Manganese enhanced MRI (MEMRI): neurophysiological applications

Neuroimaging of the effects of drug exposure or self-administration in rodents: A systematic review

A systematic review of functional neuroimaging studies on drug (self-) administration in rodents is lacking. Here, we summarized effects of acute or chronic drug administration of various classes of drugs on brain function and determined consistency with human literature. We performed a systematic literature search and identified 125 studies on in vivo rodent resting-state functional magnetic resonance imaging (n = 84) or positron emission tomography (n = 41) spanning depressants (n = 27), opioids (n = 23), stimulants (n = 72), and cannabis (n = 3). Results primarily showed alterations in the striatum, consistent with the human literature. The anterior cingulate cortex and (nonspecific) prefrontal cortex were also frequently implicated. Upregulation was most often found after shorter administration and downregulation after long chronic administration, particularly in the striatum. Importantly, results were consistent across study design, administration models, imaging method, and animal states. Results provide evidence of altered resting-state brain function in rodents upon drug administration, implicating the brain's reward network analogous to human studies. However, alterations were more dynamic than previously known, with dynamic adaptation depending on the length of drug administration.

Whole-brain mapping of increased manganese levels in welders and its association with exposure and motor function

Although manganese (Mn) is a trace metal essential for humans, chronic exposure to Mn can cause accumulation of this metal ion in the brain leading to an increased risk of neurological and neurobehavioral health effects. This is a concern for welders exposed to Mn through welding fumes. While brain Mn accumulation in occupational settings has mostly been reported in the basal ganglia, several imaging studies also revealed elevated Mn in other brain areas. Since Mn functions as a magnetic resonance imaging (MRI) T1 contrast agent, we developed a whole-brain MRI approach to map in vivo Mn deposition differences in the brains of non-exposed factory controls and exposed welders. This is a cross-sectional analysis of 23 non-exposed factory controls and 36 exposed full-time welders from the same truck manufacturer. We collected high-resolution 3D MRIs of brain anatomy and R1 relaxation maps to identify regional differences using voxel-based quantification (VBQ) and statistical parametric mapping. Furthermore, we investigated the associations between excess Mn deposition and neuropsychological and motor test performance. Our results indicate that: (1) Using whole-brain MRI relaxometry methods we can generate excess Mn deposition maps in vivo, (2) excess Mn accumulation due to occupational exposure occurs beyond the basal ganglia in cortical areas associated with motor and cognitive functions, (3) Mn likely diffuses along white matter tracts in the brain, and (4) Mn deposition in specific brain regions is associated with exposure (cerebellum and frontal cortex) and motor metrics (cerebellum and hippocampus).

Circular RNA circRest regulates manganese induced cell apoptosis by targeting the mmu-miR-6914-5p/Ephb3 axis

Overexposure to manganese (Mn) can lead to neurotoxicity, the underlying mechanisms remain incompletely understood. Circular RNAs (circRNAs) have emerged as important regulators in various biological processes. It is plausible that circRNAs may be involved in the biological mechanisms underlying Mn caused neurotoxicity. Here, circRest was downregulated in Mn-exposed mouse neuroblastoma cells (N2a cells) by RNA sequencing and quantitative real-time PCR. When circRest was overexpressed, it led to an increase in cell viability and a decrease in apoptosis following Mn exposure. Conversely, silencing circRest resulted in opposite effects in N2a cells. Further investigation revealed that circRest acts as a mmu-miR-6914-5p sponge, and mmu-miR-6914-5p could bind and inhibit Ephb3, thereby promoting apoptosis in N2a cells. This was confirmed through RNA antisense purification and dual luciferase reporter assays. Additionally, the circRest/mmu-miR-6914-5p/Ephb3 axis may influence memory and learning in mice following Mn exposure. In conclusion, our study uncovers a novel mechanism by which circRest may attenuate Mn caused neurotoxicity via the mmu-miR-6914-5p/Ephb3 axis.

Enhancement of astrocytic gap junctions Connexin43 coupling can improve long-term isoflurane anesthesia-mediated brain network abnormalities and cognitive impairment

AIM

Astrocytes are connected by gap junctions Connexin43 (GJs-Cx43) forming an extensive intercellular network and maintain brain homeostasis. Perioperative neurocognitive disorder (PND) occurs frequently after anesthesia/surgery and worsens patient outcome, but the neural circuit mechanisms remain unclear. This study aimed to determine the effects of the GJs-Cx43-mediated astrocytic network on PND and ascertain the underlying neural circuit mechanism.

METHODS

Male C57BL/6 mice were treated with long-term isoflurane exposure to construct a mouse model of PND. We also exposed primary mouse astrocytes to long-term isoflurane exposure to simulate the conditions of in vivo cognitive dysfunction. Behavioral tests were performed using the Y-maze and fear conditioning (FC) tests. Manganese-enhanced magnetic resonance imaging (MEMRI) and resting-state functional magnetic resonance imaging (rs-fMRI) were used to investigate brain activity and functional connectivity. Western blot and flow cytometry analysis were used to assess protein expression.

RESULTS

Reconfiguring the astrocytic network by increasing GJs-Cx43 expression can modulate 22 subregions affected by PND in three ways: reversed activation, reversed inhibition, and intensified activation. The brain functional connectivity analysis further suggests that PND is a brain network disorder that includes sleep-wake rhythm-related brain regions, contextual and fear memory-related subregions, the hippocampal-amygdala circuit, the septo-hippocampal circuit, and the entorhinal-hippocampal circuit. Notably, remodeling the astrocytic network by upregulation of GJs-Cx43 can partially reverse the abnormalities in the above circuits. Pathophysiological degeneration in hippocampus is one of the primary hallmarks of PND pathology, and long-term isoflurane anesthesia contributes to oxidative stress and neuroinflammation in the hippocampus. However, promoting the formation of GJs-Cx43 ameliorated cognitive dysfunction induced by long-term isoflurane anesthesia through the attenuation of oxidative stress in hippocampus.

CONCLUSION

Enhancing GJs-Cx43 coupling can improve brain network abnormalities and cognitive impairment induced by long-term isoflurane anesthesia, its mechanisms might be associated with the regulation of oxidative stress and neuroinflammation.

Brain activity studied with magnetic resonance imaging in awake rabbits

We reviewed fMRI experiments from our previous work in conscious rabbits, an experimental preparation that is advantageous for measuring brain activation that is free of anesthetic modulation and which can address questions in a variety of areas in sensory, cognitive, and pharmacological neuroscience research. Rabbits do not struggle or move for several hours while sitting with their heads restrained inside the horizontal bore of a magnet. This greatly reduces movement artifacts in magnetic resonance (MR) images in comparison to other experimental animals such as rodents, cats, and monkeys. We have been able to acquire high-resolution anatomic as well as functional images that are free of movement artifacts during several hours of restraint. Results from conscious rabbit fMRI studies with whisker stimulation are provided to illustrate the feasibility of this conscious animal model for functional MRI and the reproducibility of data gained with it.

Studying Axonal Transport in the Brain by Manganese-Enhanced Magnetic Resonance Imaging (MEMRI)

From the earliest notions of dynamic movements within the cell by Leeuwenhoek, intracellular transport in eukaryotes has been primarily explored by optical imaging. The giant axon of the squid became a prime experimental model for imaging transport due to its size, optical transparency, and physiological robustness. Even the biochemical basis of transport was identified using optical assays based on video microscopy of fractionated squid axoplasm. Discoveries about the dynamics and molecular components of the intracellular transport system continued in many model organisms that afforded experimental systems for optical imaging. Yet whether these experimental systems reflected a valid picture of axonal transport in the opaque mammalian brain was unknown.Magnetic resonance imaging (MRI) provides a non-destructive approach to peer into opaque tissues like the brain . The paramagnetic ion, manganese (MnII), gives a hyperintense signal in T₁ weighted MRI that can serve as a marker for axonal transport. Mn(II) enters active neurons via voltage-gated calcium channels and is transported via microtubule motors down their axons by fast axonal transport. Clearance of Mn(II) is slow. Scanning live animals at successive time points reveals the dynamics of Mn(II) transport by detecting Mn(II)-induced intensity increases or accumulations along a known fiber tract, such as the optic nerve or hippocampal-forebrain projections. Mn(II)-based tract tracing also reveals projections even when not in fiber bundles, such as projections in the olfactory system or from medial prefrontal cortex into midbrain and brain stem. The rate of Mn(II) accumulation, detected as increased signal intensity by MR, serves as a proxy for transport rates. Here we describe the method for measuring transport rates and projections by mangeses-enhanced magnetic resonance imaging, MEMRI.

