Local cerebral glucose utilization in perfusion-fixed rat brains

Recommended Methods for Brain Processing and Quantitative Analysis in Rodent Developmental Neurotoxicity Studies

Neuropathology methods in rodent developmental neurotoxicity (DNT) studies have evolved with experience and changing regulatory guidance. This article emphasizes principles and methods to promote more standardized DNT neuropathology evaluation, particularly procurement of highly homologous brain sections and collection of the most reproducible morphometric measurements. To minimize bias, brains from all animals at all dose levels should be processed from brain weighing through paraffin embedding at one time using a counterbalanced design. Morphometric measurements should be anchored by distinct neuroanatomic landmarks that can be identified reliably on the faced block or in unstained sections and which address the region-specific circuitry of the measured area. Common test article-related qualitative changes in the developing brain include abnormal cell numbers (yielding altered regional size), displaced cells (ectopia and heterotopia), and/or aberrant differentiation (indicated by defective myelination or synaptogenesis), but rarely glial or inflammatory reactions. Inclusion of digital images in the DNT pathology raw data provides confidence that the quantitative analysis was done on anatomically matched (i.e., highly homologous) sections. Interpreting DNT neuropathology data and their presumptive correlation with neurobehavioral data requires an integrative weight-of-evidence approach including consideration of maternal toxicity, body weight, brain weight, and the pattern of findings across brain regions, doses, sexes, and ages.

MR-Pathological Comparison in F98-Fischer Glioma Model Using a Human Gantry

OBJECT

This study reports our findings in assessing in vivo tumour growth with magnetic resonance imaging using a commercial magnet and antenna in F98 implanted Fischer rats. A comparison of T1 gadolinium-enhanced coronal MR scans and pathology specimens in corresponding animals was accomplished.

METHODS

One rat was used in serial experiments to establish adequate imaging parameters. Afterward, 12 animals implanted with F98 cells underwent a MR study following intervals spanning five, ten, 15 and 20 days on a 1.5T human Siemens. Using a small loop antenna, a coronal T1 weighted MRI scan with Gadolinium was performed. Images were analyzed and volumes of enhancing tumour were calculated. The animals were sacrificed after the imaging procedure and brain were harvested and processed in pathology. Pathology specimens and MR images were analyzed using image processing software. One hematoxylin + eosin (H&E) slide per specimen was compared to the corresponding MR slice depicting the largest area of enhancement.

RESULTS

The MR enhancement areas obtained were 2.18 mm2, 8.25 mm2, 21.6 mm2 and 23.17 mm2 at five, ten, 15 and 20 days. Tumour margin measurements on pathologic samples produced areas of 0.29 mm2, 4.43 mm2, 8.3 mm2, and 12.9 mm2 at five, ten, 15 and 20 days respectively.

CONCLUSION

The T1-enhancing images constantly overestimated the tumour bulk on H&E. This phenomenon is explained by enhancement of the brain around tumour, the extra-axial tumour growth, and a shrinking factor of 17% related to the fixation process. Nonetheless, the radiological tumour growth paralleled the histological samples. This technology is thus suitable to follow tumour growth in F98 implanted rats.

Standardization and Detailed Characterization of the Syngeneic Fischer/F98 Glioma Model

Introduction:

Adequate animal glioma models are mandatory for the pursuit of preclinical research in neuro-oncology. Many implantation models have been described, but none perfectly emulate human malignant gliomas. This work reports our experience in standardizing, optimizing and characterizing the Fischer/F98 glioma model on the clinical, pathological, radiological and metabolic aspects.

Materials and methods:

F98 cells were implanted in 70 Fischer rats, varying the quantity of cells and volume of implantation solution, and using a micro-infusion pump to minimize implantation trauma, after adequate coordinates were established. Pathological analysis consisted in hematoxylin and eosin (H&E) staining and immunohistochemistry for GFAP, vimentin, albumin, TGF-b1, TGF-b2, CD3 and CD45. Twelve animals were used for MR imaging at 5, 10, 15 and 20 days. Corresponding MR images were compared with pathological slides. Two animals underwent 18F-FDG and 11C-acetate PET studies for metabolic characterization of the tumors.

