In vivo retinotopic mapping of superior colliculus using manganese-enhanced magnetic resonance imaging

In vivo MRI evaluation of anterograde manganese transport along the visual pathway following whole eye transplantation

BACKGROUND

Since adult mammalian retinal ganglion cells cannot regenerate after injury, we have recently established a whole-eye transplantation (WET) rat model that provides an intact optical system to investigate potential surgical restoration of irreversible vision loss. However, it remains to be elucidated whether physiological axoplasmic transport exists in the transplanted visual pathway. New Method: We developed an in vivo imaging model system to assess WET integration using manganese-enhanced magnetic resonance imaging (MEMRI) in rats. Since Mn²⁺ is a calcium analogue and an active T1-positive contrast agent, the levels of anterograde manganese transport can be evaluated in the visual pathways upon intravitreal Mn²⁺ administration into both native and transplanted eyes.

RESULTS

 No significant intraocular pressure difference was found between native and transplanted eyes, whereas comparable manganese enhancement was observed between native and transplanted intraorbital optic nerves, suggesting the presence of anterograde manganese transport after WET. No enhancement was detected across the coaptation site in the higher visual areas of the recipient brain. Comparison with Existing Methods: Existing imaging methods to assess WET focus on either the eye or local optic nerve segments without direct visualization and longitudinal quantification of physiological transport along the transplanted visual pathway, hence the development of in vivo MEMRI.

CONCLUSION

 Our established imaging platform indicated that essential physiological transport exists in the transplanted optic nerve after WET. As neuroregenerative approaches are being developed to connect the transplanted eye to the recipient's brain, in vivo MEMRI is well-suited to guide strategies for successful WET integration for vision restoration. Keywords (Max 6): Anterograde transport, magnetic resonance imaging, manganese, neuroregeneration, optic nerve, whole-eye transplantation.

Mechanisms implicated in the contralateral effect in the central nervous system after unilateral injury: focus on the visual system

The retina, as part of the central nervous system is an ideal model to study the response of neurons to injury and disease and to test new treatments. During the last decade is becoming clear that unilateral lesions in bilateral areas of the central nervous system trigger an inflammatory response in the contralateral uninjured site. This effect has been better studied in the visual system where, as a rule, one retina is used as experimental and the other as control. Contralateral retinas in unilateral models of retinal injury show neuronal degeneration and glial activation. The mechanisms by which this adverse response in the central nervous system occurs are discussed in this review, focusing primarily on the visual system.

Alcohol use severity and the neural correlates of the effects of sleep disturbance on sustained visual attention

Alcohol misuse is associated with sleep disturbance and cognitive dysfunction. However, the neural processes inter-relating the severity of alcohol use, sleep disturbance and cognitive performance remain under-investigated. We addressed this issue with a dataset of 964 subjects (504 women) curated from the Human Connectome Project. Participants were assessed with the Pittsburgh Sleep Quality Index (PSQI) and fMRI while identifying relational dimension pictures and matching dimension pictures (as a control) in alternating blocks. Imaging data were analyzed with published routines and the results were evaluated at a corrected threshold. Subjects showed lower accuracy rate and longer reaction time (RT) in relational than control blocks. The difference in RT between the two blocks (RT_Rel-Con) was driven primarily by the RT and correlated positively with performance accuracy of relational trials, suggesting that a more cautious response (i.e., longer RT_Rel-Con) improved accuracy. The severity of alcohol use, identified from principal component analysis of drinking metrics, was positively correlated with sleep disturbance. Further, whole-brain regression identified activity of the superior colliculus (SC) during relational vs. control blocks in positive and negative correlation with RT_Rel-Con and PSQI score, respectively. Mediation and path analyses demonstrated a significant model: more severe alcohol use → greater sleep disturbance → diminished SC activity → impaired performance. These findings support the influences of alcohol misuse on sleep and suggest neural correlates that mediate the relationship between sleep disturbance and altered sustained attention in young adults.

Citicoline Modulates Glaucomatous Neurodegeneration Through Intraocular Pressure-Independent Control

Glaucoma is a neurodegenerative disease that causes progressive, irreversible vision loss. Currently, intraocular pressure (IOP) is the only modifiable risk factor for glaucoma. However, glaucomatous degeneration may continue despite adequate IOP control. Therefore, there exists a need for treatment that protects the visual system, independent of IOP. This study sought, first, to longitudinally examine the neurobehavioral effects of different magnitudes and durations of IOP elevation using multi-parametric magnetic resonance imaging (MRI), optokinetics and histology; and, second, to evaluate the effects of oral citicoline treatment as a neurotherapeutic in experimental glaucoma. Eighty-two adult Long Evans rats were divided into six groups: acute (mild or severe) IOP elevation, chronic (citicoline-treated or untreated) IOP elevation, and sham (acute or chronic) controls. We found that increasing magnitudes and durations of IOP elevation differentially altered structural and functional brain connectivity and visuomotor behavior, as indicated by decreases in fractional anisotropy in diffusion tensor MRI, magnetization transfer ratios in magnetization transfer MRI, T1-weighted MRI enhancement of anterograde manganese transport, resting-state functional connectivity, visual acuity, and neurofilament and myelin staining along the visual pathway. Furthermore, 3 weeks of oral citicoline treatment in the setting of chronic IOP elevation significantly reduced visual brain integrity loss and visual acuity decline without altering IOP. Such effects sustained after treatment was discontinued for another 3 weeks. These results not only illuminate the close interplay between eye, brain, and behavior in glaucomatous neurodegeneration, but also support a role for citicoline in protecting neural tissues and visual function in glaucoma beyond IOP control.

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.

Age-related Changes in Eye, Brain and Visuomotor Behavior in the DBA/2J Mouse Model of Chronic Glaucoma

Although elevated intraocular pressure (IOP) and age are major risk factors for glaucoma, their effects on glaucoma pathogenesis remain unclear. This study examined the onset and progression of glaucomatous changes to ocular anatomy and physiology, structural and physiological brain integrity, and visuomotor behavior in the DBA/2J mice via non-invasive tonometry, multi-parametric magnetic resonance imaging (MRI) and optokinetic assessments from 5 to 12 months of age. Using T2-weighted MRI, diffusion tensor MRI, and manganese-enhanced MRI, increasing IOP elevation at 9 and 12 months old coincided with anterior chamber deepening, altered fractional anisotropy and radial diffusivity of the optic nerve and optic tract, as well as reduced anterograde manganese transport along the visual pathway respectively in the DBA/2J mice. Vitreous body elongation and visuomotor function deterioration were observed until 9 months old, whereas axial diffusivity only decreased at 12 months old in diffusion tensor MRI. Under the same experimental settings, C57BL/6J mice only showed modest age-related changes. Taken together, these results indicate that the anterior and posterior visual pathways of the DBA/2J mice exhibit differential susceptibility to glaucomatous neurodegeneration observable by in vivo multi-modal examinations.

Endogenous spontaneous ultraweak photon emission in the formation of eye-specific retinogeniculate projections before birth

In 1963, it was suggested [Sperry, R.W. (1963). Chemoaffinity in the orderly growth of nerve fiber patterns and connections. Proc. Natl. Acad. Sci. USA 50, 703-710.] that molecular cues can direct the development of orderly connections between the eye and the brain (the "chemoaffinity hypothesis"). In the same year, the amazing degree of functional accuracy of the visual pathway in the absence of any external light/photon perception prior to birth [Wiesel, T.N and Hubel, D.H. (1963). Single-cell responses in striate cortex of kittens deprived of vision in one eye. J. Neurophysiol. 26, 1003-1017.] was discovered. These recognitions revealed that the wiring of the visual system relies on innate cues. However, how the eye-specific retinogeniculate pathway can be developed before birth without any visual experience is still an unresolved issue. In the present paper, we suggest that Müller cells (functioning as optical fibers), Müller cell cone (i.e. the inner half of the foveola that is created of an inverted cone-shaped zone of Müller cells), discrete retinal noise of rods, and intrinsically photosensitive retinal ganglion cells might have key functions by means of retinal spontaneous ultraweak photon emission in the development of eye-specific retinogeniculate pathways prior to birth.

