Neurodevelopment of C57B/L6 mouse brain assessed by in vivo diffusion tensor imaging

Omega-3 alleviates behavioral and molecular changes in a mouse model of stress-induced juvenile depression

Introduction

Depression is increasingly diagnosed in adolescence, necessitating specific prevention and treatment methods. However, there is a lack of animal models mimicking juvenile depression. This study explores a novel model using ultrasound (US) stress in juvenile mice.

Methods

We employed the US stress model in one-month-old C57/BL6 mice, exposing them to alternating ultrasound frequencies (20-25 kHz and 25-45 kHz) for three weeks. These frequencies correspond to negative and neutral emotional states in rodents and can induce a depressive-like syndrome. Concurrently, mice received either an omega-3 food supplement (FS) containing eicosapentaenoic acid (EPA; 0.55 mg/kg/day) and docosahexaenoic acid (DHA; 0.55 mg/kg/day) or a vehicle. Post-stress, we evaluated anxiety- and depressive-like behaviors, blood corticosterone levels, brain expression of pro-inflammatory cytokines, and conducted metabolome analysis of brain, liver and blood plasma.

Results

US-exposed mice treated with vehicle exhibited decreased sucrose preference, a sign of anhedonia, a key feature of depression, increased anxiety-like behavior, elevated corticosterone levels, and enhanced TNF and IL-1β gene expression in the brain. In contrast, US-FS mice did not display these changes. Omega-3 supplementation also reduced anxiety-like behavior in non-stressed mice. Metabolomic analysis revealed US-induced changes in brain energy metabolism, with FS increasing brain sphingomyelin. Liver metabolism was affected by both US and FS, while plasma metabolome changes were exclusive to FS. Brain glucose levels correlated positively with activity in anxiety tests.

Conclusion

Chronic omega-3 intake counteracted depressive- and anxiety-like behaviors in a US model of juvenile depression in mice. These effects likely stem from the anti-inflammatory properties of the supplement, suggesting potential therapeutic applications in juvenile depression.

White matter abnormalities in the Hdc knockout mouse, a model of tic and OCD pathophysiology

Introduction

An inactivating mutation in the histidine decarboxylase gene (Hdc) has been identified as a rare but high-penetrance genetic cause of Tourette syndrome (TS). TS is a neurodevelopmental syndrome characterized by recurrent motor and vocal tics; it is accompanied by structural and functional abnormalities in the cortico-basal ganglia circuitry. Hdc, which is expressed both in the posterior hypothalamus and peripherally, encodes an enzyme required for the biosynthesis of histamine. Hdc knockout mice (Hdc-KO) functionally recapitulate this mutation and exhibit behavioral and neurochemical abnormalities that parallel those seen in patients with TS.

Materials and methods

We performed exploratory RNA-seq to identify pathological alterations in several brain regions in Hdc-KO mice. Findings were corroborated with RNA and protein quantification, immunohistochemistry, and ex vivo brain imaging using MRI.

Results

Exploratory RNA-Seq analysis revealed, unexpectedly, that genes associated with oligodendrocytes and with myelin production are upregulated in the dorsal striatum of these mice. This was confirmed by qPCR, immunostaining, and immunoblotting. These results suggest an abnormality in myelination in the striatum. To test this in an intact mouse brain, we performed whole-brain ex vivo diffusion tensor imaging (DTI), which revealed reduced fractional anisotropy (FA) in the dorsal striatum.

Discussion

While the DTI literature in individuals with TS is sparse, these results are consistent with findings of disrupted descending cortical projections in patients with tics. The Hdc-KO model may represent a powerful system in which to examine the developmental mechanisms underlying this abnormality.

A quantitative cholinergic and catecholaminergic 3D Atlas of the developing mouse brain

The complex organization of brain regions during development requires a three-dimensional approach to facilitate the visualization and quantification of dynamic changes taking place throughout this important period. Using the tissue clearing method combined with immunohistochemistry, three-dimensional (3D) lightsheet microscopy and a multiresolution registration technique, we provide the first 3D atlases of the main cholinergic (CH) and catecholaminergic (CA) systems in the mouse brain from embryonic day 12 (E12) to post-natal day 8 (P8). We report that in several brain structures, there is a logarithmic scale increase of choline acetyltransferase and tyrosine hydroxylase positive neurons from E18 to P8. In addition, a detailed voxel-wise analysis revealed abrupt modifications in the developmental trajectory of many brain structures during the transition from E18 to P0. Our atlases will not only facilitate developmental studies aimed at quantitatively determining the fate of CH or CA neurons in utero but also be used as an anatomical reference to quantify other neuronal populations present in the annotated regions. In the future, these maps will be a reliable tool to study developmental malformations associated with neurological and psychiatric disorders.

In vivo targeting of a variant causing vanishing white matter using CRISPR/Cas9

Vanishing white matter (VWM) is a leukodystrophy caused by recessive variants in subunits of eIF2B. At present, no curative treatment is available and patients often die at young age. Due to its monogenic nature, VWM is a promising candidate for the development of CRISPR/Cas9-mediated gene therapy. Here we tested a dual-AAV approach in VWM mice encoding CRISPR/Cas9 and a DNA donor template to correct a pathogenic variant in Eif2b5. We performed sequencing analysis to assess gene correction rates and examined effects on the VWM phenotype, including motor behavior. Sequence analysis demonstrated that over 90% of CRISPR/Cas9-induced edits at the targeted locus are insertion or deletion (indel) mutations, rather than precise corrections from the DNA donor template by homology-directed repair. Around half of the CRISPR/Cas9-treated animals died prematurely. VWM mice showed no improvement in motor skills, weight, or neurological scores at 7 months of age, and CRISPR/Cas9-treated controls displayed an induced VWM phenotype. In conclusion, CRISPR/Cas9-induced DNA double-strand breaks (DSBs) at the Eif2b5 locus did not lead to sufficient correction of the VWM variant. Moreover, indel formation in Eif2b5 induced an exacerbated VWM phenotype. Therefore, DSB-independent strategies like base- or prime editing might better suited for VWM correction.

Effect of Electromagnetic Field Exposure on Mouse Brain Morphological and Histopathological Profiling

INTRODUCTION

Mobile phones (MP) and other electronic and communication devices that are used daily expose users to electromagnetic fields (EMF) and contribute to an increasing incidence of neurological disorders. Brain tissue is the closest organ to the MP as it operates, thus the influence of MP radiation on brain tissue is of particular concern, although research is still inconclusive. The present study investigated the possible effect of an EMF (1,350-1,375 megahertz (MHz)) from an MP on morphological and histopathological profiles in the mouse brain.

MATERIAL AND METHODS

Healthy BALB/c mice were assigned to three equal groups (a control and two experimental groups, n = 10 each). Experimental mice were exposed to EMFs continuously for 72 h, those of experimental group I to a 1,350 MHz field at a specific absorption rate (SAR) of 4.0 W/kg, and group II to a 1,375 MHz field EMF at an SAR of 4.0 W/kg. Brain segmentation and histopathological analysis were applied to detect changes in the morphometric parameters of the brain lobes and identify pathological lesions, respectively.

RESULTS

Histopathology results revealed shrinkage of pyramidal neurons, presence of mild perivascular and perineural oedema, and some vacuolation of neurons and glial cells derived from mouse great hemispheres. The lesions also included reduction of Purkinje cells, vacuolisation of neurons and glial cells, and interstitial oedema in the cerebellum.

CONCLUSION

MP distance of 3 cm from the cage may induce appreciable morphological changes in mouse brain structures; therefore, more comprehensive research is essential for assessment of safe distance. These pronounced effects may interfere with the results of laboratory tests on murine experimental models in veterinary or biomedical research.

