Halogenated volatile anesthetics alter brain metabolism as revealed by proton magnetic resonance spectroscopy of mice in vivo.

Halogenated volatile anesthetics (HVA) are widely used in medicine and research but their effects on brain metabolism in intact organisms are still largely unknown. Here, localized proton magnetic resonance spectroscopy (MRS) of anesthetized mice was applied to evaluate HVA effects on cerebral metabolites in vivo. Experimental protocols combined different concentrations of isoflurane, halothane, sevoflurane, and desflurane with known modulators of adrenergic, GABAergic, and glutamatergic neurotransmission. As a most striking finding, brain lactate increased in individual mice from 1.0 ± 0.6 mM (awake state) to 6.2 ± 1.5 mM (1.75% isoflurane). In addition, relative to total creatine, there were significant isoflurane-induced increases of alanine by 111%, GABA by 20%, choline-containing compounds by 20%, and myo-inositol by 10% which were accompanied by significant decreases of glucose by 51% and phosphocreatine by 9%. The elevation of lactate was most pronounced in the striatum. The HVA effects correlated with the respective minimal alveolar concentrations and were mostly reversible within minutes. The observed alterations are best explained by an HVA-induced stimulation of adrenergic pathways in conjunction with an inhibition of the respiratory chain. Apart from casting new light on cerebral energy metabolism, the present results challenge brain studies of HVA-anesthetized animals.

Assessment of lesion pathology in a new animal model of MS by multiparametric MRI and DTI.

Magnetic resonance imaging (MRI) is the gold standard for the detection of multiple sclerosis (MS) lesions. However, current MRI techniques provide little information about the structural features of a brain lesion with inflammatory cell infiltration, demyelination, gliosis, acute axonal damage and axonal loss. To identify methods for a differentiation of demyelination, inflammation, and axonal damage we developed a novel mouse model combining cuprizone-induced demyelination and experimental autoimmune encephalomyelitis. MS-like brain lesions were assessed by T1-weighted, T2-weighted, and magnetization transfer MRI as well as by diffusion tensor imaging (DTI). T2-weighted MRI differentiated control and diseased mice, while T1-weighted MRI better reflected the extent of inflammation and axonal damage. In DTI, axonal damage and cellular infiltration led to a reduction of the axial diffusivity, whereas primary demyelination after cuprizone treatment was reflected by changes in radial but not axial diffusivity. Importantly, alterations in radial diffusivity were less pronounced in mice with demyelination, inflammation, and acute axonal damage, indicating that radial diffusivity may underestimate demyelination in acute MS lesions. In conclusion, the combined information from different DTI parameters allows for a more precise identification of solely demyelinated lesions versus demyelinated and acutely inflamed lesions. These findings are of relevance for offering individualized, stage-adapted therapies for MS patients.

In vivo MRI of altered brain anatomy and fiber connectivity in adult pax6 deficient mice.

The impact of developmental ablation of Pax6 function on morphology and functional connectivity of the adult cerebrum was studied in cortex-specific Pax6 knockout mice (Pax6cKO) using structural magnetic resonance imaging (MRI), manganese-enhanced MRI, and diffusion tensor MRI in conjunction with fiber tractography. Mutants presented with decreased volumes of total brain and olfactory bulb, reduced cortical thickness, and altered layering of the piriform cortex. Tracking of major neuronal fiber bundles revealed a disorganization of callosal fibers with an almost complete lack of interhemispheric connectivity. In Pax6cKO mice intrahemispheric callosal fibers as well as intracortical fibers were predominantly directed along a rostrocaudal orientation instead of a left-right and dorsoventral orientation found in controls. Fiber disorganization also involved the septohippocampal connection targeting mostly the lateral septal nucleus. The hippocampus was rostrally extended and its volume was increased relative to that of the forebrain and midbrain. Manganese-induced MRI signal enhancement in the CA3 region suggested a normal function of hippocampal pyramidal cells. Noteworthy, several morphologic disturbances in gray and white matter of Pax6cKO mice were similar to observations in human aniridia patients. The present findings indicate an important role of Pax6 in the development of both the cortex and cerebral fiber connectivity.

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.

Rhesus monkey and human share a similar topography of the corpus callosum as revealed by diffusion tensor MRI in vivo.

A recent study of the corpus callosum (CC) in humans revealed a new topographical arrangement of the cortical connectivity pattern. To explore the CC topography in nonhuman primates, we applied magnetic resonance diffusion tensor imaging and tract tracing techniques in individual rhesus monkeys in vivo. The results demonstrate that the CC topography of primates and humans is surprisingly similar. In particular, the relatively large representation and caudal extension of commissural frontal fibers in the CC is observed in both the monkey and human brain. If evolutionary changes in relative brain volumes are reflected in the arrangement of related fibers crossing the CC, the current study is in line with the fact that the relative volume of the frontal lobe did not significantly increase after the split of the hominid line from other primates.

A remotely controlled lightweight MRI compatible ultrasonic actuator for micrometer positioning of electrodes during neuroethological primate research.

The precise positioning of microelectrodes is essential for a reliable electrophysiological exploration of anatomical structures in the brain of laboratory animals, e.g., non-human primates in systemic brain research. Despite recent advances in micromechanics, the majority of small, chronically head mounted devices for advancing and retracting electrodes in freely moving animals reported in the literature are manually operated. In this article, we present a newly developed lightweight microfeed, based on an ultrasonic actuator for micrometer positioning of recording microelectrodes. It has been designed for compatibility with magnetic resonance imaging to allow non-invasive visualization of chronically implanted electrodes. The actuator combines a teleoperation via infrared control to minimize manipulation of animals during neuroethological studies. Its design is believed to add substantially to the well-being of experimental animals.

