Compensatory functional connectome changes in a rat model of traumatic brain injury

Penetrating cortical impact injuries alter neuronal communication beyond the injury epicentre, across regions involved in affective, sensorimotor and cognitive processing. Understanding how traumatic brain injury reorganizes local and brain wide nodal interactions may provide valuable quantitative parameters for monitoring pathological progression and recovery. To this end, we investigated spontaneous fluctuations in the functional MRI signal obtained at 11.1 T in rats sustaining controlled cortical impact and imaged at 2- and 30-days post-injury. Graph theory-based calculations were applied to weighted undirected matrices constructed from 12 879 pairwise correlations between functional MRI signals from 162 regions. Our data indicate that on Days 2 and 30 post-controlled cortical impact there is a significant increase in connectivity strength in nodes located in contralesional cortical, thalamic and basal forebrain areas. Rats imaged on Day 2 post-injury had significantly greater network modularity than controls, with influential nodes (with high eigenvector centrality) contained within the contralesional module and participating less in cross-modular interactions. By Day 30, modularity and cross-modular interactions recover, although a cluster of nodes with low strength and low eigenvector centrality remain in the ipsilateral cortex. Our results suggest that changes in node strength, modularity, eigenvector centrality and participation coefficient track early and late traumatic brain injury effects on brain functional connectivity. We propose that the observed compensatory functional connectivity reorganization in response to controlled cortical impact may be unfavourable to brain wide communication in the early post-injury period.

NMR spectroscopy of single neurons

The first spatially localized NMR spectra of osmolytes and metabolites from single isolated neurons have been obtained using a combination of high magnetic field strengths and NMR radio frequency (RF) microcoils. The proton spectra display peaks at high concentrations (100-300 mM) assigned to betaine and choline, and other metabolite resonances including lactate at lower concentrations in the order of 10s of millimoles. The volumes examined were approximately 10 nl, over two orders of magnitude less than previously possible. In these initial experiments; the cells were unperfused and the signal intensities of the osmolytes decrease with time, a phenomenon consistent with cell swelling. This work demonstrates the technical feasibility of NMR spectroscopy of single cells, further broadening the scope of NMR spectroscopy of living tissues from application to entire living organisms (man and animal models) and isolated tissues (perfused organs and cultured assemblies of cells) and now to single cells. Magn Reson Med 44:19-22, 2000.

The effect of ouabain on water diffusion in the rat hippocampal slice measured by high resolution NMR imaging

High resolution NMR imaging of the isolated perfused rat hippocampal slice was used to quantitate ADC changes following ouabain-induced cell swelling. Hippocampal slices were studied in artificial cerebrospinal fluid and then in ouabain using a 600-MHz narrow bore spectrometer and a home-built perfusion chamber. The brain slices demonstrated biexponential diffusion behavior. After perfusion with 1 mMouabain, there was an increase in the fraction of slowly diffusing water. The ADCs of the two fractions did not change. These data support the hypothesis that the decrease in the ADC of brain water following an ischemic attack is caused by cell swelling. The relative amplitudes of the two diffusing fractions do not match the expected ratio of intracellular and extracellular fractions. This discrepancy may be principally due to the difference in T2 relaxation rates of the two compartments.

MR microscopy of perfused brain slices

To study the origins of signal changes in clinical MRI we have previously studied isolated single neuronal cells by MR microscopy. To account for the extracellular environment of the cells, we have developed a prototype perfusion chamber for MR microimaging of perfused rat hippocampal brain slices. To demonstrate the utility of this model, brain slices were initially perfused in isotonic solutions and then subjected to osmotic perturbations via perfusate exchange with 20% hypertonic and 20% hypotonic solutions. In diffusion weighted images, signal intensity changes of +16(sigma(n-1) = 11)% (hypotonic) and -26(sigma(n-1) = 10)% (hypertonic) were observed. No significant variation in response was observed across the slice when several subregions were examined. These observations are consistent with the view that contrast changes are driven primarily by changes in the intra- and extracellular compartmentation of water. This is the first report of MR microimaging of the isolated brain slice. The technique will enable the correlation of MR microimaging measurements with microscopic changes using other modalities and techniques to provide a better understanding of signals in clinical MRI.

