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In an effort to improve the auditory brainstem implant, a prosthesis in which user outcomesare modest, infrared neural stimulation (INS) was applied to the cochlear nucleus in a ratanimal model. Pulsed INS, delivered to the surface of the cochlear nucleus via an opticalfibre, evoked auditory brainstem responses (ABR) and generated broad neural activation inthe inferior Colliculus (IC). Varying the parameters of the laser stimulation revealed laserpeak power to be the dominating parameter for both ABR and IC responses. Strongestresponses were recorded when the fibre was placed at lateral positions on the cochlearnucleus, close to the temporal bone. After deafening by auditory nerve section, ABR andIC responses to INS disappeared, consistent with a reported "optophonic" effect, a laser-inducedacoustic artifact. Thus, for deaf individuals who use the auditory brainstemimplant, INS alone does not appear promising as a new approach.

The inferior colliculus is an auditory midbrain hub that processes both ascending and descending auditory information. Most cells within the inferior colliculus respond to sound, and for a majority of these cells, the responses can be modulated by acetylcholine. The major source of acetylcholine in the inferior colliculus comes from the pontomesencephalic tegmentum, a nucleus known for projections into the thalamus and roles in arousal and the sleep-wake cycle. Using selective viral tract tracing techniques in a ChAT-Cre Long Evans rat, we characterize the distribution of cholinergic inputs from the pontomesencephalic tegmentum to all subdivisions of the inferior colliculus and associated intercollicular nuclei. We find evidence of cholinergic inputs onto both glutamatergic and GABAergic inferior colliculus neurons. In a separate set of experiments, we seek to determine if acetylcholine modulates major inferior colliculus output pathways. Using retrograde tract tracing techniques in combination with immunohistochemistry, we determine if cholinergic boutons make synaptic contact with inferior colliculus neurons that project to the medial geniculate, cochlear nucleus, or superior olivary complex. In these experiments, we find cholinergic contacts onto GABAergic and glutamatergic inferior colliculus neurons that project to the medial geniculate, as well as cholinergic contacts onto glutamatergic inferior colliculus neurons that project to the cochlear nucleus or superior olivary complex. We found cholinergic contacts onto neurons that participate in these output pathways in all inferior colliculus subdivisions and associated intercollicular nuclei.Taken together, we show an extensive network of cholinergic inputs that modulates a wide variety of auditory and multisensory functions of the inferior colliculus.

Tinnitus is the disorder of phantom sound perception, while hyperacusis is abnormally increased loudness growth. Tinnitus and hyperacusis are both associated with hearing loss, but hearing loss does not always occur with either condition, implicating central neural activity as the basis for each disorder. Furthermore, while tinnitus and hyperacusis can co-occur, either can occur exclusively, suggesting that separate pathological neural processes underlie each disorder. Mounting evidence suggests that pathological neural activity in the cochlear nucleus, the first central nucleus in the auditory pathway, underpins hyperacusis and tinnitus. The cochlear nucleus is comprised of a ventral and dorsal subdivision, which have separate principal output neurons with distinct targets. Previous studies have shown that dorsal cochlear nucleus fusiform cells show tinnitus-related increases in spontaneous firing with minimal alterations to sound-evoked responses. In contrast, sound-evoked activity in ventral cochlear nucleus bushy cells is enhanced following noise-overexposure, putatively underlying hyperacusis. While the fusiform-cell contribution to tinnitus has been well characterized with behavioral and electrophysiological studies, the bushy-cell contribution to tinnitus or hyperacusis has been understudied. This dissertation examines how pathological neural activity in cochlear nucleus circuitry relates to tinnitus and hyperacusis in the following three chapters. In the first chapter, I characterize the development of a high-throughput tinnitus behavioral model, which combines and optimizes existing paradigms. With this model, I show that animals administered salicylate, a drug that reliably induces tinnitus at high doses in both humans and animals, show behavioral evidence of tinnitus in two separate behavioral tests. Moreover, in these same animals, I show that dorsal-cochlear-nucleus fusiform cells exhibit frequency-specific increases in spontaneous firing activity, consistent with the increased spontaneous firing observed in animal models of noise-induced tinnitus. In the second chapter, I show that following noise-overexposure, ventral-cochlear-nucleus bushy cells demonstrate hyperacusis-like neural firing patterns, but not tinnitus-specific increases in spontaneous activity. I contrast the bushy-cell neural activity with established fusiform-cell neural signatures of tinnitus, to highlight the bushy-cell, but not fusiform-cell contribution to hyperacusis. These analyses suggest that tinnitus and hyperacusis likely arise from distinct neural substrates. In the third chapter, I use computational modelling of the auditory periphery and bushy-cell circuitry to examine potential mechanisms that underlie hyperacusis-like neural firing patterns demonstrated in the second chapter. I then relate enhanced bushy-cell firing patterns to alterations in the auditory brainstem response, a sound-evoked electrical potential generated primarily by bushy cells. Findings in this chapter suggest that there are multiple hyperacusis subtypes, arising from separate mechanisms, which could be diagnosed through fine-tuned alterations to the auditory brainstem response.

