Evaluating endogenous repair of focal cartilage defects in C57BL/6 and MRL/MpJ mice using 9.4T magnetic resonance imaging: A pilot study.

The use of magnetic resonance imaging (MRI) for evaluating joint injuries is often considered superior to radiography due to the capacity of MRI for visualizing both soft and hard tissues. While longitudinal studies regarding cartilage repair have been undertaken on patients and in larger animal models, a method has yet to be developed for mouse cartilage to be repeatedly and non-invasively evaluated over time. The aim of this pilot study was to investigate if morphological changes following a focal cartilage injury in mice could be measured by 9.4T magnetic resonance imaging. Focal cartilage defects were induced in the left knee of 4-6weeks old C57BL/6 and MRL/MpJ mice. At endpoints 0, 2, and 4weeks post-injury, legs were dissected out and imaged ex vivo. The defect could be detected by MRI immediately after injury, appearing as a hyperintense focal point and with size similar to that of the surgical tool used. Defects were visible in both strains up to 4weeks post-injury, although signal intensity decreased over time. One C57BL/6 in particular, displayed extensive fibrosis in the patellar tendon at 4weeks as assessed by histology, while the MR images of the same animal displayed a clear, structural distinction between the patella and the new tissue growth. Overall, our results suggest that MRI could be used for longitudinal studies in murine cartilage injury models to evaluate certain characteristics of repair not detectable through histology.

Imaging corticospinal degeneration in neonatal rats with unilateral cerebral infarction.

Recent human studies indicate that magnetic resonance (MR) imaging, particularly diffusion weighted imaging, detects abnormalities within the descending cortico-spinal tract following stroke. Whether these changes are directly related to processes of axonal degeneration and how MR changes (e.g. apparent diffusion coefficient of water (ADC) and T(2)) vary in their diagnostic utility over time is not known. The present study demonstrates that a commonly used rat model of neonatal transient unilateral hypoxia-ischemia provides similar diffusion weighted and ADC changes in the cerebral peduncle as those observed in human neonates clinically. Imaging the descending cortico-spinal tract in this model at defined acute (1-3 days) and chronic (1 and 4 weeks) time points demonstrates increased T(2) and progressive changes in ADC within the descending cortico-spinal tract in the first days to weeks following hypoxia-ischemia with a normalization by 1 week and further increases in ispilateral cerebral cortex by 4 weeks. These imaging changes are associated with reduced axonal neurofilament staining both at the subacute and more chronic time points. This demonstrates directly the utility of ADC and T(2) MRI to detect acute changes in axons associated with early Wallerian degeneration.

Metabolic changes in rat brain following intracerebroventricular injections of streptozotocin: a model of sporadic Alzheimer's disease.

A decrease in cerebral glucose metabolic uptake is an early and characteristic sign of Alzheimer's disease (AD). Streptozotocin (STZ) is a bacterial toxin which damages insulin-producing cells and insulin receptors. Intracerebroventricular (icv) application of STZ in rats has been found to chronically decrease cerebral glucose uptake and produce other effects that bear a resemblance to several other molecular and pathological features of AD. In the present experiments in vivo (1)H MR Spectroscopy with short echo time (3 ms) was used to non-invasively obtain a neurochemical profile of rat brains, 3 weeks and 2 months after double icv injections of STZ or vehicle. Seventeen metabolites were quantified from 27 microL tissue volume which included hippocampus and a part of cerebral cortex, using the LCModel and unsuppressed water signal as an internal reference. Three weeks after icv STZ several metabolites were significantly decreased, the most prominent changes noted in glycerophosphocholine and phosphocholine (-38 +/- 5%), glutathione (-37 +/- 4%), taurine (-30 +/- 19%), glutamate (-26 +/- 14%), phosphocreatine (-23 +/- 15%) and N-acetylaspartate (-16 +/- 6%). On the contrary, the concentration of N-acetylaspartylglutamate was found significantly increased (+38 +/- 18%). After 2 months some of these changes were even more pronounced. We conclude that in vivo (1)H MRS of rat brain following icv STZ injections provides a new input into a better understanding of the critical dependency of neural function and structure on brain glucose consumption, and may be of relevance in further studies of AD pathomechanism.

Development of a model of recurrent stroke consisting of a mild transient stroke followed by a second moderate stroke in rats.

