In vivo tracing of major rat brain pathways using manganese-enhanced magnetic resonance imaging and three-dimensional digital atlasing

Neuronal dysfunction of a long projecting multisynaptic pathway in response to methamphetamine using manganese-enhanced MRI

RATIONALE

Manganese (Mn2+)-enhanced magnetic resonance imaging (MEMRI) is an emerging in vivo MR approach for pharmacological research. One new application of MEMRI in this area is to characterize functional changes of a specific neural circuit that is essential to the central effects of a drug challenge.

OBJECTIVES

To develop and validate such use of MEMRI in neuropharmacology, the current study applied MEMRI to visualize functional changes within a multisynaptic pathway originating from fasciculus retroflexus (FR) that is central to a commonly abused psychostimulant, methamphetamine (MA).

METHODS

Twelve rats were injected intraperitoneally with MA (10 mg/kg) or saline every 2 h for a total of four injections. After 6 days, Mn2+ was injected into the habenular nucleus (FR origin) of all animals, and MEMRI was repeatedly performed at certain points in time over 48 h. The evolution of Mn2+-induced signal enhancement was assessed across the FR tract, the ventral tegmental area (VTA), the striatum, the nucleus accumbens, and the prefrontal cortex (PFC), in both MA-injected animals and controls.

RESULTS

MA treatment was found to affect the complexity and efficiency of Mn2+ uptake in the VTA, via the FR tract, with significantly increased Mn2+ accumulation in the VTA, the dorsomedial part of the striatum, and the PFC.

CONCLUSIONS

MEMRI successfully visualizes disruptions in the multisynaptic pathway as the consequences of repeated MA exposure. MEMRI is potentially an important method in the future to investigate functional changes within a specific pathway under the influences of pharmacological agents, given its excellent functional, in vivo, spatial, and temporal properties.

Analysis of laminar activity in normal and injured rat spinal cord by manganese enhanced MRI

The present study provides an account of a sensitive and rapid experimental approach for MRI visualization and analysis of spinal cord (SC) laminar activity in normal and injured animals. This approach is based upon neuronal activity-dependant manganese (Mn) uptake after focal SC injection of MnCl(2), and subsequent ex-vivo magnetic resonance imaging (MRI) of activated SC pathways. The method was designed as an alternative to time-intensive histochemical and behavioral approaches typically used for analysis of spinal cord injury (SCI) and our results provide both anatomical and functional insights. We show that ex vivo imaging can determine layer-specific activity over an extended region of the rat SC. In addition, we demonstrate that the Mn concentration profile along the SC axis accurately reflects the type of SC injury. The approach is flexible since MRI analysis can be done immediately after animal sacrifice, or alternatively several days later, without a loss of sensitivity. Moreover, the integrity and functional state of SC circuitry can be analyzed in less than 1 h whereas several days and weeks are necessary to perform classical histochemical and behavioral analysis. Thus our method can be used for precise assessment of the extent of dysfunction or change in SC disorders and may facilitate the screening of molecules with therapeutic potential after SC injury.

Magnetic resonance imaging of cortical connectivity in vivo

Magnetic resonance imaging of neuronal connectivity in vivo opens up the possibility of performing longitudinal investigations on neuronal networks. This is one main reason for the attention that paramagnetic ion manganese (Mn2+) has attracted as a potential anterograde neuronal tracer for MRI experiments. However, the correct and possibly repeated use of this tracer--or of any tracer for that matter, including heavy metals--requires the development of an administration strategy that minimizes its toxic effects. Here we first investigated the conditions that maximize the tracing efficiency of Mn2+ and preserve viability and tissue architectonics in combined MRI and histology experiments in rats. We demonstrate that most common protocols for neuronal tract tracing using Mn2+ result in large neuronal and glial lesions. The toxicity of manganese is distinct during intracortical injections and blocks the transfer of the tracer. After optimizing the technique, we could show that extensive cortical connectivity maps can be generated, with no sign of neuronal damage. Importantly, preservation of tissue viability improves the efficiency of Mn2+ in tracing neuronal connections. We have successfully used this technique to trace corticofugal somatosensory and motor pathways in individual animals and describe a connectivity index (CnI) based on Mn2+ transport that quantitatively reveals cortical heterogeneities in interhemispheric communication. Finally, we have significantly improved the resolution of the technique by continuously infusing very low concentrations of Mn2+ into the target area using osmotic pumps coupled to chronically implanted brain cannulae. The specific, nontoxic and quantitative nature of the neuronal tracings described here indicates the value of this tracer for chronic studies of development and plasticity as well as for studies of brain pathology.

