A Novel Approach for Studying the Physiology and Pathophysiology of Myelinated and Non-Myelinated Axons in the CNS White Matter

Advances in brain connectomics set the need for detailed knowledge of functional properties of myelinated and non-myelinated (if present) axons in specific white matter pathways. The corpus callosum (CC), a major white matter structure interconnecting brain hemispheres, is extensively used for studying CNS axonal function. Unlike another widely used CNS white matter preparation, the optic nerve where all axons are myelinated, the CC contains also a large population of non-myelinated axons, making it particularly useful for studying both types of axons. Electrophysiological studies of optic nerve use suction electrodes on nerve ends to stimulate and record compound action potentials (CAPs) that adequately represent its axonal population, whereas CC studies use microelectrodes (MEs), recording from a limited area within the CC. Here we introduce a novel robust isolated "whole" CC preparation comparable to optic nerve. Unlike ME recordings where the CC CAP peaks representing myelinated and non-myelinated axons vary broadly in size, "whole" CC CAPs show stable reproducible ratios of these two main peaks, and also reveal a third peak, suggesting a distinct group of smaller caliber non-myelinated axons. We provide detailed characterization of "whole" CC CAPs and conduction velocities of myelinated and non-myelinated axons along the rostro-caudal axis of CC body and show advantages of this preparation for comparing axonal function in wild type and dysmyelinated shiverer mice, studying the effects of temperature dependence, bath-applied drugs and ischemia modeled by oxygen-glucose deprivation. Due to the isolation from gray matter, our approach allows for studying CC axonal function without possible "contamination" by reverberating signals from gray matter. Our analysis of "whole" CC CAPs revealed higher complexity of myelinated and non-myelinated axonal populations, not noticed earlier. This preparation may have a broad range of applications as a robust model for studying myelinated and non-myelinated axons of the CNS in various experimental models.     

Is astrocyte laminin involved in axon guidance in the mammalian CNS?

This paper provides evidence for the expression of laminin on glia in correlation with axon elongation and nerve pathway formation during embryonic development of the mouse optic nerve and other parts of the central nervous system (CNS). We show that punctate deposits of laminin on immature glial cells precede the entrance of the first optic axons into the nerve, and remain in close association with growing axons. Furthermore, we show that in one particular region of the optic pathway that the retinal ganglion cell axons avoid in normal animals (i.e., the pigmented area of the distal nerve) the punctate laminin matrix is missing. As the optic nerve matures punctate laminin deposits disappear, and laminin is reduced in the astroglial cytoplasm. The close correlation of the punctate form of laminin with early axonal growth is true not only in the optic nerve but also in some other parts of the CNS. We demonstrate such punctate laminin deposits in a model of astrocyte-induced regeneration of the corpus callosum in acallosal mice (G. Smith, R. Miller, and J. Silver, 1986, J. Comp. Neurol. 251, 23-43), and in glia associated with several normal developing axon trajectories, such as the corpus callosum, fornix, and pathways in the embryonic hindbrain. In all of these regions punctate laminin deposits are found on astroglia that are associated with early growing axons. Our results indicate that the punctate form of laminin, produced by astrocytes, may be an important factor involved in axon elongation and nerve pathway formation in the mammalian CNS.     

TRANS-SULPHURATION IN PRIMATE BRAIN: REGIONAL DISTRIBUTION OF CYSTATHIONINE SYNTHASE, CYSTATHIONINE AND TAURINE IN THE BRAIN OF THE RHESUS MONKEY AT VARIOUS STAGES OF DEVELOPMENT

—The regional distributions of cystathionine synthase, cystathionine and taurine in the brain of the Rhesus monkey were determined at various stages of foetal and postnatal development. Activity of cystathionine synthase was highest in cerebellum, cortical grey areas and globus pallidus, and lowest in subcortical white matter and corpus callosum. There was no marked change in activity in any area during development from the first-trimester foetus to the juvenile animal. In the brain of the juvenile monkey concentrations of cystathionine were highest in subcortical white matter, corpus callosum, and globus pallidus, and lowest in cortical grey matter. There was a sharp increase in concentration between late foetal life and the first 2 weeks of postnatal life and a subsequent more gradual increase during the next 2 years. Concentrations of taurine were highest in lateral cerebellum and neostriatum and lowest in brain stem areas and spinal cord. During the first 6 months of postnatal life, there was a marked decrease in concentration as the brain matured. The regional distribution of cystathionine in brain suggests that this compound may be synthesized in the perikaryon of the nerve cell and transported down axons into white matter. The changes during development suggest the further possibility that cystathionine may have some relationship to myelin and/or myelination.     