Anterograde Transport in Axons of the Retinal Ganglion Cells and its Relationship to the Intraocular Pressure during Aging in Mice with Hereditary Pigmentary Glaucoma

PURPOSE

To establish a relationship between impairment of the anterograde axonal transport (AAT) in the axons of the retinal ganglion cells and the intraocular pressure (IOP) during aging in mice with hereditary glaucoma.

METHODS

Quantitative in vivo approach based on manganese enhanced magnetic resonance imaging was developed in order to evaluate AAT in 3-, 6-, and 14-month old DBA/2J mice that develop age-dependent pigmentary glaucoma or age-matched C57Bl/6 mice that do not develop any retinal disease. Unilateral intravitreous administration of MnCl₂ solution was followed 24 h later by MRI performed to obtain spin-lattice relaxation times (T₁) for regions of interest encompassing the superior colliculi (SC) and the lateral geniculate nuclei (LGN). From the MRI scans, the estimates of Mn²⁺ concentrations in SC and LGN contralateral to the injection site, hence the efficiency of AAT in ON, were obtained. IOP and eye morphology was also monitored.

RESULTS

In C57Bl/6 mice, AAT to SC was decreasing with age, 30% decrease was noted between 3 and 14 months. The decrease in axonal transport to LGN was less pronounced in this strain. In 3-month-old DBA/2J mice, axonal transport to SC was 30% lower than in 3-month-old C57Bl/6 mice but no significant decrease was noted in 6-month-old animals. However, a decrease of over 95% in axonal transport both to SC and LGN was noted in 14-month-old DBA/2J mice. DBA/2J mice exhibited a sharp increase in IOP at 6 months, which reversed at 14 months but displayed age-dependent elongation of the eyeball and deepening of the anterior chamber.

CONCLUSION

Failure of AAT to SC of DBA/2J mice during development of pigmentary glaucoma does not follow closely changes in IOP and eye morphology. The relationship between IOP and AAT in optic nerve and tract is complex and may reflect preconditioning mechanism.

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.

Mechanisms of phosphenes in irradiated patients

Anomalous visual perceptions have been reported in various diseases of the retina and visual pathways or can be experienced under specific conditions in healthy individuals. Phosphenes are perceptions of light in the absence of ambient light, occurring independently of the physiological and classical photonic stimulation of the retina. They are a frequent symptom in patients irradiated in the region of the central nervous system (CNS), head and neck and the eyes. Phosphenes have historically been attributed to complex physical phenomena such as Cherenkov radiation. While phosphenes are related to Cherenkov radiation under high energy photon/electron irradiation conditions, physical phenomena are unlikely to be responsible for light flashes at energies used for ocular proton therapy. Phosphenes may involve a direct role for ocular photoreceptors and possible interactions between cones and rods. Other mechanisms involving the retinal ganglion cells or ultraweak biophoton emission and rhodopsin bleaching after exposure to free radicals are also likely to be involved. Despite their frequency as shown in our preliminary observations, phosphenes have been underreported probably because their mechanism and impact are poorly understood. Recently, phosphenes have been used to restore the vision and whether they might predict vision loss after therapeutic irradiation is a current field of investigation. We have reviewed and also investigated here the mechanisms related to the occurrence of phosphenes in irradiated patients and especially in patients irradiated by proton therapy for ocular tumors.

Manganese-enhanced MR imaging (MEMRI) combined with electrophysiology in the study of cross-modal plasticity in binocularly blind rats

Our study aimed to determine and verify the establishment of visual to auditory cross-modal plasticity using manganese-enhanced MR imaging (MEMRI) combined with examinations of the visual evoked potential (VEP), auditory brainstem response (ABR) and bilateral visual cortex response to a bilateral auditory stimulus (AVR). Fourteen healthy male Sprague-Dawley newborn rats were randomly divided into 2 groups (n=7 per group). Optic nerve transection was performed in the 7 rats of Group A three weeks after birth to obtain binocularly blind models, and the 7 rats of Group B were maintained as the control group. The VEP was measured to ensure complete binocular blindness. Four months after the operation, MnCl₂ was injected into the left inner ear perilymph of all rats, and an MRI examination was performed 24h after injection. Then, ABR and AVR were measured to detect the generation of auditory compensatory function. The results of the VEP have confirmed complete binocular blindness. The normalized signal intensity of the bilateral medial geniculate nucleus, auditory cortex, visual center (including the lateral geniculate nucleus, superior colliculus and visual cortex) and right hippocampus in binocularly blind rats was significantly increased compared with that in normal rats (P≤0.005), which was confirmed by measurement of the ABR and AVR. Our results suggested that MEMRI combined with electrophysiology can show changes in the auditory and visual pathways of binocularly blind rats and demonstrate the generation of an auditory compensatory pathway.

Activation of Retinotopic Visual Areas Is Central to REM Sleep Associated Dreams: Visual Dreams and Visual Imagery Possibly Co-Emerged In Evolution

The latest experimental results support that multiple retinotopic visual systems play a central role not only in the processing of visual signals but also in the integration and processing of internally represented auditory and tactile information. These retinotopic maps have access to higher levels of cognitive processing, performed by the frontal lobes, for example. The occipital cortex may have a special role in multisensory integration. There is a functional basis for the development and maturation of visual memory in association of rapid eye movement sleep (REMS) which is linked to dreams and visual imagery. Physiological and psychological processes of REMS are similar to waking visual imagery. Furthermore, visual imagery during REMS utilize a common visual neural pathway similar to that used in wakefulness. This pathway subserves visual processes accompanied with auditory experiences and intrinsic feelings. We argue that the activation of the retinotopic visual areas is central to REM sleep associated dreams and that REMS associated dreaming and visual imagery may have co-evolved in homeothermic animals during evolution. We also suggest that protoconscious state during REM sleep, as introduced by Hobson many years ago, may be a basic visual process.

Longitudinal Assessments of Normal and Perilesional Tissues in Focal Brain Ischemia and Partial Optic Nerve Injury with Manganese-enhanced MRI

Although manganese (Mn) can enhance brain tissues for improving magnetic resonance imaging (MRI) assessments, the underlying neural mechanisms of Mn detection remain unclear. In this study, we used Mn-enhanced MRI to test the hypothesis that different Mn entry routes and spatiotemporal Mn distributions can reflect different mechanisms of neural circuitry and neurodegeneration in normal and injured brains. Upon systemic administration, exogenous Mn exhibited varying transport rates and continuous redistribution across healthy rodent brain nuclei over a 2-week timeframe, whereas in rodents following photothrombotic cortical injury, transient middle cerebral artery occlusion, or neonatal hypoxic-ischemic brain injury, Mn preferentially accumulated in perilesional tissues expressing gliosis or oxidative stress within days. Intravitreal Mn administration to healthy rodents not only allowed tracing of primary visual pathways, but also enhanced the hippocampus and medial amygdala within a day, whereas partial transection of the optic nerve led to MRI detection of degrading anterograde Mn transport at the primary injury site and the perilesional tissues secondarily over 6 weeks. Taken together, our results indicate the different Mn transport dynamics across widespread projections in normal and diseased brains. Particularly, perilesional brain tissues may attract abnormal Mn accumulation and gradually reduce anterograde Mn transport via specific Mn entry routes.

