George Pollak - US grants

The proposed studies will evaluate the neural encoding and representation of binaural and spatial information within the "60 kHz" isofrequency lamina of the mustache bat's inferior colliculus (IC). In recent studies we have found that the central nucleus of the bat's IC has 3 functional divisions distingusished on both physiological and architectural criteria. The largest is the dorsoposterior division (DPD) in which all neurons have virtually the same best frequency. The DPD is a greatly enlarged, but fundamentally standard mammalian isofrequency contour, or lamina. Since the mammalian IC is constructed from serially stacked laminae, the DPD is a basic unit of neuronal architecture present in highly magnified form. I intend to exploit this adaptation in order to evaluate: 1) How binaural information is encoded by DPD neurons. Particular attention will be given to determining whether neurons can encode interaural phase or time disparities. 2) How neurons having common binaural properties are organized within the DPD. 3) What influence the pinnae have upon the directional features of 60 kHz sounds reaching the cochlea. 4) How neural processes interact with pinna effects to shape the spatial selectivity of DPD neurons. 5) What correlations exist between DPD neurons having common binaural characteristics and the specificity of projection systems originating from lower auditory centers in the brainstem.

DESCRIPTION: (Adapted From The Applicant's Abstract.) The chief goal of these studies is to evaluate how the largely GABAergic inhibitory outputs of the dorsal nuclei of the lateral lemniscus (DNLLs) shape the monaural and binaural properties of EI neurons in the DNLL and inferior colliculus (ICc). The system to be used for these studies is the enlarged 60 kHz isofrequency contour of the mustache bat's DNLL and ICc. Pharmacological studies conducted during the previous grant period prompted a number of hypotheses about the roles that the ipsi- and contralateral DNLLs play in shaping the binaural properties and azimuthal receptive fields of ICc neurons. The goals of the proposed studies are to test those hypotheses with reversible inactivation of either the ipsi- or contralateral DNLLs while monitoring the response properties of EI cells in the opposite DNLL or ICc. Four sets of experiments are proposed to test these hypotheses: 1) The role of the commissure of Probst will be evaluated by recording the monaural and binaural properties of 60 kHz DNLL neurons and then assessing the same properties after the DNLL on the opposite side has been reversibly inactivated by the iontophoretic application of muscimol, an agonist for GABAa receptors. After recovery from inactivation, bicuculline will be iontophoretically applied to the neuron under study and the same properties will be determined again. 2) The same procedures will be used to evaluate the roles of the contra- and ipsilateral DNLLs on the monaural and binaural response properties of 60 kHz EI neurons in the ICc. The influence of the contra- and ipsilateral DNLLs will be assessed in separate experiments. 3) The effects of an initial sound on the binaural properties evoked by a second sound that follows shortly thereafter will be evaluated in ICc neurons. Initially the ICc cell's normal binaural properties with a single sound presented over a wide range of IIDs will be established. Subsequently, two successive binaural tone bursts will be presented. The IID of the first stimulus will be fixed and will favor the inhibitory ear while the second stimulus will be presented with a wide range of IIDs that would be produced when sounds emanate from different regions of space. The procedure will then be repeated but the first stimulus will have a fixed IID that favors the excitatory ear. The hypothesis predicts that the binaural properties evoked by the second signals: a) will be changed, and thus the response evoked by that sound will be different from those evoked by a single sound; b) will be different when the initial signal favors the ipsi ear from the properties obtained when the initial signal favors the contra ear; and c) will be due, in part, to the inhibition of either the ipsi- or contralateral DNLL that was evoked by the first signal. 4) To test the hypothesis that inhibition at the DNLL evoked by the first signal is responsible for the changes in binaural properties at the ICc evoked by a second signal, the same properties in ICc neurons will be assessed when either the ipsi- or contralateral DNLLs are inactivated by the iontophoretic application of muscimol.

