Donald M. Caspary, Ph.D. - US grants

It is important to understand the neurochemical aspects of sensory processing since it is conceivable that a number of neurologic abnormalities such as receptive aphasia, neural prebycusis, auditory perceptual dysfunction and sensory-induced seizures involve neurochemical fault(s). This study will continue the attempt to identify and characterize the neurotransmitters which mediate synaptic transmission at specific sites in the central auditory pathway. Microiontophoresis will be used to apply putative neurotransmitters and their respective antagonists in tests of mimicry (identity of action) and antagonism (pharmacologic identity) of the synaptically released compound. Alterations of threshold, spontaneous activity, response patterns, and rate-intensity functions will be examined for neurons in brainstem auditory structures. Putative neurotransmitters to be tested are selected based on morphologic, physiologic, neurochemical, and histochemical information. Binaural neurons in the superior olivary complex (SOC) provide an opportunity to selectively control synaptic excitation and inhibition to a given neuron by manipulation of the acoustic input to each ear. Our recent findings suggest that glycine may mediate binaural inhibition in certain SOC neurons. The action of a second inhibitory neurotransmitter, GABA, and the role of excitant amino aicds at SOC synapses will be examined using selected amino acid receptor antagonists. Baclofen, a proposed inhibitor of excitant amino aicd release, reduces the response of neurons in the cochlear nucleus (CN). The effect of this drug and recently available specific antagonists of excitant amino acids will be examined in the SOC and will be used to further test the hypothesis that an excitant amino acid is the transmitter at acoustic nerve synapses. Intracellular recordings will be used as a more rigorous test of identity of action in the CN. The combination of neurophysiologic and neuropharmacologic techniques proposed should aid in the identification of auditory neurotransmitters and an elucidation of their role in the coding process.

DESCRIPTION (provided by applicant): Age-related hearing loss is a complex disorder affecting 30% of the US population aged 65 to 74 years, and 50% of the population over 75 years of age and older. Psychoacoustic studies in young and elderly subjects with similar thresholds show age-related decrements in temporal processing and speech understanding in noise. This suggests that seniors have difficulty attending to and comprehending speech in a public setting leading to a tendency to withdraw from society. The proposed studies are based on the underlying hypothesis which suggests that age-related hearing loss is, in part, a maladaptive plastic response to a slow progressive deafferentation at the auditory periphery. In many species, sound exposure, chemical or physical peripheral trauma alters physiologic responses and markers of adult inhibitory neurotransmission at multiple levels of the auditory pathway. Subcortical temporal coding studies suggest that inhibitory circuits: Allow neural responses to accurately follow the envelope/temporal fine structure of complex acoustic signals; Are involved in gain control; Are likely to provide the adaptive substrate for novelty detection (Yu et al., 2009). Studies completed during the previous grant period have shown that inhibitory neurotransmitters are critically involved in preserving temporal and spectral fidelity of coded complex acoustic signals as these signals ascend the auditory pathway. These studies found significant age-related changes in GABA- and glycinergic inhibitory pharmacology and physiology in the dorsal cochlear nucleus and primary auditory cortex. Proposal studies will examine the nature of GABAA receptors (GABAARs) at the level of the medial geniculate body (MGB) in the context of aging. The auditory thalamic nucleus receives lemniscal and extralemniscal ascending inputs as well as reticular, limbic and descending inputs from auditory and nonauditory cortices. Proposed studies seek to characterize the unique makeup of GABAAR in MGB and their functional involvement in attention/novelty detection and temporal processing. Proposed studies will examine age-related changes in the role of the inhibitory neurotransmitter GABA in the MGB. Preliminary iontophoretic, receptor binding and measures of GABAA subunit protein suggest an important role for one specific GABAA receptor subtype which is highly concentrated in young adult MGB. These extrasynaptic 14d subunit containing GABAA receptors show a profound reduction in aged animals. Specific studies will: 1) Determine age-related changes in the subunit makeup and pharmacology of GABAA receptor constructs in the MGB. 2) Determine the role of GABAA receptors in shaping responses to novel and temporally modulated stimuli in young and aged rat MGB neurons using iontophoretic techniques. 3) Determine age-related changes in response to temporally complex and novel stimuli in MGB neurons in unanesthetized rats. Collectively, these studies will begin to characterize the impact of aging on the function of inhibitory GABA circuits in the MGB and may provide a basis for development of selective agents which could potentially ameliorate certain kinds of age-related hearing loss. PUBLIC HEALTH RELEVANCE: Age-related hearing loss is arguably the second or third major malady of industrialized people, affecting 30% of the US population aged 65 to 74 and 50% of the population over 75 years of age and older. Older individuals have more difficulty understanding speech, especially in noise than younger adults. These threshold changes are at least in part, independent of inner ear hair cell loss. Previous studies supported by this grant strongly suggest that, the loss of certain inhibitory neurochemicals in the brain may be responsible for the observed age-related impairments in speech understanding. Preliminary studies suggest that one important auditory brain area, medial geniculate body, displays a profound age-related loss of one kind of inhibitory receptor. Proposed neurochemical studies will attempt to characterize the pharmacologic properties of this inhibitory receptor in young and aged animals. Electrophysiology will attempt to determine the impact of aging on the ability of brain cells in this area to process acoustic information in an unanesthetized rat model of aging. It is hoped that these studies could eventually result in the development of selective new drugs to improve speech understanding in a subset of elderly individuals.

