Age-related decline in central auditory function significantly affects quality of life in the elderly, with impaired speech perception leading to increased risk for depression, social isolation and cognitive decline. A 2017 Lancet Commission report cites hearing loss as the largest modifiable risk factor for developing cognitive decline, exceeding smoking, high blood pressure, lack of exercise and social isolation. Remarkably, a 2019 large-scale study found that even mild hearing loss, i.e., still within the normal range, produced an even closer association with cognitive decline. Currently, there is no effective therapy for age-related central auditory decline?hearing aids only address audibility?and no drug treatment. Ideally, a combination of drug treatment with hearing aids and behavioral training could restore auditory function, but the development of pharmacological treatments requires a better understanding of the mechanisms by which candidate drugs improve hearing. The goals of this proposal are to develop biomarkers of altered auditory processing in aging mice and humans, and using these biomarkers, to test the hypothesis that nicotine can normalize these age-related central auditory deficits. Because nicotine enhances cortical and cognitive function, pharmaceutical companies are developing nicotine-like drugs to target cognitive deficits in aging. These drugs are non-addictive (unlike nicotine in tobacco), yet nicotine also is non-addictive when given topically or orally. However, its clinical benefits have not been exploited except as an aid to stop smoking. We hypothesize that: 1) acute nicotine compensates for the age-related decline in inhibition by exciting the remaining inhibitory neurons; 2) chronic nicotine exposure (CNE) upregulates nicotinic acetylcholine receptors (nAChRs); and, as a result, 3) acute nicotine and/or CNE will reduce or reverse the age-related auditory decline. We will test these hypotheses in both mouse and human at the level of cells (mouse in vitro brain slice), neural systems (mouse in vivo physiology; human brain imaging and EEG) and behavior (human psychoacoustics).
Aim 1 will determine in mouse whether age-related decline in auditory spectrotemporal processing is reversed by acute nicotine or CNE, and characterize the associated cellular mechanisms.
Aim 2 will identify, in humans, age-related changes in receptive field properties in auditory cortex using novel fMRI techniques and determine if nicotine reverses these changes using psychoacoustics, fMRI and EEG. This project features a multifaceted, parallel approach in mouse and human.
Each Aim will: 1) examine auditory processing at multiple adult ages; 2) use similar acoustic stimuli in both species, accounting for species differences in hearing, to target common mechanisms; 3) test the effects of nicotine. A successful outcome will promote an integrated understanding across levels, from cellular mechanisms to perception, and facilitate translation of nicotine-based therapeutic treatments to clinical populations.
Nicotine enhances auditory and cognitive functions because it mimics the brain?s system for ?paying attention? to important sounds amid distractions (for example, understanding speech in a noisy environment). In part, nicotine does this by activating inhibitory neurons in the auditory cortex. Since age-related hearing deficits result in part from the loss of inhibitory neurons, this project will determine, in humans and mice, whether nicotine?s effects can compensate for reduced inhibition in aging and thereby restore auditory function.
