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.
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.
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