Manganese in Diagnostics: A Preformulatory Study

This investigation aims to find lipid-based nanosystems to be used as tools to deliver manganese for diagnostic purposes in multimodal imaging techniques. In particular, the study describes the production and characterization of aqueous dispersions of anionic liposomes as delivery systems for two model manganese-based compounds, namely manganese chloride and manganese acetylacetonate. Negatively charged liposomes were obtained using four different anionic surfactants, namely sodium docusate (SD), N-lauroylsarcosine (NLS), Protelan AG8 (PAG) and sodium lauroyl lactylate (SLL). Liposomes were produced by the direct hydration method followed by extrusion and characterized in terms of size, polydispersity, surface charge and stability over time. After extrusion, liposomes are homogeneous and monodispersed with an average diameter not exceeding 200 nm and a negative surface charge as confirmed by ζ potential measurement. Moreover, as indicated by atomic absorption spectroscopy analyses, the loading of manganese-based compounds was almost quantitative. Liposomes containing NLS or SLL were the most stable over time and the presence of manganese-based compounds did not affect their size distribution. Liposomes containing PAG and SD were instable and therefore discarded. The in vitro cytotoxicity of the selected anionic liposomes was evaluated by MTT assay on human keratinocyte. The obtained results highlighted that the toxicity of the formulations is dose dependent.

Complexation of Mn(II) by Rigid Pyclen Diacetates: Equilibrium, Kinetic, Relaxometric, Density Functional Theory, and Superoxide Dismutase Activity Studies

We report the Mn(II) complexes with two pyclen-based ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with acetate pendant arms at either positions 3,6 (3,6-PC2A) or 3,9 (3,9-PC2A) of the macrocyclic fragment. The 3,6-PC2A ligand was synthesized in five steps from pyclen oxalate by protecting one of the secondary amine groups of pyclen using Alloc protecting chemistry. The complex with 3,9-PC2A is characterized by a higher thermodynamic stability [log K_MnL = 17.09(2)] than the 3,6-PC2A analogue [log K_MnL = 15.53(1); 0.15 M NaCl]. Both complexes contain a water molecule coordinated to the metal ion, which results in relatively high ¹H relaxivities (r_1p = 2.72 and 2.91 mM^-1 s^-1 for the complexes with 3,6-PC2A and 3,9-PC2A, respectively, at 25 °C and 0.49 T). The coordinated water molecule displays fast exchange kinetics with the bulk in both cases; the rates (k_ex²⁹⁸) are 140 × 10⁶ and 126 × 10⁶ s^-1 for [Mn(3,6-PC2A)(H₂O)] and [Mn(3,9-PC2A)(H₂O)], respectively. The two complexes were found to be remarkably inert with respect to their dissociation, with half-lives of 63 and 21 h, respectively, at pH = 7.4 in the presence of excess Cu(II). The r_1p values recorded in blood serum remain constant at least over a period of 120 h. Cyclic voltammetry experiments show irreversible oxidation features shifted to higher potentials with respect to [Mn(EDTA)(H₂O)]^2- (H₄EDTA = ethylenediaminetetraacetic acid) and [Mn(PhDTA)(H₂O)]^2- (H₄PhDTA = phenylenediamine-N,N,N',N'-tetraacetic acid), indicating that the PC2A complexes reported here have a lower tendency to stabilize Mn(III). The superoxide dismutase activity of the Mn(II) complexes was tested using the xanthine/xanthine oxidase/p-nitro blue tetrazolium chloride assay at pH = 7.8. The Mn(II) complexes of 3,6-PC2A and 3,9-PC2A are capable of assisting decomposition of the superoxide anion radical. The kinetic rate constant of the complex of 3,9-PC2A is smaller by 1 order of magnitude than that of 3,6-PC2A.

A three-dimensional digital atlas of the Nile crocodile (Crocodylus niloticus) forebrain

The phylogenetic position of crocodilians in relation to birds and mammals makes them an interesting animal model for investigating the evolution of the nervous system in amniote vertebrates. A few neuroanatomical atlases are available for reptiles, but with a growing interest in these animals within the comparative neurosciences, a need for these anatomical reference templates is becoming apparent. With the advent of MRI being used more frequently in comparative neuroscience, the aim of this study was to create a three-dimensional MRI-based atlas of the Nile crocodile (Crocodylus niloticus) brain to provide a common reference template for the interpretation of the crocodilian, and more broadly reptilian, brain. Ex vivo MRI acquisitions in combination with histological data were used to delineate crocodilian brain areas at telencephalic, diencephalic, mesencephalic, and rhombencephalic levels. A total of 50 anatomical structures were successfully identified and outlined to create a 3-D model of the Nile crocodile brain. The majority of structures were more readily discerned within the forebrain of the crocodile with the methods used to produce this atlas. The anatomy outlined herein corresponds with both classical and recent crocodilian anatomical analyses, barring a few areas of contention predominantly related to a lack of functional data and conflicting nomenclature.

Mapping accumulative whole-brain activities during environmental enrichment with manganese-enhanced magnetic resonance imaging

An enriched environment (EE) provides multi-dimensional stimuli to the brain. EE exposure for days to months induces functional and structural neuroplasticity. In this study, manganese-enhanced magnetic resonance imaging (MEMRI) was used to map the accumulative whole-brain activities associated with a 7-day EE exposure in freely-moving adult male mice, followed by c-Fos immunochemical assessments. Relative to the mice residing in a standard environment (SE), the mice subjected to EE treatment had significantly enhanced regional MEMRI signal intensities in the prefrontal cortex, somatosensory cortices, basal ganglia, amygdala, motor thalamus, lateral hypothalamus, ventral hippocampus and midbrain dopaminergic areas at the end of the 7-day exposure, likely attributing to enhanced Mn²⁺ uptake/transport associated with brain activities at both the regional and macroscale network levels. Some of, but not all, the brain regions in the EE-treated mice showing enhanced MEMRI signal intensity had accompanying increases in c-Fos expression. The EE-treated mice were also found to have significantly increased overall amount of food consumption, decreased body weight gain and upregulated tyrosine hydroxylase (TH) expression in the midbrain dopaminergic areas. Taken together, these results demonstrated that the 7-day EE exposure was associated with elevated cumulative activities in the nigrostriatal, mesolimbic and corticostriatal circuits underpinning reward, motivation, cognition, motor control and appetite regulation. Such accumulative activities might have served as the substrate of EE-related neuroplasticity and the beneficial effects of EE treatment on neurological/psychiatric conditions including drug addiction, Parkinson's disease and eating disorder.

Decoupling the Effects of the Amyloid Precursor Protein From Amyloid-β Plaques on Axonal Transport Dynamics in the Living Brain

Amyloid precursor protein (APP) is the precursor to Aβ plaques. The cytoplasmic domain of APP mediates attachment of vesicles to molecular motors for axonal transport. In APP-KO mice, transport of Mn²⁺ is decreased. In old transgenic mice expressing mutated human (APP^SwInd) linked to Familial Alzheimer’s Disease, with both expression of APP^SwInd and plaques, the rate and destination of Mn²⁺ axonal transport is altered, as detected by time-lapse manganese-enhanced magnetic resonance imaging (MEMRI) of the brain in living mice. To determine the relative contribution of expression of APP^SwInd versus plaque on transport dynamics, we developed a Tet-off system to decouple expression of APP^SwInd from plaque, and then studied hippocampal to forebrain transport by MEMRI. Three groups of mice were compared to wild-type (WT): Mice with plaque and APP^SwInd expression; mice with plaque but suppression of APP^SwInd expression; and mice with APP^SwInd suppressed from mating until 2 weeks before imaging with no plaque. MR images were captured before at successive time points after stereotactic injection of Mn²⁺ (3–5 nL) into CA3 of the hippocampus. Mice were returned to their home cage between imaging sessions so that transport would occur in the awake freely moving animal. Images of multiple mice from the three groups (suppressed or expressed) together with C57/B6J WT were aligned and processed with our automated computational pipeline, and voxel-wise statistical parametric mapping (SPM) performed. At the conclusion of MR imaging, brains were harvested for biochemistry or histopathology. Paired T-tests within-group between time points (p = 0.01 FDR corrected) support the impression that both plaque alone and APP^SwInd expression alone alter transport rates and destination of Mn²⁺ accumulation. Expression of APP^SwInd in the absence of plaque or detectable Aβ also resulted in transport defects as well as pathology of hippocampus and medial septum, suggesting two sources of pathology occur in familial Alzheimer’s disease, from toxic mutant protein as well as plaque. Alternatively mice with plaque without APP^SwInd expression resemble the human condition of sporadic Alzheimer’s, and had better transport. Thus, these mice with APP^SwInd expression suppressed after plaque formation will be most useful in preclinical trials.

