Ellen Covey - US grants

The cell groups of the ventral lateral lemniscus (VLL) transmit information from one ear to the contralateral inferior colliculus (IC). Although they are a major component of the central auditory system in all mammals, their extreme hypertrophy and elegance of organization in echolocating bats provides a unique opportunity to discover the function of each component cell group in a highly differentiated system. The VLL was long regarded as a single anatomical and functional region, but recent evidence shows that it contains at least three readily distinguishable cell groups. These are part of a larger system of parallel pathways which converge at the IC. A likely hypothesis is that each individual pathway is specialized for the analysis of some specific feature of sound, with the outputs being integrated at the IC. The long-term goal of the proposed research is to determine which features of sound are being analyzed and to elucidate the anatomical mechanisms for sound analysis in each of the nuclei of the VLL. The specific studies proposed are: l) To identify and characterize the subdivisions of the VLL on the basis of their cytoarchitecture and immunohistochemical reactivity. This step is preliminary to the remaining experiments. 2) To determine the tonotopy in each division. The presence of a complete tonotopic representation within a cytoarchitectural division would confirm that it is, indeed, an independent nucleus. 3) To map the pattern of afferent inputs, including terminal types, in each nucleus, and to identify the location and types of cells that project there. If it can be established that each VLL nucleus receives projections from a different subset of cells having known response properties, this will represent the first step in determining their input/output functions, and in understanding the ways in which auditory information is transformed at this level for relay to the inferior colliculus. 4) To relate the anatomical findings with the physiological response properties of cells in each division of the VLL. The proposed studies should provide the initial clues for establishing the function of each of the component nuclei of the VLL. In addition, they should reveal what type of information is being transmitted to the midbrain for integration with the outputs of other more thoroughly studied brainstem auditory centers, such as the superior olivary complex. The results may, in addition, provide a basis for understanding the early steps in analyzing sequences of sounds such as speech.

The goal of this project is to develop, in the P.I.'s laboratory, techniques for whole cell patch clamp recording from neurons in the inferior colliculus (IC) of awake echolocating bats. Pilot data show that such studies are feasible, and suggest that the use of patch clamp recording to measure sound-evoked EPSCs and IPSCs in vivo will yield a wealth of new information on signal processing mechanisms in the auditory midbrain. Initially this technique will be used to investigate the hypothesis that IC cells have long periods of subthreshold facilitation or inhibition following sound- evoked discharge. Patch clamp recording is the best method available for studying subthreshold synaptic processes. Neurons remain healthy longer and the signal-to-noise ration is better than when they are impaled with intracellular microelectrodes. These factors are important when recording from small neurons such as those in the IC, and when measuring stimulus-evoked responses where lengthy testing of different variables is necessary. The in vivo patch clamp technique is an essential addition to the anatomical, electrophysiological and neuropharmacological methods currently employed by the P.I. because it will, for the first time, make natural subthreshold synaptic events amenable to investigation.

Processing by the auditory pathway in the brain passes through several stages. In mammals, information from the auditory nerve from the ear is transmitted to clusters of nerve cells (neurons) in the brainstem; these clusters are called nuclei. These cells in turn pass their output to the major auditory center of the midbrain, called the superior colliculus. Complex sound patterns present a range of important features for the brain to analyze, including timing properties that can include duration, and rate of modulation of either amplitude or frequency. Within the brain, networks of nerve cells produce excitation and inhibition in particular ways that produce effective "filters" to enhance or to eliminate signals with particular timing patterns. This work uses a novel approach of reversible, specific pharmacological blockade of individual nuclei. Such targeted blockade of the brainstem inputs, while physiologically recording neuronal activity in the superior colliculus, allows a novel way to study how "tuning" for particular dynamic auditory properties can arise by combining excitatory and inhibitory inputs. Results of this analysis of auditory circuitry will provide an advance in understanding auditory processing, but also will have an impact beyond auditory neuroscience to studies of brain circuits in general, and the way in which complex information processing can emerge in complex networks.