Aspm knockout ferret reveals an evolutionary mechanism governing cerebral cortical size

The human cerebral cortex is distinguished by its large size and abundant gyrification, or folding, yet the evolutionary mechanisms driving cortical size and structure are unknown. While genes essential for cortical developmental expansion have been identified from the genetics of human primary microcephaly (“small head”, associated with reduced brain size and intellectual disability)¹, studies of these genes in mice, whose smooth cortex is one thousand times smaller than that of humans, have provided limited insight. Mutations of abnormal spindle-like microcephaly-associated (ASPM), the most common recessive microcephaly gene, reduce cortical volume by ≥50% in humans^2–4, but have little effect in mice^5–9, likely reflecting evolutionarily divergent functions of ASPM^10,11. We used genome editing to create a germline knockout (KO) of Aspm in the ferret (Mustela putorius furo), a species with a larger, gyrified cortex and greater neural progenitor cell (NPC) diversity^12–14 than mice, and closer Aspm protein sequence homology to human. Aspm KO ferrets exhibit severe microcephaly (25–40% decreases in brain weight), reflecting reduced cortical surface area without significant change in cortical thickness, as in human patients^3,4, suggesting loss of “cortical units”. The mutant ferret fetal cortex displays a massive premature displacement of ventricular radial glial cells (VRG) to the outer subventricular zone (OSVZ), where many resemble outer radial glia (ORG), an NPC subtype essentially absent in mice and implicated in cerebral cortical expansion in primates^12–16. These data suggest an evolutionary mechanism whereby Aspm regulates cortical expansion by controlling the affinity of VRG for the ventricular surface, thus modulating the ratio of VRG, the most undifferentiated cell type, to ORG, a more differentiated progenitor.

Longitudinal Diffusion Tensor Imaging Revealed Nerve Fiber Alterations in Aspm Mutated Microcephaly Model Mice

Autosomal recessive primary microcephaly-5 (MCPH5) is characterized by congenital microcephaly and is caused by the mutation in the abnormal spindle-like, microcephaly-associated (ASPM) gene. This study aimed to demonstrate a correlation between radiological and pathological analyses in evaluating postnatal brain development using MCPH5-model mice, ASPM ortholog (Aspm) knockout (KO) mice. In vivo MRI was performed at two time points (postnatal 3 weeks; P3W and P10W) and complementary histopathological analyses of brains were done at P5W and P13W. In the MRI analysis, Aspm KO mice showed significantly decreased brain sizes (average 8.6% difference) with larger ventricles (average 136.4% difference) at both time points. Voxel-based statistics showed that the fractional anisotropy (FA) values were significantly lower in Aspm KO mice in both the cortex and white matter at both time points. Developmental changes in the FA values were less remarkable in the Aspm KO mice, compared with the controls. Histometric analyses revealed that the ratios of the horizontal to the vertical neurites were significantly higher in cortical layers IV, V and VI, with a remarkable increase according to maturation at P13W in the control mice (average 12.7% difference between control and KO), whereas the ratio in layer VI decreased at P13W in the KO mice. The myelin basic protein positive ratio in the white matter significantly decreased in Aspm KO mice at P5W. These results suggest that temporal FA changes are closely correlated with pathological findings such as abnormal neurite outgrowth and differentiation, which may be applicable for analyzing diseased human brain development.

Network evolution in mesial temporal lobe epilepsy revealed by diffusion tensor imaging

OBJECTIVE

The objective of the present study is to identify novel, time-indexed imaging biomarkers of epileptogenesis in mesial temporal lobe epilepsy (MTLE).

METHODS

We used high-resolution brain diffusion tensor imaging (DTI) of the translationally relevant methionine sulfoximine (MSO) brain infusion model of MTLE. MSO inhibits astroglial glutamine synthetase, which is deficient in the epileptogenic hippocampal formation of patients with MTLE. MSO-infused (epileptogenic) rats were compared with phosphate-buffered saline (PBS)-infused (nonepileptogenic) rats at early (3-4 days) and late (6-9 weeks) time points during epileptogenesis.

RESULTS

The epileptogenic rats exhibited significant changes in DTI-measured fractional anisotropy (FA) in numerous brain regions versus nonepileptogenic rats. Changes included decreases and increases in FA in regions such as the entorhinal-hippocampal area, amygdala, corpus callosum, thalamus, striatum, accumbens, and neocortex. The FA changes evolved over time as animals transitioned from early to late epileptogenesis. For example, some areas with significant decreases in FA early in epileptogenesis changed to significant increases late in epileptogenesis. Finally, the FA changes significantly correlated with the seizure load.

SIGNIFICANCE

Our results suggest (1) that high-resolution DTI can be used for early identification and tracking of the epileptogenic process in MTLE, and (2) that the process identified by DTI is present in multiple brain areas, even though infusion of MSO is restricted to the unilateral entorhinal-hippocampal region.

Hypofrontality and Posterior Hyperactivity in Early Schizophrenia: Imaging and Behavior in a Preclinical Model

BACKGROUND

Schizophrenia is a debilitating neuropsychiatric disorder typically diagnosed from late adolescence to adulthood. Subthreshold behavioral symptoms (e.g., cognitive deficits and substance abuse) often precede the clinical diagnosis of schizophrenia. However, these prodromal symptoms have not been consistently associated with structural and functional brain biomarkers, limiting the chance of early diagnosis of schizophrenia.

METHODS

Using an extensively multimodal range of magnetic resonance methods (for anatomy, metabolism, and function), we screened early biomarkers in a methylazoxymethanol acetate (MAM) rat model of schizophrenia and saline-treated control (SHAM) rats, in conjunction with immunohistochemistry, myelin staining, and a novel three-choice, reversal-learning task to identify early behavioral markers corresponding the subthreshold symptoms.

RESULTS

MAM (vs. SHAM) rats had lower/higher structural connectivity in anterior/posterior corpus callosum. The orbitofrontal cortex of MAM rats showed lower resting-state functional magnetic resonance imaging functional connectivity in conjunction with lower neuronal density, lower glucose oxidation, and attenuated neurotransmission (hypofrontality). In contrast, these measures were all higher in visual cortex of MAM rats (posterior hyperactivity), which might parallel perceptual problems in schizophrenia. In behavioral studies, MAM (vs. SHAM) rats displayed abnormal orbitofrontal cortex-mediated decision-making processes, resulting in a novel reward-sensitive hyperflexible phenotype, which might reflect vulnerability of prodromal patients to substance abuse.

CONCLUSIONS

We identified two novel biomarkers of early schizophrenia in a preclinical rat model: hypofrontality associated with the hyperflexible phenotype, and posterior hyperactivity. Because each of these magnetic resonance methods is clinically translatable, these markers could contribute to early diagnosis and the development of novel therapies of schizophrenia.

Excitatory Synaptic Drive and Feedforward Inhibition in the Hippocampal CA3 Circuit Are Regulated by SynCAM 1

Select adhesion proteins control the development of synapses and modulate their structural and functional properties. Despite these important roles, the extent to which different synapse-organizing mechanisms act across brain regions to establish connectivity and regulate network properties is incompletely understood. Further, their functional roles in different neuronal populations remain to be defined. Here, we applied diffusion tensor imaging (DTI), a modality of magnetic resonance imaging (MRI), to map connectivity changes in knock-out (KO) mice lacking the synaptogenic cell adhesion protein SynCAM 1. This identified reduced fractional anisotropy in the hippocampal CA3 area in absence of SynCAM 1. In agreement, mossy fiber refinement in CA3 was impaired in SynCAM 1 KO mice. Mossy fibers make excitatory inputs onto postsynaptic specializations of CA3 pyramidal neurons termed thorny excrescences and these structures were smaller in the absence of SynCAM 1. However, the most prevalent targets of mossy fibers are GABAergic interneurons and SynCAM 1 loss unexpectedly reduced the number of excitatory terminals onto parvalbumin (PV)-positive interneurons in CA3. SynCAM 1 KO mice additionally exhibited lower postsynaptic GluA1 expression in these PV-positive interneurons. These synaptic imbalances in SynCAM 1 KO mice resulted in CA3 disinhibition, in agreement with reduced feedforward inhibition in this network in the absence of SynCAM 1-dependent excitatory drive onto interneurons. In turn, mice lacking SynCAM 1 were impaired in memory tasks involving CA3. Our results support that SynCAM 1 modulates excitatory mossy fiber inputs onto both interneurons and principal neurons in the hippocampal CA3 area to balance network excitability.

SIGNIFICANCE STATEMENT

This study advances our understanding of synapse-organizing mechanisms on two levels. First, the data support that synaptogenic proteins guide connectivity and can function in distinct brain regions even if they are expressed broadly. Second, the results demonstrate that a synaptogenic process that controls excitatory inputs to both pyramidal neurons and interneurons can balance excitation and inhibition. Specifically, the study reveals that hippocampal CA3 connectivity is modulated by the synapse-organizing adhesion protein SynCAM 1 and identifies a novel, SynCAM 1-dependent mechanism that controls excitatory inputs onto parvalbumin-positive interneurons. This enables SynCAM 1 to regulate feedforward inhibition and set network excitability. Further, we show that diffusion tensor imaging is sensitive to these cellular refinements affecting neuronal connectivity.