Quantitative validation of voxel-wise statistical analyses of autoradiographic rat brain volumes: application to unilateral visual stimulation

PET scanners devoted to in vivo functional study have recently been developed, but autoradiography remains the reference technique for assessing cerebral glucose metabolism (CMRGlu) in rodents. Autoradiographs are conventionally subjected to region of interest (ROI) analysis, which is intrinsically hypothesis-driven and therefore not suitable for whole-brain investigation. Voxel-wise statistical methods of analysis have long been used to determine differences in brain activity during in vivo functional neuroimaging experiments. They have also recently been applied to 3D reconstructed autoradiographic volume images from rat brains. We present here a fully automated analysis for autoradiographic data combining (1) computerized procedures for the acquisition and 3D reconstruction of postmortem volume images and (2) spatial normalization followed by classical whole-brain voxel-wise statistical analysis. We also describe an additional procedure for characterizing functional differences between the right and left hemispheres of the brain. We compared two spatial normalization techniques and evaluated how the effect of choosing a particular normalization technique impacted on the statistical analysis. We also propose a small volume correction analysis to address the problem of multiple statistical comparisons. Lastly, we investigated the reliability of such analyses, by comparing their results qualitatively and quantitatively with those previously obtained with our semiautomated ROI-based analysis [Dubois, A., Dauguet, J., Herard, A.-S., Besret, L., Duchesnay, E., Frouin, V., Hantraye, P., Bonvento, G., Delzescaux, T., 2007. Automated three-dimensional analysis of histologic and autoradiographic rat brain sections: application to an activation study. J. Cereb. Blood Flow Metab. 27 (10), 1742-1755.]. Both voxel-wise statistical analyses led to the detection of consistent interhemispheric differences in CMRGlu. This work demonstrates the potential value and robustness of voxel-wise statistical methods for analyzing autoradiographic data sets.

Neuroaxonal ion dyshomeostasis of the normal-appearing corpus callosum in experimental autoimmune encephalomyelitis

Atrophy of the corpus callosum (CC) is a well-documented observation in clinically definite multiple sclerosis (MS) patients. One recent hypothesis for the neurodegeneration that occurs in MS is that ion dyshomeostasis leads to neuroaxonal damage. To examine whether ion dyshomeostasis occurs in the CC during MS onset, experimental autoimmune encephalomyelitis (EAE) was utilized as an animal MS model to induce autoimmunity-mediated responses. To date, in vivo investigations of neuronal ion homeostasis has not been feasible using traditional neuroscience techniques. Therefore, the current study employed an emerging MRI method, called Mn2+-enhanced MRI (MEMRI). Mn2+ dynamics is closely associated with important neuronal activity events, and is also considered to be a Ca2+ surrogate. Furthermore, when injected intracranially, Mn2+ can be used as a multisynaptic tracer. These features enable MEMRI to detect neuronal ion homeostasis within a multisynaptic circuit that is connected to the injection site. Mn2+ was injected into the visual cortex to trace the CC, and T1-weighted imaging was utilized to observe temporal changes in Mn2+-induced signals in the traced pathways. The results showed that neuroaxonal functional changes associated with ion dyshomeostasis occurred in the CC during an acute EAE attack. In addition, the pathway appeared normal, although EAE-induced immune-cell infiltration was visible around the CC. The findings suggest that ion dyshomeostasis is a major neuronal aberration underlying the deterioration of normal-appearing brain tissues in MS, supporting its involvement in neuroaxonal functioning in MS.

Three-dimensional atlas system for mouse and rat brain imaging data

Tomographic neuroimaging techniques allow visualization of functionally and structurally specific signals in the mouse and rat brain. The interpretation of the image data relies on accurate determination of anatomical location, which is frequently obstructed by the lack of structural information in the data sets. Positron emission tomography (PET) generally yields images with low spatial resolution and little structural contrast, and many experimental magnetic resonance imaging (MRI) paradigms give specific signal enhancements but often limited anatomical information. Side-by-side comparison of image data with conventional atlas diagram is hampered by the 2-D format of the atlases, and by the lack of an analytical environment for accumulation of data and integrative analyses. We here present a method for reconstructing 3-D atlases from digital 2-D atlas diagrams, and exemplify 3-D atlas-based analysis of PET and MRI data. The reconstruction procedure is based on two seminal mouse and brain atlases, but is applicable to any stereotaxic atlas. Currently, 30 mouse brain structures and 60 rat brain structures have been reconstructed. To exploit the 3-D atlas models, we have developed a multi-platform atlas tool (available via The Rodent Workbench, http://rbwb.org) which allows combined visualization of experimental image data within the 3-D atlas space together with 3-D viewing and user-defined slicing of selected atlas structures. The tool presented facilitates assignment of location and comparative analysis of signal location in tomographic images with low structural contrast.

In vivo manganese MR imaging of calcium influx in spontaneous rat pituitary adenoma

BACKGROUND AND PURPOSE

Rapid uptake of the calcium analog manganese (Mn2+) into spontaneous pituitary adenoma during MR imaging of aged rats generated the hypothesis that neuroendocrine tumors may have a corresponding increase in calcium influx required to trigger hormonal release. A goal of this study was to investigate the potential for clinical evaluation of pituitary adenoma by MR imaging combined with administration of Mn2+ (Mn-MR imaging).

MATERIALS AND METHODS

Mn-MR imaging was used to characterize the dynamic calcium influx in normal aged rat pituitary gland as well as spontaneous pituitary adenoma. To confirm the validity of Mn2+ as a calcium analog, we inhibited Mn2+ uptake into the olfactory bulb and pituitary gland of normal rats by using the calcium channel blocker verapamil. Rats with adenomas received fluorodeoxyglucose-positron-emission tomography (FDG-PET) scanning for characterization of tumor metabolism. Mn2+ influx was characterized in cultured pituitary adenoma cells.