Manganese-enhanced MRI (ME MRI) in evaluation of the auditory pathway in an experimental rat model

This study aimed to explore the optimal dose and manner of administration for visualization of the auditory pathway on manganese-enhanced MRI (ME MRI). Twenty-four healthy male Sprague-Dawley rats were randomly divided into three experimental groups (n = 8 for Groups A, B and C). The rats in Groups A, B and C were subjected to MnCl₂ injection through the tympanum, inner ear endolymph and perilymph, respectively (0.2 M for four rats and 0.4 M for the others in each group) and observed at 1, 2, 3, 4, 7 and 10 days after the operation with 3.0 T MRI. The signal intensity (SI) and dynamic changes of the auditory pathways at various times, and at two doses through three injection routes, were compared by statistical analysis. Administration of MnCl₂ through the perilymph best showed the complete auditory pathway (P < 0.01), whereas administration though the tympanum only demonstrated part of the pathway. The SI was highest at 24 h after administration of the tracer and began to decline at 48 h. The SI of the auditory cortex was higher after the injection of 0.4 M MnCl₂ than that of 0.2 M MnCl₂ . ME MRI best demonstrated the whole auditory pathway at 24 h after the injection of 0.4 M MnCl₂ through the perilymph in the rat, which provided an optimal method for the study of ME MRI of the auditory pathway in the animal model.

Advanced Functional Nanomaterials for Theranostics

Nanoscale materials have been explored extensively as agents for therapeutic and diagnostic (i.e. theranostic) applications. Research efforts have shifted from exploring new materials in vitro to designing materials that function in more relevant animal disease models, thereby increasing potential for clinical translation. Current interests include non-invasive imaging of diseases, biomarkers and targeted delivery of therapeutic drugs. Here, we discuss some general design considerations of advanced theranostic materials and challenges of their use, from both diagnostic and therapeutic perspectives. Common classes of nanoscale biomaterials, including magnetic nanoparticles, quantum dots, upconversion nanoparticles, mesoporous silica nanoparticles, carbon-based nanoparticles and organic dye-based nanoparticles, have demonstrated potential for both diagnosis and therapy. Variations such as size control and surface modifications can modulate biocompatibility and interactions with target tissues. The needs for improved disease detection and enhanced chemotherapeutic treatments, together with realistic considerations for clinically translatable nanomaterials will be key driving factors for theranostic agent research in the near future.

Glaucoma: the retina and beyond

Over 60 million people worldwide are diagnosed with glaucomatous optic neuropathy, which is estimated to be responsible for 8.4 million cases of irreversible blindness globally. Glaucoma is associated with characteristic damage to the optic nerve and patterns of visual field loss which principally involves the loss of retinal ganglion cells (RGCs). At present, intraocular pressure (IOP) presents the only modifiable risk factor for glaucoma, although RGC and vision loss can continue in patients despite well-controlled IOP. This, coupled with the present inability to diagnose glaucoma until relatively late in the disease process, has led to intense investigations towards the development of novel techniques for the early diagnosis of disease. This review outlines our current understanding of the potential mechanisms underlying RGC and axonal loss in glaucoma. Similarities between glaucoma and other neurodegenerative diseases of the central nervous system are drawn before an overview of recent developments in techniques for monitoring RGC health is provided, including recent progress towards the development of RGC specific contrast agents. The review concludes by discussing techniques to assess glaucomatous changes in the brain using MRI and the clinical relevance of glaucomatous-associated changes in the visual centres of the brain.

In Vivo Evaluation of the Visual Pathway in Streptozotocin-Induced Diabetes by Diffusion Tensor MRI and Contrast Enhanced MRI

Visual function has been shown to deteriorate prior to the onset of retinopathy in some diabetic patients and experimental animal models. This suggests the involvement of the brain's visual system in the early stages of diabetes. In this study, we tested this hypothesis by examining the integrity of the visual pathway in a diabetic rat model using in vivo multi-modal magnetic resonance imaging (MRI). Ten-week-old Sprague-Dawley rats were divided into an experimental diabetic group by intraperitoneal injection of 65 mg/kg streptozotocin in 0.01 M citric acid, and a sham control group by intraperitoneal injection of citric acid only. One month later, diffusion tensor MRI (DTI) was performed to examine the white matter integrity in the brain, followed by chromium-enhanced MRI of retinal integrity and manganese-enhanced MRI of anterograde manganese transport along the visual pathway. Prior to MRI experiments, the streptozotocin-induced diabetic rats showed significantly smaller weight gain and higher blood glucose level than the control rats. DTI revealed significantly lower fractional anisotropy and higher radial diffusivity in the prechiasmatic optic nerve of the diabetic rats compared to the control rats. No apparent difference was observed in the axial diffusivity of the optic nerve, the chromium enhancement in the retina, or the manganese enhancement in the lateral geniculate nucleus and superior colliculus between groups. Our results suggest that streptozotocin-induced diabetes leads to early injury in the optic nerve when no substantial change in retinal integrity or anterograde transport along the visual pathways was observed in MRI using contrast agent enhancement. DTI may be a useful tool for detecting and monitoring early pathophysiological changes in the visual system of experimental diabetes non-invasively.

Dose response and time course of manganese-enhanced magnetic resonance imaging for visual pathway tracing in vivo

Axonal tracing is useful for detecting optic nerve injury and regeneration, but many commonly used methods cannot be used to observe axoplasmic flow and synaptic transmission in vivo. Manganese (Mn(2+))-enhanced magnetic resonance imaging (MEMRI) can be used for in vivo longitudinal tracing of the visual pathway. Here, we explored the dose response and time course of an intravitreal injection of MnCl2 for tracing the visual pathway in rabbits in vivo using MEMRI. We found that 2 mM MnCl2 enhanced images of the optic nerve but not the lateral geniculate body or superior colliculus, whereas at all other doses tested (5-40 mM), images of the visual pathway from the retina to the contralateral superior colliculus were significantly enhanced. The images were brightest at 24 hours, and then decreased in brightness until the end of the experiment (7 days). No signal enhancement was observed in the visual cortex at any concentration of MnCl2. These results suggest that MEMRI is a viable method for temporospatial tracing of the visual pathway in vivo. Signal enhancement in MEMRI depends on the dose of MnCl2, and the strongest signals appear 24 hours after intravitreal injection.

Non-invasive MRI Assessments of Tissue Microstructures and Macromolecules in the Eye upon Biomechanical or Biochemical Modulation

The microstructural organization and composition of the corneoscleral shell (CSS) determine the biomechanical behavior of the eye, and are important in diseases such as glaucoma and myopia. However, limited techniques can assess these properties globally, non-invasively and quantitatively. In this study, we hypothesized that multi-modal magnetic resonance imaging (MRI) can reveal the effects of biomechanical or biochemical modulation on CSS. Upon intraocular pressure (IOP) elevation, CSS appeared hyperintense in both freshly prepared ovine eyes and living rat eyes using T2-weighted MRI. Quantitatively, transverse relaxation time (T2) of CSS increased non-linearly with IOP at 0-40 mmHg and remained longer than unloaded tissues after being unpressurized. IOP loading also increased fractional anisotropy of CSS in diffusion tensor MRI without apparent change in magnetization transfer MRI, suggestive of straightening of microstructural fibers without modification of macromolecular contents. Lastly, treatments with increasing glyceraldehyde (mimicking crosslinking conditions) and chondroitinase-ABC concentrations (mimicking glycosaminoglycan depletion) decreased diffusivities and increased magnetization transfer in cornea, whereas glyceraldehyde also increased magnetization transfer in sclera. In summary, we demonstrated the changing profiles of MRI contrast mechanisms resulting from biomechanical or biochemical modulation of the eye non-invasively. Multi-modal MRI may help evaluate the pathophysiological mechanisms in CSS and the efficacy of corneoscleral treatments.

Semiquantitative assessment of optic nerve injury using manganese-enhanced MRI

OBJECTIVE

To evaluate the capability of manganese (Mn(2+))-enhanced MRI (MEMRI) in a continuously semiquantitative assessment of rat optic nerve (ON) injury.

