Matthew I. Banks - US grants

Our long-term objective is to understand the role of cortical GABAA receptor-mediated inhibition in the perception of sensory stimuli, and how modulation of GABAA receptors by general anesthetics and changes in cortical inhibition in various neuropathologies alters sensory perception. In this proposal, we seek to understand how GABAergic cells control the timing of action potentials in auditory cortex (ACx). Firing patterns distributed across populations of pyramidal cells in ACx are postulated to represent certain features of acoustic stimuli. Spikes in these population codes are either time-locked to transitions in the stimulus, or comprise temporal patterns generated internally. Evidence suggests that networks of cortical interneurons are involved in establishing both types of patterns. Thalamocortical (TC) afferents activate multiple populations of 'feedforward'GABAergic interneurons in ACx. Lemniscal TC fibers target cells in layers III and IV (LIII/IV), while extralemniscal TC afferents target cells in layer I (LI). Activation of the latter afferents triggers gamma frequency oscillations in ACx. We propose that feedforward intemeurons in ACx regulate spike timing in pyramidal cells and that this capability is enhanced by network interactions among inhibitory cells. We will test the hypothesis that LI interneurons control internally generated firing patterns, while LIII/IV interneurons coordinate firing patterns that are time-locked to the stimulus. Disruption of cortical timing signals has also been postulated to underlie sensory and cognitive impairment during general anesthesia, and modulation of cortical rhythms in ACx is being pursued as a solution to the important medical application of monitoring depth of anesthesia in patients. GABAergic interneurons are central players in normal, pathological and exogenously modulated cortical processing. Thus, understanding how these cells are activated and organized and the functional implications of this structure is critical to understanding cortical information processing. Although evidence abounds that these cells play a pivotal role in setting the response properties of )rincipal cells, we seek to fill the gap in our knowledge of their organization and how they interact with )rincipal cells.

DESCRIPTION (provided by applicant): Sensory perceptions are shaped by prior experience and expectation, and integration of these top-down and bottom-up information streams enhances our ability to identify stimuli in noisy environments and speeds sensorimotor integration. Deficits in this ability are common in neuropathologies such as autism, schizophrenia and attention deficit hyperactivity disorder. Evidence suggests that feedback circuits in cerebral cortex are critical for this experience-dependent modulation of incoming sensory information, but the neural mechanisms involved are poorly understood. The importance of this process for awareness is suggested by its selective loss upon anesthetic-induced hypnosis and during slow-wave sleep. Here, we propose to investigate the cellular and circuit mechanisms of this integrative process in auditory cortex and its modulation by general anesthetics. Based on the laminar segregation of ascending and descending afferents to a column and of cell types with distinct dendritic arborization, we suggest that integration of ascending and descending inputs will be cell-type specific. The laminar position and temporal sequence of cells activated by ascending and descending inputs, as well as these inputs'synaptic physiology, are critical to understanding columnar integration, but are poorly understood for any cortical area, including auditory cortex. We predict that descending inputs will alter the spatiotemporal activity pattern induced by ascending inputs to the column, and that the dynamics of this process will depend on the synaptic physiology of ascending and descending afferents and the engagement of local inhibitory processes. We will use calcium imaging, electrophysiology, and anatomy in brain slices of primary auditory cortex (A1) to test these hypotheses. Three specific aims will be addressed. We will investigate the integration of ascending and descending inputs in pyramidal cells of layer 2/3 and layer 5, we will characterize the modulation of spatiotemporal activation patterns by descending afferents, and we will investigate the effects of hypnotic agents on ascending and descending inputs to A1. Understanding how cortical circuits integrate information from external and internal sources is fundamental to understanding the neural basis of sensory processing and sensory awareness, and has important and practical clinical implications. Traditional views that have focused on bottom-up processing and convergence only at the highest levels of the cortical hierarchy are challenged by studies showing top-down influences at all levels of the hierarchy and highlighting the importance of primary sensory regions for perceptual phenomena. Understanding cortical mechanisms of anesthetic-induced loss of consciousness will benefit research into the design of hypnotic drugs that have fewer undesirable effects on hemodynamics and other phenomena outside the CNS, and will additionally provide insight into the neural basis of sensory awareness. Perceptions are shaped by prior experience and expectation, and integration of these top-down and bottom-up information streams enhances our ability to identify stimuli in noisy environments and speeds sensorimotor integration. Deficits in this ability are common in neuropathologies such as autism, schizophrenia and attention deficit hyperactivity disorder. Evidence suggests that feedback circuits in cerebral cortex are critical for this experience- dependent modulation of incoming sensory information, but the neural mechanisms involved are poorly understood. The proposed experiments will elucidate how one such cortical feedback circuit affects the processing of incoming auditory information.