RESULTS

Volume of interest analysis of the normal aged pituitary gland versus adenoma indicated faster and increased calcium influx in adenoma at 1, 3, 11, and 48 hours. Mn2+ uptake into the olfactory bulb and pituitary gland of normal rats was inhibited by calcium channel blockers and showed dose-dependent inhibition on dynamic MR imaging. FDG-PET indicated correlation between tumor energy metabolism and Mn2+ influx as well as tumor size.

CONCLUSION

These results indicate that adenomas have increased activity-dependent calcium influx compared with normal aged pituitary glands, suggesting a potential for exploitation in the clinical work-up of pituitary and other neuroendocrine tumors by developing Mn-MR imaging for humans.

Magnetic Resonance Imaging of Functional Anatomy: Use for Small Animal Epilepsy Models

Neuroimaging has greatly assisted the diagnosis and treatment of epilepsy. Volumetric analysis, diffusion-weighted imaging, and other magnetic resonance imaging (MRI) modalities provide a clear picture of altered anatomical structures in both focal and nonfocal disease. More recently, advances in novel imaging methodologies have provided unique insights into this disease. Two examples include manganese-enhanced MRI (MEMRI) and diffusion tensor imaging (DTI). MEMRI involves injection of MnCl(2) to evaluate neuronal activity where it is actively transported. Areas of neuronal hyperactivity are expected to have altered uptake and transport. Mapping of activation along preferential uptake pathways can be confirmed by T(1)-weighted imaging. DTI uses the intrinsic preferential mobility of water movement along axonal pathways to map anatomical regions. DTI has been used to investigate white matter disease and is now being applied to clinical and, to a lesser extent, animal investigations of seizure disorders. These two diverse MRI methods can be applied to animal models to provide important information about the functional status of anatomical regions that may be altered by epilepsy.

Age-related decrease in axonal transport measured by MR imaging in vivo

Axonal transport is a crucial process for neuronal homeostasis and cell functions. In vitro studies have indicated transport rates decrease with age. Disruption of axonal transport has been implicated in age-associated neurodegenerative disorders. We hypothesized that aged rats would show decreased transport in the brain, which could be measured using in vivo manganese-enhanced MR imaging (Mn-MRI) and parametric estimation. Serial T1-weighted images were obtained at pre- and post-administration of MnCl(2) in rats scanned longitudinally (n=4) and in a separate aged group (n=3). Subtraction analysis was performed for group-wise statistical comparison on a pixel-by-pixel basis. Change in intensity over time was plotted for the olfactory bulb and anterior and posterior olfactory tract. Bulk transport of material was estimated over an initial 72 h. Tracer kinetic estimation of time-intensity data, based on a mass transport model, used intensity change in the bulb as input function for subsequent changes in the tract. Time to the peak of Mn(2+) flow was estimated for both anterior and posterior tracts. Results indicated age-related decreases in axonal transport rate and bulk transport of material in the olfactory tract of living rat brains. Longitudinally scanned, mid-age group was decreased by 58% and the aged group by 71% of young rate (neuronal transport=4.07+/-1.24 mm/h, 1.72+/-0.89 mm/h, and 1.16+/-0.18 mm/h for young, mid-age, and aged, respectively). Neuronal transport rate decreases correlated with increased age. The use of kinetic analysis combined with dynamic manganese enhanced MR imaging provides a unique opportunity to study this important neuronal process.

Current status of functional MRI on small animals: application to physiology, pathophysiology, and cognition

This review aims to make the reader aware of the potential of functional MRI (fMRI) in brain activation studies in small animal models. As small animals generally require anaesthesia for immobilization during MRI protocols, this is believed to be a serious limitation to the type of question that can be addressed with fMRI. We intend to introduce a fresh view with an in-depth overview of the surprising number of fMRI applications in a wide range of important research domains in neuroscience. These include the pathophysiology of brain functioning, the basic science of activity, and functional connectivity of different sensory circuits, including sensory brain mapping, the challenges when studying the hypothalamus as the major control centre in the central nervous system, and the limbic system as neural substrate for emotions and reward. Finally the contribution of small animal fMRI research to cognitive neuroscience is outlined. This review avoids focusing exclusively on traditional small laboratory animals such as rodents, but rather aims to broaden the scope by introducing alternative lissencephalic animal models such as songbirds and fish, as these are not yet well recognized as neuroimaging study subjects. These models are well established in many other neuroscience disciplines, and this review will show that their investigation with in vivo imaging tools will open new doors to cognitive neuroscience and the study of the autonomous nervous system in experimental animals.

Validation of in vitro probabilistic tractography

Diffusion weighted imaging (DWI) and tractography allow the non-invasive study of anatomical brain connectivity. However, a gold standard for validating tractography of complex connections is lacking. Using the porcine brain as a highly gyrated brain model, we quantitatively and qualitatively assessed the anatomical validity and reproducibility of in vitro multi-fiber probabilistic tractography against two invasive tracers: the histochemically detectable biotinylated dextran amine and manganese enhanced magnetic resonance imaging. Post mortem DWI was used to ensure that most of the sources known to degrade the anatomical accuracy of in vivo DWI did not influence the tracking results. We demonstrate that probabilistic tractography reliably detected specific pathways. Moreover, the applied model allowed identification of the limitations that are likely to appear in many of the current tractography methods. Nevertheless, we conclude that DWI tractography can be a precise tool in studying anatomical brain connectivity.