METHODS

Forty rats were divided into three groups: (I) a control group that was submitted to MEMRI or to fluorescent labeling of retinal ganglion cells (RGCs) (n = 10); (II) an ON injury group that was submitted to MEMRI (n = 15); (III) an ON injury group that was submitted to fluorescent labeling of RGCs (n = 15). Groups II and III were examined at 3, 7, and 14 days post-lesion (dpl), when the contrast-to-noise ratio (CNR) of the retina and ON was measured on MEMRI images and the RGCs were counted by fluorescence microscopy and compared between the groups.

RESULTS

In the control group, the intact visual pathway from the retina to the contralateral superior colliculus was visualized by MEMRI. In group II, continuous Mn(2+) enhancement was seen from the retina to the lesion site of the optic nerves at 3, 7, and 14 dpl. However, no Mn(2+) enhancement was observed distal to the lesion site at those time points. The observed Mn(2+) enhancement proximal to the ON lesion site declined between 7 and 14 dpl. The decrease in Mn(2+)-enhanced signal intensity at these sites at 7 and 14 dpl when compared to that at 3 dpl was significant (P < 0.05). The RGC density dropped by 6.84, 45.31, and 72.36 % at 3, 7, and 14 dpl, respectively.

CONCLUSION

MEMRI can be used to evaluate the structural changes after optic nerve injury.

In Vivo Evaluation of White Matter Integrity and Anterograde Transport in Visual System After Excitotoxic Retinal Injury With Multimodal MRI and OCT

PURPOSE

Excitotoxicity has been linked to the pathogenesis of ocular diseases and injuries and may involve early degeneration of both anterior and posterior visual pathways. However, their spatiotemporal relationships remain unclear. We hypothesized that the effects of excitotoxic retinal injury (ERI) on the visual system can be revealed in vivo by diffusion tensor magnetic resonance imagining (DTI), manganese-enhanced magnetic resonance imagining (MRI), and optical coherence tomography (OCT).

METHODS

Diffusion tensor MRI was performed at 9.4 Tesla to monitor white matter integrity changes after unilateral N-methyl-D-aspartate (NMDA)-induced ERI in six Sprague-Dawley rats and six C57BL/6J mice. Additionally, four rats and four mice were intravitreally injected with saline to compare with NMDA-injected animals. Optical coherence tomography of the retina and manganese-enhanced MRI of anterograde transport were evaluated and correlated with DTI parameters.

RESULTS

In the rat optic nerve, the largest axial diffusivity decrease and radial diffusivity increase occurred within the first 3 and 7 days post ERI, respectively, suggestive of early axonal degeneration and delayed demyelination. The optic tract showed smaller directional diffusivity changes and weaker DTI correlations with retinal thickness compared with optic nerve, indicative of anterograde degeneration. The splenium of corpus callosum was also reorganized at 4 weeks post ERI. The DTI profiles appeared comparable between rat and mouse models. Furthermore, the NMDA-injured visual pathway showed reduced anterograde manganese transport, which correlated with diffusivity changes along but not perpendicular to optic nerve.

CONCLUSIONS

Diffusion tensor MRI, manganese-enhanced MRI, and OCT provided an in vivo model system for characterizing the spatiotemporal changes in white matter integrity, the eye-brain relationships and structural-physiological relationships in the visual system after ERI.

Effect of Lycium barbarum (Wolfberry) on alleviating axonal degeneration after partial optic nerve transection

Our previous results showed that the polysaccharides extracted from Lycium barbarum (LBP) could delay secondary degeneration of retinal ganglion cell bodies and improve the function of the retinas after partial optic nerve transection (PONT). Although the common degeneration mechanisms were believed to be shared by both neuronal bodies and axons, recently published data from slow Wallerian degeneration mutant (Wld(s)) mice supported the divergence in the mechanisms of them. Therefore, we want to determine if LBP could also delay the degeneration of axons after PONT. Microglia/macrophages were thought to be a source of reactive oxygen species after central nervous system (CNS) injury. After PONT, however, oxidative stress was believed to occur prior to the activation of microglia/macrophages in the areas vulnerable to secondary degeneration both in the optic nerves (ONs) and the retinas. But the results did not take into account the morphological changes of microglia/macrophages after their activation. So we examined the morphology in addition to the response magnitude of microglia/macrophages to determine their time point of activation. In addition, the effects of LBP on the activation of microglia/macrophages were investigated. The results showed that (1) LBP reduced the loss of axons in the central ONs and preserved the g-ratio (axon diameter/fiber diameter) in the ventral ONs although no significant effect was detected in the dorsal ONs; (2) microglia/macrophages were activated in the ONs by 12 h after PONT; (3) LBP decreased the response magnitude of microglia/macrophages 4 weeks after PONT. In conclusion, our results showed that LBP could delay secondary degeneration of the axons, and LBP could also inhibit the activation of microglia/macrophages. Therefore, LBP could be a promising herbal medicine to delay secondary degeneration in the CNS via modulating the function of microglia/macrophages.

Long-term effects of neonatal hypoxia-ischemia on structural and physiological integrity of the eye and visual pathway by multimodal MRI

PURPOSE

Neonatal hypoxia-ischemia is a major cause of brain damage in infants and may frequently present visual impairments. Although advancements in perinatal care have increased survival, the pathogenesis of hypoxic-ischemic injury and the long-term consequences to the visual system remain unclear. We hypothesized that neonatal hypoxia-ischemia can lead to chronic, MRI-detectable structural and physiological alterations in both the eye and the brain's visual pathways.

METHODS

Eight Sprague-Dawley rats underwent ligation of the left common carotid artery followed by hypoxia for 2 hours at postnatal day 7. One year later, T2-weighted MRI, gadolinium-enhanced MRI, chromium-enhanced MRI, manganese-enhanced MRI, and diffusion tensor MRI (DTI) of the visual system were evaluated and compared between opposite hemispheres using a 7-Tesla scanner.

RESULTS

Within the eyeball, systemic gadolinium administration revealed aqueous-vitreous or blood-ocular barrier leakage only in the ipsilesional left eye despite comparable aqueous humor dynamics in the anterior chamber of both eyes. Binocular intravitreal chromium injection showed compromised retinal integrity in the ipsilesional eye. Despite total loss of the ipsilesional visual cortex, both retinocollicular and retinogeniculate pathways projected from the contralesional eye toward ipsilesional visual cortex possessed stronger anterograde manganese transport and less disrupted structural integrity in DTI compared with the opposite hemispheres.

CONCLUSIONS

High-field, multimodal MRI demonstrated in vivo the long-term structural and physiological deficits in the eye and brain's visual pathways after unilateral neonatal hypoxic-ischemic injury. The remaining retinocollicular and retinogeniculate pathways appeared to be more vulnerable to anterograde degeneration from eye injury than retrograde, transsynaptic degeneration from visual cortex injury.

Axonal transport rate decreased at the onset of optic neuritis in EAE mice

Optic neuritis is frequently the first symptom of multiple sclerosis (MS), an inflammatory demyelinating neurodegenerative disease. Impaired axonal transport has been considered as an early event of neurodegenerative diseases. However, few studies have assessed the integrity of axonal transport in MS or its animal models. We hypothesize that axonal transport impairment occurs at the onset of optic neuritis in experimental autoimmune encephalomyelitis (EAE) mice. In this study, we employed manganese-enhanced MRI (MEMRI) to assess axonal transport in optic nerves in EAE mice at the onset of optic neuritis. Axonal transport was assessed as (a) optic nerve Mn(2+) accumulation rate (in % signal change/h) by measuring the rate of increased total optic nerve signal enhancement, and (b) Mn(2+) transport rate (in mm/h) by measuring the rate of change in optic nerve length enhanced by Mn(2+). Compared to sham-treated healthy mice, Mn(2+) accumulation rate was significantly decreased by 19% and 38% for EAE mice with moderate and severe optic neuritis, respectively. The axonal transport rate of Mn(2+) was significantly decreased by 43% and 65% for EAE mice with moderate and severe optic neuritis, respectively. The degree of axonal transport deficit correlated with the extent of impaired visual function and diminished microtubule-associated tubulins, as well as the severity of inflammation, demyelination, and axonal injury at the onset of optic neuritis.