Standardization of Small Animal Imaging-Current Status and Future Prospects

The benefit of small animal imaging is directly linked to the validity and reliability of the collected data. If the data (regardless of the modality used) are not reproducible and/or reliable, then the outcome of the data is rather questionable. Therefore, standardization of the use of small animal imaging equipment, as well as of animal handling in general, is of paramount importance. In a recent paper, guidance for efficient small animal imaging quality control was offered and discussed, among others, the use of phantoms in setting up a quality control program (Osborne et al. 2016). The same phantoms can be used to standardize image quality parameters for multi-center studies or multi-scanners within center studies. In animal experiments, the additional complexity due to animal handling needs to be addressed to ensure standardized imaging procedures. In this review, we will address the current status of standardization in preclinical imaging, as well as potential benefits from increased levels of standardization.

Effect of early experience on neuronal and behavioral responses to con- and heterospecific odors in closely related Mus taxa: epigenetic contribution in formation of precopulatory isolation

BACKGROUND

The most effective learning occurs during sensitive periods. Olfactory plasticity to main social olfactory cues is limited to a critical period to a large degree. The objective was to evaluate the influence of early olfactory experience on the behavioral and neuronal responses of males to con- and heterospecific odors of receptive females in two species, M. musculus (subspecies musculus, wagneri) and M. spicilegus, and thus to determine the potential role of epigenetic contribution in the formation of precopulatory isolation.

RESULTS

Males were reciprocally cross-fostered shortly after the birth and were tested for response to con- and heterospecific urine odors of estrus females using two-choice tests at 70-85 days of age. Neuronal activity of non- and cross-fostered males was evaluated at 90-110 days of age in the MOB and AOB to con- and heterospecific female odor using fMRI (MEMRI). Non-cross-fostered males of three taxa demonstrated a strong preference for odor of conspecific females compared to odor of heterospecific ones. Spicilegus-nursed musculus preferred odor of heterospecific females. Wagneri-nursed spicilegus and spicilegus-nursed wagneri did not demonstrate significant choice of con - or heterospecific female odor. The level of MRI signal obtained from the evaluation of manganese accumulation in AOB neurons was significantly higher when the odor of conspecific estrus females was exposed, compared to urine exposure of heterospecific females. The response pattern changed to the opposite in males raised by heterospecific females. Response patterns of neuronal activity in the MOB to con- and heterospecific female odors were different in cross-fostered and control males.

CONCLUSION

The maternal environment, including odor, had a greater effect on the level of MRI signal in the AOB than the genetic relationships of the recipient and the donor of the odor stimulus. Behavioral and neuronal responses to con- and heterospecific odors changed in closely related Mus taxa as a result of early experience. We demonstrated the importance of early learning in mate choice in adulthood in mice and the possibility of epigenetic contribution in the formation of precopulatory reproductive isolation.

Cocaine differentially affects synaptic activity in memory and midbrain areas of female and male rats: an in vivo MEMRI study

Manganese enhanced magnetic resonance imaging (MEMRI) has been previously used to determine the effect of acute cocaine on calcium-dependent synaptic activity in male rats. However, there have been no MEMRI studies examining sex differences in the functional neural circuits affected by repeated cocaine. In the present study, we used MEMRI to investigate the effects of repeated cocaine on brain activation in female and male rats. Adult female and male rats were scanned at 4.7 Tesla three days after final treatment with saline, a single cocaine injection (15 mg kg^-1, i.p. × 1 day) or repeated cocaine injections (15 mg kg^-1, i.p. × 10 days). A day before imaging rats were provided with an i.p. injection of manganese chloride (70 mg kg^-1). Cocaine produced effects on MEMRI activity that were dependent on sex. In females, we observed that a single cocaine injection reduced MEMRI activity in hippocampal CA3, ventral tegmental area (VTA), and median Raphé, whereas repeated cocaine increased MEMRI activity in dentate gyrus and interpeduncular nucleus. In males, repeated cocaine reduced MEMRI activity in VTA. Overall, it appeared that female rats showed a general trend towards increase MEMRI activity with single cocaine and reduced activity with repeated exposure, while male rats showed a trend towards opposite effects. Our results provide evidence for sex differences in the in vivo neural response to cocaine, which involves primarily hippocampal, amygdala and midbrain areas.

Impairment of Axonal Transport in Diabetes: Focus on the Putative Mechanisms Underlying Peripheral and Central Neuropathies

Diabetes mellitus is a chronic disease with numerous complications that severely impact on the quality of life of patients. Different neuropathies may arise as complications associated with the nervous system, both peripherally and at the central level. The mechanisms behind these neuronal complications are far from being clarified, but axonal transport impairment, a vital process for neuronal physiology, has been described in the context of experimental diabetes. Alterations in neuronal cytoskeleton and motor proteins, deficits in ATP supply or neuroinflammation, as processes that disturb the effective transport of cargoes along the axon, were reported as putative causes of axonal impairment, ultimately leading to axonal degeneration. The main goal of the present review is to reunite the main studies in the literature exploring diabetes-induced alterations likely involved in axonal transport deficits, and call the attention for the uttermost importance of further exploring the field. Understanding the mechanisms underlying neuronal deficits in diabetes is crucial for the development of new therapeutic strategies to prevent neuronal degeneration in diabetes and related neuropathies.

Chemotherapy-Induced Cognitive Impairment Is Associated with Cytokine Dysregulation and Disruptions in Neuroplasticity

Chemotherapy-induced cognitive impairment, often referred to as "chemobrain," is a common side effect. In this study, mice received three intraperitoneal injections of a combination of docetaxel, adriamycin, and cyclophosphamide (DAC) at 2-day intervals. A water maze test was used to examine cognitive performance, and manganese-enhanced magnetic resonance imaging (MEMRI) was used to examine hippocampal neuronal activity. The whole brain, prefrontal cortex, hippocampus, and blood samples were then collected for cytokine measurement. The DAC-treated mice displayed a significantly shorter duration spent in and fewer entries into the target quadrant of the water maze than the control mice and a pronounced decrease in MEMRI signal intensity in the hippocampal subregions. In a separate experiment using in vivo transcranial two-photon imaging, DAC markedly eliminated dendritic spines without changing the rate of spine formation, leading to a striking loss of spines in the medial prefrontal cortex. DAC treatment resulted in significant elevations in the levels of the proinflammatory cytokines interleukin 6 (IL-6) and tumor necrosis factor-α (TNF-α) and in significant decreases in the levels of the anti-inflammatory cytokines IL-4 and IL-10 in most of the sera and brain tissues examined. The IL-6 and TNF-α levels of several sera and brain tissues showed strong inverse correlations with the duration and number of entries in the target quadrant of the water maze and with the hippocampal MEMRI signal intensity, but also showed striking positive correlations with spine elimination and loss. These results indicate that chemobrain is associated with cytokine dysregulation and disrupted neuroplasticity of the brain.

