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

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.

Neuroanatomy of the Cetacean Sensory Systems

“…It is a major integration centre for both auditory and non-auditory signals, and one of the most metabolically active areas of the dolphin brain [ 126 ]. While a basic division between a fibre-rich tectosome and a (predominantly multipolar) neuron-rich central nucleus is evident [ 117 ], subdivisions of these two areas are debated [ 102 , 127 ].…”

“…Auditory evoked potential studies place the primary auditory cortex (A1) in the supra-sylvian gyrus, with secondary auditory cortical fields (A2) lateral to it in the ecto-sylvian gyrus [ 46 , 128 ]. These areas, and an auditory receptive field in the temporal cortex, have been seen in tracing studies using dyes, auditory evoked potentials, and ethically unrepeatable experiments with direct access to the brain [ 129 ] and have been revisited using cyto-architectural studies [ 130 , 131 , 132 , 133 ] and post-mortem magnetic resonance imaging (MRI) techniques [ 49 , 117 , 133 , 134 ]. A1 is the thickest out of the main cortical fields in cetaceans [ 135 ].…”

“…For instance, Breathnach (1960) observed that Monakow's striae acousticae, which connect the posterior cochlear nucleus neurons with the lateral lemniscus and the central nucleus of the inferior colliculus, are not apparent, while Held's striae (connecting the VCN and bilateral ventral nuclei of the trapezoid body, superior olivary and perio-livary nuclei) [114] are clearly visible. More recent studies specify morphological cell types (e.g., spherical neurons in the VCN or principal neurons in the lateral superior olive) comparative to terrestrial mammals [115] and provide estimates for the number and density of neurons, and the volume of the VCN and the lateral superior olive [116,117]. See Figure 1 for an overview of the auditory pathways.…”

Open questions in marine mammal sensory research

“…Brain imaging techniques combined with postmortem assessments of central sensory processing structures (e.g. Cook et al, 2018 ; Orekhova et al, 2022 ) will most likely contribute greatly to a comprehensive understanding of how sensory information of multiple modalities is integrated and used to perform complex behaviors. Such an approach could also help to discern adaptations for sensory perception in the three-dimensional underwater environment allowing movements with all degrees of freedom ( Cook and Berns, 2022 ).…”

Investigating tissue microstructure using steady-state diffusion MRI

“…In this work, I investigate Diffusion-Weighted Steady-State Free Precession [8][9][10][11] (DW-SSFP) (Figure 1c), a powerful diffusion imaging sequence that has demonstrated high SNR-efficiency and strong diffusion-weighting [12] with minimal image distortions (no EPI readout required). Whilst in vivo use is plagued by high motion sensitivity [13] , DW-SSFP has become an established method for post-mortem imaging [14][15][16][17][18][19][20][21][22] , addressing the low-diffusivity and short T2 environment of fixed post-mortem tissue. A key challenge for DW-SSFP is signal interpretation.…”