Manganese-enhanced magnetic resonance imaging combined with electrophysiology in the evaluation of visual pathway in experimental rat models with monocular blindness

Morphometric evaluation of cerebellar structures in late monocular blindness

BACKGROUND

Only a few studies have investigated structural and functional changes in monocular blind individuals. Our aim in this study was to segmentally investigate the cerebellar structures of subjects with late-onset monocular blindness (LMB) using a Voxel-based volumetric analysis system.

METHODS

The segmental volumetric values, cortical thickness, gray matter volumes and percentage ratios of the cerebellar lobules of individuals with LMB due to trauma and in healthy individuals with bilateral sight as the control group were calculated at the volBrain CERES 1.0 website ( https://volbrain.upv.es/ ) by using brain 3D fast spoiled gradient recall acquisition in steady-state (3D T1 FSPGR) MRI sequence images in our prospective study.

RESULTS

We studied 11 subjects with LMB (8 males/3 females) and 11 healthy control subjects (8 males/3 females). The mean age was 41.45 ± 14.15 and 40 ± 11.11 years, respectively (p > 0.05). The mean duration of the LMB status was 20.8 ± 11.2 years. Cerebellar lobule crus II volume and cerebellar lobule VIIB/VIIIA volume/percentage were higher in the LMB group, and mean cerebellar cortical thickness, cerebellar lobule VI-cerebellar lobule crus I-II cortical thickness, and cerebellar lobule VI gray matter volume values were lower in the LMB group (p < 0.05).

CONCLUSION

In this study, cerebellar lobule VIIB/VIIIA volume/total percent ratio, cerebellar cortical thickness and cerebellar gray matter volume in the LMB group were found to be different from the control group. To our knowledge, this is the first study to report cerebellar anatomical changes in patients with LMB.

Radiation-induced impairment of optic nerve axonal transport in tree shrews and rats monitored by longitudinal manganese-enhanced MRI

PURPOSE

Radiation-induced optic neuropathy (RION) is a serious complication that occurs after radiation therapy of tumors in the vicinity of the optic nerve, yet its mechanism and imaging features are poorly understood. In this study, we employed manganese-enhanced MRI (MEMRI) to assess optic nerve axonal transport in tree shrews and rats after irradiation.

MATERIALS AND METHODS

A comparison of normal visual projections in tree shrews and rats was conducted by intravitreal MnCl₂ injection followed by MRI. Adult male tree shrews and rats received a total dose of 20 Gy delivered in two fractions (10 Gy per fraction) within 5 days. Longitudinal MEMRI was conducted 5, 10, 20 and 30 weeks after radiation. At the end of observation, motor proteins involved in axonal transport were detected by western blotting, and the axon cytoskeleton was assessed by immunofluorescence.

RESULTS

The eyeballs, lens sizes, vitreous volumes, optic nerves and superior colliculi of tree shrews were significantly larger than those of rats on MEMRI (P < 0.05). The Mn²⁺-enhancement of the optic nerve showed no significant changes at 5 and 10 weeks (P > 0.05) but decreased gradually from 20 to 30 weeks postirradiation (P < 0.05). The enhancement of the superior colliculus gradually decreased from 5 weeks to 30 weeks, and the decrease was most significant at 30 weeks (P < 0.05). The levels of the motor proteins cytoplasmic dynein-1, kinesin-1 and kinesin-2 in the experimental group were significantly decreased (P < 0.05). The immunofluorescence results showed that the α-tubulin, β-tubulin and SMI 31 levels in the experimental groups and control groups were not significantly different (P > 0.05).

CONCLUSION

Tree shrews show great advantages in visual neuroscience research involving MEMRI. The main cause of the decline in axonal transport in RION is an insufficient level of motor protein rather than damage to the axonal cytoskeletal structure. Longitudinal MEMRI can be used to detect changes in axonal transport function and to observe the relatively intact axon structure from the early to late stages after radiation administration.

Applications of Manganese-Enhanced Magnetic Resonance Imaging in Ophthalmology and Visual Neuroscience

Understanding the mechanisms of vision in health and disease requires knowledge of the anatomy and physiology of the eye and the neural pathways relevant to visual perception. As such, development of imaging techniques for the visual system is crucial for unveiling the neural basis of visual function or impairment. Magnetic resonance imaging (MRI) offers non-invasive probing of the structure and function of the neural circuits without depth limitation, and can help identify abnormalities in brain tissues in vivo. Among the advanced MRI techniques, manganese-enhanced MRI (MEMRI) involves the use of active manganese contrast agents that positively enhance brain tissue signals in T1-weighted imaging with respect to the levels of connectivity and activity. Depending on the routes of administration, accumulation of manganese ions in the eye and the visual pathways can be attributed to systemic distribution or their local transport across axons in an anterograde fashion, entering the neurons through voltage-gated calcium channels. The use of the paramagnetic manganese contrast in MRI has a wide range of applications in the visual system from imaging neurodevelopment to assessing and monitoring neurodegeneration, neuroplasticity, neuroprotection, and neuroregeneration. In this review, we present four major domains of scientific inquiry where MEMRI can be put to imperative use — deciphering neuroarchitecture, tracing neuronal tracts, detecting neuronal activity, and identifying or differentiating glial activity. We deliberate upon each category studies that have successfully employed MEMRI to examine the visual system, including the delivery protocols, spatiotemporal characteristics, and biophysical interpretation. Based on this literature, we have identified some critical challenges in the field in terms of toxicity, and sensitivity and specificity of manganese enhancement. We also discuss the pitfalls and alternatives of MEMRI which will provide new avenues to explore in the future.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.

Manganese-enhanced magnetic resonance imaging combined with electrophysiology in the evaluation of visual pathway in experimental rat models with monocular blindness

PURPOSE

Our study aimed to explore the feasibility of manganese-enhanced magnetic resonance imaging (MEMRI) combined with visual evoked potentials (VEP) and auditory evoked visual cortex responses (AVR) in evaluating for the establishment of visual/auditory compensatory pathways after monocular blindness.

MATERIALS AND METHODS

A total of 14 healthy neonatal male Sprague-Dawley rats were randomly divided into two groups (n = 7 for Groups A and B). Right optic nerve (ON) transection was performed on the 7 rats of Group A to obtain a monocularly blind model, and the 7 rats of Group B were chosen as the control group. Four months later, 400 mmol MnCl₂ was injected into the left eye in both groups via intravitreal injection. The changes in the visual pathways projected from the blind eye and the remaining eye in Group A and the normal eyes in Group B were compared to determine if new visual compensatory pathways were established. Additionally, VEP tests were performed to determine complete blindness, and AVR examinations were performed to help identify the generation of auditory compensatory function.

RESULTS

The VEP test indicated complete visual loss after ON transection. In the monocularly blind rats, the contrast-to-noise ratio (CNR) of ON, optic tract (OT), lateral geniculate nucleus (LGN), superior colliculus (SC), optic radiation (OR) and visual cortex (VC) of visual pathway projected from the left eye was significantly higher than that of the right pathway (p < .001). Moreover, the CNR of ON, OT, LGN, SC, OR and VC in the visual pathway projected from the left eye of monocularly blind rats was significantly lower than those of normal rats (p < .05). The AVR results revealed that the corresponding bilateral visual cortex in monocularly blind rats did not respond to the auditory stimulus or showed dissimilation with the low frequency.

CONCLUSION

MEMRI combined with electrophysiology, including VEP and AVR, may be potentially helpful in the evaluation of the possible generation of new visual/auditory compensatory pathways after monocular blindness.