PET radiometals for antibody labeling

Recent advances in molecular characterization of tumors have made possible the emergence of new types of cancer therapies where traditional cytotoxic drugs and nonspecific chemotherapy can be complemented with targeted molecular therapies. One of the main revolutionary treatments is the use of monoclonal antibodies (mAbs) that selectively target the disseminated tumor cells while sparing normal tissues. mAbs and related therapeutics can be efficiently radiolabeled with a wide range of radionuclides to facilitate preclinical and clinical studies. Non-invasive molecular imaging techniques, such as Positron Emission Tomography (PET), using radiolabeled mAbs provide useful information on the whole-body distribution of the biomolecules, which may enable patient stratification, diagnosis, selection of targeted therapies, evaluation of treatment response, and prediction of dose limiting tissue and adverse effects. In addition, when mAbs are labeled with therapeutic radionuclides, the combination of immunological and radiobiological cytotoxicity may result in enhanced treatment efficacy. The pharmacokinetic profile of antibodies demands the use of long half-life isotopes for longitudinal scrutiny of mAb biodistribution and precludes the use of well-stablished short half-life isotopes. Herein, we review the most promising PET radiometals with chemical and physical characteristics that make the appealing for mAb labeling, highlighting those with theranostic radioisotopes.

Validation strategies for the interpretation of microstructure imaging using diffusion MRI

Extracting microanatomical information beyond the image resolution of MRI would provide valuable tools for diagnostics and neuroscientific research. A number of mathematical models already suggest microstructural interpretations of diffusion MRI (dMRI) data. Examples of such microstructural features could be cell bodies and neurites, e.g. the axon's diameter or their orientational distribution for global connectivity analysis using tractography, and have previously only been possible to access through conventional histology of post mortem tissue or invasive biopsies. The prospect of gaining the same knowledge non-invasively from the whole living human brain could push the frontiers for the diagnosis of neurological and psychiatric diseases. It could also provide a general understanding of the development and natural variability in the healthy brain across a population. However, due to a limited image resolution, most of the dMRI measures are indirect estimations and may depend on the whole chain from experimental parameter settings to model assumptions and implementation. Here, we review current literature in this field and highlight the integrative work across anatomical length scales that is needed to validate and trust a new dMRI method. We encourage interdisciplinary collaborations and data sharing in regards to applying and developing new validation techniques to improve the specificity of future dMRI methods.

Development of manganese-enhanced magnetic resonance imaging of the rostral ventrolateral medulla of conscious rats: Importance of normalization and comparison with other regions of interest

Spinally projecting neurons in the rostral ventrolateral medulla (RVLM) are believed to contribute to pathophysiological alterations in sympathetic nerve activity and the development of cardiovascular disease. The ability to identify changes in the activity of RVLM neurons in conscious animals and humans, especially longitudinally, would represent a clinically important advancement in our understanding of the contribution of the RVLM to cardiovascular disease. To this end, we describe the initial development of manganese-enhanced magnetic resonance imaging (MEMRI) for the rat RVLM. Manganese (Mn²⁺ ) has been used to estimate in vivo neuronal activity in other brain regions because of both its paramagnetic properties and its entry into and accumulation in active neurons. In this initial study, our three goals were as follows: (1) to validate that Mn²⁺ enhancement occurs in functionally and anatomically localized images of the rat RVLM; (2) to quantify the dose and time course dependence of Mn²⁺ enhancement in the RVLM after one systemic injection in conscious rats (66 or 33 mg/kg, intraperitoneally); and (3) to compare Mn²⁺ enhancement in the RVLM with other regions to determine an appropriate method of normalization of T₁ -weighted images. In our proof-of-concept and proof-of-principle studies, Mn²⁺ was identified by MRI in the rat RVLM after direct microinjection or via retrograde transport following spinal cord injections, respectively. Systemic injections in conscious rats produced significant Mn²⁺ enhancement at 24 h (p < 0.05). Injections of 66 mg/kg produced greater enhancement than 33 mg/kg in the RVLM and paraventricular nucleus of the hypothalamus (p < 0.05 for both), but only when normalized to baseline scans without Mn²⁺ injection. Consistent with findings from our previous functional and anatomical studies demonstrating subregional neuroplasticity, Mn²⁺ enhancement was higher in the rostral regions of the RVLM (p < 0.05). Together with important technical considerations, our studies support the development of MEMRI as a potential method to examine RVLM activity over time in conscious animal subjects.

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.

Brain activation induced by chronic psychosocial stress in mice

Chronic psychosocial stress is a well-established risk factor for neuropsychiatric diseases. Abnormalities in brain activity have been demonstrated in patients with stress-related disorders. Global brain activation patterns during chronic stress exposure are less well understood but may have strong modifying effects on specific brain circuits and thereby influence development of stress-related pathologies. We determined neural activation induced by chronic social defeat stress, a mouse model of psychosocial stress. To assess chronic activation with an unbiased brain-wide focus we used manganese-enhanced magnetic resonance imaging (MEMRI) and immunohistochemical staining of ∆FOSB, a transcription factor induced by repeated neural activity. One week after 10-day social defeat we observed significantly more activation in several brain regions known to regulate depressive and anxiety-like behaviour, including the prefrontal cortex, bed nucleus of stria terminalis, ventral hippocampus and periaqueductal grey in stressed compared to control mice. We further established that the correlation of ∆FOSB positive cells between specific brain regions was altered following chronic social defeat. Chronic activation of these neural circuits may relate to persistent brain activity changes occurring during chronic psychosocial stress exposure, with potential relevance for the development of anxiety and depression in humans.

A Single Angiotensin II Hypertensive Stimulus Is Associated with Prolonged Neuronal and Immune System Activation in Wistar-Kyoto Rats

Activation of autonomic neural pathways by chronic hypertensive stimuli plays a significant role in pathogenesis of hypertension. Here, we proposed that even a single acute hypertensive stimulus will activate neural and immune pathways that may be important in initiation of memory imprinting seen in chronic hypertension. We investigated the effects of acute angiotensin II (Ang II) administration on blood pressure, neural activation in cardioregulatory brain regions, and central and systemic immune responses, at 1 and 24 h post-injection. Administration of a single bolus intra-peritoneal (I.P.) injection of Ang II (36 μg/kg) resulted in a transient increase in the mean arterial pressure (MAP) (by 22 ± 4 mmHg vs saline), which returned to baseline within 1 h. However, in contrast to MAP, neuronal activity, as measured by manganese-enhanced magnetic resonance (MEMRI), remained elevated in several cardioregulatory brain regions over 24 h. The increase was predominant in autonomic regions, such as the subfornical organ (SFO; ~20%), paraventricular nucleus of the hypothalamus (PVN; ~20%) and rostral ventrolateral medulla (RVLM; ~900%), among others. Similarly, systemic and central immune responses, as evidenced by circulating levels of CD4⁺/IL17⁺ T cells, and increased IL17 levels and activation of microglia in the PVN, respectively, remained elevated at 24 h following Ang II challenge. Elevated Fos expression in the PVN was also present at 24 h (by 73 ± 11%) following Ang II compared to control saline injections, confirming persistent activation of PVN. Thus, even a single Ang II hypertensive stimulus will initiate changes in neuronal and immune cells that play a role in the developing hypertensive phenotype.

Manganese-Enhanced Magnetic Resonance Imaging and Studies of Rat Behavior: Transient Motor Deficit in Skilled Reaching, Rears, and Activity in Rats After a Single Dose of MnCl2

Manganese-enhanced magnetic resonance imaging (MEMRI) has been suggested to be a useful tool to visualize and map behavior-relevant neural populations at large scale in freely behaving rodents. A primary concern in MEMRI applications is Mn²⁺ toxicity. Although a few studies have specifically examined toxicity on gross motor behavior, Mn²⁺ toxicity on skilled motor behavior was not explored. Thus, the objective of this study was to combine manganese as a functional contrast agent with comprehensive behavior evaluation. We evaluated Mn²⁺ effect on skilled reach-to-eat action, locomotion, and balance using a single pellet reaching task, activity cage, and cylinder test, respectively. The tests used are sensitive to the pathophysiology of many neurological and neurodegenerative disorders of the motor system. The behavioral testing was done in combination with a moderate dose of manganese. Behavior was studied before and after a single, intravenous infusion of MnCl₂ (48 mg/kg). The rats were imaged at 1, 3, 5, 7, and 14 days following infusion. The results show that MnCl₂ infusion resulted in detectable abnormalities in skilled reaching, locomotion, and balance that recovered within 3 days compared with the infusion of saline. Because some tests and behavioral measures could not detect motor abnormalities of skilled movements, comprehensive evaluation of motor behavior is critical in assessing the effects of MnCl₂. The relaxation mapping results suggest that the transport of Mn²⁺ into the brain is through the choroid plexus-cerebrospinal fluid system with the primary entry point and highest relaxation rates found in the pituitary gland. Relaxation rates in the pituitary gland correlated with measures of motor skill, suggesting that altered motor ability is related to the level of Mn circulating in the brain. Thus, combined MEMRI and behavioral studies that both achieve adequate image enhancement and are also free of motor skills deficits are difficult to achieve using a single systemic dose of MnCl₂.