Convulsive seizures from experimental focal cortical dysplasia occur independently of cell misplacement

Focal cortical dysplasia (FCD), a local malformation of cortical development, is the most common cause of pharmacoresistant epilepsy associated with life-long neurocognitive impairments. It remains unclear whether neuronal misplacement is required for seizure activity. Here we show that dyslamination and white matter heterotopia are not necessary for seizure generation in a murine model of type II FCDs. These experimental FCDs generated by increasing mTOR activity in layer 2/3 neurons of the medial prefrontal cortex are associated with tonic-clonic seizures and a normal survival rate. Preventing all FCD-related defects, including neuronal misplacement and dysmorphogenesis, with rapamycin treatments from birth eliminates seizures, but seizures recur after rapamycin withdrawal. In addition, bypassing neuronal misplacement and heterotopia using inducible vectors do not prevent seizure occurrence. Collectively, data obtained using our new experimental FCD-associated epilepsy suggest that life-long treatment to reduce neuronal dysmorphogenesis is required to suppress seizures in individuals with FCD.

Diffusion tensor imaging as a predictor of locomotor function after experimental spinal cord injury and recovery

Traumatic spinal cord injury (SCI) causes long-term disability with limited functional recovery linked to the extent of axonal connectivity. Quantitative diffusion tensor imaging (DTI) of axonal integrity has been suggested as a potential biomarker for prognostic and therapeutic evaluation after trauma, but its correlation with functional outcomes has not been clearly defined. To examine this application, female Sprague-Dawley rats underwent midthoracic laminectomy followed by traumatic spinal cord contusion of differing severities or laminectomy without contusion. Locomotor scores and hindlimb kinematic data were collected for 4 weeks post-injury. Ex vivo DTI was then performed to assess axonal integrity using tractography and fractional anisotropy (FA), a numerical measure of relative white matter integrity, at the injury epicenter and at specific intervals rostral and caudal to the injury site. Immunohistochemistry for tissue sparing was also performed. Statistical correlation between imaging data and functional performance was assessed as the primary outcome. All injured animals showed some recovery of locomotor function, while hindlimb kinematics revealed graded deficits consistent with injury severity. Standard T2 magnetic resonance sequences illustrated conventional spinal cord morphology adjacent to contusions while corresponding FA maps indicated graded white matter pathology within these adjacent regions. Positive correlations between locomotor (Basso, Beattie, and Bresnahan score and gait kinematics) and imaging (FA values) parameters were also observed within these adjacent regions, most strongly within caudal segments beyond the lesion. Evaluation of axonal injury by DTI provides a mechanism for functional recovery assessment in a rodent SCI model. These findings suggest that focused DTI analysis of caudal spinal cord should be studied in human cases in relationship to motor outcome to augment outcome biomarkers for clinical cases.

Progressive volume loss and white matter degeneration in cstb-deficient mice: a diffusion tensor and longitudinal volumetry MRI study

Unverricht-Lundborg type progressive myoclonus epilepsy (EPM1, OMIM 254800) is an autosomal recessive disorder characterized by onset at the age of 6 to 16 years, incapacitating stimulus-sensitive myoclonus and tonic-clonic epileptic seizures. It is caused by mutations in the gene encoding cystatin B. Previously, widespread white matter changes and atrophy has been detected both in adult EPM1 patients and in 6-month-old cystatin B-deficient mice, a mouse model for the EPM1 disease. In order to elucidate the spatiotemporal dynamics of the brain atrophy and white matter changes in EPM1, we conducted longitudinal in vivo magnetic resonance imaging and ex vivo diffusion tensor imaging accompanied with tract-based spatial statistics analysis to compare volumetric changes and fractional anisotropy in the brains of 1 to 6 months of age cystatin B-deficient and control mice. The results reveal progressive but non-uniform volume loss of the cystatin B-deficient mouse brains, indicating that different neuronal populations possess distinct sensitivity to the damage caused by cystatin B deficiency. The diffusion tensor imaging data reveal early and progressive white matter alterations in cystatin B-deficient mice affecting all major tracts. The results also indicate that the white matter damage in the cystatin B-deficient brain is most likely secondary to glial activation and neurodegenerative events rather than a primary result of CSTB deficiency. The data also show that diffusion tensor imaging combined with TBSS analysis provides a feasible approach not only to follow white matter damage in neurodegenerative mouse models but also to detect fractional anisotropy changes related to normal white matter maturation and reorganisation.

Quantitative magnetic susceptibility of the developing mouse brain reveals microstructural changes in the white matter

Cerebral development involves a complex cascade of events which are difficult to visualize and quantify in vivo. In this study we combine information from Diffusion Tensor Imaging (DTI) and Quantitative Susceptibility Mapping (QSM) to analyze developing mouse brains at five stages up to 56days postnatal. Susceptibility maps were calculated using frequency shifts in gradient echo MR images acquired at 9.4T. The mean apparent magnetic susceptibility and magnetic susceptibility anisotropy of major white matter tracts were evaluated as a function of age. During the first two weeks, susceptibility of white matter appeared paramagnetic relative to surrounding gray matter; it then gradually became more diamagnetic. While diffusion anisotropy was already apparent and high at postnatal day 2, susceptibility anisotropy only became significant during the third week. This mismatch indicated different microstructural underpinnings for diffusion anisotropy and susceptibility anisotropy. Histological exams were also performed to evaluate myelin and iron content. It is confirmed that the main source of susceptibility contrast in WM is the myelin content. The ability to quantify the magnetic properties of white matter will provide valuable information on the architecture of the brain during development and potentially a more specific indicator for myelin degenerative diseases.

Mapping phosphorylation rate of fluoro-deoxy-glucose in rat brain by (19)F chemical shift imaging

(19)F magnetic resonance spectroscopy (MRS) studies of 2-fluoro-2-deoxy-d-glucose (FDG) and 2-fluoro-2-deoxy-d-glucose-6-phosphate (FDG-6P) can be used for directly assessing total glucose metabolism in vivo. To date, (19)F MRS measurements of FDG phosphorylation in the brain have either been achieved ex vivo from extracted tissue or in vivo by unusually long acquisition times. Electrophysiological and functional magnetic resonance imaging (fMRI) measurements indicate that FDG doses up to 500 mg/kg can be tolerated with minimal side effects on cerebral physiology and evoked fMRI-BOLD responses to forepaw stimulation. In halothane-anesthetized rats, we report localized in vivo detection and separation of FDG and FDG-6P MRS signals with (19)F 2D chemical shift imaging (CSI) at 11.7 T. A metabolic model based on reversible transport between plasma and brain tissue, which included a non-saturable plasma to tissue component, was used to calculate spatial distribution of FDG and FDG-6P concentrations in rat brain. In addition, spatial distribution of rate constants and metabolic fluxes of FDG to FDG-6P conversion were estimated. Mapping the rate of FDG to FDG-6P conversion by (19)F CSI provides an MR methodology that could impact other in vivo applications such as characterization of tumor pathophysiology.

Brain development in rodents and humans: Identifying benchmarks of maturation and vulnerability to injury across species

Hypoxic-ischemic and traumatic brain injuries are leading causes of long-term mortality and disability in infants and children. Although several preclinical models using rodents of different ages have been developed, species differences in the timing of key brain maturation events can render comparisons of vulnerability and regenerative capacities difficult to interpret. Traditional models of developmental brain injury have utilized rodents at postnatal day 7-10 as being roughly equivalent to a term human infant, based historically on the measurement of post-mortem brain weights during the 1970s. Here we will examine fundamental brain development processes that occur in both rodents and humans, to delineate a comparable time course of postnatal brain development across species. We consider the timing of neurogenesis, synaptogenesis, gliogenesis, oligodendrocyte maturation and age-dependent behaviors that coincide with developmentally regulated molecular and biochemical changes. In general, while the time scale is considerably different, the sequence of key events in brain maturation is largely consistent between humans and rodents. Further, there are distinct parallels in regional vulnerability as well as functional consequences in response to brain injuries. With a focus on developmental hypoxic-ischemic encephalopathy and traumatic brain injury, this review offers guidelines for researchers when considering the most appropriate rodent age for the developmental stage or process of interest to approximate human brain development.

