave a talk as part of a series of group reports in my home institute. Inspired by the approaches I learned during the Outgoing Phase, I had formulated a research program and successfully applied for a national grant. I was also awarded by a mentoring grant that enabled the training of a summer intern. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) In recent years, considerable amount of information has been accumulated about how the brain represents the environment. We know that in the hippocampus, the brain's memory center, a large proportion of neurons carry information either about the location of the animal, or their activity is related to certain features or objects of the environment. The question of how this complex code emerges or is still unanswered. The transformation of a silent neuron to a coding neuron is strictly coupled to time windows determined by the theta oscillation generated by the medial septum (MS), the main modulator of hippocampal function. Our hypothesis moves beyond the traditional view of the MS as the pacemaker of theta by raising the possibility of its contribution to the formation of the hippocampal code. Our experimental approach was based on the location-contingent activation of the MS in a closed-loop configuration, to test if we were able to artificially create or erase the place code corresponding to the location of activation. At the end of the project, we expect to reveal if and how the MS controls the hippocampal coding process. Deciphering the mechanisms by which the hippocampus codes the environment and forms memories can facilitate the understanding of diseases connected to hippocampal dysfunction. Significance of our work is underlined by the increasing prevalence of neurpsychiatric syndromes, especially Alzheimer's disease, associated with hippocampal pathologies and the lesion of the basal forebrain neural network of which the MS is a key component.Additionally, we have found an unexpectedly rich diversity of context-dependent activity coding trajectory the animal followed in different trials. Based on our findings, we expect to enrich our understanding of the components the hippocampal representation of the environment is composed of. Figure about the main finding of the data analysis carried out during the return phase. Figure demonstrating the main finding of the outgoing phase.