Ability of Mn(2+) to Permeate the Eye and Availability of Manganese-enhanced Magnetic Resonance Imaging for Visual Pathway Imaging via Topical Administration

Background:

Manganese-enhanced magnetic resonance imaging (MEMRI) for visual pathway imaging via topical administration requires further research. This study investigated the permeability of the corneal epithelium and corneal toxicity after topical administration of Mn²⁺ to understand the applicability of MEMRI.

Methods:

Forty New Zealand rabbits were divided into 0.05 mol/L, 0.10 mol/L, and 0.20 mol/L groups as well as a control group (n = 10 in each group). Each group was further subdivided into epithelium-removed and epithelium-intact subgroups (n = 5 in each subgroup). Rabbits were given 8 drops of MnCl₂ in 5 min intervals. The Mn²⁺ concentrations in the aqueous and vitreous humors were analyzed using inductively coupled plasma-mass spectrometry at different time points. MEMRI scanning was carried out to image the visual pathway after 24 h. The corneal toxicity of Mn²⁺ was evaluated with corneal imaging and pathology slices.

Results:

Between the aqueous and vitreous humors, there was a 10 h lag for the peak Mn²⁺ concentration times. The intraocular Mn²⁺ concentration increased with the concentration gradients of Mn²⁺ and was higher in the epithelium-removed subgroup than that in the epithelium-intact subgroup. The enhancement of the visual pathway was achieved in the 0.10 mol/L and 0.20 mol/L epithelium-removed subgroups. The corresponding peak concentrations of Mn²⁺ were 5087 ± 666 ng/ml, 22920 ± 1188 ng/ml in the aqueous humor and 884 ± 78 ng/ml, 2556 ± 492 ng/ml in the vitreous body, respectively. Corneal injury was evident in the epithelium-removed and 0.20 mol/L epithelium-intact subgroups.

Conclusions:

The corneal epithelium is a barrier to Mn²⁺, and the iris and lens septum might be another intraocular barrier to the permeation of Mn²⁺. An elevated Mn²⁺ concentration contributes to the increased permeation of Mn²⁺, higher MEMRI signal, and corneal toxicity. The enhancement of the visual pathway requires an effective Mn²⁺ concentration in the vitreous body.

Multi-system state shifts and cognitive deficits induced by chronic morphine during abstinence

Chronic morphine administration induces neural plasticity followed by withdraw. And clinic observation indicates that obvious cognitive deficits are found during withdrawal. However, current neural substrates that regulate dysfunction in withdrawal are unknown. In our studies, chronic morphine administration was used to induce the spontaneous withdrawal model in rats. A series of cognitive abilities was tested to explore brain function. To further evaluate the neural substrates of dysfunction, Manganese-enhanced MRI(MEMRI) was used to map the dysfunctional regions in vivo.We observed that chronic morphine administration could induce obvious withdrawal behaviors in abstinence followed by cognitive impairments, such as impairments in working memory, reward, interaction and enhancement of anxiety. Our in-vivo MEMRI data using the voxel-wise comparisons showed that the manganese-enhanced signal intensity (VMI) within morphine withdrawal groups was increased in cingulate cortex (Cg), secondary motor cortex (M2), CA3 subfield of hippocampus, dorsal striatum (D-striatum), retrosplenial cortex (RS), shell subregion of NAc (AcbSh), core subregion of NAc (AcbC), central nucleus of amygdala (CeC), basolateral amygdaloid nucleus (BLA), central amygdaloid nucleus (CeM), anterior hypothalamic area, central (AHC), ventral tegmental area (VTA) and scaphoid thalamic nucleus (SC).However, decreasing of VMI was found in the ventrolateral striatum (V-striatum) and lateral posterior thalamic nucleus (LP) compared to the control group. These brain regions were beleived to be components of the memory, executive, limbic and regulatory systems. Therefore, our present studies indicate that withdrawal induced by chronic morphine adiministration could disturb brain function leading to multi-systems state shifts and cognitive deficits in abstinence.

In vivo MEMRI characterization of brain metastases using a 3D Look-Locker T1-mapping sequence

Although MEMRI (Manganese Enhanced MRI) informations were obtained on primary tumors in small animals, MEMRI data on metastases are lacking. Thus, our goal was to determine if 3D Look-Locker T1 mapping was an efficient method to evaluate Mn ions transport in brain metastases in vivo. The high spatial resolution in 3D (156 × 156 × 218 μm) of the sequence enabled to detect metastases of 0.3 mm³. In parallel, the T1 quantitation enabled to distinguish three populations of MDA-MB-231 derived brain metastases after MnCl2 intravenous injection: one with a healthy blood-tumor barrier that did not internalize Mn²⁺ ions, and two others, which T1 shortened drastically by 54.2% or 24%. Subsequent scans of the mice, enabled by the fast acquisition (23 min), demonstrated that these T1 reached back their pre-injection values in 24 h. Contrarily to metastases, the T1 of U87-MG glioma remained 26.2% shorter for one week. In vitro results supported the involvement of the Transient Receptor Potential channels and the Calcium-Sensing Receptor in the uptake and efflux of Mn²⁺ ions, respectively. This study highlights the ability of the 3D Look-Locker T1 mapping sequence to study heterogeneities (i) amongst brain metastases and (ii) between metastases and glioma regarding Mn transport.

Eliminating Nox2 reactive oxygen species production protects dystrophic skeletal muscle from pathological calcium influx assessed in vivo by manganese-enhanced magnetic resonance imaging

KEY POINTS

Inhibiting Nox2 reactive oxygen species (ROS) production reduced in vivo calcium influx in dystrophic muscle. The lack of Nox2 ROS production protected against decreased in vivo muscle function in dystrophic mice. Manganese-enhanced magnetic resonance imaging (MEMRI) was able to detect alterations in basal calcium levels in skeletal muscle and differentiate disease status. Administration of Mn2+ did not affect muscle function or the health of the animal, and Mn2+ was cleared from skeletal muscle rapidly. We conclude that MEMRI may be a viable, non-invasive technique to monitor molecular alterations in disease progression and evaluate the effectiveness of potential therapies for Duchenne muscular dystrophy.

ABSTRACT

Duchenne muscular dystrophy (DMD) is an X-linked progressive degenerative disease resulting from a mutation in the gene that encodes dystrophin, leading to decreased muscle mechanical stability and force production. Increased Nox2 reactive oxygen species (ROS) production and sarcolemmal Ca2+ influx are early indicators of disease pathology, and eliminating Nox2 ROS production reduces aberrant Ca2+ influx in young mdx mice, a model of DMD. Various imaging modalities have been used to study dystrophic muscle in vivo; however, they are based upon alterations in muscle morphology or inflammation. Manganese has been used for indirect monitoring of calcium influx across the sarcolemma and may allow detection of molecular alterations in disease progression in vivo using manganese-enhanced magnetic resonance imaging (MEMRI). Therefore, we hypothesized that eliminating Nox2 ROS production would decrease calcium influx in adult mdx mice and that MEMRI would be able to monitor and differentiate disease status in dystrophic muscle. Both in vitro and in vivo data demonstrate that eliminating Nox2 ROS protected against aberrant Ca2+ influx and improved muscle function in dystrophic muscle. MEMRI was able to differentiate between different pathological states in vivo, with no long-term effects on animal health or muscle function. We conclude that MEMRI is a viable, non-invasive technique to differentiate disease status and might provide a means to monitor and evaluate the effectiveness of potential therapies in dystrophic muscle.