Results:

Implantation with 1x104 cells produced a median survival of 26 days and a tumor take of 100%. Large infiltrative neoplasms with a necrotic core were seen on H&E. Numerous mitosis, peritumoral infiltrative behavior, and neovascular proliferation were also obvious. GFAP and vimentin staining was positive inside the tumor cells. Albumin staining was observed in the extracellular space around the tumors. CD3 staining was negligible. The MR images correlated the pathologic findings. 18F-FDG uptake was strong in the tumors.

Conclusion:

The standardized model described in this study behaves in a predictable and reproducible fashion, and could be considered for future pre-clinical studies. It adequately mimics the behavior of human malignant astrocytomas.

Imaging brain deoxyglucose uptake and metabolism by glucoCEST MRI

2-Deoxy-D-glucose (2DG) is a known surrogate molecule that is useful for inferring glucose uptake and metabolism. Although (13)C-labeled 2DG can be detected by nuclear magnetic resonance (NMR), its low sensitivity for detection prohibits imaging to be performed. Using chemical exchange saturation transfer (CEST) as a signal-amplification mechanism, 2DG and the phosphorylated 2DG-6-phosphate (2DG6P) can be indirectly detected in (1)H magnetic resonance imaging (MRI). We showed that the CEST signal changed with 2DG concentration, and was reduced by suppressing cerebral metabolism with increased general anesthetic. The signal changes were not affected by cerebral or plasma pH, and were not correlated with altered cerebral blood flow as demonstrated by hypercapnia; neither were they related to the extracellular glucose amounts as compared with injection of D- and L-glucose. In vivo (31)P NMR revealed similar changes in 2DG6P concentration, suggesting that the CEST signal reflected the rate of glucose assimilation. This method provides a new way to use widely available MRI techniques to image deoxyglucose/glucose uptake and metabolism in vivo without the need for isotopic labeling of the molecules.

Zero net flux estimates of septal extracellular glucose levels and the effects of glucose on septal extracellular GABA levels

Although hippocampal infusions of glucose enhance memory, we have found repeatedly that septal glucose infusions impair memory when gamma-aminobutyric acid (GABA) receptors are activated. For instance, hippocampal glucose infusions reverse the memory-impairing effects of co-infusions of the GABA agonist muscimol, whereas septal glucose infusions exacerbate memory deficits produced by muscimol. One potential explanation for these deleterious effects of glucose in the septum is that there are higher levels of endogenous extracellular fluid glucose concentrations in the septum than in the hippocampus. Another hypothesis is that septal glucose infusions impair memory by increasing septal GABA synthesis or release, which is possible because elevating glucose increases GABA levels in other brain regions. To test these hypotheses, Experiment 1 quantified extracellular fluid glucose levels in the septum and hippocampus using zero net flux in vivo microdialysis procedures in conscious, freely moving rats. Experiment 2 determined whether septal infusions of glucose would increase GABA concentrations in dialysates obtained from the septum. The results of Experiment 1 indicated that extracellular fluid glucose levels in the hippocampus and septum are comparable. The results of Experiment 2 showed that co-infusions of glucose with muscimol, at doses that did not affect memory on their own, decreased percent alternation memory scores. However, none of the infusions significantly affected GABA levels. Collectively, these findings suggest that the memory-impairing effects of septal infusions of glucose are not likely due to regional differences in basal extracellular fluid glucose concentrations and are not mediated via an increase in septal GABA release.