Glutamate transport,glutamine synthetase and phosphate-activated glutaminase in rat CNS white matter. A quantitative study

Glutamatergic signal transduction occurs in CNS white matter, but quantitative data on glutamate uptake and metabolism are lacking. We report that the level of the astrocytic glutamate transporter GLT in rat fimbria and corpus callosum was approximately 35% of that in parietal cortex; uptake of [3H]glutamate was 24 and 43%, respectively, of the cortical value. In fimbria and corpus callosum levels of synaptic proteins, synapsin I and synaptophysin were 15-20% of those in cortex; the activities of glutamine synthetase and phosphate-activated glutaminase, enzymes involved in metabolism of transmitter glutamate, were 11-25% of cortical values, and activities of aspartate and alanine aminotransferases were 50-70% of cortical values. The glutamate level in fimbria and corpus callosum was 5-6 nmol/mg tissue, half the cortical value. These data suggest a certain capacity for glutamatergic neurotransmission. In optic and trigeminal nerves, [3H]glutamate uptake was < 10% of the cortical uptake. Formation of [14C]glutamate from [U-14C]glucose in fimbria and corpus callosum of awake rats was 30% of cortical values, in optic nerve it was 13%, illustrating extensive glutamate metabolism in white matter in vivo. Glutamate transporters in brain white matter may be important both physiologically and during energy failure when reversal of glutamate uptake may contribute to excitotoxicity.     

Developmental and regional distribution of aspartoacylase in rat brain tissue

The function of N-acetyl-aspartate (NAA), a predominant molecule in the brain, has not yet been determined. However, NAA is commonly used as a putative marker of viable neurones. To investigate the possible function of NAA, we determined the anatomical, developmental and cellular distribution of aspartoacylase, which catalyses the hydrolysis of NAA. Levels of aspartoacylase activity were measured during postnatal development in several brain regions. The differential distribution of aspartoacylase activity in purified populations of cells derived from the rat CNS was also investigated. The developmental and anatomical distribution of aspartoacylase correlated with the maturation of white matter tracts in the rat brain. Activity increased markedly after 7 days and coincided with the time course for the onset of myelination in the rat brain. Gray matter showed little activity or developmental trend. There was a 60-fold excess in optic nerve (a white matter tract) when compared with cortex at 21 days of development. In the adult brain there was a 18-fold difference in corpus callosum compared with cortex (stripped of corpus callosum). Cellular studies demonstrated that purified cortical neurons and cerebellar granular neurones have no activity. Primary O-2A progenitor cells had moderate activity, with three-fold higher activity in immature oligodendrocyte and 13-fold increase in mature oligodendrocytes (myelinating cells of the CNS). The highest activity was seen in type-2 astrocytes (20-fold difference compared with O-2A progenitors) derived from the same source. Aspartoacylase activity increased with time in freshly isolated astrocytes, with significantly higher activity after 15 days in culture. We conclude that aspartoacylase activity in the developing postnatal brain corresponds with maturation of myelination, and that the cellular distribution is limited to glial cells.     

Age-related white matter microstructural differences partly mediate age-related decline in processing speed but not cognition

Aging is associated with declining cognitive performance as well as structural changes in brain gray and white matter (WM). The WM deterioration contributes to a disconnection among distributed brain networks and may thus mediate age-related cognitive decline. The present diffusion tensor imaging (DTI) study investigated age-related differences in WM microstructure and their relation to cognition (episodic memory, visuospatial processing, fluency, and speed) in a large group of healthy subjects (n = 287) covering 6 decades of the human life span. Age related decreases in fractional anisotropy (FA) and increases in mean diffusivity (MD) were observed across the entire WM skeleton as well as in specific WM tracts, supporting the WM degeneration hypothesis. The anterior section of the corpus callosum was more susceptible to aging compared to the posterior section, lending support to the anterior-posterior gradient of WM integrity in the corpus callosum. Finally, and of critical interest, WM integrity differences were found to mediate age-related reductions in processing speed but no significant mediation was found for episodic memory, visuospatial ability, or fluency. These findings suggest that compromised WM integrity is not a major contributing factor to declining cognitive performance in normal aging. This article is part of a Special Issue entitled: Imaging Brain Aging and Neurodegenerative disease.     

