Abstract
IntroductionThis study aimed to assess the top-down control of sound processing in the auditory brainstem of rats. Short latency evoked responses were analyzed after unilateral or bilateral ablation of auditory cortex. This experimental paradigm was also used towards analyzing the long-term evolution of post-lesion plasticity in the auditory system and its ability to self-repair.MethodAuditory cortex lesions were performed in rats by stereotactically guided fine-needle aspiration of the cerebrocortical surface. Auditory Brainstem Responses (ABR) were recorded at post-surgery day (PSD) 1, 7, 15 and 30. Recordings were performed under closed-field conditions, using click trains at different sound intensity levels, followed by statistical analysis of threshold values and ABR amplitude and latency variables. Subsequently, brains were sectioned and immunostained for GAD and parvalbumin to assess the location and extent of lesions accurately.ResultsAlterations in ABR variables depended on the type of lesion and post-surgery time of ABR recordings. Accordingly, bilateral ablations caused a statistically significant increase in thresholds at PSD1 and 7 and a decrease in waves amplitudes at PSD1 that recover at PSD7. No effects on latency were noted at PSD1 and 7, whilst recordings at PSD15 and 30 showed statistically significant decreases in latency. Conversely, unilateral ablations had no effect on auditory thresholds or latencies, while wave amplitudes only decreased at PSD1 strictly in the ipsilateral ear.ConclusionPost-lesion plasticity in the auditory system acts in two time periods: short-term period of decreased sound sensitivity (until PSD7), most likely resulting from axonal degeneration; and a long-term period (up to PSD7), with changes in latency responses and recovery of thresholds and amplitudes values. The cerebral cortex may have a net positive gain on the auditory pathway response to sound.
Highlights
This study aimed to assess the top-down control of sound processing in the auditory brainstem of rats
Bilateral ablations caused a statistically significant increase in thresholds at PSD1 and 7 and a decrease in waves amplitudes at PSD1 that recover at PSD7
Unilateral ablations had no effect on auditory thresholds or latencies, while wave amplitudes only decreased at PSD1 strictly in the ipsilateral ear
Summary
This study aimed to assess the top-down control of sound processing in the auditory brainstem of rats. The end effectors of this chain are the olivocochlear neurons, which in turn regulate the inner ear responses This efferent pathway feedback on cochlear responses was masterfully shown in electrophysiological experiments performed by Robert Galambos, who found that electrical stimulation of olivo-cochlear fibres causes a drop in the electrically evoked activation of the cochlea after sound stimulation [2]. Anterograde tracers injected in the temporal cortex showed terminal fields from the auditory cortex (AC) in the LSO and dorsal periolivary region [3], wherein lateral olivo-cochlear (LOC) neurons are located These findings and the presumably excitatory character of the corticofugal pathway [6] suggest that the cerebral cortex may activate both MOC and LOC neurons and its efferent circuits to the cochlea and cochlear nuclei
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