Sara J. Aton - US grants

[unreadable] DESCRIPTION (provided by applicant): In mammals, a circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus mediates daily rhythms in behavior and physiology. Individual SCN neurons fire rhythmically with near-24-hour periodicity. It is not known how these neurons communicate within the SCN network to produce synchronous circadian rhythmicity. Vasoactive intestinal polypeptide (VIP) and GABA, implicated in tissue level rhythms, may be required for rhythmicity in, or synchrony among, individual SCN neurons. Determining how these transmitters affect the rhythms of individual SCN neurons is critical to understanding how coordinated rhythmic output is generated by the SCN network. We will employ multi-electrode array technology to record firing rate rhythms of single SCN neurons, in conjunction with genetic and pharmacologic manipulations aimed at blocking or mimicking GABA and VIP signaling. These experiments will directly identify which SCN neurons have cell-autonomous pacemaking ability, and test whether GABA or VIP signaling is needed to maintain synchrony between them or drive their rhythmicity. Our findings will facilitate the development of novel drug interventions for the treatment of circadian rhythm disorders.

[unreadable] DESCRIPTION (provided by applicant): Recent studies have revealed a critical role for sleep in learning, memory, and cortical plasticity. We have previously shown that sleep enhances ocular dominance plasticity (OOP), a form of in vivo synaptic remodeling triggered by monocular deprivation (MD) during a critical period of development. Postsynaptic activity in the primary visual cortex during post-MD sleep is critical for this process. However, the relative contributions of rapid eye movement (REM) and non-REM (NREM) sleep to OOP are unknown, as are the precise intracellular mechanisms underlying sleep-mediated ODP. The early stages of ODP share key features with long-term depression (LTD) and long-term potentiation (LTP) in the visual cortex during the critical period. It is possible that sleep enhances ODP through mechanisms similar to LTD, LTP, or both. The goals of the proposed studies are first, to clarify the roles of REM and NREM sleep in this process, and second, to investigate the contribution of LTP-like and LTD-like plasticity mechanisms to sleep-dependent ODP. To test the roles of REM and NREM sleep in ODP, we will selectively manipulate these sleep stages following a period of MD. To determine whether sleep enhances ODP via LTD-like or LTP-like mechanisms, we will test the effects of blocking protein phosphatase or kinase pathways - critical for LTD or LTP, respectively - in the visual cortex during post-MD sleep. While the primary function of sleep is still unknown, its role in learning and memory has become an area of increasing interest. It is likely that the mechanisms underlying sleep enhancement of ODP also underlie sleep effects on other types of learning and memory; understanding the role of sleep in ODP will further our understanding of its role in cognitive function. Our findings may have important implications for patients taking medications that affect sleep architecture, and those affected by insomnia and other increasingly prevalent sleep disorders. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Abstract: The long-term goal of this project is to identify thalamocortical network mechanisms involved in consolidating experience-dependent plasticity in the visual system. Sleep has beneficial effects for processes dependent upon synaptic plasticity, such as memory consolidation. Recent studies have shown that cortical areas engaged by waking sensory experience are "reactivated" during subsequent slow wave sleep (SWS), with local changes in electroencephalogram (EEG) oscillatory activity. Because these EEG oscillations are generated by rhythmic, synchronous firing of thalamic and cortical neurons, one untested hypothesis is that SWS thalamocortical activity leads to potentiation or depression of synaptic targets. Orientation-specific response potentiation (OSRP) in the mouse visual system involves potentiation of neuronal responses to visual stimuli of a specific orientation. OSRP is initiated by brief exposure to an oriented grating stimulus, and is consolidated "offline" in the hours immediately following visual experience. My preliminary data suggest that thalamocortical spindle (7-14 Hz) activity during SWS may play a critical role in OSRP consolidation. In the mentored phase of the proposed award (Aim 1), I will: (a) test whether SWS and SWS spindle oscillations are required for OSRP, and (b) assess whether during consolidation, SWS spindles 1) activate thalamocortical connections in a non-specific manner, or 2) mediate "reactivation" of thalamocortical connections in a manner consistent with prior visual experience. I will do this by recording ongoing activity and visual response properties in populations of neurons in the visual cortex and lateral geniculate nucleus of freely-behaving mice during baseline, waking visual experience, and a subsequent consolidation period of either: ad lib sleep, total sleep deprivation, rapid eye movement sleep (REM) deprivation, or selective interruption of SWS spindles. These studies will build upon the my prior research experience with multielectrode recording and data analysis, under the co-mentorship of Drs. Marcos Frank (my current postdoctoral advisor and an expert in the areas of sleep and visual cortex plasticity) and Diego Contreras (an expert in the areas of state-dependent thalamocortical network properties and network mechanisms involved in vision). During the mentored phase of the award, I will also develop expertise in using optogenetic techniques in combination with multielectrode recording in freely-behaving mice, in preparation for experiments outlined in Aim 2. In the independent phase of the award (Aim 2), I will use this combination of state of the art techniques to silence defined populations of thalamocortical, reticular thalamic, or corticothalamic neurons during particular states (wake, REM, or SWS), to test the necessity of thalamocortical activity within each state for OSRP consolidation. I hypothesize that generation and coordination of spindles by these neuronal populations during SWS is critical for this process. Together, these studies will reveal state-dependent network mechanisms necessary for consolidating plasticity following visual experience. PUBLIC HEALTH RELEVANCE: Relevance: The proposed studies will provide new insights into how sleep and wake states uniquely contribute to synaptic plasticity in the visual system. Because cognitive processes such as memory formation rely on similar plasticity mechanisms, findings from these experiments may ultimately lead to novel treatments for disorders where both cognition and sleep patterns are adversely affected - such as Alzheimer's disease, schizophrenia, and autism.