Recurrent stroke often consists of a transient ischemic attack or mild stroke followed by a moderate stroke. Lacking is knowledge of the mechanisms of interaction of such multiple ischemic insults. Our aim was to develop a rat model of recurrent stroke and to test whether such multiple insults would enhance brain injury. A mild focal ischemic insult was produced by transient (40min) occlusion of the middle cerebral artery (MCAO) and this resulted in scattered necrosis and areas of increased labeling of astrocytes with glial fibrillary acidic protein. Additional animals were subjected to a moderate stroke alone or a recurrent stroke-a mild stroke followed 3 days later by a moderate stroke (60min MCAO). Damage was dependent on the proximal or distal cerebral cortical location from the occlusion (P<0.007) and the type of stroke insult (mild, moderate or recurrent, P<0.002). Following recurrent stroke, the cumulative injury score was similar to a mild stroke in distal parietal cortex but enhanced proximally. Recurrent stroke also resulted in changes in magnetic resonance imaging T(2), in neuronal microtubule associated protein2, in reactive astrocytes and in microglia/macrophages that were enhanced in proximal but not distal parietal cortex. This model demonstrates that when a minor stroke is combined with a second stroke, both distributed within the same middle cerebral artery territory, there are different injury processes regionally. Proximally, damage exceeds that of the first insult whereas distally the response is consistent with a tolerance to the second insult.

Simultaneous functional magnetic resonance imaging in the rat spinal cord and brain.

Functional magnetic resonance imaging (fMRI) method was developed to investigate the pattern and temporal relationship in neuronal pathways of brain and spinal cord. Signal intensity changes correlating with stimulation patterns were observed simultaneously in the rat spinal cord and brain using fMRI at 9.4 T. Electrical stimulation of the forepaw was used to elicit activity. A quadrature volume RF coil covering both brain and the cervical spinal cord was used. Sets of fast spin echo (FSE) images were acquire simultaneously for both brain and spinal cord fMRI. Experiments were repeated in single animal and across animals. Activities within the dorsal horn of the spinal cord and within the somatosensory cortex were observed consistently within each animal as well as across animals.

Cerebral blood flow response to a hypoxic-ischemic insult differs in neonatal and juvenile rats.

To compare the cerebral blood flow (CBF) response to a transient episode of hypoxia-ischemia producing damage in neonatal and juvenile rats. One- and four-week-old rats were subjected to unilateral carotid artery occlusion plus hypoxia (8% oxygen). Perfusion MR images were acquired either in sham controls or in hypoxic-ischemic rats before, during, 1 h and 24 h after hypoxia-ischemia. At 24 h post hypoxia-ischemia, T2 maps and histology were used to assess damage. In sham controls, CBF increased twofold between the age of one and four weeks. Reductions in CBF ipsilateral to the occlusion occurred during hypoxia-ischemia followed by a substantial recovery at 1 h post in both age groups. However, contralaterally, hyperemia occurred during hypoxia-ischemia in four-week but not one-week-old rats. Similarly, hyperemia occurred ipsilaterally at 24 h post hypoxia-ischemia in four-week but not one-week-olds, corresponding to the distribution of elevations in T2. Despite CBF differences, extensive cell death occurred ipsilaterally in both age groups. The CBF responses to hypoxia-ischemia and reperfusion differ depending on postnatal age, with hyperemia occurring in juvenile but not neonatal rats. The results suggest a greater CBF responsiveness and differential relationship between post-ischemic vascular perfusion and tissue injury in older compared with immature animals.

Functional magnetic resonance imaging in rats subjected to intense electrical and noxious chemical stimulation of the forepaw.

We examined whether cerebral activation to two different intense and painful stimuli could be detected using functional magnetic resonance imaging (fMRI) in alpha-chloralose anesthetized rats. Experiments were performed using a 9.4 T magnet and a surface coil centered over the forebrain. A set of gradient echo images were acquired and analyzed using our software based on fuzzy cluster analysis (EvIdent). Following the injection of 50 microl of formalin (5%) into the forepaw we observed a regional increase in signal intensity in the MR images in all animals. Anterior cingulate cortex, frontal cortex and sensory-motor cortex were some of the regions that activated frequently and often bilaterally. Surprisingly, activation appeared sequentially, often occurring first in either the right or the left hemisphere with a separation of seconds to minutes between peak activations. Morphine pre-treatment (1 mg/kg, i. v.) delayed and/or reduced the intensity of the activation resulting in a decrease in the overall response. Following episodes of intense electrical stimulation, produced by two brief stimulations (15 V, 0. 3 ms, 3 Hz) of the forepaw, activation was observed consistently in the sensory-motor cortex contralateral to the stimulation. Activation also occurred frequently in the anterior cingulate cortex, ipsilateral sensory-motor cortex and frontal cortical regions. All these regions of activation were markedly reduced during nitrous oxide inhalation. Treatment with morphine resulted in an inhibition of the activation response to electrical stimulation in most regions except for sensory-motor cortex. Thus, electrical and chemical noxious stimuli activated regions that are known to be involved in the central processing of pain and morphine modified the activation observed. fMRI combined with appropriate exploratory data analysis tools could provide an effective new tool with which to study novel analgesics and their effects on the CNS processing of pain in animal models.