In vivo assessment of aqueous humor dynamics upon chronic ocular hypertension and hypotensive drug treatment using gadolinium-enhanced MRI

PURPOSE

Although glaucoma treatments alter aqueous humor (AH) dynamics to lower intraocular pressure, the regulatory mechanisms of AH circulation and their contributions to the pathogenesis of ocular hypertension and glaucoma remain unclear. We hypothesized that gadolinium-enhanced magnetic resonance imaging (Gd-MRI) can visualize and assess AH dynamics upon sustained intraocular pressure elevation and pharmacologic interventions.

METHODS

Gadolinium contrast agent was systemically administered to adult rats to mimic soluble AH components entering the anterior chamber (AC) via blood-aqueous barrier. Dynamic Gd-MRI was applied to examine the signal enhancement in AC and vitreous body upon microbead-induced ocular hypertension and unilateral topical applications of latanoprost, timolol maleate, and brimonidine tartrate to healthy eyes.

RESULTS

Gadolinium signal time courses in microbead-induced hypertensive eyes possessed faster initial gadolinium uptake and higher peak signals in AC than control eyes, reflective of reduced gadolinium clearance upon microbead occlusion. Opposite trends were observed in latanoprost- and timolol-treated eyes, indicative of their respective drug actions on increased uveoscleral outflow and reduced AH production. The slowest initial gadolinium uptake but strongest peak signals were found in AC of both brimonidine-treated and untreated fellow eyes. These findings drew attention to the systemic effects of topical hypotensive drug treatment. Gadolinium leaked into the vitreous of microbead-induced hypertensive eyes and brimonidine-treated and untreated fellow eyes, suggestive of a compromise of aqueous-vitreous or blood-ocular barrier integrity.

CONCLUSIONS

Gadolinium-enhanced MRI allows spatiotemporal and quantitative evaluation of altered AH dynamics and ocular tissue permeability for better understanding the physiological mechanisms of ocular hypertension and the efficacy of antiglaucoma drug treatments.

In vivo visuotopic brain mapping with manganese-enhanced MRI and resting-state functional connectivity MRI

The rodents are an increasingly important model for understanding the mechanisms of development, plasticity, functional specialization and disease in the visual system. However, limited tools have been available for assessing the structural and functional connectivity of the visual brain network globally, in vivo and longitudinally. There are also ongoing debates on whether functional brain connectivity directly reflects structural brain connectivity. In this study, we explored the feasibility of manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administration for visuotopic brain mapping and understanding of physiological transport in normal and visually deprived adult rats. In addition, resting-state functional connectivity MRI (RSfcMRI) was performed to evaluate the intrinsic functional network and structural-functional relationships in the corresponding anatomical visual brain connections traced by MEMRI. Upon intravitreal, subcortical, and intracortical Mn(2+) injection, different topographic and layer-specific Mn enhancement patterns could be revealed in the visual cortex and subcortical visual nuclei along retinal, callosal, cortico-subcortical, transsynaptic and intracortical horizontal connections. Loss of visual input upon monocular enucleation to adult rats appeared to reduce interhemispheric polysynaptic Mn(2+) transfer but not intra- or inter-hemispheric monosynaptic Mn(2+) transport after Mn(2+) injection into visual cortex. In normal adults, both structural and functional connectivity by MEMRI and RSfcMRI was stronger interhemispherically between bilateral primary/secondary visual cortex (V1/V2) transition zones (TZ) than between V1/V2 TZ and other cortical nuclei. Intrahemispherically, structural and functional connectivity was stronger between visual cortex and subcortical visual nuclei than between visual cortex and other subcortical nuclei. The current results demonstrated the sensitivity of MEMRI and RSfcMRI for assessing the neuroarchitecture, neurophysiology and structural-functional relationships of the visual brains in vivo. These may possess great potentials for effective monitoring and understanding of the basic anatomical and functional connections in the visual system during development, plasticity, disease, pharmacological interventions and genetic modifications in future studies.

Manganese-enhanced MRI (MEMRI) via topical loading of Mn(2+) significantly impairs mouse visual acuity: a comparison with intravitreal injection

Manganese-enhanced MRI (MEMRI) with topical loading of MnCl2 provides optic nerve enhancement comparable to that seen by intravitreal injection. However, the impact of this novel and non-invasive Mn(2+) loading method on visual function requires further assessments. The objective of this study is to determine the optimal topical Mn(2+) loading dosage for MEMRI and to assess visual function after MnCl2 loading. Intravitreal administration was performed to compare the two approaches of MnCl2 loading. Twenty-four hours after topical loading of 0, 0.5, 0.75, and 1 M MnCl2 , T1 -weighted, T2-weighted, diffusion tensor imaging and visual acuity (VA) assessments were performed to determine the best topical loading dosage for MEMRI measurements and to assess the integrity of retinas and optic nerves. Mice were perfusion fixed immediately after in vivo experiments for hematoxylin and eosin and immunohistochemistry staining. Topical loading of 1 M MnCl2 damaged the retinal photoreceptor layer with no detectable damage to retina ganglion cell layers or prechiasmatic optic nerves. For the topical loading, 0.75 M MnCl2 was required to see sufficient enhancement of the optic nerve. At this concentration the visual function was significantly affected, followed by a slow recovery. Intravitreal injection (0.25 μL of 0.2 M MnCl2 ) slightly affected VA, with full recovery a day later. To conclude, intravitreal MnCl2 injection provides more reproducible results with less adverse side-effects than topical loading.

Effect of lycium barbarum (wolfberry) polysaccharides on preserving retinal function after partial optic nerve transection

Lycium Barbarum Polysaccharides (LBP) are the active components of Wolfberry (a traditional Chinese medicine) which has long been used for improving visual function. This study aims to investigate localized changes of retinal function in a partial optic nerve transection (PONT) model, and effects of LBP on visual function. The multifocal electroretinograms (mfERG) were obtained from 30 eyes of 30 Sprague-Dawley rats. The rats were divided into 6 groups (five treatment groups and one control group). Starting from the first day of the experiment, the rats in the (PONT+LBP) group and the (LBP) group were dosed with LBP; rats in the (PONT+PBS (phosphate buffered saline)) group and the (PBS) group were dosed with PBS via nasogastric tube every day until euthanized. The dorsal part of the optic nerve was transected in the (PONT), (PONT+LBP) and (PONT+PBS) groups at the end of week 1 (day 7 after LBP or PBS feeding began). The mfERG was measured at three time points: week 2, week 3 and week 5. Significant reduction of P1 and PhNR amplitudes of the mfERG were observed in all retinal regions a week after PONT. Feeding with LBP prior to PONT preserved retinal function. All mfERG responses returned to the normal range in the superior retina, which corresponds to the transected dorsal region of the optic nerve, while most of the inferior retinal responses were significantly increased at week 4 after PONT. The ventral part of the retina had secondary degeneration which was not only limited to the ganglion cell layer, but is a widespread effect affecting the outer retina. LBP altered the functional reduction caused by PONT by regulating the signal from the outer retina.