Live imaging of neuronal connections by magnetic resonance: Robust transport in the hippocampal-septal memory circuit in a mouse model of Down syndrome

Connections from hippocampus to septal nuclei have been implicated in memory loss and the cognitive impairment in Down syndrome (DS). We trace these connections in living mice by Mn(2+) enhanced 3D MRI and compare normal with a trisomic mouse model of DS, Ts65Dn. After injection of 4 nl of 200 mM Mn(2+) into the right hippocampus, Mn(2+) enhanced circuitry was imaged at 0.5, 6, and 24 h in each of 13 different mice by high resolution MRI to detect dynamic changes in signal over time. The pattern of Mn(2+) enhanced signal in vivo correlated with the histologic pattern in fixed brains of co-injected 3kD rhodamine-dextran-amine, a classic tracer. Statistical parametric mapping comparing intensity changes between different time points revealed that the dynamics of Mn(2+) transport in this pathway were surprisingly more robust in DS mice than in littermate controls, with statistically significant intensity changes in DS appearing at earlier time points along expected pathways. This supports reciprocal alterations of transport in the hippocampal-forebrain circuit as being implicated in DS and argues against a general failure of transport. This is the first examination of in vivo transport dynamics in this pathway and the first report of elevated transport in DS.

Neuroanatomical affiliation visualization-interface system

A number of knowledge management systems have been developed to allow users to have access to large quantity of neuroanatomical data. The advent of three-dimensional (3D) visualization techniques allows users to interact with complex 3D object. In order to better understand the structural and functional organization of the brain, we present Neuroanatomical Affiliations Visualization-Interface System (NAVIS) as the original software to see brain structures and neuroanatomical affiliations in 3D. This version of NAVIS has made use of the fifth edition of "The Rat Brain in Stereotaxic coordinates" (Paxinos and Watson, 2005). The NAVIS development environment was based on the scripting language name Python, using visualization toolkit (VTK) as 3D-library and wxPython for the graphic user interface. The following manuscript is focused on the nucleus of the solitary tract (Sol) and the set of affiliated structures in the brain to illustrate the functionality of NAVIS. The nucleus of the Sol is the primary relay center of visceral and taste information, and consists of 14 distinct subnuclei that differ in cytoarchitecture, chemoarchitecture, connections, and function. In the present study, neuroanatomical projection data of the rat Sol were collected from selected literature in PubMed since 1975. Forty-nine identified projection data of Sol were inserted in NAVIS. The standard XML format used as an input for affiliation data allows NAVIS to update data online and/or allows users to manually change or update affiliation data. NAVIS can be extended to nuclei other than Sol.

Toward a workbench for rodent brain image data systems architecture and design

We present a novel system for storing and manipulating microscopic images from sections through the brain and higher-level data extracted from such images. The system is designed and built on a three-tier paradigm and provides the research community with a web-based interface for facile use in neuroscience research. The Oracle relational database management system provides the ability to store a variety of objects relevant to the images and provides the framework for complex querying of data stored in the system. Further, the suite of applications intimately tied into the infrastructure in the application layer provide the user the ability not only to query and visualize the data, but also to perform analysis operations based on the tools embedded into the system. The presentation layer uses extant protocols of the modern web browser and this provides ease of use of the system. The present release, named Functional Anatomy of the Cerebro-Cerebellar System (FACCS), available through The Rodent Brain Workbench (http:// rbwb.org/), is targeted at the functional anatomy of the cerebro-cerebellar system in rats, and holds axonal tracing data from these projections. The system is extensible to other circuits and projections and to other categories of image data and provides a unique environment for analysis of rodent brain maps in the context of anatomical data. The FACCS application assumes standard animal brain atlas models and can be extended to future models. The system is available both for interactive use from a remote web-browser client as well as for download to a local server machine.

In vivo imaging of functional disruption, recovery and alteration in rat olfactory circuitry after lesion

Compensatory changes following disruption of neuronal circuitry have been indicated by previous imaging studies of stroke and other brain injury, but evidence of the pathways involved in such dynamic changes has not been shown in vivo. We imaged rats before and after lesion-induced disruption of the lateral olfactory tract to investigate the subsequent recovery and/or reorganization of functional neuronal circuitry. Serial magnetic resonance imaging was performed following intranasal administration of a paramagnetic track tracer Mn(2+). Images were analyzed using statistical mapping techniques in the stereotactic coordinate system. At 1 week post-lesion, Mn(2+) transport caudal to lesion was reduced as expected, and more importantly, increased transport through the anterior commissure was seen. At 4 weeks post-lesion, there was recovery of transport caudal to lesion, and increased transport through the anterior commissure extended to the contralateral olfactory cortex. Correlation analysis of regional Mn(2+) transport indicated that contralateral enhancement was not simply due to septal window spillover. This study demonstrates for the first time in vivo evidence of compensatory changes in functional neuronal activity to a contralateral pathway through the commissure following brain injury.