Astrocytes as gate-keepers in optic nerve regeneration--a mini-review

Animals that develop without extra-embryonic membranes (anamniotes--fish, amphibians) have impressive regenerative capacity, even to the extent of replacing entire limbs. In contrast, animals that develop within extra-embryonic membranes (amniotes--reptiles, birds, mammals) have limited capacity for regeneration as adults, particularly in the central nervous system (CNS). Much is known about the process of nerve development in fish and mammals and about regeneration after lesions in the CNS in fish and mammals. Because the retina of the eye and optic nerve are functionally part of the brain and are accessible in fish, frogs, and mice, optic nerve lesion and regeneration (ONR) has been extensively used as a model system for study of CNS nerve regeneration. When the optic nerve of a mouse is severed, the axons leading into the brain degenerate. Initially, the cut end of the axons on the proximal, eye-side of the injury sprout neurites which begin to grow into the lesion. Simultaneously, astrocytes of the optic nerve become activated to initiate wound repair as a first step in reestablishing the structural integrity of the optic nerve. This activation appears to initiate a cascade of molecular signals resulting in apoptotic cell death of the retinal ganglion cells axons of which make up the neural component of the optic nerve; regeneration fails and the injury is permanent. Evidence specifically implicating astrocytes comes from studies showing selective poisoning of astrocytes at the optic nerve lesion, along with activation of a gene whose product blocks apoptosis in retinal ganglion cells, creates conditions favorable to neurites sprouting from the cut proximal stump, growing through the lesion and into the distal portion of the injured nerve, eventually reaching appropriate targets in the brain. In anamniotes, astrocytes ostensibly present no such obstacle since optic nerve regeneration occurs without intervention; however, no systematic study of glial involvement has been done. In fish, vigorously growing neurites sprout from the cut axons and within a few days begin to re-enervate the brain. This review offers a new perspective on the role of glia, particularly astrocytes, as "gate-keepers;" i.e., as being permissive or inhibitory, by comparison between fish and mammals of glial function during ONR.     

Schwann cells in the regenerating fish optic nerve: Evidence that CNS axons,not the glia,determine when myelin formation begins

Fish optic nerve fibres quickly regenerate after injury, but the onset of remyelination is delayed until they reach the brain. This recapitulates the timetable of CNS myelinogenesis during development in vertebrate animals generally, and we have used the regenerating fish optic nerve to obtain evidence that it is the axons, not the myelinating glial cells, that determine when myelin formation begins. In fish, the site of an optic nerve injury becomes remyelinated by ectopic Schwann cells of unknown origin. We allowed these cells to become established and then used them as reporters to indicate the time course of pro-myelin signalling during a further round of axonal outgrowth following a second upstream lesion. Unlike in the mammalian PNS, the ectopic Schwann cells failed to respond to axotomy and to the initial outgrowth of new optic axons. They only began to divide after the axons had reached the brain. Shortly afterwards, small numbers of Schwann cells began to leave the dividing pool and form myelin sheaths. More followed gradually, so that by 3 months remyelination was almost completed and few dividing cells were left. Moreover, remyelination occurred synchronously throughout the optic nerve, with the same time course in the pre-existing Schwann cells, the new ones that colonised the second injury, and the CNS oligodendrocytes elsewhere. The optic axons are the only common structures that could synchronise myelin formation in these disparate glial populations. The responses of the ectopic Schwann cells suggest that they are controlled by the regenerating optic axons in two consecutive steps. First, they begin to proliferate when the growing axons reach the brain. Second, they leave the cell cycle to differentiate individually at widely different times during the ensuing 2 months, during the critical period when the initial rough pattern of axon terminals in the optic tectum becomes refined into an accurate map. We suggest that each axon signals individually for myelin ensheathment once it completes this process.     