Volume reconstruction techniques improve the correlation between histological and in vivo tumor volume measurements in mouse models of human gliomas

Assessment of therapy efficacy using animal models of tumorigenic cancer requires the ability to accurately measure changes in tumor volume over the duration of disease course. In order to be meaningful, in vivo tumor volume measurements by non-invasive techniques must correlate with tumor volume measurements from endpoint histological analysis. Tumor volume is frequently assessed by endpoint histological analyses approximating the tumor volume with geometric primitives such as spheroids and ellipsoids. In this study we investigated alternative techniques for quantifying histological volume measurements of tumors in a xenograft orthotopic mouse model of human glioblastoma multiforme, and compared these to in vivo tumor volume measurements based on magnetic resonance imaging (MRI) data. Two techniques leveraging three-dimensional (3D) image analysis methods were investigated. The first technique involves the reconstruction of a smoothed polygonal model representing the tumor volume from histological section images and is intended for accuracy and qualitative assessment of tumor burden by visualization, while a second technique which approximates the tumor volume as a series of slabs is presented as an abbreviated process intended to produce quantitatively similar volume measurements with a minimum of effort required on behalf of the investigator. New software (QuickVol) designed for use in the first technique, is also discussed. In cases where tumor growth is asymmetric and invasive, we found that 3D analysis techniques using histological section images produced volume measurements more consistent with in vivo volume measurements based on MRI data, than approximation of tumor volume using geometric primitives. Visualizations of the volumes represented by each of these techniques qualitatively support this finding, and suggest that future research using mouse models of glioblastoma multiforme (genetically engineered or xenograft) will benefit from the use of these or similar alternative tumor volume measurement techniques.

Regional pattern of metabolic activation is reflected in the sleep EEG after sleep deprivation combined with unilateral whisker stimulation in mice

Regional differences in EEG slow wave activity (SWA) during sleep after sleep deprivation (SD) may be a consequence of differential metabolic activation of cortical areas. We investigated the relationship between the regional EEG dynamics and 2-deoxyglucose (DG) uptake after SD in mice. Six hours' SD were combined with natural unilateral whisker stimulation in an enriched environment to selectively activate the barrel cortex and motor areas. As expected, an interhemispheric asymmetry of 2-DG uptake was found in the barrel cortex immediately after SD. To test whether sleep contributes to recovery of the asymmetry, the stimulation was followed by either undisturbed sleep or by an additional SD. The asymmetry vanished after recovery sleep but also after the additional period of wakefulness without stimulation. In addition, relative 2-DG uptake in the primary motor cortex and retrosplenial area was significantly higher immediately after the SD than after the additional sleep or wakefulness, whereas no other region differed between the groups. Whisker stimulation elicited a greater increase in EEG SWA during non rapid eye movement sleep in the stimulated hemisphere than in the control hemisphere; this increase lasted for 10 h. Within a hemisphere, the initial increase in SWA was higher in the frontal than in the parietal derivation. We conclude that the regional SWA differences during sleep are use-dependent and may be related to the regional pattern of metabolism during the previous waking episode. However, the regional metabolic recovery is not dependent on sleep, and is not directly reflected in changes in SWA during sleep.

Imaging of experimental rat gliomas using a clinical MR scanner

BACKGROUND

Studies of brain tumor development in experimental animal models have to date mostly been based on post-mortem histological examinations. The use of magnetic resonance imaging (MRI) may provide a non-invasive technique for studying tumor growth and treatment effects in such animal models. However, most of these studies have been performed on purpose-dedicated small bore magnetic resonance (MR) systems, of high cost and limited availability. The purpose of this study was thus to obtain high-resolution images of experimental gliomas in the rat brain, using a clinical 1.5 T MR scanner.

METHODS

Anesthesized rats bearing BT4C brain tumors were positioned into a specially designed immobilizing device, and a small circular coil was positioned onto the skulls. Two T1 weighted series were acquired before and after subcutaneous contrast injections. A T2 weighted series was also obtained. The rats were then sacrified, the brains removed, and the histological tumor volumes were compared to the volumes obtained on MRI.

RESULTS

There were visible tumors in 10 of 13 animals scanned on MR. The rim of the tumors were visualized on T1 weighted series without contrast. On T1 images with contrast, the tumors were seen as high signal intensity areas. The T2 weighted images showed peritumoral edema. No necrosis or cystic parts of the tumors were detected. There was a consistency between the MR and the histology findings, showing a high degree of correlation between the two volume determination methods.

CONCLUSIONS

High-resolution images of experimental rat gliomas can be obtained using a clinical MR scanner and a commercially available RF coil. This MRI technique may also be expanded to extraneural rat tumor models, for studies of tumor development and treatment.