In vivo high-resolution diffusion tensor imaging of the mouse brain

Diffusion tensor imaging (DTI) of the laboratory mouse brain provides important macroscopic information for anatomical characterization of mouse models in basic research. Currently, in vivo DTI of the mouse brain is often limited by the available resolution. In this study, we demonstrate in vivo high-resolution DTI of the mouse brain using a cryogenic probe and a modified diffusion-weighted gradient and spin echo (GRASE) imaging sequence at 11.7 T. Three-dimensional (3D) DTI of the entire mouse brain at 0.125 mm isotropic resolution could be obtained in approximately 2 h. The high spatial resolution, which was previously only available with ex vivo imaging, enabled non-invasive examination of small structures in the adult and neonatal mouse brains. Based on data acquired from eight adult mice, a group-averaged DTI atlas of the in vivo adult mouse brain with 60 structure segmentations was developed. Comparisons between in vivo and ex vivo mouse brain DTI data showed significant differences in brain morphology and tissue contrasts, which indicate the importance of the in vivo DTI-based mouse brain atlas.

Functional MRI and neural responses in a rat model of Alzheimer's disease

Based on the hypothesis that brain plaques and tangles can affect cortical function in Alzheimer's disease (AD), we investigated functional responses in an AD rat model (called the Samaritan Alzheimer's rat achieved by ventricular infusion of amyloid peptide) and age-matched healthy control. High-field functional magnetic resonance imaging (fMRI) and extracellular neural activity measurements were applied to characterize sensory-evoked responses. Electrical stimulation of the forepaw led to BOLD and neural responses in the contralateral somatosensory cortex and thalamus. In AD brain we noted much smaller BOLD activation patterns in the somatosensory cortex (i.e., about 50% less activated voxels compared to normal brain). While magnitudes of BOLD and neural responses in the cerebral cortex were markedly attenuated in AD rats compared to normal rats (by about 50%), the dynamic coupling between the BOLD and neural responses in the cerebral cortex, as assessed by transfer function analysis, remained unaltered between the groups. However thalamic BOLD and neural responses were unaltered in AD brain compared to controls. Thus cortical responses in the AD model were indeed diminished compared to controls, but the thalamic responses in the AD and control rats were quite similar. Therefore these results suggest that Alzheimer's disease may affect cortical function more than subcortical function, which may have implications for interpreting altered human brain functional responses in fMRI studies of Alzheimer's disease.

Diffusion tensor magnetic resonance histology reveals microstructural changes in the developing rat brain

The postnatal period is a remarkably dynamic phase of brain growth and development characterized by large-scale macrostructural changes, as well as dramatic microstructural changes, including myelination and cortical layering. This crucial period of neurodevelopment is uniquely susceptible to a wide variety of insults that may lead to neurologic disease. MRI is an important tool for studying both normal and abnormal neurodevelopmental changes, and quantitative imaging strategies like diffusion tensor imaging (DTI) allow visualization of many of the complex microstructural changes that occur during postnatal life. Diffusion tensor magnetic resonance histology (DT-MRH) provides particularly unique insight into cytoarchitectural changes in the developing brain. In this study, we used DT-MRH to track microstructural changes in the rat brain throughout normal postnatal neurodevelopment. We provide examples of diffusion tensor parameter changes in both white matter and gray matter structures, and correlate these changes with changes in cytoarchitecture. Finally, we provide a comprehensive database of image sets as a foundation for future studies using DT-MRH to characterize abnormal neurodevelopment in rodent models of neurodevelopmental disease.

Maternal separation with early weaning: a rodent model providing novel insights into neglect associated developmental deficits

Child neglect is the most prevalent form of child maltreatment in the United States, and poses a serious public health concern. Children who survive such episodes go on to experience long-lasting psychological and behavioral problems, including higher rates of post-traumatic stress disorder symptoms, depression, alcohol and drug abuse, attention-deficit/hyperactivity disorder, and cognitive deficits. To date, most research into the causes of these life-long problems has focused on well-established targets such as stress responsive systems, including the hypothalamus-pituitary-adrenal axis. Using the maternal separation and early weaning model, we have attempted to provide comprehensive molecular profiling of a model of early-life neglect in an organism amenable to genomic manipulation: the mouse. In this article, we report new findings generated with this model using chromatin immunoprecipitation sequencing, diffuse tensor magnetic resonance imaging, and behavioral analyses. We also review the validity of the maternal separation and early weaning model, which reflects behavioral deficits observed in neglected humans including hyperactivity, anxiety, and attentional deficits. Finally, we summarize the molecular characterization of these animals, including RNA profiling and label-free proteomics, which highlight protein translation and myelination as novel pathways of interest.

MR diffusion tensor imaging detects rapid microstructural changes in amygdala and hippocampus following fear conditioning in mice

BACKGROUND

Following fear conditioning (FC), ex vivo evidence suggests that early dynamics of cellular and molecular plasticity in amygdala and hippocampal circuits mediate responses to fear. Such altered dynamics in fear circuits are thought to be etiologically related to anxiety disorders including posttraumatic stress disorder (PTSD). Consistent with this, neuroimaging studies of individuals with established PTSD in the months after trauma have revealed changes in brain regions responsible for processing fear. However, whether early changes in fear circuits can be captured in vivo is not known.

METHODS

We hypothesized that in vivo magnetic resonance diffusion tensor imaging (DTI) would be sensitive to rapid microstructural changes elicited by FC in an experimental mouse PTSD model. We employed a repeated measures paired design to compare in vivo DTI measurements before, one hour after, and one day after FC-exposed mice (n=18).

RESULTS

Using voxel-wise repeated measures analysis, fractional anisotropy (FA) significantly increased then decreased in amygdala, decreased then increased in hippocampus, and was increasing in cingulum and adjacent gray matter one hour and one day post-FC respectively. These findings demonstrate that DTI is sensitive to early changes in brain microstructure following FC, and that FC elicits distinct, rapid in vivo responses in amygdala and hippocampus.

CONCLUSIONS

Our results indicate that DTI can detect rapid microstructural changes in brain regions known to mediate fear conditioning in vivo. DTI indices could be explored as a translational tool to capture potential early biological changes in individuals at risk for developing PTSD.

Structural insights into the rodent CNS via diffusion tensor imaging

Diffusion tensor imaging (DTI) is a useful tool for studying anatomy and pathology in the rodent central nervous system (CNS).The unique tissue contrasts provided by DTI are well suited for monitoring disease progression, studying brain development, and characterizing anatomical phenotypes. Recent technical developments have vastly improved the speed and resolution of rodent DTI. Ongoing research efforts exploring the microstructural basis of DTI signals have provided useful insights into its capabilities to delineate brain structures and detect neuropathology. Significant progress has also been made in combining DTI results with data acquired using other imaging modalities to enhance our understanding of the rodent CNS.

The translational role of diffusion tensor image analysis in animal models of developmental pathologies

Diffusion tensor magnetic resonance imaging (DTI) has proven itself a powerful technique for clinical investigation of the neurobiological targets and mechanisms underlying developmental pathologies. The success of DTI in clinical studies has demonstrated its great potential for understanding translational animal models of clinical disorders, and preclinical animal researchers are beginning to embrace this new technology to study developmental pathologies. In animal models, genetics can be effectively controlled, drugs consistently administered, subject compliance ensured, and image acquisition times dramatically increased to reduce between-subject variability and improve image quality. When pairing these strengths with the many positive attributes of DTI, such as the ability to investigate microstructural brain organization and connectivity, it becomes possible to delve deeper into the study of both normal and abnormal development. The purpose of this review is to provide new preclinical investigators with an introductory source of information about the analysis of data resulting from small animal DTI studies to facilitate the translation of these studies to clinical data. In addition to an in-depth review of translational analysis techniques, we present a number of relevant clinical and animal studies using DTI to investigate developmental insults in order to further illustrate techniques and to highlight where small animal DTI could potentially provide a wealth of translational data to inform clinical researchers.

Adolescent hyperactivity and impaired coordination after neonatal hyperoxia

In preterm infants, the risk to develop attention-deficit/hyperactivity disorder is 3 to 4-fold higher than in term infants. Moreover, preterm infants exhibit deficits in motor coordination and balance. Based on clinical data, higher oxygen levels in preterm infants lead to worse neurological outcome, and experimental hyperoxia causes wide-ranging cerebral changes in neonatal rodents. We hypothesize that hyperoxia in the immature brain may affect motor activity in preterm infants. We subjected newborn mice from P6 to P8 to 48 h of hyperoxia (80% O(2)) and tested motor activity in running wheels starting at adolescent age P30. Subsequently, from P44 to P53, regular wheels were replaced by complex wheels with variable crossbar positions to assess motor coordination deficits. MRI with diffusion tensor imaging was performed in the corpus callosum to determine white matter diffusivity in mice after hyperoxia at ages P30 and P53 in comparison to control animals. Adolescent mice after neonatal hyperoxia revealed significantly higher values for maximum velocity and mean velocity in regular wheels than controls (P<0.05). In the complex running wheels, however, maximum velocity was decreased in animals after hyperoxia, as compared to controls (P<0.05). Decreased fractional anisotropy and increased radial diffusion coefficient were observed in the corpus callosum of P30 and P53 mice after neonatal hyperoxia compared to control mice. Hyperoxia in the immature brain causes hyperactivity, motor coordination deficits, and impaired white matter diffusivity in adolescent and young adult mice.