Brain resting-state functional MRI connectivity: morphological foundation and plasticity

Despite the immense ongoing efforts to map brain functional connections and organizations with resting-state functional MRI (rsfMRI), the mechanisms governing the temporally coherent rsfMRI signals remain unclear. In particular, there is a lack of direct evidence regarding the morphological foundation and plasticity of these rsfMRI derived connections. In this study, we investigated the role of axonal projections in rsfMRI connectivity and its plasticity. Well-controlled rodent models of complete and posterior corpus callosotomy were longitudinally examined with rsfMRI at 7T in conjunction with intracortical EEG recording and functional MRI tracing of interhemispheric neuronal pathways by manganese (Mn(2+)). At post-callosotomy day 7, significantly decreased interhemispheric rsfMRI connectivity was observed in both groups in the specific cortical areas whose callosal connections were severed. At day 28, the disrupted connectivity was restored in the partial callosotomy group but not in the complete callosotomy group, likely due to the compensation that occurred through the remaining interhemispheric axonal pathways. This restoration - along with the increased intrahemispheric functional connectivity observed in both groups at day 28 - highlights the remarkable adaptation and plasticity in brain rsfMRI connections. These rsfMRI findings were paralleled by the intracortical EEG recording and Mn(2+) tracing results. Taken together, our experimental results directly demonstrate that axonal connections are the indispensable foundation for rsfMRI connectivity and that such functional connectivity can be plastic and dynamically reorganized atop the morphological connections.

In vivo manganese-enhanced MRI for visuotopic brain mapping

This study explored the feasibility of localized manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administrations for visuotopic brain mapping of retinal, callosal, cortico-subcortical, transsynaptic and horizontal connections in normal adult rats. Upon fractionated intravitreal Mn(2+) injection, Mn enhancements were observed in the contralateral superior colliculus (SC) and lateral geniculate nucleus (LGN) by 45-60% at 1-3 days after initial Mn(2+) injection and in the contralateral primary visual cortex (V1) by about 10% at 2-3 days after initial Mn(2+) injection. Direct, single-dose Mn(2+) injection to the LGN resulted in Mn enhancement by 13-21% in V1 and 8-11% in SC of the ipsilateral hemisphere at 8 to 24 hours after Mn(2+) administration. Intracortical, single-dose Mn(2+) injection to the visual cortex resulted in Mn enhancement by 53-65% in ipsilateral LGN, 15-26% in ipsilateral SC, 32-34% in the splenium of corpus callosum and 17-25% in contralateral V1/V2 transition zone at 8 to 24 hours after Mn(2+) administration. Notably, some patchy patterns were apparent near the V1/V2 border of the contralateral hemisphere. Laminar-specific horizontal cortical connections were also observed in the ipsilateral hemisphere. The current results demonstrated the sensitivity of MEMRI for assessing the neuroarchitecture of the visual brains in vivo without depth-limitation, and may possess great potentials for studying the basic neural components and connections in the visual system longitudinally during development, plasticity, pharmacological interventions and genetic modifications.

Regional specificity of manganese accumulation and clearance in the mouse brain: implications for manganese-enhanced MRI

Manganese-enhanced MRI has recently become a valuable tool for the assessment of in vivo functional cerebral activity in animal models. As a result of the toxicity of manganese at higher dosages, fractionated application schemes have been proposed to reduce the toxic side effects by using lower concentrations per injection. Here, we present data on regional-specific manganese accumulation during a fractionated application scheme over 8 days of 30 mg/kg MnCl2 , as well as on the clearance of manganese chloride over the course of several weeks after the termination of the whole application protocol supplying an accumulative dose of 240 mg/kg MnCl2 . Our data show most rapid accumulation in the superior and inferior colliculi, amygdala, bed nucleus of the stria terminalis, cornu ammonis of the hippocampus and globus pallidus. The data suggest that no ceiling effects occur in any region using the proposed application protocol. Therefore, a comparison of basal neuronal activity differences in different animal groups based on locally specific manganese accumulation is possible using fractionated application. Half-life times of manganese clearance varied between 5 and 7 days, and were longest in the periaqueductal gray, amygdala and entorhinal cortex. As the hippocampal formation shows one of the highest T1 -weighted signal intensities after manganese application, and manganese-induced memory impairment has been suggested, we assessed hippocampus-dependent learning as well as possible manganese-induced atrophy of the hippocampal volume. No interference of manganese application on learning was detected after 4 days of Mn(2+) application or 2 weeks after the application protocol. In addition, no volumetric changes induced by manganese application were found for the hippocampus at any of the measured time points. For longitudinal measurements (i.e. repeated manganese applications), a minimum of at least 8 weeks should be considered using the proposed protocol to allow for sufficient clearance of the paramagnetic ion from cerebral tissue.

NAP (davunetide) modifies disease progression in a mouse model of severe neurodegeneration: protection against impairments in axonal transport

NAP (davunetide) is a novel neuroprotective compound with mechanism of action that appears to involve microtubule (MT) stabilization and repair. To evaluate, for the first time, the impact of NAP on axonal transport in vivo and to translate it to neuroprotection in a severe neurodegeneration, the SOD1-G93A mouse model for amyotrophic lateral sclerosis (ALS) was used. Manganese-enhanced magnetic resonance imaging (MRI), estimating axonal transport rates, revealed a significant reduction of the anterograde axonal transport in the ALS mice compared to healthy control mice. Acute NAP treatment normalized axonal transport rates in these ALS mice. Tau hyperphosphorylation, associated with MT dysfunction and defective axonal transport, was discovered in the brains of the ALS mice and was significantly reduced by chronic NAP treatment. Furthermore, in healthy wild type (WT) mice, NAP reversed axonal transport disruption by colchicine, suggesting drug-dependent protection against axonal transport impairment through stabilization of the neuronal MT network. Histochemical analysis showed that chronic NAP treatment significantly protected spinal cord motor neurons against ALS-like pathology. Sequential MRI measurements, correlating brain structure with ALS disease progression, revealed a significant damage to the ventral tegmental area (VTA), indicative of impairments to the dopaminergic pathways relative to healthy controls. Chronic daily NAP treatment of the SOD1-G93A mice, initiated close to disease onset, delayed degeneration of the trigeminal, facial and hypoglossal motor nuclei as was significantly apparent at days 90-100 and further protected the VTA throughout life. Importantly, protection of the VTA was significantly correlated with longevity and overall, NAP treatment significantly prolonged life span in the ALS mice.

Retrograde signaling in the optic nerve is necessary for electrical responsiveness of retinal ganglion cells

PURPOSE

We investigated the role of retrograde signaling in the optic nerve on retinal ganglion cell (RGC) electrical responsiveness in the mouse model.

METHODS

Electrical response of RGC was measured by pattern electroretinogram (PERG) in 43 C57BL/6J mice 4 to 6 months old under ketamine/xylazine anesthesia. PERGs were recorded before and at different times after blockade of axon transport with lidocaine at either the retrobulbar level (2 μL, 40 μg/μL) or at level of the superior colliculus (SC, 1 μL, 40 μg/μL). PERGs also were recorded before and at different times after optic nerve crush 1.5 mm behind the eye, followed by TUJ1-positive RGC counts of excised retinas. As controls, PERGs also were recorded after either saline injections or sham optic nerve surgery. The photopic flash electroretinogram (FERG) and visual evoked potential (FVEP) also were recorded before lidocaine and at relevant times afterwards.

RESULTS

Lidocaine injection caused rapid (retrobulbar ~10 minutes, SC 1 hour), reversible reduction of PERG amplitude (≥50%). Optic nerve crush caused rapid (10-20 minutes), irreversible reduction of PERG amplitude (70-75%), increase of PERG latency (>25%), as well as RGC loss (88%) 1 month after crush. FVEP was unaltered by lidocaine. For all procedures, the FERG was unaltered.

CONCLUSIONS

As experimental interventions were made at postretinal level(s), PERG changes were likely associated with altered supply of retrogradely-delivered material from the SC. This implies that retrograde transport of target-derived molecules is necessary for normal RGC electrical responsiveness. The time course of early PERG changes is consistent with the speed of fast retrograde axon transport.