Electrical stimulation of cortex improves corticospinal tract tracing in rat spinal cord using manganese-enhanced MRI

Following bilateral injection of manganese (Mn) into the rat's motor cortex, electrical stimulation of the cortex is shown to increase the transport, uptake and accumulation of Mn in the corticospinal tract (CST), as assessed by manganese-enhanced magnetic resonance imaging (MEI). T(1)-weighted gradient echo images were acquired in 3-D and displayed in different orientations to anatomically delineate the CST pathway from cortex to spinal cord (SC) at the thoracic level. T(1)-maps of the SC were produced from spin-echo based image data to demonstrate the distribution of the T(1) properties of the SC tissue and to quantitatively assess the T(1)-change occurring in the CST due to the presence of Mn therein. Implications for improving the tract tracing ability with the proposed in vivo approach and its application to spinal cord injury (SCI) research are discussed in terms of aiding future experimental investigations of neuroplasticity following an injury.

A stereotaxic MRI template set for the rat brain with tissue class distribution maps and co-registered anatomical atlas: Application to pharmacological MRI

We describe a stereotaxic rat brain MRI template set with a co-registered digital anatomical atlas and illustrate its application to the analysis of a pharmacological MRI (phMRI) study of apomorphine. The template set includes anatomical images and tissue class probability maps for brain parenchyma and cerebrospinal fluid (CSF). These facilitate the use of standard fMRI software for spatial normalisation and tissue segmentation of rat brain data. A volumetric reconstruction of the Paxinos and Watson rat brain atlas is also co-localised with the template, enabling the atlas structure and stereotaxic coordinates corresponding to a feature within a statistical map to be interactively reported, facilitating the localisation of functional effects. Moreover, voxels falling within selected brain structures can be combined to define anatomically based 3D volumes of interest (VOIs), free of operator bias. As many atlas structures are small relative to the typical resolution of phMRI studies, a mechanism for defining composite structures as agglomerations of individual atlas structures is also described. This provides a simple and robust means of interrogating structures that are otherwise difficult to delineate and an objective framework for comparing and classifying compounds based on an anatomical profile of their activity. These developments allow a closer alignment of pre-clinical and clinical analysis techniques.

Improved neuronal tract tracing using manganese enhanced magnetic resonance imaging with fast T(1) mapping

There has been growing interest in using manganese-enhanced MRI (MEMRI) to detect neuronal activation, neural architecture, and neuronal connections. Usually Mn(2+) produces a very wide range of T(1) change. In particular, in neuronal tract tracing experiments the site of Mn(2+) injection can have very short T(1) while distant regions have small T(1) reductions, primarily due to dilution of Mn(2+). Most MEMRI studies use T(1)-weighted sequences, which can only give optimal contrast for a narrow range of T(1) changes. To improve sensitivity to the full extent of Mn(2+) concentrations and to optimize detection of low concentrations of Mn(2+), a fast T(1) mapping sequence based on the Look and Locker technique was implemented. Phantom studies demonstrated less than 6.5% error in T(1) compared to more conventional T(1) measurements. Using center-out segmented EPI, whole-brain 3D T(1) maps with 200-microm isotropic resolution were obtained in 2 h from rat brain. Mn(2+) transport from the rat olfactory bulb through appropriate brain structures could be detected to the amygdala in individual animals. The method reliably detected less than 7% reductions in T(1). With this quantitative imaging it should be possible to study more extensive pathways using MEMRI and decrease the dose of Mn(2+) used.

The prefrontal cortex in the Göttingen minipig brain defined by neural projection criteria and cytoarchitecture

In an attempt to delineate the prefrontal cortex (PFC) in the Göttingen minipig brain the distribution of reciprocal thalamocortical projections was investigated using anterograde and retrograde tracing techniques and evaluated in relation to the specific cytoarchitectonic organization. Tracers were visualized using standard immunohistochemistry or evaluated in vivo using manganese (Mn2+) as an MRI paramagnetic tracer. The in vivo tract tracing turned out to be very sensitive with a high correspondence to the histological labelling. Tracers injected into the mediodorsal thalamus labelled the medial and rostral pole of the frontal lobe as well as the anterior cingulate, anterior insular and dorsomedial frontal cortices. Subsequently, the reciprocity and specificity of these connections were tested from injections into the traced frontal cortices indicating that the PFC has cortical connections to different parts of the MD nucleus. Although the granular layer IV, characteristic of primate PFC could not be identified, both cytoarchitectonic and connectional data suggests that the Göttingen minipig has a structurally divided prefrontal cortex. Stereological estimates of PFC volume showed that the Göttingen minipig PFC constitutes about 24% of the total neocortex volume and 10% of the total brain volume.

Neural lineages of the Drosophila brain: a three-dimensional digital atlas of the pattern of lineage location and projection at the late larval stage

The late larval brain consists of embryonically produced primary neurons forming a deep core cortex, surrounded at the surface by approximately 100 secondary lineages. Each secondary lineage forms a tract (secondary lineage tract) with an invariant and characteristic trajectory. Within the neuropile, tracts of neighboring lineages bundle together to form secondary tract systems. In this paper, we visualized secondary lineages by the global marker BP106 (neurotactin), as well as green fluorescent protein-labeled clones and thereby establish a comprehensive digital atlas of secondary lineages. The information contained in this atlas is the location of the lineage within the cortex, the neuropile compartment contacted by the lineage tract, and the projection pattern of the lineage tract within the neuropile. We have digitally mapped the expression pattern of three genes, sine oculis, period, and engrailed into the lineage atlas. The atlas will enable us and others to analyze the phenotype of mutant clones in the larval brain. Mutant clones can only be interpreted if the corresponding wild-type clone is well characterized, and our lineage atlas, which visualizes all wild-type lineages, will provide this information. Secondly, secondary lineage tracts form a scaffold of connections in the neuropile that foreshadows adult nerve connections. Thus, starting from the larval atlas and proceeding forward through pupal development, one will be able to reconstruct adult brain connectivity at a high level of resolution. Third, the atlas can serve as a repository for genes expressed in lineage-specific patterns.

