Date of Award
Doctor of Philosophy
Tinnitus, ringing in the ears, is a phantom sound percept affecting roughly 10-20% of the total world population. Tinnitus severely impacts the quality of life of 10% of tinnitus sufferers, affecting their sleep, concentration, emotion, social enjoyment, and sometimes leading to depression and suicidal tendencies. In humans, most forms of tinnitus are associated with noise-exposure, leading to compensatory maladaptive plasticity of central auditory neurons. Human and animal studies suggest that tinnitus alters normal adult attentional resources. Human studies by McKenna, Hallam and Surlock 1996, suggested tinnitus-related impairment in sustained attention, vigilance, visual conceptualization and visuo-motor memory. Additionally, tinnitus may impact aspects of selective or divided attention as well as working and long-term memory. The involvement of primary auditory cortex and nicotinic signaling in selective attention, working and long-term memory has been well established. Neuronal nicotinic acetylcholine receptors (nAChRs) are present on presynaptic and postsynaptic inputs that innervate neurons across layers of primary auditory cortex (A1). Layer 5 pyramidal neurons (PNs) in the A1 are major output neurons, conveying auditory information to corticocortical and subcortical nuclei. The excitation of PNs is regulated by a complex microcircuitory of inhibitory neurons with vasointestinal peptide positive (VIP) neurons playing a key role in regulating the excitation. The focus of present studies was to: 1) Characterize tinnitus-related changes in the physiology and nAChR signaling of layer 5 PNs and VIP neurons in the A1 and 2) Determine the ability of nAChR partial/desensitizing agonists to ameliorate tinnitus pathology in subcellular studies. Wild-type, ChAT-Cre and VIP-Cre:Rosa26-loxP-stop-loxP-tdTomato (VIP-Cre:Rosa-tdTomato Long-Evans rats were used in the present study. CHAT-Cre rats allowed us to selectively express cre-inducible AAV-EF1a-DIO-hChR2(H134R)-EYFP and stimulate the cholinergic neurons of basal forebrain (BF). VIP-Cre:Rosa-tdTomato express fluorescent tdTomato protein in the VIP positive neurons allowing us to identify them under fluorescence microscopy using 550 nm wavelength. An established noise-exposure (one hour of 116 dB narrowband noise centered at 16 kHz) was used to induce behavioral tinnitus in a rat model. Approximately 50-60% noise-exposed animals (53/92) exhibited behavioral evidence of tinnitus with significant shifts in hearing threshold contiguous to the exposure frequency. Animals were classified as control, exposed tinnitus and non-tinnitus. In vitro whole-cell patch clamp recordings were performed in control and tinnitus animals. Results: Numerous tinnitus-related changes in the physiology of layer 5 PNs and VIP neurons, and changes in the activity of excitatory and inhibitory input neurons were observed. The resting membrane potential of A1 layer 5 PNs from tinnitus animals was significantly depolarized compared to PNs from unexposed controls. PNs from the A1 of animals with behavioral evidence of tinnitus showed increases in the frequency of excitatory and decreases in frequency of inhibitory spontaneous postsynaptic currents, which directly correlated with the rat’s tinnitus score. Optical stimulation of thalamocortical terminals from PNs in tinnitus animals evoked significantly larger excitatory/inward currents than in currents evoked in PNs from controls. A1 layer 5 PNs showed tinnitus-related decreases in postsynaptic gamma-amino butyric acid (GABA) signaling suggestive of GABA-A receptors (GABA-ARs) subunit switches or loss of GABA-ARs. VIP neurons favoring excitation of layer 5 PNs via disinhibition, were depolarized with significantly lower current to evoke action potentials (rheobase current). The excitability of VIP neurons was significantly increased, with this increase being strongly correlated to the rat’s tinnitus score. Tinnitus-related changes in nAChR signaling were then tested in layer 5 PNs and VIP neurons. Both PNs and VIP neurons receive cholinergic input from basal forebrain and were highly sensitive to nicotinic stimulation. Optical stimulation of basal forebrain (BF) terminals evoked a depolarizing current from VIP neurons. In tinnitus animals, layer 5 PNs showed a significant loss of nAChR signaling, while, VIP neurons showed tinnitus-related increase in responses to nicotinic stimulation. Most of the nAChR responses in auditory cortex are believed to be mediated via volume transmission of acetylcholine (ACh). Continuous voltage clamped recordings were used to examine the activity of excitatory and inhibitory neurons impacting PNs in the presence of bath applied ACh. We observed significant tinnitus-related changes in nAChR signaling with layer 5 PNs showing significantly larger GABAergic input after prolonged bath application of ACh. This led us to hypothesize that desensitization of nAChRs could increase/normalize the activity of GABAergic input neurons. To test this hypothesis, nAChR partial desensitizing agonists sazetidine-A and varenicline were used in cellular and behavioral studies. Immediately after bath application of sazetidine-A or varenicline, a dramatic increase in the activity of inhibitory input neurons onto PNs was observed. In a behavioral tinnitus test, both sazetidine-A and varenicline were effective in lowering the tinnitus-like behavior. In conclusion, we identified a significant tinnitus-related disruption in intrinsic physiology of layer 5 PNs and VIP neurons, with strong evidence of dysregulated cholinergic signaling. Partial/desensitizing agonists sazetidine-A and varenicline increased the activity of inhibitory input neurons, showing therapeutic potential in both subcellular and behavioral studies.
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