DESCRIPTION: (Adapted from the Investigator's Abstract) There is evidence that lower brainstem auditory pathways perform important initial steps in analyzing the temporal pattern of sound. The long-term goal of the proposed research is to understand the unique contribution of each lower brainstem auditory pathway to the process of analysis, transformation and integration that results in temporal pattern perception. The specific aims of the proposed project focus on the broad question of how intrinsic properties of auditory neurons interact with synaptic inputs to shape sound-evoked responses. The specific aims are 1) To elucidate the roles of ascending input pathways in shaping sound-evoked responses of neurons in the nuclei of the lateral lemniscus (NLL); 2) To characterize the distribution and functional roles of voltage-gated ion channel in the NLL; 3) To correlate sound -evoked response patterns with neurons' morphology and ion channel distributions; and 4) To characterize the contribution of the NLL to shaping sound-evoked responses of the inferior colliculus (IC) neurons. Responses of NLL and IC neurons to sound will be recorded extracellularly and intracellularly. Stimuli will be pure tones of sound with simple temporal patterns. To examine the role of lower brainstem nuclei in shaping the responses of NLL or IC neurons, activity of inputs will be selectively and reversibly blocked. In addition, blockers of specific neurontransmitters or voltage-gated ion channels will be applied locallly to the neuron from which we are recording. It is likely that the results of these experiments will be widely applicable to mammalian hearing. All animals that hear must analyze the temporal patterns of sound; for humans, temporal patterns are the basis of both music and speech perception. Understanding how temporal patterns are analyzed in the auditory system could ultimately have implications for diagnosis and treatment of hearing and language disorders, ad could provide insight into general mechanisms that operate in all neural systems with patterns of time-varying information.

DESCRIPTION (provided by applicant): The University of Washington (UW) has a long-standing history of commitment to research in the areas of hearing, speech and language studies, and communication disorders. In addition, it has an outstanding interdisciplinary community of investigators in all of the subdisciplines of basic neuroscience. At the intersection of these communities is a diverse and highly productive group of investigators who study the fundamental neural mechanisms that underlie hearing and communication. One important mission of the auditory neuroscience community at UW is to mentor the trainees who will carry on this line of research and advance our knowledge of the field in the future. The Auditory Neuroscience Training Program, established in 2002, helps train the basic neuroscience researchers whose work will form the foundation for research in the clinical disciplines. It, therefore, complements the UW clinical training programs in otolaryngology and speech and hearing sciences. The training experience at UW currently includes six predoctoral training slots, since this is the area in which strong support during the early stages of training is most crucial. This application requests the addition of three postdoctoral training slots which would support trainees transitioning to auditory neuroscience from other disciplines. Trainees participate in active research programs in neuroanatomy, development, genetics, cell and molecular biology, neuropharmacology, and electrophysiology of the peripheral and central auditory system, as well as psychoacoustics, language perception and processing, and communication behavior. Trainees also have the opportunity to combine research in more than one area through collaborative efforts. Program trainees are exposed to a wide range of research techniques, enabling them to conduct technologically and conceptually sophisticated programs of research. Importantly, continued support through the training program should greatly enhance the ability of the UW to attract and retain high-caliber minority trainees in auditory neuroscience.

