Coutino, D. | bioRxiv Science

bioRxiv2025

End-to-end evaluation of white matter microstructure of the visual pathway in asymmetric glaucoma

Diffusion magnetic resonance imaging is a non-invasive neuroimaging technique that enables in vivo evaluation of white matter microstructure, providing sensitivity to tissue abnormalities caused by disease. Glaucoma, the second leading cause of blindness worldwide, is characterized by progressive loss of retinal ganglion cells and axonal damage in the optic nerve, leading to degeneration along the entire visual pathway. This degeneration includes secondary effects on fiber crossings within the optic chiasm, which are challenging to characterize with conventional diffusion models. In this study, we evaluated 31 patients with asymmetric glaucoma and 31 healthy controls using advanced diffusion magnetic resonance imaging methods, including Diffusion Tensor Imaging, Constrained Spherical Deconvolution, multi-tensor fit via Multi-Resolution Discrete Search method, and Fixel-Based Analysis. We found significant differences of diffusion metrics in white matter tracts of the visual system, including the optic nerve, optic chiasm, optic tracts, and optic radiations. Moreover, diffusion metrics correlated with clinical ophthalmological parameters such as cup-to-disc ratio, visual field mean deviation, and retinal nerve fiber layer thickness. These findings support the use of advanced diffusion magnetic resonance imaging models as sensitive tools for detecting Wallerian degeneration and resolving complex white matter architecture in the human visual pathway, and demonstrate their utility to study other fiber-crossing regions throughout the brain.

neuroscience↗

bioRxiv2023

Selective medial septum lesions in healthy rats induce longitudinal changes in microstructure of limbic regions, behavioral alterations, and increased susceptibility to status epilepticus

Septo-hippocampal pathway is crucial for physiological functions and is involved in epilepsy. Its clinical monitoring during epileptogenesis is complicated. We aim to evaluate tissue changes after lesioning the medial septum of normal rats and assess how the depletion of specific neuronal populations alters the animals behavior and susceptibility to establishing a pilocarpine-induced status epilepticus. A total of 64 young-adult male Sprague-Dawley rats were injected into the medial septum with vehicle or saporins (GAT1 or 192-IgG for GABAergic or cholinergic depletion, respectively; n=16 per group). Thirty-two animals were used for diffusion tensor imaging (DTI); they were scanned before surgery and 14 and 49 days post-injection. Fractional anisotropy and apparent diffusion coefficient were evaluated in the fimbria, dorsal hippocampus, ventral hippocampus, dorso-medial thalamus and amygdala. Between scans 2 and 3, animals were submitted to the elevated plus-maze, open-field test, rotarod test, Y-maze and water-maze. Timm, toluidine and Nissl staining were used to analyze tissue alterations. Twenty-four different animals received pilocarpine to evaluate the latency and severity of the status epilepticus two weeks after surgery. Eight animals were only used to evaluate the extent of neuronal damage inflicted on the medial septum one week after the molecular surgery. Progressive changes in DTI parameters in both the white and gray matter structures of the four evaluated groups were observed. Behaviorally, the GAT1-saporin injection impacted spatial memory formation, while 192-IgG-saporin triggered anxiety-like behaviors. Histologically, the GABAergic toxin also induced aberrant mossy fiber sprouting, tissue damage and neuronal death. Regarding the pilocarpine-induced status epilepticus, this agent provoked an increased mortality rate. Selective septo-hippocampal modulation impacts the integrity of limbic regions crucial for certain behavioral skills and could represent a precursor for epilepsy development. Significance statementThe medial septum is believed to be involved in epilepsy. However, whether and how defects in the integrity of each neuronal subpopulation conforming this structure affect gray and white matter structures remains unclear. Here we examine whether the injection of vehicle or partially-selective saporins into medial septum of normal rats play a role in the integrity of specific brain regions relevant to memory formation, anxiety-like behaviors, and susceptibility to status epilepticus induction and survival. We find that lesioning the medial septum GABAergic or cholinergic neurons can represent a precursor for behavioral deficits or epilepsy development. Therefore, these results strongly support the idea that modulation of medial septum can be a potential target to improve cognition or reduce seizure frequency.

neuroscience↗