Manganese-enhanced MRI optic nerve tracking: effect of intravitreal manganese dose on retinal toxicity

The aim of this study was to provide data on the dose dependence of manganese-enhanced MRI (MEMRI) in the visual pathway of experimental rats and to study the toxicity of MnCl₂ to the retina. Sprague-Dawley rats were intravitreally injected with 2 μL of 0, 10, 25, 50, 75, 100, 150 and 300 mM MnCl₂, respectively. The contrast-to-noise ratio (CNR) of MEMRI for optic nerve enhancement was measured at different concentrations of MnCl₂. Simultaneously, the toxicity of manganese was evaluated by counting retinal ganglion cells and by retinal histological examination using light microscopy and transmission electron microscopy. The CNR increased with increasing concentration of MnCl₂ up to 75 mM. Retinal ganglion cell densities were reduced significantly when the concentration of MnCl₂ in the intravitreal injection was equal to or greater than 75 mM. Increasing numbers of ribosomes in retinal ganglion cells were first detected at 25 mM of MnCl₂. The retinal toxicity of MnCl₂ at higher concentration also included mitochondrial pathology and cell disruption of retinal ganglion cells, as well as abnormalities of photoreceptor and retinal pigment epithelium cells. It can be concluded that intravitreal injection of MnCl₂ induces retinal cell damage that appears to start from 25 mM. The concentration of MnCl₂ should not exceed 25 mm through intravitreal injection for visual pathway MEMRI in the rat.

Functional magnetic resonance imaging of sound pressure level encoding in the rat central auditory system

Intensity is an important physical property of a sound wave and is customarily reported as sound pressure level (SPL). Invasive techniques such as electrical recordings, which typically examine one brain region at a time, have been used to study neuronal encoding of SPL throughout the central auditory system. Non-invasive functional magnetic resonance imaging (fMRI) with large field of view can simultaneously examine multiple auditory structures. We applied fMRI to measure the hemodynamic responses in the rat brain during sound stimulation at seven SPLs over a 72 dB range. This study used a sparse temporal sampling paradigm to reduce the adverse effects of scanner noise. Hemodynamic responses were measured from the central nucleus of the inferior colliculus (CIC), external cortex of the inferior colliculus (ECIC), lateral lemniscus (LL), medial geniculate body (MGB), and auditory cortex (AC). BOLD signal changes generally increase significantly (p<0.001) with SPL and the dependence is monotonic in CIC, ECIC, and LL. The ECIC has higher BOLD signal change than CIC and LL at high SPLs. The difference between BOLD signal changes at high and low SPLs is less in the MGB and AC. This suggests that the SPL dependences of the LL and IC are different from those in the MGB and AC and the SPL dependence of the CIC is different from that of the ECIC. These observations are likely related to earlier observations that neurons with firing rates that increase monotonically with SPL are dominant in the CIC, ECIC, and LL while non-monotonic neurons are dominant in the MGB and AC. Further, the IC's SPL dependence measured in this study is very similar to that measured in our earlier study using the continuous imaging method. Therefore, sparse temporal sampling may not be a prerequisite in auditory fMRI studies of the IC.

In vivo manganese-enhanced MRI and diffusion tensor imaging of developing and impaired visual brains

This study explored the feasibility of high-resolution Mn-enhanced MRI (MEMRI) and diffusion tensor imaging (DTI) for in vivo assessments of the development and reorganization of retinal and visual callosal pathways in normal neonatal rodent brains and after early postnatal visual impairments. Using MEMRI, intravitreal Mn(2+) injection into one eye resulted in maximal T1-weighted hyperintensity in neonatal contralateral superior colliculus (SC) 8 hours after administration, whereas in adult contralateral SC signal increase continued at 1 day post-injection. Notably, mild but significant Mn(2+) enhancement was observed in the ipsilateral SC in normal neonatal rats, and in adult rats after neonatal monocular enucleation (ME) but not in normal adult rats. Upon intracortical Mn(2+) injection to the visual cortex, neonatal binocularly-enucleated (BE) rats showed an enhancement of a larger projection area, via the splenium of corpus callosum to the V1/V2 transition zone of the contralateral hemisphere in comparison to normal rats. For DTI, the retinal pathways projected from the enucleated eyes possessed lower fractional anisotropy (FA) 6 weeks after BE and ME. Interestingly, in the optic nerve projected from the remaining eye in ME rats a significantly higher FA was observed compared to normal rats. The results of this study are potentially important for understanding the axonal transport, microstructural reorganization and functional activities in the living visual brain during early postnatal development and plasticity in a global and longitudinal setting.

Impact of repeated topical-loaded manganese-enhanced MRI on the mouse visual system

PURPOSE

Optic nerve degeneration in diseases such as glaucoma and multiple sclerosis evolves in months to years. The use of Mn(2+)-Enhanced Magnetic Resonance Imaging (MEMRI) in a time-course study may provide new insights into the disease progression. Previously, we demonstrated the feasibility of using a topical administration for Mn(2+) delivery to the visual system. This study is to evaluate the impact of biweekly or monthly repeated Mn(2+) topical administration and the pH levels of the Mn(2+) solutions for MEMRI on the mouse visual pathway.

METHODS

Using groups of mice, the MEMRI with an acidic or pH neutralized 1 M MnCl(2) solution was performed. To evaluate the feasibility of repeated MEMRIs, topical-loaded MEMRI was conducted biweekly seven times or monthly three times. The enhancement of MEMRI in the visual system was quantified. After repeated MEMRIs, the corneas were examined by optical coherence tomography. The retinal ganglion cells (RGCs) and optic nerves were examined by histology.

RESULTS

All mice exhibited consistent enhancements along the visual system following repeated MEMRIs. The acidic Mn(2+) solution induced a greater MEMRI enhancement as compared with a neutral pH Mn(2+) solution. Significant 20% RGC loss was found after three biweekly Mn(2+) inductions, but no RGC loss was found after three monthly Mn(2+) treatments. The corneal thickness was found increased after seven biweekly topical-loaded MEMRI.

CONCLUSIONS

Acidic Mn(2+) solutions enhanced the uptake of Mn(2+) observed on the MEMRI. Increasing the time intervals of repeated Mn(2+) topical administration reduced the adverse effects caused by MEMRI.

In vivo MRI of olfactory ensheathing cell grafts and regenerating axons in transplant mediated repair of the adult rat optic nerve

The purpose of the present study was to use magnetic resonance imaging (MRI) as a tool for monitoring transplant-mediated repair of the adult rat visual pathway. We labelled rat olfactory ensheathing cells (OECs) using micron-sized particles of iron oxide (MPIO) and transplanted them by: i) intravitreal injection (ivit) and ii) intra-optic nerve (ON) injection (iON) in adult rats with ON crush (ONC) injury. We applied T(2)-weighted MRI and manganese-enhanced MRI (MEMRI) to visualise transplanted cells and ON axons at specific times after injury and cell engraftment. Our findings demonstrate that ivit MPIO-labelled OECs are unequivocally detected by T(2)-weighted MRI in vivo and that the T(1)-weighted 3D FLASH sequence applied for MEMRI facilitates simultaneous visualisation of Mn(2+-) enhanced regenerating retinal ganglion cell (RGC) axons and MPIO-labelled OEC grafts. Furthermore, analysis of MRI data and ultrastructural findings supports the hypothesis that iON OEC transplants mediate regeneration and remyelination of RGC axons post injury.

MR imaging of papilledema and visual pathways: effects of increased intracranial pressure and pathophysiologic mechanisms

Papilledema, defined as swelling of the optic disc, frequently occurs in the setting of increased ICP and in a variety of medical conditions, including pseudotumor cerebri, sinus thrombosis, intracerebral hemorrhage, frontal lobe neoplasms, and Chiari malformation. Noninvasive imaging of the ON is possible by using MR imaging, with a variety of findings occurring in the setting of papilledema, including flattening of the posterior sclera, protrusion of the optic disc, widening of the ONS, and tortuosity of the ON. Early recognition of papilledema and elevated ICP is of paramount importance for ensuring restoration of vision. Newer advanced MR imaging techniques such as fMRI and DTI may prove useful in the future to assess the potential effects of papilledema on retinal and visual pathway integrity.