Diffusion-prepared fast imaging with steady-state free precession (DP-FISP): a rapid diffusion MRI technique at 7 T

Diffusion MRI is a useful imaging technique with many clinical applications. Many diffusion MRI studies have utilized echo-planar imaging (EPI) acquisition techniques. In this study, we have developed a rapid diffusion-prepared fast imaging with steady-state free precession MRI acquisition for a preclinical 7T scanner providing diffusion-weighted images in less than 500 ms and diffusion tensor imaging assessments in ∼1 min with minimal image artifacts in comparison with EPI. Phantom apparent diffusion coefficient (ADC) and fractional anisotropy (FA) assessments obtained from the diffusion-prepared fast imaging with steady-state free precession (DP-FISP) acquisition resulted in good agreement with EPI and spin echo diffusion methods. The mean apparent diffusion coefficient was 2.0 × 10(-3) mm(2) /s, 1.90 × 10(-3) mm(2) /s, and 1.97 × 10(-3) mm(2) /s for DP-FISP, diffusion-weighted spin echo, and diffusion-weighted EPI, respectively. The mean fractional anisotropy was 0.073, 0.072, and 0.070 for diffusion-prepared fast imaging with steady-state free precession, diffusion-weighted spin echo, and diffusion-weighted EPI, respectively. Initial in vivo studies show reasonable ADC values in a normal mouse brain and polycystic rat kidneys.

Fractal analysis of spontaneous fluctuations of the BOLD signal in rat brain

Analysis of task-evoked fMRI data ignores low frequency fluctuations (LFF) of the resting-state the BOLD signal, yet LFF of the spontaneous BOLD signal is crucial for analysis of resting-state connectivity maps. We characterized the LFF of resting-state BOLD signal at 11.7T in α-chloralose and domitor anesthetized rat brain and modeled the spontaneous signal as a scale-free (i.e., fractal) distribution of amplitude power (|A|²) across a frequency range (f) compatible with an |A(f)|² ∝ 1/f(β) model where β is the scaling exponent (or spectral index). We compared β values from somatosensory forelimb area (S1FL), cingulate cortex (CG), and caudate putamen (CPu). With α-chloralose, S1FL and CG β values dropped from ~0.7 at in vivo to ~0.1 at post mortem (p<0.0002), whereas CPu β values dropped from ~0.3 at in vivo to ~0.1 at post mortem (p<0.002). With domitor, cortical (S1FL, CG) β values were slightly higher than with α-chloralose, while subcortical (CPu) β values were similar with α-chloralose. Although cortical and subcortical β values with both anesthetics were significantly different in vivo (p<0.002), at post mortem β values in these regions were not significantly different and approached zero (i.e., range of -0.1 to 0.2). Since a water phantom devoid of susceptibility gradients had a β value of zero (i.e., random), we conclude that deoxyhemoglobin present in voxels post-sacrifice still impacts tissue water diffusion. These results suggest that in the anesthetized rat brain the LFF of BOLD signal at 11.7T follow a general 1/f(β) model of fractality where β is a variable responding to physiology. We describe typical experimental pitfalls which may elude detection of fractality in the resting-state BOLD signal.

Magnetic resonance-based imaging in animal models of fetal alcohol spectrum disorder

Magnetic resonance imaging (MRI) techniques, such as magnetic resonance microscopy (MRM), diffusion tensor imaging (DTI), and magnetic resonance spectroscopy (MRS), have recently been applied to the study of both normal and abnormal structure and neurochemistry in small animals. Herein, findings from studies in which these methods have been used for the examination of animal models of Fetal Alcohol Spectrum Disorder (FASD) are discussed. Emphasis is placed on results of imaging studies in fetal and postnatal mice that have highlighted the developmental stage dependency of prenatal ethanol exposure-induced CNS defects. Consideration is also given to the promise of methodological advances to allow in vivo studies of aberrant brain and behavior relationships in model animals and to the translational nature of this work.

Use of High Resolution 3D Diffusion Tensor Imaging to Study Brain White Matter Development in Live Neonatal Rats

High resolution diffusion tensor imaging (DTI) can provide important information on brain development, yet it is challenging in live neonatal rats due to the small size of neonatal brain and motion-sensitive nature of DTI. Imaging in live neonatal rats has clear advantages over fixed brain scans, as longitudinal and functional studies would be feasible to understand neuro-developmental abnormalities. In this study, we developed imaging strategies that can be used to obtain high resolution 3D DTI images in live neonatal rats at postnatal day 5 (PND5) and PND14, using only 3 h of imaging acquisition time. An optimized 3D DTI pulse sequence and appropriate animal setup to minimize physiological motion artifacts are the keys to successful high resolution 3D DTI imaging. Thus, a 3D rapid acquisition relaxation enhancement DTI sequence with twin navigator echoes was implemented to accelerate imaging acquisition time and minimize motion artifacts. It has been suggested that neonatal mammals possess a unique ability to tolerate mild-to-moderate hypothermia and hypoxia without long term impact. Thus, we additionally utilized this ability to minimize motion artifacts in magnetic resonance images by carefully suppressing the respiratory rate to around 15/min for PND5 and 30/min for PND14 using mild-to-moderate hypothermia. These imaging strategies have been successfully implemented to study how the effect of cocaine exposure in dams might affect brain development in their rat pups. Image quality resulting from this in vivo DTI study was comparable to ex vivo scans. fractional anisotropy values were also similar between the live and fixed brain scans. The capability of acquiring high quality in vivo DTI imaging offers a valuable opportunity to study many neurological disorders in brain development in an authentic living environment.

MR diffusion kurtosis imaging for neural tissue characterization

In conventional diffusion tensor imaging (DTI), water diffusion distribution is described as a 2nd-order three-dimensional (3D) diffusivity tensor. It assumes that diffusion occurs in a free and unrestricted environment with a Gaussian distribution of diffusion displacement, and consequently that diffusion weighted (DW) signal decays with diffusion factor (b-value) monoexponentially. In biological tissue, complex cellular microstructures make water diffusion a highly hindered or restricted process. Non-monoexponential decays are experimentally observed in both white matter and gray matter. As a result, DTI quantitation is b-value dependent and DTI fails to fully utilize the diffusion measurements that are inherent to tissue microstructure. Diffusion kurtosis imaging (DKI) characterizes restricted diffusion and can be readily implemented on most clinical scanners. It provides a higher-order description of water diffusion process by a 2nd-order 3D diffusivity tensor as in conventional DTI together with a 4th-order 3D kurtosis tensor. Because kurtosis is a measure of the deviation of the diffusion displacement profile from a Gaussian distribution, DKI analyses quantify the degree of diffusion restriction or tissue complexity without any biophysical assumption. In this work, the theory of diffusion kurtosis and DKI including the directional kurtosis analysis is revisited. Several recent rodent DKI studies from our group are summarized, and DKI and DTI compared for their efficacy in detecting neural tissue alterations. They demonstrate that DKI offers a more comprehensive approach than DTI in describing the complex water diffusion process in vivo. By estimating both diffusivity and kurtosis, it may provide improved sensitivity and specificity in MR diffusion characterization of neural tissues.