The bHLH transcription factor Math5 (Atoh7) is required for retinal ganglion cell and optic nerve development. Using Math5-lacZ knockout mice, we have identified additional expression domains for Math5 in functionally connected structures of the central auditory system. In the adult hindbrain, the cytoplasmic Math5-lacZ reporter is expressed in the ventral cochlear nucleus (CN), within a subpopulation of neurons that project to medial nucleus of the trapezoid body, lateral superior olive, and lateral lemniscus. These cells were identified as globular and small spherical bushy cells based on their morphology and distribution within the CN, molecular characteristics, and projection patterns. During development, Math5-lacZ was detected in precursor cells emerging from the caudal rhombic lip from E 12 onwards, consistent with the time course of CN neurogenesis. The hindbrains of Math5 mutants appear grossly normal, with the exception of the CN. Although overall CN dimensions are unchanged, the lacZ positive cells are significantly smaller in Math5 -/- mice compared to Math5 +/- mice, suggesting neuronal dysfunction. The Auditory Brainstem Response (ABR) of Math5 mutants was evaluated in a BALB/cJ congenic background. ABR thresholds of Math5 -/- mice were similar to those of wild-type and heterozygous mice, but interpeak latencies for Peaks II-IV were significantly altered. These temporal changes are consistent with a higher level auditory processing disorder involving the CN, potentially affecting the integration of binaural sensory information. BAC transgenic mice that express diphtheria toxin A in the endogenous Math5 expression pattern were generated to selectively ablate Math5-expressing cells in the CN, thereby enhancing the auditory phenotype observed in Math5 null mice. Preliminary histological analysis of one M5-DTA transgenic line at postnatal day 1 indicates selective elimination of most Math5-expressing cells in the retina and CN. Remaining Math5-expressing CN neurons have abnormal projection patterns that terminate shortly after exiting the CN and are likely non-functional. Math5-lacZ null mice and M5-DTA transgenic mice represent two new models of central auditory dysfunction. Future studies with these models will allow a more detailed understanding of mechanisms underlying anomalies in central auditory processing and may lead to a better understanding and clinical diagnosis of human central auditory pathology.

The aim of this dissertation was to provide some biochemical evidence relevant to previous functional studies by measuring muscarinic receptor binding and choline acetyltransferase (ChAT) activity following cochlear ablation and intense tone exposure. The ChAT activity in the rat cochlear nucleus (CN) had a characteristic distribution pattern. Significant gradients of ChAT activity were found in all dissected regions. These chemical gradients suggest that descending cholinergic input to the CN is widely distributed and tonotopically organized. After cochlear ablation, bilateral increases in ChAT were found in the anteroventral CN (AVCN) and dorsolateral granular region (G-AVCN) 1 mo after lesion. However, there was no change in the dorsal CN (DCN). In the lateral superior olive, there was a sustained decrease ipsilateral to the lesion after cochlear ablation, By contrast, ChAT activity in the ventral nucleus of the trapezoid body had a tendency to decrease bilaterally in 1 mo and 2 mo survival animals. Except for similar ChAT activities in granular regions, ChAT activity was 2-3-fold lower in the hamster CN than in the rat. Eight days after tone exposure, significant increases over control were found in the AVCN and fusiform soma layer of DCN on the exposed side. In addition, ChAT activities in the G-AVCN, in AVCN, and in deep layer of DCN (DCNd) ipsilateral to the exposure were higher than contralateral values. Two months after tone exposure, ChAT activity in the ipsilateral DCNd was still higher than the contralateral value. Changes of ChAT in the CN may underlie pathophysiological conditions. Analysis of 1-[N-methyl-3H]scopolamine binding in the ventral CN after cochlear ablation demonstrated significant asymmetry in muscarinic receptor binding in 2 mo survival rats, with higher density on the lesion side. The lesion side was 67% higher in the AVCN and 70% higher in the posteroventral CN. In the DCN, increased binding on the lesion side relative to contralateral was significant in DCNf and DCNd in the 1 mo and 2 mo survival animals. The asymmetry was more obvious in the DCNd. The [3H]NMS binding pattern in the DCNf changed from a patchy pattern in control rats to a more diffuse pattern on the lesion side.

The application of multi-channel recording electrodes is expanding rapidly, as neuroscientists seek methods to understand how multiple neurons carry out brain functions. In this dissertation, methods are described to improve the recording properties of micro-machined silicon-substrate electrodes. The methods are based on multi-channel signal processing techniques and empirical determination of optimal recording surface area. Also described is the analysis of neurons recorded in the dorsal cochlear nucleus, a region of the auditory system, revealing a possible inhibitory interaction between two specific cell types. Background noise was found to obscure action potentials of neurons recorded in the auditory system, particularly in the presence of a synchronizing stimulus. Using silicon electrodes with closely-spaced recording elements, the signal-to-noise ratio was substantially improved by application of a linear array signal processing technique. The denoising process resulted in more easily detected spike events, with signal-to-noise ratio improvements as large as 12 dB. The spikes were subsequently classified into individual neural sources by multi-channel template or principal component methods. Refinements to the array processing method were made by expansion to the spatio-temporal domain and by an adaptive processing procedure. Design optimization experiments involved the use of a custom electrode having recording elements of various surface areas along its length. The electrode was moved slowly past a neuron, allowing an empirical determination of the electrode size yielding the best signal-to-noise ratio of detected spikes. The optimal electrode surface area was found to be significantly larger than that currently in use. Iridium activation of the electrodes did not improve the signal-to-noise ratio. The optimized recording methods were applied to neural ensembles of the guinea pig dorsal cochlear nucleus. To verify previous theories that an inhibitory interaction between cartwheel and fusiform cell types exists, we recorded simultaneously from these units using a 16-channel electrode. Cartwheel cells were identified by their tendency to fire spikes in bursts, and fusiform cells by their high spontaneous rate and response to noise and tones. A small number of putative cartwheel-fusiform pairs exhibited a significant feature in cross-correlograms derived from spontaneous activity, consistent with a monosynaptic inhibitory connection.