Age-related cognitive decline coincides with accelerated volume loss of the dorsal but not ventral hippocampus in mice

Even in the absence of neurodegenerative diseases, progressing age often coincides with cognitive decline and morphological changes. However, longitudinal studies that directly link these two processes are missing. In this proof-of-concept study we therefore performed repeated within-subject testing of healthy male R26R mice in a spatial learning task in combination with manganese-enhanced volumetric MRI analyses at the ages of 8, 16, and 24 months. We grouped the mice into good and poor performers (n = 6, each), based on their spatial learning abilities at the age of 24 months. Using this stratification, we failed to detect a priori volume differences, but observed a significant decrease in total hippocampal volume over time for both groups. Interestingly, this volume decrease was specific for the dorsal hippocampus and significantly accelerated in poor performers between 16 and 24 months of age. This is the first time that individual changes in hippocampal volume were traced alongside cognitive performance within the same subjects over 1½ years. Our study points to a causal link between volume loss of the dorsal hippocampus and cognitive impairments. In addition, it suggests accelerated degenerative processes rather than a priori volume differences as determining trajectories of age-related cognitive decline. Despite the relatively small sample sizes, the strong behavioral and moderate morphological alterations demonstrate the general feasibility of longitudinal studies of age-related decline in cognition and hippocampus integrity. © 2016 Wiley Periodicals, Inc.

Molecular fMRI

Comprehensive analysis of brain function depends on understanding the dynamics of diverse neural signaling processes over large tissue volumes in intact animals and humans. Most existing approaches to measuring brain signaling suffer from limited tissue penetration, poor resolution, or lack of specificity for well-defined neural events. Here we discuss a new brain activity mapping method that overcomes some of these problems by combining MRI with contrast agents sensitive to neural signaling. The goal of this "molecular fMRI" approach is to permit noninvasive whole-brain neuroimaging with specificity and resolution approaching current optical neuroimaging methods. In this article, we describe the context and need for molecular fMRI as well as the state of the technology today. We explain how major types of MRI probes work and how they can be sensitized to neurobiological processes, such as neurotransmitter release, calcium signaling, and gene expression changes. We comment both on past work in the field and on challenges and promising avenues for future development.

SIGNIFICANCE STATEMENT

Brain researchers currently have a choice between measuring neural activity using cellular-level recording techniques, such as electrophysiology and optical imaging, or whole-brain imaging methods, such as fMRI. Cellular level methods are precise but only address a small portion of mammalian brains; on the other hand, whole-brain neuroimaging techniques provide very little specificity for neural pathways or signaling components of interest. The molecular fMRI techniques we discuss have particular potential to combine the specificity of cellular-level measurements with the noninvasive whole-brain coverage of fMRI. On the other hand, molecular fMRI is only just getting off the ground. This article aims to offer a snapshot of the status and future prospects for development of molecular fMRI techniques.

Neuroimaging in Alzheimer's disease: preclinical challenges toward clinical efficacy

The scope of this review focuses on recent applications in preclinical and clinical magnetic resonance imaging (MRI) toward accomplishing the goals of early detection and responses to therapy in animal models of Alzheimer's disease (AD). Driven by the outstanding efforts of the Alzheimer's Disease Neuroimaging Initiative (ADNI), a truly invaluable resource, the initial use of MRI in AD imaging has been to assess changes in brain anatomy, specifically assessing brain shrinkage and regional changes in white matter tractography using diffusion tensor imaging. However, advances in MRI have led to multiple efforts toward imaging amyloid beta plaques first without and then with the use of MRI contrast agents. These technological advancements have met with limited success and are not yet appropriate for the clinic. Recent developments in molecular imaging inclusive of high-power liposomal-based MRI contrast agents as well as fluorine 19 ((19)F) MRI and manganese enhanced MRI have begun to propel promising advances toward not only plaque imaging but also using MRI to detect perturbations in subcellular processes occurring within the neuron. This review concludes with a discussion about the necessity for the development of novel preclinical models of AD that better recapitulate human AD for the imaging to truly be meaningful and for substantive progress to be made toward understanding and effectively treating AD. Furthermore, the continued support of outstanding programs such as ADNI as well as the development of novel molecular imaging agents and MRI fast scanning sequences will also be requisite to effectively translate preclinical findings to the clinic.

Manganese-Enhanced Magnetic Resonance Imaging for Detection of Vasoactive Intestinal Peptide Receptor 2 Agonist Therapy in a Model of Parkinson's Disease

Neuroprotective immunity is defined by transformation of T-cell polarity for therapeutic gain. For neurodegenerative disorders and specifically for Parkinson's disease (PD), granulocyte-macrophage colony stimulating factor or vasoactive intestinal peptide receptor 2 (VIPR2) agonists elicit robust anti-inflammatory microglial responses leading to neuronal sparing in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mice. While neurotherapeutic potential was demonstrated for PD, there remain inherent limitations in translating these inventions from the laboratory to patients. One obstacle in translating such novel neurotherapeutics centers on the availability of suitable noninvasive methods to track disease progression and therapeutic efficacy. To this end, we developed manganese-enhanced magnetic resonance imaging (MEMRI) assays as a way to track a linkage between glial activation and VIPR2 agonist (LBT-3627)-induced neuroprotective immunity for MPTP-induced nigrostriatal degeneration. Notably, LBT-3627-treated, MPTP-intoxicated mice show reduced MEMRI brain signal intensities. These changes paralleled reduced astrogliosis and resulted in sparing of nigral tyrosine hydroxylase neurons. Most importantly, the data suggest that MEMRI can be developed as a biomarker tool to monitor neurotherapeutic responses that are relevant to common neurodegenerative disorders used to improve disease outcomes.

Manganese-Enhanced Magnetic Resonance Imaging as a Diagnostic and Dispositional Tool after Mild-Moderate Blast Traumatic Brain Injury

Traumatic brain injury (TBI) caused by explosive munitions, known as blast TBI, is the signature injury in recent military conflicts in Iraq and Afghanistan. Diagnostic evaluation of TBI, including blast TBI, is based on clinical history, symptoms, and neuropsychological testing, all of which can result in misdiagnosis or underdiagnosis of this condition, particularly in the case of TBI of mild-to-moderate severity. Prognosis is currently determined by TBI severity, recurrence, and type of pathology, and also may be influenced by promptness of clinical intervention when more effective treatments become available. An important task is prevention of repetitive TBI, particularly when the patient is still symptomatic. For these reasons, the establishment of quantitative biological markers can serve to improve diagnosis and preventative or therapeutic management. In this study, we used a shock-tube model of blast TBI to determine whether manganese-enhanced magnetic resonance imaging (MEMRI) can serve as a tool to accurately and quantitatively diagnose mild-to-moderate blast TBI. Mice were subjected to a 30 psig blast and administered a single dose of MnCl2 intraperitoneally. Longitudinal T1-magnetic resonance imaging (MRI) performed at 6, 24, 48, and 72 h and at 14 and 28 days revealed a marked signal enhancement in the brain of mice exposed to blast, compared with sham controls, at nearly all time-points. Interestingly, when mice were protected with a polycarbonate body shield during blast exposure, the marked increase in contrast was prevented. We conclude that manganese uptake can serve as a quantitative biomarker for TBI and that MEMRI is a minimally-invasive quantitative approach that can aid in the accurate diagnosis and management of blast TBI. In addition, the prevention of the increased uptake of manganese by body protection strongly suggests that the exposure of an individual to blast risk could benefit from the design of improved body armor.

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.

Modulation of nucleus accumbens connectivity by alcohol drinking and naltrexone in alcohol-preferring rats: A manganese-enhanced magnetic resonance imaging study

The nonselective opioid receptor antagonist naltrexone is now used for the treatment of alcoholism, yet naltrexone's central mechanism of action remains poorly understood. One line of evidence suggests that opioid antagonists regulate alcohol drinking through interaction with the mesolimbic dopamine system. Hence, our goal here was to examine the role of the nucleus accumbens connectivity in alcohol reinforcement and naltrexone's actions using manganese-enhanced magnetic resonance imaging (MEMRI). Following long-term free-choice drinking of alcohol and water, AA (Alko Alcohol) rats received injections of MnCl2 into the nucleus accumbens for activity-dependent tracing of accumbal connections. Immediately after the accumbal injections, rats were imaged using MEMRI, and then allowed to drink either alcohol or water for the next 24h. Naltrexone was administered prior to the active dark period, and the second MEMRI was performed 24h after the first scan. Comparison of signal intensity at 1 and 24h after accumbal MnCl2 injections revealed an ipsilateral continuum through the ventral pallidum, bed nucleus of the stria terminalis, globus pallidus, and lateral hypothalamus to the substantia nigra and ventral tegmental area. Activation was also seen in the rostral part of the insular cortex and regions of the prefrontal cortex. Alcohol drinking resulted in enhanced activation of these connections, whereas naltrexone suppressed alcohol-induced activity. These data support the involvement of the accumbal connections in alcohol reinforcement and mediation of naltrexone's suppressive effects on alcohol drinking through their deactivation.