Neurochemical changes in the cerebral cortex of treated and untreated hydrocephalic rat pups quantified with in vitro 1H-NMR spectroscopy

The pathophysiology of infantile hydrocephalus is poorly understood, and shunt treatment does not always lead to a normal neurological outcome. To investigate some of the neurochemical changes in infantile hydrocephalus and the response to shunt treatment, we have used high-resolution 1H-NMR spectroscopy to analyze extracts of cerebral cortex from H-Tx rats, which have inherited hydrocephalus with an onset in late gestation. Hydrocephalic rats and rats with shunts placed at either 4 or 12 days after birth were studied at 21 days after birth, together with age-matched control littermates. In hydrocephalic rats there was a 46-62% reduction in the following compounds: myo-inositol, creatine, choline-containing compounds, N-acetyl aspartate, taurine, glutamine, glutamate, aspartate, and alanine. Phosphocreatine, glycine, GABA, and lactate were also reduced but not significantly. These changes are consistent with neuronal atrophy rather than ischemic damage. In hydrocephalic rats that received shunt treatment at 4 days, there were no significant reductions in any chemicals, indicating a normal complement of neurons. However, some compounds, particularly taurine, were elevated above control. After treatment at 12 days, N-acetyl aspartate and aspartate remained significantly reduced, suggesting continued neuronal deficiency.

Metabolite changes in the cerebral cortex of treated and untreated infant hydrocephalic rats studied using in vitro 31P-NMR spectroscopy

The effect of hydrocephalus on cerebral energy metabolites and on intermediates of membrane phospholipid metabolism has been studied in H-Tx rats with inherited infantile hydrocephalus. Hydrocephalic rats and rats with shunts placed at 4-5 days or at 10 days after birth were subjected to magnetic resonance imaging in vivo before 21 days of age to determine the dimensions of the ventricles and cortex. At 21 days, the brains from the three groups of rats, together with age-matched control littermates, were frozen in situ, and chloroform/methanol extracts of cerebral cortex were prepared for high-resolution 31P-NMR spectroscopy. Hydrocephalus resulted in modest decreases in most metabolites quantified. Levels of phosphocreatine, ATP, and diphosphodiesters plus NAD were significantly reduced by 23-32%, and inorganic phosphate content was reduced but not significantly. Levels of the membrane phospholipid intermediates phosphorylethanolamine, glycerophosphorylethanolamine, and glycerophosphorylcholine were also significantly reduced by 30-33%, indicating changes in membrane metabolism. These general decreases are consistent with a loss of cell contents, possibly due to changes in dendrite structure in hydrocephalus. Rats shunt-treated at 4-5 days were similar to control rats for all energy metabolites, but those treated later at 10 days had reduced phosphocreatine and ATP levels. Shunt-treated rats also had reductions in levels of membrane phospholipids, some of which occurred in sham-operated rats. It is concluded that hydrocephalus leads to reductions in levels of energy metabolites and in levels of membrane phospholipids and that the changes in energy metabolites can be reversed by early, but not by later, shunt treatment.

Metabolism of 17alpha-ethynylestradiol by intact liver parenchymal cells isolated from mouse and rat

Liver parenchymal cells isolated by perfusion from female C3H/HeN-MTV+Nctr mice and Sprague-Dawley rats were incubated with [6,7-3H] 17alpha-ethynylestradiol (EE2). The incubates were individually fractionated into free steroid (organic phase), steroid conjugates (aqueous), and bound steroids (macromolecular pellet). The rat had significantly less total free radioactive steroid but significantly more total conjugated and irreversibly bound radioactivity than the mouse. However, when the metabolic conversion of EE2 was compared in the rat and the mouse on a cellular basis (metabolic clearance per 10(6) cells), the rat was found to be less efficient than the mouse. The two species were essentially equivalent in their covalent binding when expressed on a per 10(6) cell basis. Purification of the free radiolabeled steriods on LH-20 demonstrated the mouse to have the parent compound and on identifiable 2-OH-EE2 fraction. The rat had EE2 and an identifiable 2-methoxy-EE2 fraction. A major metabolite fraction for both species was very nonpolar and, although not identified, was found to be ethynylated as demonstrated by silver-sulfoethylcellulose chromatography. The conjugate fractions of the mouse were indicative of glucuronide conjugation, whereas the rat had additional conjugate fractions suggestive of sulfoconjugation.