White matter atrophy and cognitive dysfunctions in neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory disease of central nervous system characterized by optic neuritis and longitudinally extensive acute transverse myelitis. NMO patients have cognitive dysfunctions but other clinical symptoms of brain origin are rare. In the present study, we aimed to investigate cognitive functions and brain volume in NMO. The study population consisted of 28 patients with NMO and 28 healthy control subjects matched for age, sex and educational level. We applied a French translation of the Brief Repeatable Battery (BRB-N) to the NMO patients. Using SIENAx for global brain volume (Grey Matter, GM; White Matter, WM; and whole brain) and VBM for focal brain volume (GM and WM), NMO patients and controls were compared. Voxel-level correlations between diminished brain concentration and cognitive performance for each tests were performed. Focal and global brain volume of NMO patients with and without cognitive impairment were also compared. Fifteen NMO patients (54%) had cognitive impairment with memory, executive function, attention and speed of information processing deficits. Global and focal brain atrophy of WM but not Grey Matter (GM) was found in the NMO patients group. The focal WM atrophy included the optic chiasm, pons, cerebellum, the corpus callosum and parts of the frontal, temporal and parietal lobes, including superior longitudinal fascicle. Visual memory, verbal memory, speed of information processing, short-term memory and executive functions were correlated to focal WM volumes. The comparison of patients with, to patients without cognitive impairment showed a clear decrease of global and focal WM, including brainstem, corticospinal tracts, corpus callosum but also superior and inferior longitudinal fascicles. Cognitive impairment in NMO patients is correlated to the decreased of global and focal WM volume of the brain. Further studies are needed to better understand the precise origin of cognitive impairment in NMO patients, particularly in the WM.     

Differential distribution of the 5-alpha-reductase in the central nervous system of the rat and the mouse: are the white matter structures of the brain target tissue for testosterone action?

In the brain of several animal species testosterone is converted into a series of 5-alpha-reduced metabolites, and especially into 17-beta-hydroxy-5-alpha-androstan-3-one (DHT), by the action of the enzyme 5-alpha-reductase. The formation of DHT has never been evaluated in the white matter structures of the brain, which are composed mainly of myelinated axons. The experiments here described were performed in order to study, in the rat and the mouse, the DHT forming activity of several white matter structures, in comparison with that of the cerebral cortex and of the hypothalamus. Two sampling techniques were used in the rat: microdissection under a stereo-microscope from frozen brain sections of fragments of corpus callosum, optic chiasm and cerebral cortex; fresh tissue macrodissection of subcortical white matter, cerebral cortex and hypothalamus. Only macrodissection was used in the mice. The data show that, independently from the sampling technique used, there are considerable quantitative differences in the distribution pattern of the 5-alpha-reductase activity within different brain structures. Both in the rat and in the mouse, the enzyme appears to be present in higher concentrations in the white matter structures, than in the cerebral cortex and in the hypothalamus. The present results clearly show that the subcortical white matter and the corpus callosum are at least three times as potent as the cerebral cortex in converting testosterone into DHT. An even higher 5-alpha-reductase activity has been found in the optic chiasm. Further work is needed in order to understand the possible physiological role of DHT formation in the white matter structures.     

Irradiation in vitro discriminates between different O-2A progenitor cell subpopulations in the perinatal central nervous system of rats

The effects of X irradiation on oligodendrocyte-type-2-astrocyte (O-2A) progenitor cells derived from different regions of the perinatal central nervous system (CNS) of rats were investigated in vitro. The O-2A progenitor cells can differentiate into either oligodendrocytes or type-2 astrocytes. The depletion of these cells could lead to demyelination, seen as a delayed reaction after irradiation of the CNS in vivo. To quantify cell survival, O-2A progenitor cells were grown on monolayers of type-1 astrocytes. Monolayers of type-1 astrocytes stimulate O-2A progenitor cells to divide. O-2A progenitor cells were irradiated in vitro and clonogenic cell survival was measured. The O-2A progenitor cells derived from perinatal optic nerve were quite radiosensitive in contrast to O-2A progenitor cells derived from perinatal spinal cord and perinatal corpus callosum. Furthermore, O-2A progenitor cells derived from the optic nerve formed smaller colonies, with most colonies showing early differentiation into oligodendrocytes. In contrast, more than half of the colonies derived from corpus callosum did not show any differentiation after 2 weeks in vitro and kept growing. These differences support the view that perinatal O-2A progenitor cells derived from the optic nerve are committed progenitor cells while the O-2A progenitor cells derived from the perinatal corpus callosum and the perinatal spinal cord have more stem cell properties.     