A decrease in cell size accompanies a loss of docosahexaenoate in the rat hippocampus

Rats raised on n-3 essential fatty acid deficient diets demonstrate spatial memory deficits. To investigate neuroanatomical correlates of these deficits, morphological analysis of the hippocampus were carried out. Adult, female rats were raised for three generations on n-3 deficient or n-3 supplemented diets. Two n-3 deficient diets contained adequate linoleic acid (LA), or high linoleic acid (high LA), and two supplemented diets contained LA supplemented with alpha-linolenic acid (+LNA), or linoleic supplementation with alpha-linolenic and docosahexaenoic acids (+LNA/DHA). The total fatty acid composition of the hippocampus revealed a profound loss (90%) in docosahexaenoic acid (DHA) in the hippocampi of LA and high LA animals compared to those on +LNA and +LNA/DHA diets with a reciprocal increase in docosapentaenoic acid (DPAn-6) in all phospholipid species. The volume, density, total number, and cell body size of neurons in CA1-3, granular and hilar layers of the hippocampus were measured at septal and temporal locations using unbiased stereology. No differences were detected in any of these measures except for in cell body size; CA1 pyramidal neurons in the LA group were significantly (p < 0.04) smaller than neurons in the +LNA/DHA group at the septal location.

Immunohistochemical study of the hnRNP A2 and B1 in the rat forebrain

Immunohistochemical techniques were employed to examine the distribution of RNA-binding proteins A2 and B1 in the rat forebrain. Intense A2 and B1 immunolabeling were observed in the nucleoplasm of the neurons in the cerebral cortices, hippocampal formation, olfactory regions, caudate-putamen as well as the supraoptic nucleus of hypothalamus. In contrast, within the bed nucleus of the stria terminalis, as well as the medial and lateral habenular nucleus of thalamus, immunoreactivity for both proteins was weak. Within the globus pallidus and thalamic nucleus immunoreactivity for A2 was hardly detectable despite of intense B1 immunolabeling, while within the endopiriform nucleus and lateral and basolateral nucleus of amygdala intensity of B1 immunolabeling was relatively weak compared to A2. Our study suggests that the distribution of A2 and B1 are not constant throughout the forebrain and this diversity may reflect the post-transcriptional regulation of cell-specific gene expression of neuronal cells.

Increased asymmetries in 2-deoxyglucose uptake in the brain of freely moving congenitally acallosal mice

To investigate the role of the corpus callosum in the expression of functional brain asymmetries, we compared left and right uptake of [14C]2-deoxyglucose in 43 brain regions measured in 10 C57B1/6 mice with a normal corpus callosum and in 12 congenitally acallosal mice, after 45 min of free activity in a novel, large open-field arena. The metabolic patterns across the brain appeared to be similar in the two groups of mice, as well as the average direction of asymmetry in tracer incorporation, which was higher at right in most of the brain regions for both acallosals and controls. However, the direction of the metabolic asymmetries of any given region was not consistent across individual animals. The largest asymmetries were found in the central auditory nuclei in both groups of mice, with extreme values in some acallosals. Significantly larger asymmetries were found in acallosal mice for the brain and the cortex as a whole, as well as for the lateral geniculate and pretectal nuclei, the olfactory tubercles, and retrosplenial, infrarhinal and perirhinal cortices. The metabolic asymmetries of the thalamic sensory nuclei were correlated with the asymmetries of the corresponding sensory cortical fields in the acallosal, but not in control mice. On the other hand, asymmetries of the cortical regions were largely intercorrelated in control mice, resulting in a general activation of one hemisphere over the other, while in acallosals they were more independent, resulting in a "patchy" pattern of cortical asymmetries. These results suggest that callosal agenesis, combined with the occurrence of ipsilateral Probst bundles, leads to a loss of co-ordination in the activation of different sensory and motor areas. The impaired co-ordination might then be distributed through cortico-subcortical loops, resulting in larger asymmetries throughout the brain. Thus, a normal corpus callosum appears to balance and synchronize metabolic brain activity, perhaps by smoothing the effects of asymmetrically activated ascending systems.