Multimodal Assessment of Bottlenose Dolphin Auditory Nuclei Using 7-Tesla MRI, Immunohistochemistry and Stereology

SIMPLE SUMMARY: Monitoring cetacean health is important considering the high strain from both natural and human-related threats. Most of these, such as Cetacean morbillivirus and man-made pollution in form of toxins and noise, affect nervous tissues and brain function. Neuropathological research in cetaceans has been qualitative or semiquantitative. Here, we use stereology to quantify protein expression in neurons, axons, and glial cells in the auditory nuclei of a bottlenose dolphin and correlate the values to their pre-processed volumes from MRI scans. This multimodal approach is aims to avoid artifacts that may arise using either methodology. Similarities in protein expression between a healthy dolphin and a human with brain trauma implies that dolphins might have a baseline neurochemical buffer against low oxygen (hypoxia). This study will help expand our quantitative understanding of health and disease in cetacean brains. ABSTRACT: The importance of assessing neurochemical processes in the cetacean brain as a tool for monitoring their cognitive health and to indirectly model human neurodegenerative conditions is increasingly evident, although available data are largely semiquantitative. High-resolution MRI for post-mortem brains and stereology allow for quantitative assessments of the cetacean brain. In this study, we scanned two brains of bottlenose dolphins in a 7-Tesla (7T) MR scanner and assessed the connectivity of the inferior colliculi and ventral cochlear nuclei using diffusion tensor imaging (DTI). Serial thick sections were investigated stereologically in one of the dolphins to generate rigorous quantitative estimates of identifiable cell types according to their morphology and expression of molecular markers, yielding reliable cell counts with most coefficients of error <10%. Fibronectin immunoreactivity in the dolphin resembled the pattern in a human chronic traumatic encephalopathy brain, suggesting that neurochemical compensation for insults such as hypoxia may constitute a noxious response in humans, while being physiological in dolphins. These data contribute to a growing body of knowledge on the morphological and neurochemical properties of the dolphin brain and highlight a stereological and neuroimaging workflow that may enable quantitative and translational assessment of pathological processes in the dolphin brain in the future.