Compensatory neural reorganization in Tourette syndrome

Children with neurological disorders may follow unique developmental trajectories whereby they undergo compensatory neuroplastic changes in brain structure and function that help them gain control over their symptoms. We used behavioral and brain imaging techniques to investigate this conjecture in children with Tourette syndrome (TS). Using a behavioral task that induces high levels of intermanual conflict, we show that individuals with TS exhibit enhanced control of motor output. Then, using structural (diffusion-weighted imaging) brain imaging techniques, we demonstrate widespread differences in the white matter (WM) microstructure of the TS brain that include alterations in the corpus callosum and forceps minor (FM) WM that significantly predict tic severity in TS. Most importantly, we show that task performance for the TS group (but not for controls) is strongly predicted by the WM microstructure of the FM pathways that lead to the prefrontal cortex and by the functional magnetic resonance imaging blood oxygen level-dependent response in prefrontal areas connected by these tracts. These results provide evidence for compensatory brain reorganization that may underlie the increased self-regulation mechanisms that have been hypothesized to bring about the control of tics during adolescence.     

Reduced Ventral Cingulum Integrity and Increased Behavioral Problems in Children with Isolated Optic Nerve Hypoplasia and Mild to Moderate or No Visual Impairment

Objectives

To assess the prevalence of behavioral problems in children with isolated optic nerve hypoplasia, mild to moderate or no visual impairment, and no developmental delay. To identify white matter abnormalities that may provide neural correlates for any behavioral abnormalities identified.

Patients and Methods

Eleven children with isolated optic nerve hypoplasia (mean age 5.9 years) underwent behavioral assessment and brain diffusion tensor imaging, Twenty four controls with isolated short stature (mean age 6.4 years) underwent MRI, 11 of whom also completed behavioral assessments. Fractional anisotropy images were processed using tract-based spatial statistics. Partial correlation between ventral cingulum, corpus callosum and optic radiation fractional anisotropy, and child behavioral checklist scores (controlled for age at scan and sex) was performed.

Results

Children with optic nerve hypoplasia had significantly higher scores on the child behavioral checklist (p<0.05) than controls (4 had scores in the clinically significant range). Ventral cingulum, corpus callosum and optic radiation fractional anisotropy were significantly reduced in children with optic nerve hypoplasia. Right ventral cingulum fractional anisotropy correlated with total and externalising child behavioral checklist scores (r = −0.52, p<0.02, r = −0.46, p<0.049 respectively). There were no significant correlations between left ventral cingulum, corpus callosum or optic radiation fractional anisotropy and behavioral scores.

Conclusions

Our findings suggest that children with optic nerve hypoplasia and mild to moderate or no visual impairment require behavioral assessment to determine the presence of clinically significant behavioral problems. Reduced structural integrity of the ventral cingulum correlated with behavioral scores, suggesting that these white matter abnormalities may be clinically significant. The presence of reduced fractional anisotropy in the optic radiations of children with mild to moderate or no visual impairment raises questions as to the pathogenesis of these changes which will need to be addressed by future studies.     

Hypoxic damage to the periventricular white matter in neonatal brain: role of vascular endothelial growth factor, nitric oxide and excitotoxicity

The present study examined factors that may be involved in the development of hypoxic periventricular white matter damage in the neonatal brain. Wistar rats (1-day old) were subjected to hypoxia and the periventricular white matter (corpus callosum) was examined for the mRNA and protein expression of hypoxia-inducible factor-1alpha (HIF-1alpha), endothelial, neuronal and inducible nitric oxide synthase (eNOS, nNOS and iNOS), vascular endothelial growth factor (VEGF) and N-methyl-D-aspartate receptor subunit 1 (NMDAR1) between 3 h and 14 days after hypoxic exposure by real-time RT-PCR, western blotting and immunohistochemistry. Up-regulated mRNA and protein expression of HIF-1alpha, VEGF, NMDAR1, eNOS, nNOS and iNOS in corpus callosum was observed in response to hypoxia. NMDAR1 and iNOS expression was found in the activated microglial cells, whereas VEGF was localized to astrocytes. An enzyme immunoassay showed that the VEGF concentration in corpus callosum was significantly higher up to 7 days after hypoxic exposure. NO levels, measured by colorimetric assay, were also significantly higher in hypoxic rats up to 14 days after hypoxic exposure as compared with the controls. A large number of axons undergoing degeneration were observed between 3 h and 7 days after the hypoxic exposure at electron-microscopic level. Our findings point towards the involvement of excitotoxicity, VEGF and NO in periventricular white matter damage in response to hypoxia.     