DESCRIPTION: Studies completed in this laboratory during the past several years have contributed to our understanding of two closely related questions in auditory research: 1) How do inhibitory amino acids function within known brainstem auditory circuits to encode acoustic information? 2) What is the impact of aging on inhibitory amino acid neurotransmission in the auditory brainstem? Studies of this nature have made significant inroads into the nature of presbycusis, a major disorder among the aged and may lead to pharmacological treatment strategies for age-related hearing loss. In vivo iontophoretic and in vitro studies in cochlear nucleus (CN) and inferior colliculus (IC) have revealed that inhibitory amino acid neurotransmitters are critically involved in the control of dynamic range, echo suppression and detection of signal in noise. Recent studies from this laboratory suggest that circuits within the CN and IC undergo age-related changes associated with inhibitory neurotransmission. The current proposal will extend this strong foundation of work by carrying out the following proposed studies: 1) microiontophoresis to examine the role of the inhibitory amino acid GABA in shaping responses to amplitude modulated sinusoids in the IC; 2) neurochemical techniques to further characterize age-related changes in the molecular nature of the GABAA receptor; and 3) single unit physiology to examine age-related changes in the response properties of DCN fusiform cells.

? DESCRIPTION (provided by applicant): Age-related hearing loss is a complex disorder affecting at least 30-50% of the United States population aged 65 or older. In public settings, seniors frequently have difficulty understanding speech, which can lead to withdrawal from social activities and depression. Elderly individuals also show deficits in their ability to attendto speech in complex auditory environments. Aging studies suggest that the elderly can maintain speech understanding as they age despite degraded ascending temporal information by using attentional and cognitive resources. Studies completed during the previous grant period detailed the makeup and function of receptors mediating inhibition in young and aged thalamocortical circuits likely to underpin coding of acoustic information. Using in vitro slice recordings and in vivo recordings from awake rat medial geniculate body (MGB), these studies found significant age-related changes in the makeup, function, and pharmacology of GABAA receptors in MGB and in auditory cortex. In young adults, difficult-to-understand speech and novel stimuli result in downstream signals sent from cortical areas, including auditory cortex and hippocampus, to activate brainstem cholinergic arousal/attentional circuits. In turn, brainstem cholinergic neurons project to auditory structures including MGB. In sensory thalamus, the neurotransmitter acetylcholine plays a critical role in waking attention and in establishing the salience of important stimuli. High levels of nicotinic cholinergic receptors (nAChRs) are found in MGB, yet much remains to be learned about their subunit composition, location within MGB circuits and how aging impacts these systems. Humans and rats show a 30+% age-related loss of one key nAChR subunit in whole thalamus. Our preliminary MGB assays support and extend these findings. These data suggest, for the first time, the presence of significant age-related changes in the subunit makeup of nAChRs resulting in an age-related loss in receptor affinity. Preliminary patch-clamp slice data show reduced pre- and postsynaptic nAChR responses supporting this age-related loss in affinity. In vitro and in vivo approaches will: SA1. Characterize heteromeric nAChRs in the major subdivisions and cell types of young-adult MGB via: 1A) pharmacology, subunit composition and location, and 1B) physiology and cellular location. SA2. Characterize age-related changes that occur in nAChRs for the major subdivisions and cell types in MGB via: 2A) pharmacology, subunit composition and location, and 2B) physiology and cellular location. SA3. 3A) Determine age-related changes that occur in processing novel, temporally rich stimuli recorded from MGB units in awake animals. 3B) Iontophoretic studies will examine the impact of nAChR agonist and antagonists on these temporal responses properties. Understanding pharmacologic and functional nAChR changes associated with aging would inform development of selective pharmacotherapy that could ameliorate age-related loss of speech understanding.