Micro3D: computer program for three-dimensional reconstruction visualization, and analysis of neuronal populations and barin regions

This article presents a computer program, Micro3D, designed for 3-D reconstruction, visualization, and analysis of coordinate-data (points and lines) recorded from serial sections. The software has primarily been used for studying shapes and dimension of brain regions (contour line data) and distributions of cellular elements such as neuronal cell bodies or axonal terminal fields labeled with tract-tracing techniques (point data). The tissue elements recorded could equally well be labeled with use of other techniques, the only requirement being that the data collected are saved as x,y,z coordinates. Data are typically imported from image-combining computerized microscopy systems or image analysis systems, such as Neurolucida (MicroBrightField, Colchester, VT) or analySIS (Soft Imaging System, Gmbh, Münster, Germany). System requirements are a PC running LINUX. Reconstructions in Micro3D may be rotated and zoomed in real-time, and submitted to perspective viewing and stereo-imaging. Surfaces are re-synthesized on the basis of stacks of contour lines. Clipping is used for defining section-independent subdivisions of the reconstruction. Flattening of curved sheets of points layers (e.g., neurons in a layer) facilitates inspection of complicated distribution patterns. Micro3D computes color-coded density maps. Opportunities for translation of data from different reconstructions into common coordinate systems are also provided. This article demonstrates the use of Micro3D for visualization of complex neuronal distribution patterns in somatosensory and auditory systems. The software is available for download on conditions posted at the NeSys home pages (http://www.nesys.uio.no/) and at The Rodent Brain Workbench (http://www.rbwb.org/).

Imaging in cell-based therapy for neurodegenerative diseases

Fetal cell transplantation for the treatment of Parkinson's and Huntington's diseases has been developed over the past two decades and is now in early clinical testing phase. Direct assessment of the graft's survival, integration into the host brain and impact on neuronal functions requires advanced in vivo neuroimaging techniques. Owing to its high sensitivity, positron emission tomography is today the most widely used tool to evaluate the viability and function of the transplanted tissue in the brain. Nuclear magnetic resonance techniques are opening new possibilities for imaging neurochemical events in the brain. The ultimate goal will be to use the combination of multiple imaging modalities for complete functional monitoring of the repair processes in the central nervous system.

Manganese-enhanced MRI of the optic visual pathway and optic nerve injury in adult rats

PURPOSE

To evaluate manganese (Mn2+)-enhanced MRI in a longitudinal study of normal and injured rat visual projections.

MATERIALS AND METHODS

MRI was performed 24 hours after unilateral intravitreal injection of MnCl2 (150 nmol) into adult Fischer rats that were divided into four groups: 1) controls (N = 5), 2) dose-response (N = 10, 0.2-200 nmol), 3) time-response with repeated MRI during 24-168 hours post injection (N = 4), and 4) optic nerve crush (ONC) immediately preceding the MnCl2 injection (N = 7). Control and ONC animals were reinjected with MnCl2 20 days after the first injection, and MRI was performed 24 hours later.

RESULTS

In the control group, the optic projection was visualized from the retina to the superior colliculus, with indications of transsynaptic transport to the cortex. There was a semilogarithmic relationship between the Mn2+ dose and Mn2+ enhancement from 4 to 200 nmol, and the enhancement decayed gradually to 0 by 168 hours. No Mn2+-enhanced signal was detected distal to the ON crush site. In the control group, similar enhancement was obtained after the first and second MnCl2 injections, while in the ONC group the enhancement proximal to the crush site was reduced 20 days post lesion (20 dpl).

CONCLUSION

Mn2+-enhanced MRI is a viable method for temporospatial visualization of normal and injured ON in the adult rat. The observed reduction in the Mn2+ signal proximal to the ONC is probably a result of retrograde damage to the retinal ganglion cells, and not of Mn2+ toxicity.

Manganese-enhanced magnetic resonance imaging for in vivo assessment of damage and functional improvement following spinal cord injury in mice

In past decades, much effort has been invested in developing therapies for spinal injuries. Lack of standardization of clinical read-out measures, however, makes direct comparison of experimental therapies difficult. Damage and therapeutic effects in vivo are routinely evaluated using rather subjective behavioral tests. Here we show that manganese-enhanced magnetic resonance imaging (MEMRI) can be used to examine the extent of damage following spinal cord injury (SCI) in mice in vivo. Injection of MnCl2 solution into the cerebrospinal fluid leads to manganese uptake into the spinal cord. Furthermore, after injury MEMRI-derived quantitative measures correlate closely with clinical locomotor scores. Improved locomotion due to treating the detrimental effects of SCI with an established therapy (neutralization of CD95Ligand) is reflected in an increase of manganese uptake into the injured spinal cord. Therefore, we demonstrate that MEMRI is a sensitive and objective tool for in vivo visualization and quantification of damage and functional improvement after SCI. Thus, MEMRI can serve as a reproducible surrogate measure of the clinical status of the spinal cord in mice, potentially becoming a standard approach for evaluating experimental therapies.

