Assessment of metabolic changes in the striatum of a rat model of parkinsonism: an in vivo (1)H MRS study

Degeneration of the dopaminergic neurons of the substantia nigra pars compacta in Parkinson's disease induces an abnormal activation of the glutamatergic neurotransmission system within the basal ganglia network and related structures. The aim of this study was to use proton MRS to show metabolic changes in the striatum of 6-hydroxydopamine-lesioned rats, a rodent animal model of Parkinson's disease. Animals were examined before and after extensive lesioning of the nigral dopaminergic neurons and after acute administration of L-3,4-dihydroxyphenylalanine. No significant alterations in glutamate concentrations, assessed by the MR signal dominated by glutamate with minor contributions from glutamine and gamma-aminobutyric acid, could be measured. The total choline/total creatine ratio was found to be reduced in the striatum of the ipsilateral hemisphere.

Vessel size imaging using low intravascular contrast agent concentrations

Vessel size index (VSI) measurements have been validated on rats bearing a glioma with high doses of contrast agent. The aim of this study was to evaluate the impact of using a reduced dose of contrast agent, necessary for clinical trials. Experiments were performed on rats at three doses of AMI-227 and clearly show higher VSI values at lower doses, whatever the region of interest (contralateral, peritumoral and intratumoral tissue). These results are in good correlation with Monte-Carlo simulations on healthy brain and suggest that only relative values can be obtained at clinical contrast agent doses and magnetic fields.

Assessment of vascular reactivity in rat brain glioma by measuring regional blood volume during graded hypoxic hypoxia

While morphological and molecular events during angiogenesis in brain glioma have been extensively studied, the functional properties of tumour vessels have yet received little attention. We have determined changes in regional blood volume (BV) during graded hypoxic hypoxia using susceptibility contrast magnetic resonance imaging in a model of rat brain glioma. Nine anaesthetised and ventilated rats with C6 glioma were subjected to incremental reduction in the fraction of inspired oxygen (FiO₂): 0.35, 0.25, 0.15, 0.12, 0.10 and reoxygenation to 0.35. At each episode, BV was determined in peritumoral, intratumoral and contralateral regions. Baseline BV values (FiO₂ of 0.35) were higher in peritumoral than in the contralateral and intratumoral regions. Progressive hypoxia resulted in a graded increase in BV in contralateral and peritumoral regions. At FiO₂ of 0.10, BV increases were comparable between these two regions: 49±22% (s.d.) and 28±17% with respect of control values, respectively. These BV changes reversed during the reoxygenation episode. By contrast, the intratumoral region had a significant increase in BV at FiO₂ of 0.10 only, with no evidence of return to the basal value during reoxygenation. Immunohistochemical staining of α-smooth muscle actin confirmed reactivity of vessels in the peritumoral region. Our findings indicate that peritumoral vessels present a vascular reactivity to hypoxia, which is comparable to that of nontumoral vessels. A method is thus available for noninvasively demonstrating whether any particular vascular modifying strategy results in the desired outcome in terms of tumour blood volume changes.

In vivo assessment of tumoral angiogenesis

Vessel size imaging (VSI) for brain tumor characterization was evaluated and the vessel size index measured by MRI (VSIMRI) was correlated with VSI obtained by histology (VSIhisto). Blood volume (BV) and VSI maps were obtained on 12 rats by simultaneous measurements of R2* and R2, before and after the injection of a macromolecular contrast agent, AMI-227. Immunostaining of collagen IV in vessels was performed. An expression was derived for evaluating VSI from stereologic measurements on histology data (VSIhisto). On BV and VSI images obtained from large-size tumors (n = 9), three regions could be distinguished and correlated well with histological sections: a high BV region surrounding the tumor, a necrotic area where BV is very low, and a viable tumor tissue region showing lower BV but higher VSI than the normal rat cortex, with the presence of larger vessels. The quantitative analysis showed a good correlation (Spearman rank's rho = 0.74) between VSIhisto and VSIMRI with a linear regression coefficient of 1.17. The good correlation coefficient supports VSI imaging as a quantitative method for tumor vasculature characterization.