White matter microstructure in bipolar disorder is influenced by the serotonin transporter gene polymorphism 5‐HTTLPR

Bipolar disorder (BD) is associated with signs of widespread disruption of white matter (WM) integrity. A polymorphism in the promoter of the serotonin transporter (5‐HTTLPR) influenced functional cortico‐limbic connectivity in healthy subjects and course of illness in BD, with the short (s) allele being associated with lower functional connectivity, and with earlier onset of illness and poor response to treatment. We tested the effects of 5‐HTTLPR on diffusion tensor imaging (DTI) measures of WM microstructure in 140 inpatients, affected by a major depressive episode in course of BD, of Italian descent. We used whole brain tract‐based spatial statistics in the WM skeleton with threshold‐free cluster enhancement of DTI measures of WM microstructure: axial, radial and mean diffusivity and fractional anisotropy. Compared with l/l homozygotes, 5‐HTTLPR*s carriers showed significantly increased radial and mean diffusivity in several brain WM tracts, including corpus callosum, cingulum bundle, uncinate fasciculus, corona radiata, thalamic radiation, inferior and superior longitudinal fasciculus and inferior fronto‐occipital fasciculus. An increase of mean and radial diffusivity, perpendicular to the main axis of the WM tract, is thought to signify increased space between fibers, thus suggesting demyelination or dysmyelination, or loss of bundle coherence. The effects of 5‐HTTLPR on the anomalous emotional processing in BD might be mediated by changes of WM microstructure in key WM tracts contributing to the functional integrity of the brain.     

Microstructure and Cerebral Blood Flow within White Matter of the Human Brain: A TBSS Analysis

Background

White matter (WM) fibers connect different brain regions and are critical for proper brain function. However, little is known about the cerebral blood flow in WM and its relation to WM microstructure. Recent improvements in measuring cerebral blood flow (CBF) by means of arterial spin labeling (ASL) suggest that the signal in white matter may be detected. Its implications for physiology needs to be extensively explored. For this purpose, CBF and its relation to anisotropic diffusion was analyzed across subjects on a voxel-wise basis with tract-based spatial statistics (TBSS) and also across white matter tracts within subjects.

Methods

Diffusion tensor imaging and ASL were acquired in 43 healthy subjects (mean age = 26.3 years).

Results

CBF in WM was observed to correlate positively with fractional anisotropy across subjects in parts of the splenium of corpus callosum, the right posterior thalamic radiation (including the optic radiation), the forceps major, the right inferior fronto-occipital fasciculus, the right inferior longitudinal fasciculus and the right superior longitudinal fasciculus. Furthermore, radial diffusivity correlated negatively with CBF across subjects in similar regions. Moreover, CBF and FA correlated positively across white matter tracts within subjects.

Conclusion

The currently observed findings on a macroscopic level might reflect the metabolic demand of white matter on a microscopic level involving myelination processes or axonal function. However, the exact underlying physiological mechanism of this relationship needs further evaluation.     