Manganese-enhanced MRI of rat spinal cord injury

The potential of the manganese-enhanced MRI (MEI) technique in labeling the intact neuronal circuitry of rat spinal cord was examined. Experiments were conducted on normal and injured cords at 9.4-T magnetic field strength using an implantable rf coil. The contrast agent manganese (Mn) was locally delivered within the parenchyma at a dose of 25 mmol/L in 10 nL. The transport, uptake and accumulation of Mn in tissue were then followed remotely on T1-weighted images that were acquired serially from the cord. In MEIs of normal cord, Mn was observed to be transported in directions both rostral and caudal to the site of injection. In the cord that was subjected to hemisection, signal enhancement was on the contralesional side of the cord, but not at the ipsilesional side. The sensitivity and specificity of the MEI technique in labeling the neurons that are functional were also validated with a traditional track-tracing method using biotinylated dextran amine.

In vivo measurements of T1 relaxation times in mouse brain associated with different modes of systemic administration of manganese chloride

PURPOSE

To measure regional T1 and T2 values for normal C57Bl/6 mouse brain and changes in T1 after systemic administration of manganese chloride (MnCl2) at 9.4 T.

MATERIALS AND METHODS

C57Bl/6 mice were anesthetized and baseline T1 and T2 measurements obtained prior to measurement of T1 after administration of MnCl2 at 9.4 T. MnCl2 was administered systemically either by the intravenous (IV), intraperitoneal (IP), or subcutaneous (SC) routes. T1 and T2 maps for each MRI transverse slice were generated using commercial software, and T1 and T2 values of white matter (WM), gray matter (GM), pituitary gland, and lateral ventricle were obtained.

RESULTS

When compared with baseline values at low-field, significant lengthening of the T1 values was shown at 9.4 T, while no significant change was seen for T2 values. Significant T1 shortening of the normal mouse brain was observed following IV, IP, and SC administration of MnCl2, with IV and IP showing similar acute effects. Significant decreases in T1 values were seen for the pituitary gland and the ventricles 15 minutes after either IV or IP injection. GM showed greater uptake of the contrast agent than WM at 15 and 45 minutes after either IV or IP injections. Although both structures are within the blood-brain barrier (BBB), GM and WM revealed a steady decrease in T1 values at 24 and 72 hours after MnCl2 injection regardless of the route of administration.

CONCLUSION

Systemic administration of MnCl2 by IV and IP routes induced similar time-course of T1 changes in different regions of the mouse brain. Acute effects of MnCl2 administration were mainly influenced by either the presence or absence of BBB. SC injection also provided significant T1 change at subacute stage after MnCl2 administration.

Functional circuits mediating sensorimotor integration: Quantitative comparisons of projections from rodent barrel cortex to primary motor cortex, neostriatum, superior colliculus, and the pons

Motor performance depends on somatosensory feedback, and consistent with this finding, primary somatosensory (SI) cortex projects to several regions involved in motor control. Although the pathways mediating sensorimotor integration are known, few studies have compared their projection patterns. Therefore, in each animal, we injected two anterograde tracers into SI barrel cortex and compared the relative density and spatial extent of the labeled projections to the primary motor (MI) cortex, neostriatum, superior colliculus, and basal pons. Quantitative analysis revealed that these projections terminated most extensively in the neostriatum, to a lesser extent in MI cortex, and innervated the least amount of neuropil in the superior colliculus and pontine nuclei. Tracer overlap in the pontine nuclei was significantly higher than in the other three brains regions, and was strongly correlated with overlap in the superior colliculus, presumably because some projections to these two brain regions represent collaterals of the same neurons. The density of labeled varicosities was highest in the pons and lowest in MI. As a proportion of total labeling, densely packed clusters of labeled terminals were most prevalent in the pons, less prevalent in neostriatum and superior colliculus, and least prevalent in MI cortex. These results are consistent with physiological evidence indicating strong coherence between SI barrel cortex and the cerebellum during whisking behavior.

Magnetic resonance imaging at 9.4 T as a tool for studying neural anatomy in non-vertebrates

This report describes magnetic resonance imaging (MRI) methods we have developed at 9.4 T for observing internal organs and the nervous system of an invertebrate organism, the crayfish, Cherax destructor. We have compared results acquired using two different pulse sequences, and have tested manganese (Mn(2+)) as an agent to enhance contrast of neural tissues in this organism. These techniques serve as a foundation for further development of functional MRI and neural tract-tracing methods in non-vertebrate systems.