Pimonidazole binding in C6 rat brain glioma: relation with lipid droplet detection

In C6 rat brain glioma, we have investigated the relation between hypoxia and the presence of lipid droplets in the cytoplasm of viable cells adjacent to necrosis. For this purpose, rats were stereotaxically implanted with C6 cells. Experiments were carried out by the end of the tumour development. A multifluorescence staining protocol combined with digital image analysis was used to quantitatively study the spatial distribution of hypoxic cells (pimonidazole), blood perfusion (Hoechst 33342), total vascular bed (collagen type IV) and lipid droplets (Red Oil) in single frozen sections. All tumours (n=6) showed necrosis, pimonidazole binding and lipid droplets. Pimonidazole binding occurred at a mean distance of 114 microm from perfused vessels mainly around necrosis. Lipid droplets were principally located in the necrotic tissue. Some smaller droplets were also observed in part of the pimonidazole-binding cells surrounding necrosis. Hence, lipid droplets appeared only in hypoxic cells adjacent to necrosis, at an approximate distance of 181 microm from perfused vessels. In conclusion, our results show that severe hypoxic cells accumulated small lipid droplets. However, a 100% colocalisation of hypoxia and lipid droplets does not exist. Thus, lipid droplets cannot be considered as a surrogate marker of hypoxia, but rather of severe, prenecrotic hypoxia.

Mapping extracellular pH in rat brain gliomas in vivo by 1H magnetic resonance spectroscopic imaging: comparison with maps of metabolites

The value of extracellular pH (pH(e)) in tumors is an important factor in prognosisand choice of therapy. We demonstrate here that pH(e) can be mappedin vivo in a rat brain glioma by (1)H magnetic resonance spectroscopic imaging (SI) of the pH buffer (+/-)2-imidazole-1-yl-3-ethoxycarbonylpropionic acid (IEPA). (1)H SI also allowed us to map metabolites, and, to better understand the determinants of pH(e), we compared maps of pH(e), metabolites, and the distribution of the contrast agent gadolinium1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraaceticacid (Gd-DOTA). C6 cells injected in caudate nuclei of four Wistar rats gave rise to gliomas of approximately 10 mm in diameter. Three mmols of IEPA were injected in the right jugular vein from t = 0 to t = 60 min. From t = 50 min to t = 90 min, spin-echo (1)H SI was performed with an echo time of 40 ms in a 2.5-mm slice including the glioma (nominal voxel size, 2.2 microl). IEPA resonances were detected only within the glioma and were intense enough for pH(e) to be calculated from the chemical shift of the H2 resonance in almost all voxels of the glioma. (1)H spectroscopic images with an echo time of 136 ms were then acquired to map metabolites: lactate, choline-containing compounds (tCho), phosphocreatine/creatine, and N-acetylaspartate. Finally, T(1)-weighted imaging after injection of a bolus of Gd-DOTA gave a map indicative of extravasation. On average, the gradient of pH(e) (measured where sufficient IEPA was present) from the center to the periphery was not statistically significant. Mean pH(e) was calculated for each of the four gliomas, and the average was 7.084 +/- 0.017 (+/- SE; n = 4 rats), which is acid with respect to pH(e) of normal tissue. After normalization of spectra to their water peak, voxel-by-voxel comparisons of peak areas showed that N-acetylaspartate, a marker of neurons, correlated negatively with IEPA (P < 0.0001) and lactate (P < 0.05), as expected of a glioma surrounded by normal tissue. tCho (which may indicate proliferation) correlated positively with pH(e) (P < 0.0001). Lactate correlated positively with tCho (P < 0.0001), phosphocreatine/creatine (P < 0.001), and Gd-DOTA (P < 0.0001). Although lactate is exported from cells in association with protons, within the gliomas, no evidence was observed that pH(e) was significantly lower where lactate concentration was higher. These results suggest that lactate is produced mainly in viable, well-perfused, tumoral tissue from which proton equivalents are rapidly cleared.