In vivo chromium-enhanced MRI of the retina

Chromium (Cr) has been used histologically to stabilize lipid fractions in the retina and is suggested to enhance oxidizable lipids in brain MRI. This study explored the feasibility, sensitivity, and specificity of in vivo chromium-enhanced MRI of retinal lipids by determining its spatiotemporal profiles and toxic effect after intravitreal Cr(VI) injection to normal adult rats. One day after 3 μL Cr(VI) administration at 1-100 mM, the retina exhibited a dose-dependent increase in T1-weighted hyperintensity until 50 mM. Time-dependently, significant T1-weighted hyperintensity persisted up to 2 weeks after 10 mM Cr(VI) administration. Three-dimensional chromium-enhanced MRI of ex vivo normal eyes at isotropic 50-μm resolution showed at least five alternating bands across retinal layers, with the outermost layer being the brightest. This agreed with histology indicating alternating lipid contents with the highest level in the photoreceptor layer of the outer retina. Although Cr(VI) reduction may induce oxidative stress and depolymerize microtubules, manganese-enhanced MRI after chromium-enhanced MRI showed a dose-dependent effect of Cr toxicity on manganese uptake and axonal transport along the visual pathway. These results potentiated future longitudinal chromium-enhanced MRI studies on retinal lipid metabolism upon further optimization of Cr doses with visual cell viability.

Balanced steady-state free precession fMRI with intravascular susceptibility contrast agent

One major challenge in echo planar imaging-based functional MRI (fMRI) is the susceptibility-induced image distortion. In this study, a new cerebral blood volume-weighted fMRI technique using distortion-free balanced steady-state free precession (bSSFP) sequence was proposed and its feasibility was investigated in rat brain at 7 Tesla. After administration of intravascular susceptibility contrast agent (monocrystalline iron oxide nanoparticle [MION] at 15 mg/kg), unilateral visual stimulation was presented using a block-design paradigm. With repetition time/echo time = 3.8/1.9 ms and α = 18°, bSSFP fMRI was performed and compared with the conventional cerebral blood volume-weighted fMRI using post-MION gradient echo and spin echo echo planar imaging. The results showed that post-MION bSSFP fMRI provides comparable sensitivity but with no severe image distortion and signal dropout. Robust negative responses were observed during stimulation and activation patterns were in excellent agreement with known neuroanatomy. Furthermore, the post-MION bSSFP signal was observed to decrease significantly during hypercapnia challenge, indicating its sensitivity to cerebral blood volume changes. These findings demonstrated that post-MION bSSFP fMRI is a promising alternative to conventional cerebral blood volume-weighted fMRI. This technique is particularly suited for fMRI investigation of animal models at high field.

In vivo evaluation of retinal and callosal projections in early postnatal development and plasticity using manganese-enhanced MRI and diffusion tensor imaging

The rodents are an excellent model for understanding the development and plasticity of the visual system. In this study, we explored the feasibility of Mn-enhanced MRI (MEMRI) and diffusion tensor imaging (DTI) at 7 T for in vivo and longitudinal assessments of the retinal and callosal pathways in normal neonatal rodent brains and after early postnatal visual impairments. Along the retinal pathways, unilateral intravitreal Mn2+ injection resulted in Mn2+ uptake and transport in normal neonatal visual brains at postnatal days (P) 1, 5 and 10 with faster Mn2+ clearance than the adult brains at P60. The reorganization of retinocollicular projections was also detected by significant Mn2+ enhancement by 2%-10% in the ipsilateral superior colliculus (SC) of normal neonatal rats, normal adult mice and adult rats after neonatal monocular enucleation (ME) but not in normal adult rats or adult rats after monocular deprivation (MD). DTI showed a significantly higher fractional anisotropy (FA) by 21% in the optic nerve projected from the remaining eye of ME rats compared to normal rats at 6 weeks old, likely as a result of the retention of axons from the ipsilaterally uncrossed retinal ganglion cells, whereas the anterior and posterior retinal pathways projected from the enucleated or deprived eyes possessed lower FA after neonatal binocular enucleation (BE), ME and MD by 22%-56%, 18%-46% and 11%-15% respectively compared to normal rats, indicative of neurodegeneration or immaturity of white matter tracts. Along the visual callosal pathways, intracortical Mn2+ injection to the visual cortex of BE rats enhanced a larger projection volume by about 74% in the V1/V2 transition zone of the contralateral hemisphere compared to normal rats, without apparent DTI parametric changes in the splenium of corpus callosum. This suggested an adaptive change in interhemispheric connections and spatial specificity in the visual cortex upon early blindness. The results of this study may help determine the mechanisms of axonal uptake and transport, microstructural reorganization and functional activities in the living visual brains during development, diseases, plasticity and early interventions in a global and longitudinal setting.

Manganese-enhanced MRI reveals multiple cellular and vascular layers in normal and degenerated retinas

PURPOSE

To use manganese-enhanced magnetic resonance imaging (MEMRI) at 25 × 25 × 800 μm(3) to image different retinal and vascular layers in the rat retinas.

MATERIALS AND METHODS

Manganese-chloride was injected intraocularly in normal (n = 5) and Royal College of Surgeons (RCS, an model of photoreceptor degeneration) (n = 5) rats at postnatal day 90. MEMRI at 4.7 T was performed 24 hours later. MRI was repeated following intravenous Gd-DTPA in the same animals to highlight the vasculatures. Layer assignment and thickness were compared to histology.

RESULTS

MEMRI 24 hours after intravitreal manganese-chloride injection revealed seven bands of alternating hyper- and hypointensities, corresponding histologically to the ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, photoreceptor-segment layer, and choroidal vascular layer. Intravenous Gd-DTPA-which does not cross the blood-retinal barrier and the retinal pigment epithelium-further enhanced the two layers bounding the retina, corresponding to the retinal and choroidal vascular layers, but not the avascular outer nuclear layer and the photoreceptor-segment layer. MEMRI of the RCS retinas revealed the loss of the outer plexiform layer, outer nuclear layer, and photoreceptor-segment layer. Histological analysis corroborated the MRI laminar assignments and thicknesses.

CONCLUSION

Lamina-specific retinal structures neurodegenerative changes to structure in retinal diseases can be detected using MEMRI.

Noninvasive topical loading for manganese-enhanced MRI of the mouse visual system

PURPOSE

To evaluate topical loading as an alternative to intravitreal injection for Mn(2+)-enhanced magnetic resonance imaging (MEMRI) of the visual system.

METHODS

Topical administration of 0.5 to 1.5 M MnCl(2) and intravitreal injections with 0.5 μL 100 mM and 2 μL 1 M MnCl(2) for mouse MEMRI were conducted, followed by immunohistochemistry. In another mouse group, two topical administrations of 1 M Mn(2+) were applied to the same animals 7 days apart, to evaluate the use of MEMRI in a time course study. Dynamic imaging was also conducted to reveal how Mn(2+) travels to the retina. MEMRI with topically loaded MnCl(2) was also conducted in eyes with retinal ischemia, to evaluate whether the enhancements required healthy neurons.

RESULTS

After 1 day, topical administration of 1 M and 1.5 M MnCl(2) rendered significant signal enhancement (up to 20%) in the superior colliculus (P < 0.05) that was equivalent to that of the 2-μL 1 M injection. Repeated exposure to Mn(2+) showed reproduced enhancement. Dynamic imaging showed significant enhancement in the iris, retina, and lens boundary, but not in the vitreous space. In retinal ischemic eyes, no enhancement of MEMRI was detected in the optic nerves. The immunohistochemistry of the optic nerve (1.5 mm anterior to the chiasm) and retina showed no injury 1 week after Mn(2+) topical administrations to each mouse.

CONCLUSIONS

The results demonstrated the feasibility of using topical administration of Mn(2+) for MEMRI. Topically loaded Mn(2+) did not diffuse into the vitreous space, but was it may have been absorbed into the iris to diffuse or travel via the capillary circulation to reach the retina.