Kamran Diba - US grants

?PROJECT SUMMARY (See instructions): The hippocampus is integral in the formation of episodic memory. Interestingly, neurons in the rodent hippocampus exhibit sequential activity on multiple timescales; such activity may represent the neuronal basis of episodes to be encoded and stored in memory. When the animal is within the place field of a given neuron, the neuron begins to spike, while outside of the place field, the firing rate is essentially zero. Since their discovery by O'Keefe and Dostrovsky (1) a wealth of research has followed, resulting in the following observations. The spiking activity of place-cells relative to that of the remaining population provides an additional channel of information regarding the animal's position, particularly in relation to oscillations in the local field potential at the 5-12 Hz theta frequency. The spiking phase relative to this latter oscillation begins at the peak of theta, and decreases as the animal advances and ultimately leaves the place-field. This phase precession demonstrates the remarkable degree of temporal coordination between cells in the hippocampus and may provide the basis for brief sequential patterns within each cycle of theta. These sequences are again reactivated when the animal is resting or sleeping, both in the forward and reverse order. The sequential spiking of large populations of neurons occur within a 100-400 ms time-window and are accompanied by network events called sharp-wave ripples. In summary, place-field activity in the hippocampus results in temporal sequences observed: 1) at the behavioral timescale, as animals run through sequences of place fields, 2) at the timescale of hippocampal theta oscillations, as cells fire with location-dependent phases in a theta cycle, and 3) at the timescale of sharp-wave ripples, when large populations of neurons fire with fine temporal structure. The goal of the research outlined here is to uncover the mechanisms underlying the generation of these apparently different sequences within a unified framework. To this end the proposed work employs an innovative combination of state-of-the-art computational modeling of the hippocampal network, large-scale electrophysiology in the CA 1 and CA3 regions of freely-behaving and sleeping rats, with optogenetic silencing of CA3 during behavior and sleep.

? DESCRIPTION (provided by applicant): The hippocampal circuit of the brain is considered to be responsible for storage and recall of episodic memory. Within this circuit, activity propagates through at least two distinct pathways. Comprising the input to CA1 neurons, neural activity arrives either directly from layer III of the entorhinal cortex, or through a multi-synaptic chain involving the recurrently connected CA3 region. A number of important experimental and theoretical studies have targeted the role and effect of each of these pathways on neural activity observable at CA1 but, due to various limitations, have resulted in incomplete and often contradictory viewpoints. In this project, we aim to untangle the postsynaptic effect of these two input pathways to region CA1 by performing optogenetic inactivation of region CA3 pyramidal neurons in the rat hippocampus during a specific set of in vivo network states while recording large-scale neuronal activity in CA3 and CA1. Our working hypothesis is that the multi-synaptic pathway through CA3 represents a major source of input to CA1 and as such should have a significant postsynaptic effect during key phases of oscillatory activity. We will test this hypothesis in a number of specific aims to determine and characterize the effect of region CA3 on CA1 activity separately during theta, gamma, and sharp-wave ripple oscillations, in urethane-anesthetized rats (Aims 1a and 1b), during behavior in a delayed alternation task (Aim 2), and to correlate these with behavioral effects on performance in a spatial alternation task (Aim 3). The neuronal firing patterns of CA1 neurons during theta, gamma and ripple oscillations are widely considered to play critical roles in serving the mnemonic function of the hippocampus. Thus, we expect that this work will provide a major advance forward on two fronts: in our understanding of the role of the hippocampal multi-synaptic pathway and in our understanding of how the convergence of input at different phases of theta and gamma oscillations subserves the functions of the hippocampus.

Project Summary  Sleep deprivation is a chronic and widespread hallmark of life in modern America. One of the strongest and  most costly effects of sleep deprivation is its effect on memory. Multiple lines of evidence indicate that sleep  promotes the formation and stabilization of memories through changes in the synapses of individual neurons  mediated by cyclic adenosine 3?, 5? monophosphate (cAMP) protein signaling (a process known as ?cellular  memory consolidation?), and through changes in the oscillatory activities of neuronal circuits during which other  brain regions synchronize with the hippocampus (a process known as systems memory consolidation). When  animals are deprived of sleep, they show both an impaired ability to form new memories and a reduction in  cAMP-­dependent forms of synaptic plasticity in the hippocampus. However, the causal link between cellular  and systems consolidation of memory remains unknown. Recently, Havekes, Abel, and colleagues developed  a viral vector which introduces a Gs-­coupled Drosophila octopamine receptor to increase cAMP levels in  excitatory neurons in specific regions of the brain (1). By infusing this vector into the hippocampus of mice and  activating it with octopamine injections during sleep deprivation, Havekes et al were able to prevent the usual  effects of sleep deprivation on both synaptic plasticity and object location memory, a task which is known to  depend on the hippocampus and on sleep after learning. In this proposal, we aim to combine this novel  chemogenetic manipulation for enhancing neuronal cAMP signaling with large-­scale extracellular recordings  from the hippocampus and prefrontal cortex of rats during training, sleep deprivation, recovery sleep, and  memory retrieval. We will detect and measure the network oscillations which have been implicated in systems  memory consolidation during sleep as well as waking rest, including hippocampal sharp-­wave ripples, sleep  spindles, cortical slow-­wave activity, and theta oscillations. Based on our own and others? previous work, we  hypothesize that sharp-­wave ripples are the most important mechanism for the consolidation of memories, and  that enhanced cAMP signaling will specifically increase the rate and replay content of these events during both  sleep and waking rest. These experiments will be critically valuable for scientists working to explain how  molecular changes in the sleeping brain translate into changes in the behavior of neurons which consolidate  memories, and for researchers developing pharmacological interventions to overcome the detrimental effects  of sleep deprivation in humans.       