A new gadolinium-based contrast agent for magnetic resonance imaging of brain tumors: kinetic study on a C6 rat glioma model

T1-weighted magnetic resonance imaging (MRI) was used to evaluate the potential interest of a new Gd-based contrast agent, termed P760, to characterize brain tumor heterogeneity and vascularization and to delineate regions containing permeable vessels. The C6 rat glioma model was used as a model of high-grade glioblastoma. The signal enhancement was measured as a function of time in the vascular compartment and in different regions of interest (ROIs) within the tumor after the injection of 0.02 mmol x kg(-1) of P760. The results were compared to those obtained after the injection of 0.1 mmol x kg(-1) of Gd-DOTA. We showed that P760, in spite of a Gd concentration five times smaller, produces an enhancement in the blood pool similar to that produced by Gd-DOTA. It was shown that P760 makes possible an excellent delineation of regions containing vessels with a damaged blood-brain barrier (BBB). Images acquired 5-10 minutes after P760 injection showed the location of permeable vessels more accurately than Gd-DOTA-enhanced images. The enhancement produced in the tumor by P760 was, however, less than that produced by Gd-DOTA. The extravasation and/or diffusion rate of P760 in the interstitial medium were found to be strongly reduced, compared to those found with Gd-DOTA. This study suggests that the new contrast agent has promising capabilities in clinical imaging of brain tumors.

In vivo measurement of the size of lipid droplets in an intracerebral glioma in the rat

Pulsed field gradient NMR was used to measure the root mean square displacement lambda of the NMR visible lipid molecules in C6 brain tumors in the rat at different diffusion times. For a distribution of spherical droplets of diameter with volume fraction xi(Phi(i)), the mean characteristic droplet diameter Phi(c) = square root of Sigma(i)xi(Phi(i)Phi(i)(2) was shown to be related to the root mean square displacement at long diffusion times by the simple relationship Phi(c)(2) = 10 lambda(2). In the range of diffusion times 100--530 msec, lambda was found to be independent of the diffusion time and equal to 1.35 +/- 0.22 microm and Phi(c) to 4.27 +/- 0.71 microm. The data reinforce the notion that the presence of lipid resonances in NMR spectra of tumors is due to lipid droplets. Light microscopy of histologic slices showed the presence of lipid droplets mainly in the necrotic region and in a layer of tumor cells surrounding the necrosis. Magn Reson Med 45:409-414, 2001.

In vivo K-edge imaging with synchrotron radiation

We present in this paper two imaging techniques using contrast agents assessed with in vivo experiments. Both methods are based on the same physical principle, and were implemented at the European Synchrotron Radiation Facility medical beamline. The first one is intravenous coronary angiography using synchrotron radiation X-rays. This imaging technique has been planned for human studies in the near future. We describe the first experiments that were carried out with pigs at the ESRF. The second imaging mode is computed tomography using synchrotron radiation on rats bearing brain tumors. Owing to synchrotron radiation physical properties, these new imaging methods provide additional information compared to conventional techniques. After infusion of the contrast agent, it is possible to derive from the images the concentration of the contrast agent in the tumor area for the computed tomography and in any visible vessel for the angiography method.

Quantification and distribution of neovascularization following microinjection of C6 glioma cells in rat brain

BACKGROUND

It has been suggested that the switch to an angiogenic phenotype can separate the development of a tumor into two stages: the prevascular phase and the vascular phase. The purpose of the present work is to demonstrate the existence of an angiogenic switch in a longitudinal study of a brain tumor model during tumor growth by means of microvessel density measurements.

MATERIALS AND METHODS

The study was performed on 32 rats bearing C6 glioma. At different stages of tumor growth, the histological aspects were described and sections were immunostained for factor VIII-related antigen in order to highlight microvessel endothelial cells. Microvessels were counted at 400 magnification for different areas (central non necrotic area, peripheral area, contralateral grey and white matter area), using image analysis software.

RESULTS

Vessel density was significantly higher at the tumor-brain interface than in the center of the tumor or in the contralateral cortex. The vessel density remains stable in the tumor during the first 3 weeks after cell implantation, after which a clear increase of vessel density can be observed.

CONCLUSION

The present study demonstrates the presence of an angiogenic switch which is concomitant with the development of necrosis and pseudopalisading pattern.