Facile synthesis of ternary CdMnS QD-based hollow nanospheres as fluorescent/magnetic probes for bioimaging

Fluorescent/magnetic dual-functional CdMnS hollow nanospheres with bright tunable emission and strong MR signal were synthesized via a facile Ostwald-ripening process with promising applications in bioimaging.

Activity-induced manganese-dependent MRI (AIM-MRI) and functional MRI in awake rabbits during somatosensory stimulation

Activity-induced manganese-dependent MRI (AIM-MRI) is a powerful tool to track system-wide neural activity using high resolution, quantitative T1-weighted MRI in animal models and has significant advantages for investigating neural activity over other modalities including BOLD fMRI. With AIM-MRI, Mn(2+) ions enter neurons via voltage-gated calcium channels preferentially active during the time of experimental exposure. A broad range of AIM-MRI studies using different species studying different phenomena have been performed, but few of these studies provide a systematic evaluation of the factors influencing the detection of Mn(2+) such as dosage and the temporal characteristics of Mn(2+) uptake. We identified an optimal dose of Mn(2+) (25 mg/kg, s.c.) in order to characterize the time-course of Mn(2+) accumulation in active neural regions in the rabbit. T1-weighted MRI and functional MRI were collected 0-3, 6-9, and 24-27 h post-Mn(2+) injection while the vibrissae on the right side were vibrated. Significant BOLD activation in the left somatosensory (SS) cortex and left ventral posteromedial (VPM) thalamic nucleus was detected during whisker vibration. T1-weighted signal intensities were extracted from these regions, their corresponding contralateral regions and the visual cortex (to serve as controls). A significant elevation in T1-weighted signal intensity in the left SS cortex (relative to right) was evident 6-9 and 24-27 h post-Mn(2+) injection while the left VPM thalamus showed a significant enhancement (relative to the right) only during the 24-27 h session. Visual cortex showed no hemispheric difference at any timepoint. Our results suggest that studies employing AIM-MRI would benefit by conducting experimental manipulations 6-24 h after subcutaneous MnCl2 injections to optimize the concentration of contrast agent in the regions active during the exposure.

Brain activation induced by voluntary alcohol and saccharin drinking in rats assessed with manganese-enhanced magnetic resonance imaging

The neuroanatomical and neurochemical basis of alcohol reward has been studied extensively, but global alterations of neural activity in reward circuits during chronic alcohol use remain poorly described. Here, we measured brain activity changes produced by long-term voluntary alcohol drinking in the alcohol-preferring AA (Alko alcohol) rats using manganese-enhanced magnetic resonance imaging (MEMRI). MEMRI is based on the ability of paramagnetic manganese ions to accumulate in excitable neurons and thereby enhance the T1-weighted signal in activated brain areas. Following 6 weeks of voluntary alcohol drinking, AA rats were allowed to drink alcohol for an additional week, during which they were administered manganese chloride (MnCl2 ) with subcutaneous osmotic minipumps before MEMRI. A second group with an identical alcohol drinking history received MnCl2 during the abstinence week following alcohol drinking. For comparing alcohol with a natural reinforcer, MEMRI was also performed in saccharin-drinking rats. A water-drinking group receiving MnCl2 served as a control. We found that alcohol drinking increased brain activity extensively in cortical and subcortical areas, including the mesocorticolimbic and nigrostriatal dopamine pathways and their afferents. Remarkably similar activation maps were seen after saccharin ingestion. Particularly in the prelimbic cortex, ventral hippocampus and subthalamic nucleus, activation persisted into early abstinence. These data show that voluntary alcohol recruits an extensive network that includes the ascending dopamine systems and their afferent connections, and that this network is largely shared with saccharin reward. The regions displaying persistent alterations after alcohol drinking could participate in brain networks underlying alcohol seeking and relapse.

Pharmocologic treatment with histone deacetylase 6 inhibitor (ACY-738) recovers Alzheimer's disease phenotype in amyloid precursor protein/presenilin 1 (APP/PS1) mice

Introduction

Current therapy for Alzheimer's disease (AD) focuses on delaying progression, illustrating the need for more effective therapeutic targets. Histone deacetylase 6 (HDAC6) modulates tubulin acetylation and has been implicated as an attractive target. HDAC6 is also elevated in postmortem tissue samples from patients. However, HDAC6 inhibitors have had limited success preclinically due to low blood-brain barrier penetration.

Method

We investigated a specific, potent HDAC6 inhibitor (ACY-738) in a mouse model of AD. We determined the effects of ACY-738 treatment on axonal transport, behavior, and pathology in amyloid precursor protein/presenilin 1 mice.

Results

We demonstrated improvements in in vivo axonal transport in two treatment groups as a result of ACY-738 brain levels. We also demonstrated recovery of short-term learning and memory deficits, hyperactivity, and modifications of tau and tubulin.

Discussion

Our findings implicate specific, targeted HDAC6 inhibitors as potential therapeutics and demonstrate that further investigations are warranted into effects of HDAC6 inhibitors in AD.

Effect of MDMA-Induced Axotomy on the Dorsal Raphe Forebrain Tract in Rats: An In Vivo Manganese-Enhanced Magnetic Resonance Imaging Study

3,4-Methylenedioxymethamphetamine (MDMA), also known as “Ecstasy”, is a common recreational drug of abuse. Several previous studies have attributed the central serotonergic neurotoxicity of MDMA to distal axotomy, since only fine serotonergic axons ascending from the raphe nucleus are lost without apparent damage to their cell bodies. However, this axotomy has never been visualized directly in vivo. The present study examined the axonal integrity of the efferent projections from the midbrain raphe nucleus after MDMA exposure using in vivo manganese-enhanced magnetic resonance imaging (MEMRI). Rats were injected subcutaneously six times with MDMA (5 mg/kg) or saline once daily. Eight days after the last injection, manganese ions (Mn²⁺) were injected stereotactically into the raphe nucleus, and a series of MEMRI images was acquired over a period of 38 h to monitor the evolution of Mn²⁺-induced signal enhancement across the ventral tegmental area, the medial forebrain bundle (MFB), and the striatum. The MDMA-induced loss of serotonin transporters was clearly evidenced by immunohistological staining consistent with the Mn²⁺-induced signal enhancement observed across the MFB and striatum. MEMRI successfully revealed the disruption of the serotonergic raphe-striatal projections and the variable effect of MDMA on the kinetics of Mn²⁺ accumulation in the MFB and striatum.

Imaging organelle transport in primary hippocampal neurons treated with amyloid-β oligomers

We describe a strategy for fluorescent imaging of organelle transport in primary hippocampal neurons treated with amyloid-β (Aβ) peptides that cause Alzheimer's disease (AD). This method enables careful, rigorous analyses of axonal transport defects, which are implicated in AD and other neurodegenerative diseases. Moreover, we present and emphasize guidelines for investigating Aβ-induced mechanisms of axonal transport disruption in the absence of nonspecific, irreversible cellular toxicity. This approach should be accessible to most laboratories equipped with cell culture facilities and a standard fluorescent microscope and may be adapted to other cell types.

Manganese-Enhanced MRI: Biological Applications in Neuroscience

Magnetic resonance imaging (MRI) is an excellent non-invasive tool to investigate biological systems. The administration of the paramagnetic divalent ion manganese (Mn²⁺) enhances MRI contrast in vivo. Due to similarities between Mn²⁺ and calcium (Ca²⁺), the premise of manganese-enhanced MRI (MEMRI) is that the former may enter neurons and other excitable cells through voltage-gated Ca²⁺ channels. As such, MEMRI has been used to trace neuronal pathways, define morphological boundaries, and study connectivity in morphological and functional imaging studies. In this article, we provide a brief overview of MEMRI and discuss recently published data to illustrate the usefulness of this method, particularly in animal models.