Telemetrically recorded neuronal activity in the inferior colliculus and bordering tegmentum during vocal communication in squirrel monkeys (Saimiri sciureus).

In order to find out whether the inferior colliculus, in addition to its auditory decoding function, also has an auditory gating function in the sense that it treats self-produced sounds differently from external ones, we have explored the inferior colliculus and bordering tegmentum for neurones reacting differently to self-produced vocalizations and vocalizations produced by conspecifics. The experiments were made in the squirrel monkey (Saimiri sciureus), using a telemetric extracellular recording technique which allowed to register neuronal activity in freely moving animals during natural vocal communication. The results show that the neurones of the central nucleus of the inferior colliculus do not react differently to self-produced and group mate vocalizations of the same type. In the external nucleus of the inferior colliculus, in addition to classical auditory neurones, neurones are found which react to the vocalizations of group mates, but not to self-produced vocalizations. In the paralemniscal area just below the inferior colliculus, there are neurones which are active during self-produced vocalization, but not during vocalization produced by other animals. The results suggest that the external nucleus of the inferior colliculus and bordering tegmentum are involved in vocalization-dependent auditory gating processes.

Assessment of disintegrant efficacy with fractal dimensions from real-time MRI.

An efficient disintegrant is capable of breaking up a tablet in the smallest possible particles in the shortest time. Until now, comparative data on the efficacy of different disintegrants is based on dissolution studies or the disintegration time. Extending these approaches, this study introduces a method, which defines the evolution of fractal dimensions of tablets as surrogate parameter for the available surface area. Fractal dimensions are a measure for the tortuosity of a line, in this case the upper surface of a disintegrating tablet. High-resolution real-time MRI was used to record videos of disintegrating tablets. The acquired video images were processed to depict the upper surface of the tablets and a box-counting algorithm was used to estimate the fractal dimensions. The influence of six different disintegrants, of different relative tablet density, and increasing disintegrant concentration was investigated to evaluate the performance of the novel method. Changing relative densities hardly affect the progression of fractal dimensions, whereas an increase in disintegrant concentration causes increasing fractal dimensions during disintegration, which are also reached quicker. Different disintegrants display only minor differences in the maximal fractal dimension, yet the kinetic in which the maximum is reached allows a differentiation and classification of disintegrants.

Tablet disintegration studied by high-resolution real-time magnetic resonance imaging.

The present work employs recent advances in high-resolution real-time magnetic resonance imaging (MRI) to investigate the disintegration process of tablets containing disintegrants. A temporal resolution of 75 ms and a spatial resolution of 80 × 80 µm with a section thickness of only 600 µm were achieved. The histograms of MRI videos were quantitatively analyzed with MATLAB. The mechanisms of action of six commercially available disintegrants, the influence of relative tablet density, and the impact of disintegrant concentration were examined. Crospovidone seems to be the only disintegrant acting by a shape memory effect, whereas the others mainly swell. A higher relative density of tablets containing croscarmellose sodium leads to a more even distribution of water within the tablet matrix but hardly impacts the disintegration kinetics. Increasing the polacrilin potassium disintegrant concentration leads to a quicker and more thorough disintegration process. Real-time MRI emerges as valuable tool to visualize and investigate the process of tablet disintegration.

Chromium(VI) as a novel MRI contrast agent for cerebral white matter: preliminary results in mouse brain in vivo.

This work demonstrates that intraventricular microinjections of a low dose of potassium dichromate (0.4 microL of 10 mM solution) yield a specific contrast enhancement of white matter (WM) tracts in T1-weighted 3D MRI of mouse brain in vivo. Pronounced and persistent signal increases (40-100% at 24 hr after injection) were observed in the corpus callosum, anterior commissure, fornix, and stria medullaris, as well as in the mammillothalamic tract and fasciculus retroflexus. These results suggest that the extracellular diffusion of diamagnetic chromium(VI) (Cr(VI)) after injection is followed by a tissue-specific reduction to paramagnetic Cr(V) and (III), which relies predominantly on the oxidation of myelin lipids. Because Cr(VI)-induced contrast leads to only a mild unspecific enhancement (10-20%) of gray matter (GM) structures, such as the hippocampal formation, the method reveals novel information that differs from that obtainable using other paramagnetic ions, such as manganese.

Monitoring of EAE onset and progression in the common marmoset monkey by sequential high-resolution 3D MRI.

Experimental autoimmune encephalomyelitis (EAE) induced by myelin-oligodendrocyte glycoprotein (MOG) in common marmosets (Callithrix jacchus) is a model for multiple sclerosis. Here, EAE was induced in four common marmosets by 250-300 microg recombinant rat MOG. In addition to a detailed disability scoring, T2- and T1-weighted high-resolution 3D MRI was performed to assess the onset and development of cerebral lesions. The findings were confirmed by histopathology in all animals. Although the animals exhibited a large heterogeneity with regard to onset and localization of lesions and also to disease duration and severity of disability signs, none of the animals revealed any evidence of recovery. A specification of the disability scoring system to account for different aspects of the disease led to a good concurrence of the first MRI-detectable lesion and the onset of central nervous system (CNS) symptoms. The results suggest that MRI monitoring of white matter lesions in conjunction with disability scores that focus on CNS symptoms may be a suitable method to evaluate novel therapeutic interventions even in the presence of pronounced interindividual heterogeneity.