In vivo diffusion tensor magnetic resonance imaging and fiber tracking of the mouse brain

Until very recently, the study of neural architecture using fixed tissue has been a major scientific focus of neurologists and neuroanatomists. A non-invasive detailed insight into the brain's axonal connectivity in vivo has only become possible since the development of diffusion tensor magnetic resonance imaging (DT-MRI). This unique approach of analyzing axonal projections in the living brain was used in the present study to describe major white matter fiber tracts of the mouse brain and also to identify for the first time non-invasively the rich connectivity between the amygdala and different target regions. To overcome the difficulties associated with high spatially and temporally resolved DT-MRI measurements a 4-shot diffusion weighted spin echo (SE) echo planar imaging (EPI) protocol was adapted to mouse brain imaging at 9.4T. Diffusion tensor was calculated from data sets acquired by using 30 diffusion gradient directions while keeping the acquisition time at 91 min. Two fiber tracking algorithms were employed. A deterministic approach (fiber assignment by continuous tracking - FACT algorithm) allowed us to identify and generate the 3D representations of various neural pathways. A probabilistic approach was further used for the generation of probability maps of connectivity with which it was possible to investigate - in a statistical sense - all possible connecting pathways between selected seed points. We show here applications to determine the connection probability between regions belonging to the visual or limbic systems. This method does not require a priori knowledge about the projections' trajectories and is shown to be efficient even if the investigated pathway is long or three-dimensionally complex. Additionally, high resolution images of rotational invariant parameters of the diffusion tensor, such as fractional anisotropy, volume ratio or main eigenvalues allowed quantitative comparisons in-between regions of interest (ROIs) and showed significant differences between various white matter regions.

Three-dimensional diffusion tensor microimaging for anatomical characterization of the mouse brain

Diffusion tensor imaging is gaining increasing importance for anatomical imaging of the developing mouse brain. However, the application of diffusion tensor imaging to mouse brain imaging at microscopic levels is hindered by the limitation on achievable spatial resolution. In this study, fast diffusion tensor microimaging of the mouse brain, based on a diffusion-weighted gradient and spin echo technique with twin-navigator echo phase correction, is presented. Compared to echo planar and spin echo acquisition, the diffusion-weighted gradient and spin echo acquisition resulted in significant reduction in scan time and had minimal image distortion, thereby allowing acquisition at higher spatial resolution. In this study, three-dimensional diffusion tensor microimaging of the mouse brains at spatial resolutions of 50-60 microm revealed unprecedented anatomical details. Thin fiber bundles in the adult striatum and white matter tracts in the embryonic day 12 mouse brains were visualized for the first time. The study demonstrated that data acquired using the diffusion tensor microimaging technique allow three-dimensional mapping of gene expression data and can serve as a platform to study gene expression patterns in the context of neuroanatomy in the developing mouse brain.

Normobaric hyperoximia increases hypoxia-induced cerebral injury: DTI study in rats

Perinatal hypoxia affects normal neurological development and can lead to motor, behavioral and cognitive deficits. A common acute treatment for perinatal hypoxia is oxygen resuscitation (hyperoximia), a controversial treatment. Magnetic resonance imaging (MRI), including diffusion tensor imaging (DTI), was performed in a P7 rat model of perinatal hypoxia to determine the effect of hyperoximia. These studies were performed on two groups of animals: 1) animals which were subjected to ischemia followed by hypoxia (HI), and 2) HI followed by hyperoximic treatment (HHI). Lesion volumes on high resolution MRI and DTI derived measures, fractional anisotropy (FA), mean diffusivity (MD), and axial and radial diffusivities (lambda(l) and lambda(t), respectively) were measured in vivo one day, one week, and three weeks after injury. Most significant differences in the MRI and DTI measures were found at three weeks after injury. Specifically, three weeks after HHI injury resulted in significantly larger hyperintense lesion volumes (95.26 +/- 50.42 mm(3)) compared to HI (22.25 +/- 17.62 mm(3)). The radial diffusivity lambda(t) of the genu of corpus callosum was significantly larger in HHI (681 +/- 330 x 10(-6) mm(2)/sec) than in HI (486 +/- 96 x 10(-6) mm(2)/sec). Over all, most significant differences in all the DTI metrics (FA, MD, lambda(t), lambda(l)) at all time points were found in the corpus callosum. Our results suggest that treatment of perinatal hypoxia with normobaric oxygen does not ameliorate, but exacerbates damage.

Cellular-level diffusion tensor microscopy and fiber tracking in mammalian nervous tissue with direct histological correlation

Magnetic resonance imaging techniques have literally revolutionized neuroimaging with an unprecedented ability to explore tissue structure and function. Over the last three decades, the sensitivity and array of imaging techniques available have improved providing ever finer structural information and more sensitive functional techniques. Among these methods, diffusion imaging techniques have facilitated the generation of fiber-tract maps of the brain enabling an examination of issues related to brain structure and neural connectivity. Despite the potential utility of the techniques described, validation has not yet been achieved on biological samples. Recently, using newly developed surface microcoils on small samples at high magnetic fields, we demonstrated the ability of MR microscopy to image individual neurons in mammalian brain tissue. In the present work, we combine MR microscopy with the highest resolution (15microm) fiber tracking yet reported and demonstrate the accuracy of the fiber tract maps with direct histological validation. Thus it becomes possible to delineate fiber structure in tissues at the cellular level. A semi-quantitative approach was used to estimate the cell overlap fraction (cOF) and fiber tract overlap fraction (tOF), with cOFs of 94%, 92% and 100%, and tOFs of 84%, 86% and 100%, in rat cervical, rat lumbar, and pig spinal cord tissue, respectively. These methods provide a way to directly validate fiber tracking techniques with histology so that contemporary tracking techniques may be compared and refined using the microstructural details of a biological template as a ground truth.

Magnetic resonance imaging and micro-computed tomography combined atlas of developing and adult mouse brains for stereotaxic surgery

Stereotaxic atlases of the mouse brain are important in neuroscience research for targeting of specific internal brain structures during surgical operations. The effectiveness of stereotaxic surgery depends on accurate mapping of the brain structures relative to landmarks on the skull. During postnatal development in the mouse, rapid growth-related changes in the brain occur concurrently with growth of bony plates at the cranial sutures, therefore adult mouse brain atlases cannot be used to precisely guide stereotaxis in developing brains. In this study, three-dimensional stereotaxic atlases of C57BL/6J mouse brains at six postnatal developmental stages: postnatal day (P) 7, P14, P21, P28, P63 and in adults (P140-P160) were developed, using diffusion tensor imaging (DTI) and micro-computed tomography (CT). At present, most widely-used stereotaxic atlases of the mouse brain are based on histology, but the anatomical fidelity of ex vivo atlases to in vivo mouse brains has not been evaluated previously. To account for ex vivo tissue distortion due to fixation as well as individual variability in the brain, we developed a population-averaged in vivo magnetic resonance imaging adult mouse brain stereotaxic atlas, and a distortion-corrected DTI atlas was generated by nonlinearly warping ex vivo data to the population-averaged in vivo atlas. These atlas resources were developed and made available through a new software user-interface with the objective of improving the accuracy of targeting brain structures during stereotaxic surgery in developing and adult C57BL/6J mouse brains.

Mild hypoxic-ischemic injury in the neonatal rat brain: longitudinal evaluation of white matter using diffusion tensor MR imaging

BACKGROUND AND PURPOSE

Selective white matter (WM) damage is a known sequela of mild hypoxic-ischemic (HI) injury in the neonatal rat model. The aim of this study was to evaluate longitudinally mild HI-induced WM damage (represented by the external capsule [EC]) by diffusion tensor MR imaging (DTI) and to correlate the findings with histology.

MATERIALS AND METHODS

Seven-day-old Sprague-Dawley rats (n = 19) underwent unilateral ligation of the left common carotid artery followed by hypoxia for 50 minutes to create mild HI injury. DTI was performed longitudinally at 5 time points from day 1 to day 90 postinjury (n = 19, 16, 13, 11, 9, respectively), and fractional anisotropy (FA), trace, radial diffusivity (lambda( perpendicular)), and axial diffusivity (lambda(//)) of the injury and control contralateral ECs were quantified. Rats were randomly sacrificed (n = 15, in total), and the corresponding ECs were stained with hematoxylin-eosin, Luxol fast blue (LFB), and neurofilament (NF) to evaluate morphologic changes, amount of myelin, and axonal count at every time point. A paired t test was applied to evaluate statistical differences between both ECs, and the Pearson correlation test was used to evaluate the relationships between DTI indices and histologic evaluations. In addition, longitudinal changes in DTI indices and histologic evaluations were analyzed by a linear mixed model and an analysis of variance test, respectively.

RESULTS

We demonstrated significantly decreased FA, increased lambda( perpendicular), and similar lambda(//) in the injury compared with the control EC, which was persistent through all time points. Histologic evaluation by LFB and NF staining showed reduced myelin stain intensity in the injury EC and similar axonal counts in both ECs. Longitudinally, there was an increase in FA, a decrease in lambda( perpendicular) and trace, and stability in lambda(//) in both ECs. Also, there was progressive reduction in the differences in FA, trace, and lambda( perpendicular) between the injury and control EC, especially between day 1 and day 7 postinjury and in tandem with changes in myelin stain. FA was significantly correlated with myelin stain (r = 0.681, P < .01) and axonal count (r = 0.673, P < .01), whereas lambda( perpendicular) was significantly correlated with myelin stain only (r = -0.528, P < .01), and lambda(//), with axonal count only (r = 0.372, P = .043).

