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The neuronal organization and network of the inferior colliculus and the auditory behavior Open Access

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The auditory system enables animals to perceive and react to the acoustic stimulus abundant in the world, from social communication with vocalization to threatening signals from predators. Located in the midbrain, the inferior colliculus, especially its central nucleus, is considered a major center of integration as all ascending auditory projections converge in this region. Also emerged in the inferior colliculus is complex sound processing, including amplitude and frequency modulation. Complex sound processing is essential for the perception of real-world sounds, such as voice or music, which are always complex. However, the neuronal organizations in the inferior colliculus for postsynaptic current inputs, as well as the behavioral network and relevance involving the inferior colliculus, have not been well described. In this dissertation, three separate papers investigated various unknown problems regarding inferior colliculus and auditory behavior. Firstly, the tonal receptive field in the inferior colliculus was thoroughly dissected. While the frequency-intensity tonal receptive field is regarded as the fundamental property of auditory neurons, how it is constructed in the synaptic level had not been well understood. In this study, whole-cell voltage-clamping of the central nucleus of inferior colliculus neurons revealed that the balance and imbalance of the excitatory and inhibitory synaptic inputs construct the sensitiveness and selectiveness of the spike receptive field. Secondly, the role of the inferior colliculus in emotional processing was investigated. Mice underwent Pavlovian fear conditioning with auditory cues had enlarged the tonal receptive field in the inferior colliculus compared to the controls. Pharmacological silencing the amygdala by injecting muscimol shrank the size of the receptive field, while subsequent silencing the auditory cortex rescued the effect. Retrograde tracing at the inferior colliculus did not show neuronal labeling in the amygdala, unlike in bats, suggesting that the amygdala influence over the inferior colliculus is indirect, presumably through the auditory cortex. Lastly, social communication between mice with ultrasonic vocalizations (USVs) was examined. Mice emit USVs in various situations, namely pup isolation, male-male or female-female competition, and male-female courtship. An automatic ultrasound vocalization extractor and classifier was developed for the analysis of mouse USV ‘syllables,’ which had superior noise resilience and low false positive and negative rates. In this study, the USVs emitted from male mice during male-female courtship were compared between wild-type mice and Mecp2-null mice. Mecp2-null mice emitted reduced quantity as well as the compromised quality of the USVs compared to the wild type mice. Overall, the papers presented in this dissertation will provide a further understanding of the neuronal organization and network of the inferior colliculus, as well as the social communication of the mice using USV and the effect of genetic mutations on USVs.

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