Cellular MR Imaging

Cellular MR imaging is a young field that aims to visualize targeted cells in living organisms. In order to provide a different signal intensity of the targeted cell, they are either labeled with MR contrast agents in vivo or prelabeled in vitro. Either (ultrasmall) superparamagnetic iron oxide [(U)SPIO] particles or (polymeric) paramagnetic chelates can be used for this purpose. For in vivo cellular labeling, Gd3+- and Mn2+- chelates have mainly been used for targeted hepatobiliary imaging, and (U)SPIO-based cellular imaging has been focused on imaging of macrophage activity. Several of these magneto-pharmaceuticals have been FDA-approved or are in late-phase clinical trials. As for prelabeling of cells in vitro, a challenge has been to induce a sufficient uptake of contrast agents into nonphagocytic cells, without affecting normal cellular function. It appears that this issue has now largely been resolved, leading to an active research on monitoring the cellular biodistribution in vivo following transplantation or transfusion of these cells, including cell migration and trafficking. New applications of cellular MR imaging will be directed, for instance, towards our understanding of hematopoietic (immune) cell trafficking and of novel guided (stem) cell-based therapies aimed to be translated to the clinic in the future.

Statistical mapping of functional olfactory connections of the rat brain in vivo

The olfactory pathway is a unique route into the brain. To better characterize this system in vivo, rat olfactory functional connections were mapped using magnetic resonance (MR) imaging and manganese ion (Mn2+) as a transport-mediated tracer combined with newly developed statistical brain image analysis. Six rats underwent imaging on a 1.5-T MR scanner at pre-administration, and 6, 12, 24, 36, 48, and 72 h and 5.5, 7.5, 10.5, and 13.5 days post-administration of manganese chloride (MnCl2) into the right nasal cavity. Images were coregistered, pixel-intensity normalized, and stereotactically transformed to the Paxinos and Watson rat brain atlas, then averaged across subjects using automated image analysis software (NEUROSTAT). Images at each time point were compared to pre-administration using a one-sample t statistic on a pixel-by-pixel basis in 3-D and converted to Z statistic maps. Statistical mapping and group averaging improved signal to noise ratios and signal detection sensitivity. Significant transport of Mn2+ was observed in olfactory structures ipsilateral to site of Mn2+ administration including the bulb, lateral olfactory tract (lo) by 12 h and in the tubercle, piriform cortex, ventral pallidum, amygdala, and in smaller structures such as the anterior commissure after 24 h post-administration. MR imaging with group-wise statistical analysis clearly demonstrated bilateral transsynaptic Mn2+ transport to secondary and tertiary neurons of the olfactory system. The method permits in vivo investigations of functional neuronal connections within the brain.

Magnetic Resonance Imaging Contrast Agents in the Study of Development

The use of magnetic resonance imaging (MRI) as a research tool in the study of development has increased in recent years due in part to improvements in spatial resolution, new imaging agents, and increased availability of MRI scanners. This chapter describes how contrast agent-enhanced MRI can be used to study development. In addition, we highlight some novel applications of contrast agent-enhanced MRI in biology that may be useful as tools for the study of development.

In vivo, trans-synaptic tract-tracing utilizing manganese-enhanced magnetic resonance imaging (MEMRI)

It is well established that manganese ion (Mn2+) can access neurons through voltage-gated calcium (Ca2+) channels. Based upon this fundamental principle, Mn2+ has long been used in biomedical research as an indicator of Ca2+ influx in conjunction with fluorescent microscopy. Additionally, after entry into neurons, Mn2+ is transported down axons via microtubule based fast axonal transport. Furthermore, Mn2+ is paramagnetic, resulting in a shortening of the spin-lattice relaxation time-constant, T1, which yields positive contrast enhancement in T1-weighted MRI images, specific to tissues where the ion has accumulated. Manganese-enhanced MRI (MEMRI) utilizes a combination of these properties of Mn2+ to trace neuronal pathways in an MRI-detectable manner. The focus of this review will detail some of the current MEMRI tract-tracing methodologies in mice and non-human primates as well as biological applications of MEMRI tract-tracing.

Applications of manganese-enhanced magnetic resonance imaging (MEMRI) to image brain plasticity in song birds

The song control system of song birds is an excellent model for studying brain plasticity and has thus far been extensively analyzed by histological and electrophysiological methods. However, these approaches do not provide a global view of the brain and/or do not allow repeated measures, which are necessary to establish correlations between alterations in neural substrate and behavior. Application of in vivo manganese-enhanced MRI enabled us for the first time to visualize the song control system repeatedly in the same bird, making it possible to quantify dynamically the volume changes in this circuit as a function of seasonal and hormonal influences. In this review, we introduce and explore the song control system of song birds as a natural model for brain plasticity to validate a new cutting edge technique, which we called 'repeated dynamic manganese enhanced MRI' or D-MEMRI. This technique is based on the use of implanted permanent cannulae--for accurate repeated manganese injections in a defined target area--and the subsequent MRI acquisition of the dynamics of the accumulation of manganese in projection brain targets. A compilation of the D-MEMRI data obtained thus far in this system demonstrates the usefulness of this new method for studying brain plasticity. In particular it is shown to be a perfect tool for long-term studies of morphological and functional responses of specific brain circuits to changes in endocrine conditions. The method was also successfully applied to obtain quantitative measures of changes in activity as a function of auditory stimuli in different neuronal populations of a same nucleus that project to different targets. D-MEMRI, combined with other MRI techniques, clearly harbors potential for unraveling seasonal, hormonal, pharmacological or even genetically driven changes in a neuronal circuit, by simultaneously measuring changes in morphology, activity and connectivity.