DESCRIPTION (provided by applicant): The consolidation of recent experiences into long-lasting memories is a fundamental function of the brain and essential for survival. Sleep has long been thought to be important for memory consolidation, and sleep deprivation has adverse consequences for cognitive function. Recent data suggest that sleep promotes the adaptive changes in connections between neurons which are thought to underlie memory formation. However, progress in understanding how sleep promotes these processes has been slow, due to the number and diversity of changes occurring simultaneously in the sleeping brain - including changes in neuronal firing patterns, intercellular signaling between neurons, and neuronal protein synthesis. The goal of the proposed research is to define which features of sleep are necessary and sufficient for a simple form of sleep-dependent memory consolidation - contextual fear memory. We will use recently-developed optogenetic and pharmacogenetic tools to manipulate neuronal activity in the sleeping brain in a cell type-specific manner. This will allow us to test whether features of network activity (e.g., low-frequency oscillations in hippocampal circuits) and neurotransmission (e.g., changes in noradrenaline and acetylcholine release) unique to the sleeping brain play a causal role in sleep-dependent memory formation. We will also use newly-developed molecular tools to profile active protein translation in specific cell types within the hippocampus during sleep- dependent memory consolidation. Finally, we will combine these tools to determine for the first time how sleep- associated changes in neuronal activity and neurotransmission impact intracellular processes such as protein translation in neural circuits.

While the exact physiological function of sleep remains unknown, there is mounting evidence that it plays an important role in the consolidation of long-term memories. In particular, it appears that sleep promotes the consolidation of declarative memories that require a functionally intact hippocampus, including memories of place. In rodents, place-dependent fear memory is promoted by sleep and disrupted by sleep deprivation. Sleep deprivation also disrupts a number biochemical and neurophysiological processes that are thought to be involved in memory consolidation. These studies have led many to suggest that sleep promotes long-term memory consolidation by modulating neural network dynamics and synaptic plasticity within the hippocampus. In experimental studies, the co-PIs have recently identified changes in hippocampal neural network dynamics during sleep that are induced by place-dependent fear learning. In computational modeling studies, the co-PIs have shown that acetylcholine, a modulatory chemical whose levels vary across sleep states, can change neural network dynamics in a similar way. The proposed projects take a multidisciplinary, multi-scale approach to bridge the gap between experimental and computational results, to identify how effects of acetylcholine on neurons lead to changes in neural network dynamics to promote learning, ultimately leading to learning and memory behavior. While the focus is on fear learning and memory consolidation, the fundamental knowledge of learning-related and sleep-related brain network dynamics gained by the proposed experiments and computations will provide valuable insights into mechanisms for all types of learning.

At present, it is unclear how sleep-related changes in hippocampal network dynamics might promote contextual fear memory consolidation. The team's recent experimental studies have shown that contextual fear conditioning produces long-lasting, sleep-dependent increases in the stability of hippocampal network functional connectivity patterns. These results, coupled with the team's recent computational studies describing a role for acetylcholine in network-wide activity and synaptic plasticity patterning, have led to the hypothesis that sleep promotes memory consolidation, at least in part, by dynamically shifting patterns in hippocampal neural network activity during naturally-occurring rapid eye movement and slow wave sleep brain states. Sleep-dependent acetylcholine has a primary role in driving these shifts in network activity through its effects on cellular excitability properties. The proposed projects use behavioral, physiological, and computational approaches to tackle the missing links that will show the hypothesized network mechanisms occur in brain hippocampal networks and participate in fear learning and memory. Hippocampal acetylcholine levels will be manipulated across wake and sleep states while recording multi-unit activity in hippocampus to quantify changes in spike timing dynamics in the context of fear and subsequent sleep or sleep deprivation. The team has developed a suite of quantitative measures to identify learning-related changes in network dynamics. In addition, acetylcholine-induced changes in cellular membrane properties that affect network dynamics will be measured in hippocampal pyramidal cell and inhibitory interneuron populations. The results will be used to inform details of biophysical neural network models to identify specific dynamical mechanisms by which acetylcholine-mediated changes in network activity dynamics promote network stability, structural changes and synaptic reorganization associated with learning and consolidation.

Project summary: Consolidating transient sensory experiences into long-lasting memories is a fundamental function of the brain, linked to synaptic plasticity. The importance of sleep for promoting this process, and the disruptive effect of sleep deprivation on it, have been appreciated for nearly a century. However, it remains unclear how sleep-associated changes in the activity of specific brain circuits contribute to sensory plasticity. Using a combination of longitudinal recordings of neuronal activity in freely-behaving mice, recently-developed optogenetic strategies, novel computational tools for characterizing network activity patterns, we will test the necessity and sufficiency of sleep-associated patterns of thalamocortical activity in consolidating a simple form of experience dependent plasticity. We will test the hypothesis that coherent firing during network oscillations unique NREM sleep plays a causal role in promoting plasticity between the thalamic lateral geniculate nucleus (LGN) and the primary visual cortex (V1) following presentation of a novel visual stimulus. Here we will selectively manipulate cortical, thalamocortical and corticothalamic neuronal populations in a state specific manner. We will measure both response changes in individual V1 and LGN neurons to the presented stimulus, and behavioral responses to the presented stimulus in the context of a visual discrimination task. We will test whether neurons that are selectively responsive to the visual stimulus play a critical role in guiding network activity patterns during subsequent sleep, acting as an instructive mechanism for circuit plasticity. Finally, we will test whether following visual experience, sleep-dependent communication between V1 and the perirhinal cortex (essential for visual recognition memory) is responsible sleep-dependent discrimination learning.