CONCLUSIONS

Diffusion indices can reflect dysmyelination in mild HI injury, continual myelination of both injury and control ECs with growth, and the partial recovery of myelin postinjury. We propose that diffusion indices may be used as biomarkers to monitor noninvasively the longitudinal changes of mild HI-induced WM damage.

Consequences of intraventricular hemorrhage in a rabbit pup model

BACKGROUND AND PURPOSE

Intraventricular hemorrhage (IVH) is a common complication of prematurity that results in neurological sequelae, including cerebral palsy, posthemorrhagic hydrocephalus, and cognitive deficits. Despite this, there is no standardized animal model exhibiting neurological consequences of IVH in prematurely delivered animals. We asked whether induction of moderate-to-severe IVH in premature rabbit pups would produce long-term sequelae of cerebral palsy, posthemorrhagic hydrocephalus, reduced myelination, and gliosis.

METHODS

The premature rabbit pups, delivered by cesarean section, were treated with intraperitoneal glycerol at 2 hours postnatal age to induce IVH. The development of IVH was diagnosed by head ultrasound at 24 hours of age. Neurobehavioral, histological, and ultrastructural evaluation and diffusion tensor imaging studies were performed at 2 weeks of age.

RESULTS

Although 25% of pups with IVH (IVH pups) developed motor impairment with hypertonia and 42% developed posthemorrhagic ventriculomegaly, pups without IVH (non-IVH) were unremarkable. Immunolabeling revealed reduced myelination in the white matter of IVH pups compared with saline- and glycerol-treated non-IVH controls. Reduced myelination was confirmed by Western blot analysis. There was evidence of gliosis in IVH pups. Ultrastructural studies in IVH pups showed that myelinated and unmyelinated fibers were relatively preserved except for focal axonal injury. Diffusion tensor imaging showed reduction in fractional anisotropy and white matter volume confirming white matter injury in IVH pups.

CONCLUSION

The rabbit pups with IVH displayed posthemorrhagic ventriculomegaly, gliosis, reduced myelination, and motor deficits, like humans. The study highlights an instructive animal model of the neurological consequences of IVH, which can be used to evaluate strategies in the prevention and treatment of posthemorrhagic complications.

Therapeutic administration of plasminogen activator inhibitor-1 prevents hypoxic-ischemic brain injury in newborns

Disruption of the integrity of the blood-brain barrier (BBB) is an important mechanism of cerebrovascular diseases, including neonatal cerebral hypoxia-ischemia (HI). Although both tissue-type plasminogen activator (tPA) and matrix metalloproteinase-9 (MMP-9) can produce BBB damage, their relationship in neonatal cerebral HI is unclear. Here we use a rodent model to test whether the plasminogen activator (PA) system is critical for MMP-9 activation and HI-induced brain injury in newborns. To test this hypothesis, we examined the therapeutic effect of intracerebroventricular injection of plasminogen activator inhibitor-1 (PAI-1) in rat pups subjected to unilateral carotid artery occlusion and systemic hypoxia. We found that the injection of PAI-1 greatly reduced the activity of both tPA and urokinase-type plasminogen activator after HI. It also blocked HI-induced MMP-9 activation and BBB permeability at 24 h of recovery. Furthermore, magnetic resonance imaging and histological analysis showed the PAI-1 treatment reduced brain edema, axonal degeneration, and cortical cell death at 24-48 h of recovery. Finally, the PAI-1 therapy provided a dose-dependent decrease of brain tissue loss at 7 d of recovery, with the therapeutic window at 4 h after the HI insult. Together, these results suggest that the brain PA system plays a pivotal role in neonatal cerebral HI and may be a promising therapeutic target in infants suffering hypoxic-ischemic encephalopathy.

QSI and DTI of excised brains of the myelin-deficient rat

High b-value q-space diffusion imaging (QSI) and conventional DTI methodologies were used to study the MRI diffusion characteristics of excised brains of 21-day-old myelin-deficient (md) rats and their age-matched controls. Three different indices were calculated from the QSI data, i.e., Displacement, Probability and Kurtosis, for the purpose of evaluating the effect of the myelin sheaths on the MR diffusion characteristics in white matter (WM) ROIs of the md versus control brains. The examined WM ROIs were the corpus callosum, the external capsule, and the internal capsule. In all examined WM ROIs, significant differences were observed between the md and control brains for all QSI indices. These differences reveal that myelin sheaths surrounding the axons in WM ROIs mostly affect the component exhibiting restricted diffusion, which is manifested by low mean displacement values and high probability and kurtosis values. Such differences were found to be more pronounced in long diffusion times, i.e., Delta=200 ms. Conventional DTI performed with relatively low b-values (b<1500 s/mm2) was also used to study md versus control brains. Interestingly, the fractional anisotropy (FA) index, which was calculated from DTI data, did not reveal any significant difference between the groups in the examined WM ROIs. However, some distinctions were revealed by the three eigenvalues (lambda1, lambda2, and lambda3) obtained from the tensor analysis. These findings were supported by Voxel-based analysis using SPM. Finally, MRI-guided histology showed very good agreement between myelin-stained regions and regions with highly restricted diffusion detected by QSI.

MRI of cellular layers in mouse brain in vivo

Noninvasive imaging of the brain of animal models demands the detection of increasingly smaller structures by in vivo MRI. The purpose of this work was to elucidate the spatial resolution and structural contrast that can be obtained for studying the brain of C57BL/6J mice by optimized T2-weighted fast spin-echo MRI at 9.4 T. As a prerequisite for high-resolution imaging in vivo, motion artifacts were abolished by combining volatile anesthetics and positive pressure ventilation with a specially designed animal bed for fixation. Multiple substructures in the cortex, olfactory bulb, hippocampus, and cerebellum were resolved at 30 to 40 microm in-plane resolution and 200 to 300 microm section thickness as well as for relatively long echo times of 65 to 82 ms. In particular, the approach resulted in the differentiation of up to five cortical layers. In the olfactory bulb the images unraveled the mitral cell layer which has a thickness of mostly single cells. In the hippocampus at least five substructures could be separated. The molecular layer, Purkinje layer, and granular layer of the cerebellum could be clearly differentiated from the white matter. In conclusion, even without the use of a contrast agent, suitable adjustments of a widely available T2-weighted MRI sequence at high field allow for structural MRI of living mice at near single-cell layer resolution.

DTI abnormalities in anterior corpus callosum of rats with spike-wave epilepsy

OBJECTIVE

Absence epilepsy is a common seizure disorder in children which can produce chronic psychosocial sequelae. Human patients and rat absence models show bilateral spike-wave discharges (SWD) in cortical regions. We employed diffusion tensor imaging (DTI) in rat absence models to detect abnormalities in white matter pathways connecting regions of seizure activity.

METHODS

We studied Wistar albino Glaxo rats of Rijswijk (WAG/Rij), genetic absence epilepsy rats of Strasbourg (GAERS), and corresponding nonepileptic control strains. Ex vivo DTI was performed at 9.4 T with diffusion gradients applied in 16 orientations. We compared fractional anisotropy (FA), perpendicular (lambda(perpendicular)) and parallel (lambda(||)) diffusivity between groups using t-maps and region of interest (ROI) measurements.

RESULTS

Adult epileptic WAG/Rij rats exhibited a localized decrease in FA in the anterior corpus callosum. This area was confirmed by tractography to interconnect somatosensory cortex regions most intensely involved in seizures. This FA decrease was not present in young WAG/Rij rats before onset of SWD. GAERS, which have more severe SWD than WAG/Rij, exhibited even more pronounced callosal FA decreases. Reduced FA in the epileptic animals originated from an increased lambda(perpendicular) with no significant changes in lambda(||).

INTERPRETATION

Reduced FA with increased lambda(perpendicular) suggests that chronic seizures cause reduction in myelin or decreased axon fiber density in white matter pathways connecting regions of seizure activity. These DTI abnormalities may improve the understanding of chronic neurological difficulties in children suffering with absence epilepsy, and may also serve as a noninvasive biomarker for monitoring beneficial effects of treatment.