DESCRIPTION (provided by applicant): Elucidating the mechanism by which anesthetics cause loss of consciousness (LOC) will benefit patient care and provide insight into the neural basis of consciousness. In this proposal, we will test two competing hypotheses, the thalamic switch hypothesis (TSH) and the information integration theory of consciousness (IITC). In the former, disruption of thalamo-cortical information transfer is thought critical for LOC. The latter proposes that anesthetics act across wide areas of cortex to reduce the repertoire of network states (information) and connectivity (integration). We postulate that propofol, isoflurane and dexmedetomidine, acting at diverse molecular loci, share a common cortical mechanism for producing LOC: degradation of stimulus representation and suppression of cortico-cortical connectivity at just-hypnotic doses (i.e. those just causing LOC), which prevent incorporation of sensory information into cortical hierarchical processing. We will test these competing hypotheses by recording unit activity and local field potentials (LFPs) in rats chronically implanted with multisite electrodes in auditory thalamus and auditory and visual cortex. A practical benefit to public health will be assays of consciousness based on population codes and cortical connectivity derived from cortical surface recordings, which are readily obtained in clinical settings. The absence of sensory awareness is a manifestation of LOC that reflects degraded information transfer between the periphery and high order cortex, but where and how this breakdown occurs is unclear. In the first Aim, we will focus on how much information responses of cells in auditory cortex carry about sensory stimuli, both at the single cell level an at the population level, and how this information changes upon loss and recovery of consciousness (LOC/ROC). By recording auditory responses in two thalamic areas, MGv and MGd, and their respective hierarchically connected cortical targets, A1 and PAF, we can determine whether anesthetics block information transfer from thalamus to cortex, as predicted by the TSH, or whether even in the face of maintained thalamic input cortical responses become impoverished upon LOC due to observed changes in local network activity caused by anesthetics, consistent with the IITC. In the second and third Aims, we will investigate connectivity along the ascending and descending thalamo-cortical pathway. Here we will record synaptic and spiking activity in entire cortical columns in response to microstimulation and auditory and visual sensory stimuli to determine if connectivity changes upon LOC/ROC at thalamo-cortical synapses, as predicted by the TSH, or at cortico-cortical synapses, consistent with the IITC. We will use the information from these experiments to aid in seeking electrophysiological correlates of the state transitions manifested in LOC/ROC, and we will derive clinically accessible measures of sensory awareness based on population coding and cortical connectivity using state of the art analysis and modeling techniques.

? DESCRIPTION (provided by applicant): We have proposed that general anesthetics, irrespective of their precise mechanism of action, induce loss of consciousness when they bring about the breakdown of information integration within the corticothalamic system. Here, we will test this proposal using animal models in which we can investigate the role of different neuronal populations in anesthetic unconsciousness. Specifically, it is unclear whether anesthetic loss/recovery of consciousness (LOC/ROC) relies on a thalamic switch or a direct action on cortical cells. Within cortex, it is not known whether anesthetic LOC/ROC is a global phenomenon or is due to specific neural populations and fiber pathways. We hypothesize that pyramidal cells in supragranular (SG) layers, which form a highly integrated network both within and across cortical areas, are ideally poised to support information integration and thereby consciousness. The roles of different thalamic populations in modulating cortical interactions are also unknown. We hypothesize that thalamic matrix cells, with their widespread cortical projections focused especially in SG layers, enable cortical information integration. In addition t actions on specific cell types, we propose that anesthetics target specific synaptic pathways, suppressing cortico-cortical (CC) and matrix thalamo-cortical (TC) synaptic connections, while leaving core TC connections intact. To test these hypotheses, we will take advantage of recent developments in laminar multiunit recordings in freely- moving rodents to examine the neural correlates of LOC/ROC, and recordings of network activity in brain slices to investigate the cellular and circuit mechanisms of these correlates. Moreover, we will use optogenetic and pharmacogenetic methods to transiently activate/inactivate specific cortical layers and thalamic populations and explore their causal involvement in LOC/ROC.

PROJECT SUMMARY The long-term objective of this proposal is to understand the mechanisms responsible for loss of consciousness (LOC) under general anesthesia. Previous invasive studies in animal models have identified candidate mechanisms, but translating those findings to human consciousness remains approximate. More precise determinations of states of consciousness are feasible in human subjects, but non-invasive measures of brain activity (fMRI, EEG, MEG) can only indirectly assess the underlying neural circuitry. This proposal will overcome these limitations by taking advantage of the unique opportunity to directly record from the human brain. Using electrocorticography in neurosurgical patients, we will investigate neural networks modulated by general anesthesia. Our overarching goal is to identify patterns of activity and connectivity in cortical networks that track changes in contents of consciousness (i.e. awareness) under anesthesia and during sleep. Our approach is to identify changes in the networks underlying auditory predictive coding that occur upon LOC. Predictive coding minimizes the differences between internally generated constructs and empirical data, subserved by ongoing interaction between sensory and higher-order cortical regions. This model is ideal for this project because it engages the crucial interplay between predictions of the world and sensory observations of the world, a fundamental function of consciousness. The scientific premise of this project is that at a systems level, disruption of predictive coding subserved by large-scale cortical networks represents a signature of anesthetic-induced unconsciousness. To accomplish our goals, we will pursue three specific aims. The first aim seeks to refine our understanding of the cortical networks involved in auditory predictive coding in awake behaving subjects. This aim will focus on identifying the connectivity of the networks subserving predictive coding over short and long time scales, as effects of LOC on these networks are believed to be distinct. The second aim examines changes in network structure upon anesthesia LOC. This will be achieved by recording brain activity from subjects during induction of general anesthesia. The generality of the findings will be tested using two different anesthetic agents. Aim 3 seeks to identify common electrophysiological signatures for LOC under anesthesia and during sleep. This will be achieved by measuring brain activity in the same subjects during natural sleep. The results will have broad clinical applicability to defining and interpreting prognostic signs in patients with altered mental status (e.g. chronic vegetative and minimally conscious states), mental illness (e.g. delirium and schizophrenia) and development of novel algorithms for use in monitoring depth of anesthesia.