REGIONAL AND SUBCELLULAR DISTRIBUTION OF AMINOTRANSFERASES IN RAT BRAIN

Abstract— Aminotransferase activity was measured in various areas of the nervous system of the rat (cortical grey matter, midbrain, corpus callosum, spinal cord and sciatic nerve) and in subcellular fractions of rat brain (nuclei, mitochondria and cytosol). Activity was low or absent in the sciatic nerve relative to that in the other areas, with the exception of incubation of glutamate with oxaloacetate (25 per cent of the activity found in brain) and of asparagine with 2-oxoglutarate (65 per cent of the activity found in brain). The distribution of enzymic activity was not homogeneous; alanine-2-oxoglutarate aminotransferase was highest in cortical grey matter; leucine- and GABA-2-oxoglutarate aminotransferases were highest in midbrain. Incubation of phenylalanine or tyrosine with 2-oxoglutarate gave similar activities in grey matter and midbrain. Activity generally was higher in the grey matter than in corpus callosum or spinal cord. However, incubations of methionine with 2-oxoglutarate, or glutamine with glyoxylate, gave similar activities in the three areas studied from the brain, whereas incubations of glutamate with glyoxylate gave highest activity in the corpus callosum. Only incubations of asparagine with 2-oxoglutarate, and glutamate with glyoxylate, gave significant activity in the nuclear subcellular fraction. Aminotransferase activity of phenylalanine, tyrosine or GABA with 2-oxoglutarate, or ornithine or glutamine with glyoxylate, was localized to mitochondria. The remaining reactions studied (glutamate with oxaloacetate; leucine, alanine, methionine or asparagine with 2-oxoglutarate and glutamate with glyoxylate) demonstrated activity in both the mitochondrial fraction and the soluble supernatant fraction.     

Different Patterns of White Matter Degeneration Using Multiple Diffusion Indices and Volumetric Data in Mild Cognitive Impairment and Alzheimer Patients

Alzheimeŕs disease (AD) represents the most prevalent neurodegenerative disorder that causes cognitive decline in old age. In its early stages, AD is associated with microstructural abnormalities in white matter (WM). In the current study, multiple indices of diffusion tensor imaging (DTI) and brain volumetric measurements were employed to comprehensively investigate the landscape of AD pathology. The sample comprised 58 individuals including cognitively normal subjects (controls), amnestic mild cognitive impairment (MCI) and AD patients. Relative to controls, both MCI and AD subjects showed widespread changes of anisotropic fraction (FA) in the corpus callosum, cingulate and uncinate fasciculus. Mean diffusivity and radial changes were also observed in AD patients in comparison with controls. After controlling for the gray matter atrophy the number of regions of significantly lower FA in AD patients relative to controls was decreased; nonetheless, unique areas of microstructural damage remained, e.g., the corpus callosum and uncinate fasciculus. Despite sample size limitations, the current results suggest that a combination of secondary and primary degeneration occurrs in MCI and AD, although the secondary degeneration appears to have a more critical role during the stages of disease involving dementia.     

Transient presence and functional interaction of endogenous GABA and GABAA receptors in developing rat optic nerve.

GABA (gamma-aminobutyric acid) is a major inhibitory synaptic neurotransmitter with widespread distribution in the central nervous system (CNS). GABA can also modulate axonal excitability by activation of GABAA receptors in CNS white matter regions where synapses and neuronal cell bodies are not present. Studies on cultured glia cells have revealed the synthesis of GABA in rat optic nerve O-2A progenitor cells that give rise to oligodendrocytes and type 2 astrocytes in vitro. We report here that: (i) GABA is detected by immuno-electron microscopy in intact rat optic nerve and is localized to glia and pre-myelinated axons during the first few weeks of postnatal development, but is markedly reduced or absent in the adult; and (ii) neonatal optic nerve is depolarized by GABAA receptor agonists or by the inhibition of GABA uptake. These results demonstrate the presence of functional GABAA receptors, and GABA uptake and release mechanisms in developing rat optic nerve, and suggest that excitability of developing axons can be modulated by endogenous neurotransmitter at non-synaptic sites.     

The correlation of changes of the optic nerve diameter in the acute retrobulbar neuritis with the brain changes in multiple sclerosis

The aim of this paper is to compare diameter of healthy and affected optic nerve determined by ultrasound with brain lesions in acute retrobulbar neuritis in patients with multiple sclerosis. In this prospective study 20 patients with multiple sclerosis and acute retrobulbar neuritis were examined. Optic nerve diameter was measured by ultrasound. Brain lesions were detected by magnetic resonance. Correlation between demyelinating lesions of the brain in multiple sclerosis and optic nerve diameter was tested by Kruskal-Wallis test. Significant difference in diameter between healthy and affected optic nerve in acute retrobulbar neuritis was found. Demyelinating brain changes examined by magnetic resonance revealed periventricular lesions, subcortical lesions and lesions in corpus callosum. There is statistically significant correlation between optic nerve diameter and number of brain lesions in multiple sclerosis, p < 0.05. Diameter of optic nerve in retrobulbar neuritis measured by ultrasound correlates with brain lesions detected by magnetic resonance in multiple sclerosis.