DESCRIPTION (provided by applicant): Chronic tinnitus is a phantom auditory sensation experienced by up to 15% of the population. Between 2 and 10% of the tinnitus population typically experience a continuous high frequency ringing or hiss sufficiently loud to impact severely on their quality of life (Cooper, 1994). Tinnitus has been found to increase with aging. Thus, the incidence of tinnitus is likely to increase due to increased longevity and recreational noise exposure. Much of the present thinking regarding the generation of tinnitus revolves around the appearance of spontaneous activity and neural hyperactivity in certain central auditory structures. A working hypothesis which parallels findings in other sensory systems suggests that partial deafferentation leads to central changes, generally involving altered inhibitory neurotransmission. Partial peripheral auditory deafferentation may produce tinnitus, coincident with a selective loss of inhibitory glycinergic function in the dorsal cochlear nucleus (DCN). Proposed studies will examine the functional and molecular neurochemical impact of aging in rats with behavioral evidence of tinnitus. Preliminary results find age and noise-exposure related changes in glycine receptor and neurotrophic factor neurochemistry in DCN of rats with behavioral evidence of tinnitus. Proposed studies will compare functional and molecular neurochemical changes between young and aged controls, and young and aged sound-exposed rats with behavioral evidence of tinnitus. Specifically: 1) quantitative measures of message and protein will be used to examine changes in the subunit make up of the glycine receptor, the scaffolding protein, gephyrin, and the protective neurotrophin BDNF;2) recordings from DCN fusiform cells will parallel behavioral studies/gap detection and examine inhibitory response properties and glycine receptor sensitivity and 3) quantitative receptor binding will be used to examine changes in glycine and neurotrophin receptor pharmacology. Findings from these studies will provide new information regarding tinnitus-related changes in glycine neurotransmission and possible neurotrophin protection in an animal model of tinnitus. Understanding the impact of aging on tinnitus-related changes in neurochemistry related to glycine and the BDNF receptor (TrkB) function might help define unique targets for the development and testing of novel selective drugs for the treatment of tinnitus.

Abstract The 8th installment of the Midwest Auditory Research Conference (MARC) will take place at Southern Illinois University School of Medicine in Springfield, Illinois from July 11-13, 2019. We expect ~150 energized auditory and vestibular scientists from a broad swath of the country to visit the home town of Abe Lincoln. The goals of this meeting are to foster collaborations among basic, clinical, and translational hearing/vestibular scientists and to provide an inexpensive opportunity for graduate students and postdoctoral fellows to present their work as posters or talks. MARC 2019 focus areas conform to the mission of the NIDCD including: Inner ear development; Vestibular hair cell regeneration; Hidden hearing loss; Synaptic transmission between hair cells and neurons; Age-related hearing loss; Central auditory function/dysfunction/plasticity; Mechanisms of tinnitus; and Improving speech understanding across all populations of ages. To this end, confirmed keynote speakers include: Gabriel Corfas-University of Michigan, Judy Dubno-Medical University of South Carolina, Paul Fuchs-Johns Hopkins University School of Medicine, Andy Groves-Baylor College of Medicine, Nina Kraus-Northwestern University, Dan Polley-Harvard Medical School, Jenny Stone-University of Washington, and Thanos Tzounopoulos- University of Pittsburgh School of Medicine. MARC 2019 will provide an informal setting that encourages graduate students and postdoctoral fellows to discuss their work amongst each other, and with nationally regarded Principal Investigators. Sessions will include a broad mix of talks that focus on peripheral and central auditory neuroscience, therefore encouraging attendance across the spectrum of auditory/vestibular research. We also plan to include a Friday night event at the Abraham Lincoln Presidential Museum with an engaging keynote speaker as part of the MARC meeting. Graduate students and postdoctoral fellows will be selected for travel awards with registration waivers, based on the quality and relevance of their abstracts. Selected abstracts from trainees and early stage investigators will be given preference for talks that are slotted into the keynote appropriately themed sessions. Since we anticipate that greater than half of the attendees will be trainees, MARC 2019 will also include a career development session where strategies to improve grant writing will be discussed in a round table format.