DESCRIPTION (Investigator's Abstract): The chief goal of these studies is to evaluate how inputs that originate from multiple lower auditory nuclei shape the response properties of collicular neurons in the mammalian auditory system. Specifically the investigators will: 1) Determine which of the cells that project to the E1 region of the colliculus, and to the 60 kHz regions of the DNLL and INLL, are GABAergic and which are glycinergic. The DNLL and INLL are the two nuclei of the lateral lemniscus that project to the 60 kHz region of the colliculus. This will be accomplished by first, in an individual bat, identifying the appropriate region in the nucleus of interest with electrophysiological recordings, and then making a focal injection of HRP. Subsequent processing will co-localize the retrogradely transported HRP with antibodies directed against glycine, GABA and glutamate decarboxylase (GAD), the synthetic enzyme for GABA. 2) Determine the monaural and binaural response properties of INLL and DNLL neurons by recording single unit activity while presenting 60 kHz tone bursts with speakers inserted into the ears. 3) Determine the functional consequences of the inhibitory inputs to the colliculus by constructing the spatial receptive fields of 60 kHz E1 neurons from their monaural and binaural response properties, which will then be compared to the changes in receptive field properties induced by iontophoretic application of bicuculling and/or strychnine. Thus, they will assess the differential roles of GABAergic and glycinergic innervation in shaping the response properties and spatial receptive fields of collicular E1 neurons, and identify the source or sources, of the GABAergic (or glycinergic) influence. Finally, by evaluating the response properties of 60 kHz neurons in DNLL and INLL and reconstructing their receptive fields, they will obtain further insights into the transformations that occur along the neuraxis, and how lower nuclei may be exerting their inhibitory influence on their targets in the 60 kHz contour of the inferior colliculus.

DESCRIPTION (provided by applicant): The inferior colliculus (IC) is a nexus in the auditory system since it processes and integrates almost all ascending acoustic information from lower centers and thereby determines what form of information is conveyed to higher regions in the forebrain. Consistent with the large convergence of projections, the response properties of IC neurons are highly diverse. The diversity is due to the particular complement of inputs that innervate each neuron and to their intrinsic ion channels. Inhibition is particularly important, because inhibition sculpts a wide range of emergent response properties from the backdrop of more expansive and less specific excitatory innervation. Because inhibition cannot be directly seen with extracellular recordings, I will use in vivo whole cell recordings from IC neurons to obtain a more direct and detailed view of sound evoked inhibition, and generate a unique and more comprehensive picture of the response properties of IC neurons, the mechanisms with which they process acoustic information and their morphologic features. The response features include their intrinsic properties, revealed by responses to injected current steps, the tuning of their tone evoked synaptic responses and discharges with particular attention directed at sound evoked inhibition, their responses to natural communication calls, and their directional preferences for upward or downward FM sweeps. What makes these experiments unique is that this constellation of response features has never been recorded in individual IC cells, and they will be used to evaluate how intrinsic channels interact with ligand-gated events to shape the response properties in IC cells. The proposed studies focus on the roles of inhibition, and inhibition in the IC is important for a number of pathological conditions and hearing impairments. The roles of inhibition may be especially relevant for the decline of speech perception in the elderly, which is correlated with age related loss of GABAergic neurons in the auditory system. Since the proposed studies will evaluate the role of inhibition in the processing of communication calls, they may provide insights into the processing of speech, and hence into why the elderly often experience a decline in speech perception.