Project Summary. The demands of modern society, career pressures, and technological advances in personal electronics and communication have increased sleep deprivation and sleep disorders across age groups. Sleep deprivation adversely impacts individual health with increased disease incidence and decreased cognitive function resulting in increased medical care, as well as occupational and traffic accidents. The negative impact of sleep deprivation on memory can be observed across species suggesting that the sleep deprivation may alter highly conserved molecular and cellular mechanisms to impact memory. The hippocampus is an excellent model to investigate the neural impacts of sleep deprivation as hippocampal activity is necessary for spatial memory and this type of memory is particularly susceptible to sleep deprivation. Current evidence indicates that sleep loss impairs the formation and stability of memories at the cellular level through changes in the synapses of individual neurons. However, the specific basis of how sleep deprivation adversely affects memory and how the brain can be rendered resilient to these effects remains poorly understood. It is critical to define the cellular, molecular and network mechanisms through which sleep deprivation impacts neural function given not only the rising incidence of sleep deprivation but also the aggravating impact of sleep loss on many neuropsychiatric, neurological and neurodegenerative disorders. Our previous research has identified decreased second messenger signaling, suppression of protein synthesis and changes in neuron dendritic structure as pathways through which sleep deprivation affects memory. However, it remains unknown if sleep deprivation separately impacts targets in each of these pathways or if the effects of sleep deprivation are mediated through a central molecular node. The objective of this proposal is to identify the molecular and neuronal mechanisms through which sleep loss impairs synaptic plasticity and memory formation by focusing on the molecular, cellular and network mechanisms through which resilience to sleep deprivation can occur. In Specific Aim 1, we use a novel transgenic approach we developed to spatially and temporally manipulate a second messenger signaling pathway. This will allow us to investigate the molecular mechanisms which underlie neuronal resilience to the detrimental effects of sleep loss. In Specific Aim 2, we investigate several types of hippocampal synaptic plasticity targeted by sleep deprivation at the neuronal level to identify which is associated with resilience. In Specific Aim 3, we identify the network and circuit properties of neurons affected by sleep deprivation using in vivo recordings from large neuronal populations. The results from our comprehensive experimental approach at the behavioral, biochemical, molecular, and electrophysiological levels will provide significant insights into the molecular signature that promotes resilience to the negative impact of sleep deprivation on memory. As such, our work may potentially lead to the development of interventions to overcome the detrimental effects of sleep deprivation on cognition.

In unit recordings from large populations of neurons, fast compressed sequential firing of neurons during rest and early sleep have been found to replay patterns first observed in active awake experience. These remarkable patterns have sparked widespread interest in the scientific community and beyond. Sequence replay is now considered to play a critical role in the long-term stabilization and storage of mnemonically important information. However, despite the general acknowledgement of the importance of the sequential structure, very little is known about the null background against which replay is compared. Specifically, are apparently 'non-replaying' spike patterns, as seen in late sleep, just simply noise? Because replay is typically assessed by comparison against a fixed known template, most methods can only determine whether the resemblance to the template is more than what might be expected from random spike trains. But these methods cannot appraise whether other patterns remain in the nonsignificant events. Recently, the Diba and Kemere labs successfully collaborated to address precisely this issue. We developed methods based on hidden Markov models (HMMs) to uncover temporal structure in spike trains of neurons in an unsupervised template-free manner. In this proposal, we aim to further improve these methods and to evaluate the hidden structure of spike trains in hippocampal neuronal populations during sleep. In our second specific aim, we will use HMMs to determine both co-active ensemble (contextual) and temporal patterns (sequential) structure in hippocampal spike trains in both pre- and post-task sleep. In the third specific aim, we will probe the essence of sleep replay further, by exposing animals to multiple novel and familiar maze environments prior to long durations of sleep. In the fourth specific aim, we will perform closed-loop disruption of neuronal population patterns to examine the causal interplay and reverberation of these patterns from early to late sleep. In summary, our proposal is designed to provide strongest characterization to date of the structure of noise in replay events. RELEVANCE (See instructions): This study will provide an opening to evaluate the role of sleep in reorganizing information in the brain and help to identify critical time windows and neuronal activities during sleep which are particularly important for information storage and stabilization. Our assumptions and deductions about the nature and purpose of sleep implicitly inform all manner of public policy, from the durations of shifts for hospital and relief workers, to morning start times of public schools. Understanding the function and mechanisms of sleep H