DESCRIPTION (provided by applicant): The general goal of the proposed research is to understand how the function of the mammalian auditory midbrain, inferior colliculus (IC), relates to behavior. The IC integrates information from multiple auditory centers in the lower brainstem, from non-ascending auditory inputs from the opposite IC and auditory cortex and from motor-related structures. This integration is a first step in selecting for behaviorally relevant sounds. The non-ascending auditory inputs may modulate the sound-evoked responses of IC neurons, or even create specialized response properties. Although the inputs from all sources overlap to some degree, there are differences in their gradients of terminal density, which probably create functional gradients. We propose three specific aims, each designed to test a broad hypothesis related to the integration of the various auditory and motor inputs to the IC. Hypothesis 1: Response properties of IC neurons are determined by the ratios of input from different sources along a dorsoventral gradient. Specific Aim 1: We will use extracellular and intracellular recording of sound-evoked responses to examine the relation between selectivity for behaviorally relevant sound parameters, general response properties, gradients in input patterns, and immunocytochemical markers. Hypothesis 2: Sound-evoked responses of at least some IC neurons are modulated or created by input from non-ascending pathways. Specific Aim 2: To evaluate the contribution of these inputs to auditory processing, we will record sound-evoked responses of IC neurons before, during, and after reversible inactivation or electrical stimulation of the opposite IC or auditory cortex. Hypothesis 3: Sensory processing in the IC is modulated by motor-related inputs. Specific Aim 3: We will trace the projections of substantia nigra and globus pallidus to the IC, determine whether the inputs are excitatory or inhibitory, and record sound-evoked responses of IC neurons before, during and after reversible inactivation of substantia nigra. It is likely that the results of these experiments will be widely applicable to mammalian hearing. All mammals appear to have similar input gradients in the IC as well as inputs from motor pathways. Understanding how information from different sources is integrated in the IC could ultimately have implications for diagnosis and treatment of central hearing and language disorders.

All animals that hear must be able to ignore common sounds that are repetitive and uninteresting and be alerted by novel sounds that may require attention. Based on noninvasive recordings of summed electrical brain activity in humans, it has long been thought that detection of novel sounds is a higher cognitive function performed by the cerebral cortex. However, work in the PI's laboratory has shown that a subcortical auditory center located in the midbrain (the inferior colliculus) contains neurons that are unresponsive to a repeated sound and respond exclusively to novel sounds. These neurons are termed ?novelty detectors?. The process by which they become unresponsive to a commonly occurring sound is termed ?stimulus-specific adaptation?. The mechanism(s) underlying stimulus-specific adaptation are not known, so one goal of the proposed research is to discover the underlying processes that allow neurons to ignore an expected stimulus but respond to an unexpected one. Although novelty detector neurons constitute a specialized population, new evidence from the PI's lab suggests that all neurons in the inferior colliculus exhibit some degree of stimulus-specific adaptation and enhancement of responses to novel stimuli. The proposed work will investigate the hypothesis that novelty detection is built up in stages through connections among neurons that exhibit different degrees of stimulus-specific adaptation, and that neural inhibition may somehow be involved in suppressing responses to an expected sound. The proposed experiments will use a paradigm in which a ?standard? stimulus is presented most of the time and an ?oddball? stimulus is presented on rare occasions. The specific aims are to 1) determine how ubiquitous stimulus-specific adaptation and enhanced novelty responses are in the inferior colliculus; 2) to characterize the conditions under which individual neurons show stimulus-specific adaptation and novelty responses, specifically examining the effect of stimulus repetition rate, probability of the oddball stimulus occurring, and the amount of contrast (i.e., difference between the oddball and standard stimuli along a specific dimension such as pitch); 3) to determine whether there is a map or gradient in the tendency of inferior colliculus neurons to show stimulus-specific adaptation and novelty responses and 4) to determine whether local blocking of the action of inhibitory neurotransmitters on the neuron being studied will reduce stimulus-specific adaptation and allow the neuron to respond to all stimuli regardless of how common or uncommon they are. These experiments will be conducted in the big brown bat, a mammal whose auditory system is highly specialized for processing sound patterns used in echolocation, and the mouse, a mammal with a generalized auditory system. This research will provide new insights into the broad issues of how mammals focus attention on novel sounds, and how this ability has evolved to serve different behaviors in different species. The findings of these studies should also provide convincing evidence that at least some processes that are considered higher cognitive functions occur at a level prior to the cerebral cortex. It is likely that the results of these experiments will be generalizable to auditory processing and attention mechanisms in humans as well as in the specific animals studied, and to novelty detection in sensory systems other than hearing. The research will have a broader impact in providing excellent training opportunities for graduate, undergraduate, and high school students.