DESCRIPTION (provided by applicant): Funds are requested to help support the Ninth International Congress of Neuroethology (ICN) to held on August 2-7, 2010 in Salamanca, Spain. The ICN, which is organized by the International Society for Neuroethology, has been held every three years since 1986 and attracts hundreds or researchers from around the world. The focus of these meetings is to bring together diverse neuroscientists who are investigating the neural basis of behavior across a broad spectrum of both vertebrates and invertebrates. This meeting provides an outstanding venue for discussions among international scientists who have a broad range of perspectives, but who focus on common basic-science questions that have important implications for neural function and disease. The program for 2010 includes 17 invited symposia, four Participant Symposia in which the speakers will be selected from submitted abstracts where special emphasis will be given to selecting graduate students and postdoctoral fellows as presenters. There also will be 8 plenary sessions on topics such as bird song learning, neural networks for olfactory behavior, acoustic communication, molecular evolution of vision, behaviors guided by pheromones, among others. While a wide variety of areas in neurobiology will be addressed in the Congress, many of the topics are relevant to the mission of the NIDCD. Specifically, 28 talks in the 17 symposia focus on chemosensation, hearing, audio- visual interactions, or comparative aspects of language acquisition or comprehension and its neural underpinnings. The list will almost certainly grow when the speakers in the Participant Symposia are chosen. Support is requested to fund travel and registration for winners of the Young Investigator Awards, for student and minority neuroscientists, and for the dissemination of the proceedings of the conference. The benefits to American scientists in general, and to young investigators in particular, will be the opportunity for exposure to cutting-edge research and techniques from around the world in a format and venue that encourage interactions between students and investigators at all levels of experience. PUBLIC HEALTH RELEVANCE: The conference will address major themes in the neural mechanisms that underlie normal behavior with an emphasis on understanding the pathways in the brain that perceive sensory inputs, make major decisions and encode the appropriate movements. Many neural diseases result from defects in these processing steps. To understand the functional basis of abnormal neural function and disease states, multiple perspectives drawn from a variety of organisms are often critical. The comparative viewpoints and emerging technologies that are emphasized at the ICN therefore have the potential to contribute to the diagnosis of nervous system diseases and the development of treatments.

DESCRIPTION (provided by applicant): The studies in this proposal will use in vivo whole cell recordings to determine the various circuits that form EI cells in the inferior colliculus (IC), an the functional impacts of those circuits. We showed in previous extracellular studies that EI cells comprise a diverse group due to the various circuits that generate the same EI property among the IC population [4,10-12,18]. We recently used whole cell recordings to confirm that the various monaural and binaural inputs proposed in our previous extracellular studies could be observed in the sound evoked post-synaptic potentials (PSPs) [25]. In most EI cells, the patterns of sound evoked PSPs showed projections that were unexpected, since extracellular recordings gave no hint of their presence. In a pilot study we also evaluated the circuitry in a new way, by computing the excitatory and inhibitory conductances that underlie monaural and binaural responses. That circuitry was even more surprising since additional inputs were revealed that were not apparent even from the sound evoked PSPs. One of the unexpected inputs is from the dorsal nucleus of the lateral lemniscus (DNLL). The DNLL is special because its cells express a unique pattern of ipsilateral evoked inhibition and it provides a strong inhibitory innervation to the IC, which can have profound influences on IC responses to dynamic IIDs [12, 25]. The circuitry revealed by both PSPs and conductances suggests that most EI cells should respond selectively to dynamic binaural signals with IIDs that change over time. It was these surprising results, especially the unexpected circuitry revealed by conductances and the exciting response features they suggest, that prompt the studies in this proposal. The studies will identify the inputs that evoke both monaural and binaural responses in each type of EI cell by recording spikes, postsynaptic potentials (PSPs) and by computing the excitatory and inhibitory conductances that generate each response. The circuits derived from the conductances show how and why each EI type responds to both IIDs presented one at a time (static IIDs) and suggest how each EI type should respond to dynamic IIDs that change over time, such as moving sound sources or multiple sounds that emanate from different regions of space. The cells will then be tested with those dynamic binaural stimuli and the responses will be compared to the responses predicted from the circuitry derived from conductances. In addition, simple tests will be administered that provide insights into whether or not each cell receives inputs from one or both DNLLs. This information will provide insights into how the auditory system handles binaural information in complex acoustic environments, including how it deals with multiple sound sources in space. Such information could be translated into algorithms that might prove useful in robotics and/or in the construction of hearing aids for the hearing impaired. PUBLIC HEALTH RELEVANCE: The proposed studies focus on how the auditory system handles binaural information in complex acoustic environments, including how it deals with multiple sound sources in space. Such information could be translated into algorithms that might prove useful in robotics and/or in the construction of hearing aids for the hearing impaired.