Hypoxic injury during neonatal development in murine brain: correlation between in vivo DTI findings and behavioral assessment

Preterm birth results in significant neurodevelopmental disability. A neonatal rodent model of chronic sublethal hypoxia (CSH), which mimics effects of preterm birth, was used to characterize neurodevelopmental consequences of prolonged exposure to hypoxia using tissue anisotropy measurements from diffusion tensor imaging. Corpus callosum, cingulum, and fimbria of the hippocampus revealed subtle, yet significant, hypoxia-induced modifications during maturation (P15-P51). Anisotropy differences between control and CSH mice were greatest at older ages (>P40) in these regions. Neither somatosensory cortex nor caudate putamen revealed significant differences between control and CSH mice at any age. We assessed control and CSH mice using tests of general activity and cognition for behavioral correlates of morphological changes. Open-field task revealed greater locomotor activity in CSH mice early in maturation (P16-P18), whereas by adolescence (P40-P45) differences between control and CSH mice were insignificant. These results may be associated with lack of cortical and subcortical anisotropy differences between control and CSH mice. Spatial-delayed alternation and free-swim tasks in adulthood revealed lasting impairments for CSH mice in spatial memory and behavioral laterality. These differences may correlate with anisotropy decreases in hippocampal and callosal connectivities of CSH mice. Thus, CSH mice revealed developmental and behavioral deficits that are similar to those observed in low birth weight preterm infants.

MRI in small brains displaying extensive plasticity

Manganese-enhanced magnetic resonance imaging (ME-MRI), blood oxygen-level-dependent functional MRI (BOLD fMRI) and diffusion tensor imaging (DTI) can now be applied to animal species as small as mice or songbirds. These techniques confirmed previous findings but are also beginning to reveal new phenomena that were difficult or impossible to study previously. These imaging techniques will lead to major technical and conceptual advances in systems neurosciences. We illustrate these new developments with studies of the song control and auditory systems in songbirds, a spatially organized neuronal circuitry that mediates the acquisition, production and perception of complex learned vocalizations. This neural system is an outstanding model for studying vocal learning, brain steroid hormone action, brain plasticity and lateralization of brain function.

Region-specific susceptibilities to cuprizone-induced lesions in the mouse forebrain: Implications for the pathophysiology of schizophrenia

Cuprizone (CPZ) is a neurotoxic agent acting as a copper chelator. In our recent study, C57BL/6 mice given dietary CPZ (0.2%) showed impairments in spatial working memory, social interaction, and prepulse inhibition. These abnormalities are reminiscent of certain schizophrenia symptoms and are not likely due to damage in the whole brain or in any single white matter tract/brain region. We hypothesized that white matter damage resulting from CPZ-treatment may be site-specific rather than universal. We examined the forebrains of C57BL/6 mice given the CPZ-containing diet and compared them with those of controls. We assessed CPZ-induced demyelination in main white matter tracts of the forebrain, evaluated myelin break down in the neuropil of the main olfactory bulb (MOB), cerebral cortex (CTX), caudate putamen (CP), hippocampus (HP), thalamus (TH), and hypothalamus (HY), and counted the number of myelin sheath forming oligodendrocytes (OLs) in CTX, CP, TH, and HY. Obvious demyelination was observed in the corpus callosum, external capsule, CP, and dorsal hippocampal commissure whereas other tracts seemed to be unaffected. The neuropil of CTX, HP and MOB showed myelin break down, which was mild in TH and HY. The number of OLs was decreased in all the above regions of CPZ-treated mice although the degree of OL loss was not consistent across regions. The data provide further support for white matter abnormalities contributing to schizophrenia-like behaviors in mice.

Oxidative neuroenergetics in event-related paradigms

Energetic basis of neural activity provides a solid foundation for noninvasive neuroimaging with calibrated functional magnetic resonance imaging (fMRI). Calculating dynamic changes in cerebral oxidative energy utilization (CMR(O(2))) is limited by uncertainties about whether or not the conventional blood oxygenation level-dependent (BOLD) model can be applied transiently using multimodal measurements of blood flow (CBF) and volume (CBV) that affect the BOLD signal. A prerequisite for dynamic calibrated fMRI is testing the linearity of multimodal signals within a temporal regimen, as assessed by signal strength (i.e., both intensity and width). If each hyperemic component (BOLD, CBV, CBF) is demonstrated to be linear with neural activity under various experimental conditions, then the respective transfer functions generated by deconvolution with neural activity should be time invariant and thus could potentially be used for calculating CMR(O(2)) transients. Hyperemic components were investigated at 11.7 T in alpha-chloralose-anesthetized rats and combined with electrophysiological recordings of local field potential (LFP) and multiunit activity (MUA) from the cortex during forepaw stimulation, in which stimulus number and frequency were varied. Although relationships between neural activity and stimulus features ranged from linear to nonlinear, associations between hyperemic components and neural activity were linear. Specific to each hyperemic component, a universal transfer function (with LFP or MUA) yielded predictions in agreement with experimental measurements. The results identified a component of the BOLD signal that can be attributed to significant changes in CMR(O(2)), even for temporal events separated by <200 ms.

Measuring diffusion parameters in the brain: comparing the real-time iontophoretic method and diffusion-weighted magnetic resonance

The extracellular space (ECS) diffusion parameters influence the movement of ions, neuroactive substances, hormones and metabolites in the nervous tissue. They also affect extrasynaptic transmission, a mode of signal transmission dependent solely on diffusion. This review compares in detail two methods for studying diffusion in the brain: the real-time iontophoretic tetramethylammonium method for ECS volume fraction and tortuosity measurements and diffusion weighted-magnetic resonance imaging for measuring the apparent diffusion coefficient of water. The results obtained using both methods under physiological conditions (post-natal development, ageing) or in pathologies (brain injury, ischaemia) and their similarities and differences are discussed.

Does diffusion kurtosis imaging lead to better neural tissue characterization? A rodent brain maturation study

Diffusion kurtosis imaging (DKI) can be used to estimate excess kurtosis, which is a dimensionless measure for the deviation of water diffusion profile from Gaussian distribution. Several recent studies have applied DKI to probe the restricted water diffusion in biological tissues. The directional analysis has also been developed to obtain the directionally specific kurtosis. However, these studies could not directly evaluate the sensitivity of DKI in detecting subtle neural tissue alterations. Brain maturation is known to involve various biological events that can affect water diffusion properties, thus providing a sensitive platform to evaluate the efficacy of DKI. In this study, in vivo DKI experiments were performed in normal Sprague-Dawley rats of 3 different ages: postnatal days 13, 31 and 120 (N=6 for each group). Regional analysis was then performed for 4 white matter (WM) and 3 gray matter (GM) structures. Diffusivity and kurtosis estimates derived from DKI were shown to be highly sensitive to the developmental changes in these chosen structures. Conventional diffusion tensor imaging (DTI) parameters were also computed using monoexponential model, yielding reduced sensitivity and directional specificity in monitoring the brain maturation changes. These results demonstrated that, by measuring directionally specific diffusivity and kurtosis, DKI offers a more comprehensive and sensitive detection of tissue microstructural changes. Such imaging advance can provide a better MR diffusion characterization of neural tissues, both WM and GM, in normal, developmental and pathological states.

Early postnatal development of rat brain: in vivo diffusion tensor imaging

Perinatal hypoxia is a major cause of neurodevelopmental deficits. Neuronal migration patterns are particularly sensitive to perinatal hypoxia/ischemia and are associated with the clinical deficits. The rat model of hypoxia/ischemia at P7 mimics that of perinatal injury in humans. Before assessing the effects of postnatal injury on brain development, it is essential to determine the normal developmental trajectories of various brain structures in individual animals. In vivo longitudinal diffusion tensor imaging (DTI) was performed from postnatal day 0 (P0) to P56 on Wistar rats. The DTI metrics, mean diffusivity (MD), fractional anisotropy (FA), axial (lambdal) and radial (lambdat) diffusivities, were determined for four gray matter and eight white matter structures. The FA of the cortical plate and the body of corpus callosum decreased significantly during the first 3 weeks after birth. The decrease in the cortical plate's FA value was associated mainly with an increase in lambdat. The initial decrease in FA of corpus callosum was associated with a significant decrease in lambdal. The FA of corpus callosum increased during the rest of the observational period, which was mainly associated with a decrease in lambdat. The FA of gray matter structures, hippocampus, caudate putamen, and cortical mantle did not show significant changes between P0 and P56. In contrast, the majority of white matter structures showed significant changes between P0 and P56. These temporal changes in the DTI metrics were related to the neuronal and axonal pruning and myelination that are known to occur in the developing brain.