Abstract:
The transmission of sound along the auditory pathway involves the transduction of sound waves into an electrical signal at the sensory hair cells and subsequent transmission to the auditory cortex via a number of auditory nuclei so that sound information can be processed, perceived, and integrated with other systems. Deficits in the development and maturation of this pathway is the main cause of hearing loss and of individuals with hearing loss, 8% are diagnosed with auditory neuropathy spectrum disorder (ANSD). ANSD results in dysynchronous transmission of information along the auditory pathway when sensory receptors are intact and these deficits in temporal coding causes difficulties in speech detection, which is crucial for language acquisition in childhood. Extreme prematurity, hypoxia/ischaemia, and hyperbilirubinaemia, are risk factors for ANSD and contribute up to 50% of newly diagnosed ANSD cases. Diagnosis of ANSD is somewhat reliable; however current rehabilitative and management strategies are not. Thus identifying the pathophysiology of ANSD is crucial for further development of therapeutic avenues. The present study investigated the effects of neonatal hypoxia/ischaemia on the structure and function of the lower brainstem auditory pathway. Male Sprague-Dawley rats were exposed to humidified 1.5% O2, 5% CO2, and 93.5% N2 at 37ºC, every 2h for a total of 12.25h.day-1 from P1 to P3. Auditory brainstem responses (ABR) were recorded at P14, P21, and P98, while tissue was sacrificed at P21 and P98 for histological analyses. ABRs were obtained using click stimuli at a normal presentation rate of 21s-1 and high presentation rates of 91s-1 and 227s-1, as well as tone burst stimuli. No significant differences were observed between the experimental and control group. Histological analyses assessed the degree of myelination in the auditory nerve and lateral lemniscus. Fluoromyelin staining of the auditory nerve and lateral lemniscus revealed no significant differences in myelination between the experimental and control group. Our results indicate that there is no effect of hypoxia on the gross function of the cochlea and auditory nuclei in the brainstem, nor on the myelination of the auditory nerve and brainstem nerve tracts. Future studies will examine whether cortical pathologies that have been previously described (Oorschot et al., 2013) affect processing of auditory information in this model by performing behavioural testing on these rats.