Developmental Plasticity of Tonotopic Maps in Chinchilla Auditory Midbrain

Sensory areas of the brain have the remarkable ability to reorganize following changes in peripheral input, especially during early development. Cortex of altricious species (post-natal hearing onset) is often investigated. It remains unclear whether neural connections in auditory midbrain reorganize when precocious subjects (hearing onset in utero; e.g., chinchilla laniger) are reared in an enhanced acoustic environment. Neonatal chinchillas were chronically exposed to a moderately-intense (70 dB SPL) narrowband (2 ± 0.25 kHz) sound stimulus for 4 weeks. No difference in tone-specific auditory brainstem response thresholds, hair cell morphology, neural thresholds, and bandwidth 10 dB above threshold were observed around 2 kHz, suggesting the exposure stimulus was non-traumatic. Sound frequency maps in central nucleus of inferior colliculus (IC) were defined with microelectrode recordings. We observed a significant decrease in the proportion of neurons dedicated to octave bands centered at 2- and 8- kHz. Changes were not limited to sound-exposure frequencies: increases in low-frequency representation (below 1 kHz) were observed. We describe a c-fos immunolabelling protocol for chinchilla. Bands of neurons observed following 90-min, 6-kHz stimulation lay ventro-medial to those present following 90-min, 2-kHz stimulation, consistent with the known tonotopic organization of IC, and verified herein by electrophysiological recordings. Interestingly, we observed decreased labeling adjacent to these bands, which we suggest represent inhibitory regions. Following sound exposure, then 90-min, 2-kHz sound stimulation, the number of labeled cells both in the 2-kHz region and throughout IC was increased. Sound-exposed subjects who received no further sound stimulation had c-fos expression patterns similar to silence controls, suggesting sound-exposure does not alter basal levels of c-fos expression, but changes the way neurons respond to prolonged (90-min) tone stimulation. This study contributes to a growing body of literature that suggests that sound frequency representation in auditory midbrain is altered following passive peripheral input during development.Ph.D.2018-02-08 00:00:0