Using Structural Magnetic Resonance Imaging (MRI) to Image the Central Auditory Pathway in Rats Following Extreme Perinatal Hypoxia

Abstract

Magnetic Resonance Imaging was used to identify potential changes in auditory nuclei and the auditory cortex following hypoxia at birth in a rat model of extreme prematurity. Using the model, we’re hoping that it produces characteristics of hearing loss that resemble Auditory Neuropathy Spectrum Disorder, a disorder where extreme hypoxia (anoxia) at birth and prematurity are considered risk factors. Our intermittent hypoxia protocol replicates that developed by Oorschot et al. (Oorschot et al., 2013), which includes exposure to 1.5% O2 for 15 minutes, every 2 hours, over a 12 hour period, from P1 to P3. Rats were weighed between P0 and P21, and at P126. At P126, rats were fixed with 4% Paraformaldehyde using whole animal perfusion, then decapitated, and the heads were placed in an appropriate vessel for scanning. Each rat head was scanned with two sequences: T1-weighted with inversion recovery and T2-weighted. In addition, one high resolution scan was carried out. Auditory nuclei were identified using stereotaxic coordinates from a histology atlas (Paxinos & Watson, 1997). Using 3D Slicer (Fedorov et al., 2012), these nuclei were labelled and volumes were measured and presented as a percentage of intracranial volume. The images were further analysed using Voxel-Based Morphometry and Volume of Interest analysis using Statistical Parametric Mapping. These brains were subsequently cryo-sectioned for histology (40 μm slice thickness) and stained with Luxol-Fast-Blue and Cresyl-Violet. As the histology is preliminary for future studies, only qualitative observations were made. Results showed significant differences in body weight between P0 and P21, and at P126. However, there were no significant differences in volume or mean signal of the auditory structures. Our analyses suggest that extreme hypoxia may induce some morphological change in the brain but not within the auditory structures. One possible reason could be the high level of plasticity in young mammals and their adaptive mechanisms to triggers of abnormal development, such as hypoxia. While Oorschot’s model is a good model of extreme prematurity, it doesn’t appear to produce changes in the central auditory pathway. This is possibly due to the central auditory pathway and/or subcortical structures being less vulnerable to hypoxia.