Manganese-Induced Parkinsonism and Parkinson's Disease: Shared and Distinguishable Features

Manganese (Mn) is an essential trace element necessary for physiological processes that support development, growth and neuronal function. Secondary to elevated exposure or decreased excretion, Mn accumulates in the basal ganglia region of the brain and may cause a parkinsonian-like syndrome, referred to as manganism. The present review discusses the advances made in understanding the essentiality and neurotoxicity of Mn. We review occupational Mn-induced parkinsonism and the dynamic modes of Mn transport in biological systems, as well as the detection and pharmacokinetic modeling of Mn trafficking. In addition, we review some of the shared similarities, pathologic and clinical distinctions between Mn-induced parkinsonism and Parkinson’s disease. Where possible, we review the influence of Mn toxicity on dopamine, gamma aminobutyric acid (GABA), and glutamate neurotransmitter levels and function. We conclude with a survey of the preventive and treatment strategies for manganism and idiopathic Parkinson’s disease (PD).

Manganese-Enhanced Magnetic Resonance Imaging Reflects Brain Pathology During Progressive HIV-1 Infection of Humanized Mice

Progressive human immunodeficiency viral (HIV) infection commonly leads to a constellation of cognitive, motor, and behavioral impairments. These are collectively termed HIV-associated neurocognitive disorders (HAND). While antiretroviral therapy (ART) reduces HAND severity, it does not affect disease prevalence. Despite decades of research, there remain no biomarkers for HAND and all potential comorbid conditions must first be excluded for a diagnosis to be made. To this end, we now report that manganese (Mn(2+))-enhanced magnetic resonance imaging (MEMRI) can reflect brain region-specific HIV-1-induced neuropathology in chronically virus-infected NOD/scid-IL-2Rγc(null) humanized mice. MEMRI diagnostics mirrors the abilities of Mn(2+) to enter and accumulate in affected neurons during disease. T1 relaxivity and its weighted signal intensity are proportional to Mn(2+) activities in neurons. In 16-week virus-infected humanized mice, altered MEMRI signal enhancement was easily observed in affected brain regions. These included, but were not limited to, the hippocampus, amygdala, thalamus, globus pallidus, caudoputamen, substantia nigra, and cerebellum. MEMRI signal was coordinated with levels of HIV-1 infection, neuroinflammation (astro- and micro-gliosis), and neuronal injury. MEMRI accurately demonstrates the complexities of HIV-1-associated neuropathology in rodents that reflects, in measure, the clinical manifestations of neuroAIDS as it is seen in a human host.

4D MEMRI atlas of neonatal FVB/N mouse brain development

The widespread use of the mouse as a model system to study brain development has created the need for noninvasive neuroimaging methods that can be applied to early postnatal mice. The goal of this study was to optimize in vivo three- (3D) and four-dimensional (4D) manganese (Mn)-enhanced MRI (MEMRI) approaches for acquiring and analyzing data from the developing mouse brain. The combination of custom, stage-dependent holders and self-gated (motion-correcting) 3D MRI sequences enabled the acquisition of high-resolution (100-μm isotropic), motion artifact-free brain images with a high level of contrast due to Mn-enhancement of numerous brain regions and nuclei. We acquired high-quality longitudinal brain images from two groups of FVB/N strain mice, six mice per group, each mouse imaged on alternate odd or even days (6 3D MEMRI images at each day) covering the developmental stages between postnatal days 1 to 11. The effects of Mn-exposure, anesthesia and MRI were assessed, showing small but significant transient effects on body weight and brain volume, which recovered with time and did not result in significant morphological differences when compared to controls. Metrics derived from deformation-based morphometry (DBM) were used for quantitative analysis of changes in volume and position of a number of brain regions. The cerebellum, a brain region undergoing significant changes in size and patterning at early postnatal stages, was analyzed in detail to demonstrate the spatiotemporal characterization made possible by this new atlas of mouse brain development. These results show that MEMRI is a powerful tool for quantitative analysis of mouse brain development, with great potential for in vivo phenotype analysis in mouse models of neurodevelopmental diseases.

Manganese-Enhanced MRI Reflects Both Activity-Independent and Activity-Dependent Uptake within the Rat Habenulomesencephalic Pathway

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful technique for assessing the functional connectivity of neurons within the central nervous system. Despite the widely held proposition that MEMRI signal is dependent on neuronal activity, few studies have directly tested this implicit hypothesis. In the present series of experiments, MnCl2 was injected into the habenula of urethane-anesthetized rats alone or in combination with drugs known to alter neuronal activity by modulating specific voltage- and/or ligand-gated ion channels. Continuous quantitative T1 mapping was used to measure Mn2+ accumulation in the interpeduncular nucleus, a midline structure in which efferents from the medial habenula terminate. Microinjection of MnCl2 into the habenular complex using a protocol that maintained spontaneous neuronal activity resulted in a time-dependent increase in MEMRI signal intensity in the interpeduncular nucleus consistent with fast axonal transport of Mn2+ between these structures. Co-injection of the excitatory amino-acid agonist AMPA, increased the Mn2+-enhanced signal intensity within the interpeduncular nucleus. AMPA-induced increases in MEMRI signal were attenuated by co-injection of either the sodium channel blocker, TTX, or broad-spectrum Ca2+ channel blocker, Ni2+, and were occluded in the presence of both channel blockers. However, neither Ni2+ nor TTX, alone or in combination, attenuated the increase in signal intensity following injection of Mn2+ into the habenula. These results support the premise that changes in neuronal excitability are reflected by corresponding changes in MEMRI signal intensity. However, they also suggest that basal rates of Mn2+ uptake by neurons in the medial habenula may also occur via activity-independent mechanisms.

Manganese-enhanced magnetic resonance imaging reveals differential long-term neuroadaptation after methamphetamine and the substituted cathinone 4-methylmethcathinone (mephedrone)

BACKGROUND

In recent years there has been a large increase in the use of substituted cathinones such as mephedrone (4-methylmethcathinone, 4-MMC), a psychostimulant drug that shows a strong resemblance to methamphetamine (METH). Unlike METH, which can produce clear long-term effects, the effects of 4-MMC have so far remained elusive. We employ manganese-enhanced magnetic resonance imaging (MEMRI), a highly sensitive method for detecting changes in neuronal activation, to investigate the effects of METH and 4-MMC on the brain.

METHODS

In Wistar rats we performed a MEMRI scan two weeks after binge treatments (twice daily for 4 consecutive days) of METH (5 mg/kg) or 4-MMC (30 mg/kg). Furthermore, locomotor activity measurements and novel object recognition tests were performed.

RESULTS

METH produced a widespread pattern of decreased bilateral activity in several regions, including the nucleus accumbens, caudate putamen, globus pallidus, thalamus, and hippocampus, as well as several other cortical and subcortical areas. Conversely, 4-MMC produced increased bilateral activity, anatomically limited to the hypothalamus and hippocampus. Drug treatments did not affect the development of locomotor sensitization or novel object recognition performance.

CONCLUSIONS

The pattern of decreased brain activity seen after METH corresponds closely to regions known to be affected by this drug and confirms the validity of MEMRI for detecting neuroadaptation two weeks after amphetamine binge treatment. 4-MMC, unlike METH, produced increased activity in a limited number of different brain regions. This highlights an important difference in the long-term effects of these drugs on neural function and shows precisely the anatomical localization of 4-MMC-induced neuroadaptation.

Electron paramagnetic resonance: a powerful tool to support magnetic resonance imaging research

The purpose of this paper is to describe some of the areas where electron paramagnetic resonance (EPR) has provided unique information to MRI developments. The field of application mainly encompasses the EPR characterization of MRI paramagnetic contrast agents (gadolinium and manganese chelates, nitroxides) and superparamagnetic agents (iron oxide particles). The combined use of MRI and EPR has also been used to qualify or disqualify sources of contrast in MRI. Illustrative examples are presented with attempts to qualify oxygen sensitive contrast (i.e. T1 - and T2 *-based methods), redox status or melanin content in tissues. Other areas are likely to benefit from the combined EPR/MRI approach, namely cell tracking studies. Finally, the combination of EPR and MRI studies on the same models provides invaluable data regarding tissue oxygenation, hemodynamics and energetics. Our description will be illustrative rather than exhaustive to give to the readers a flavour of 'what EPR can do for MRI'.