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Z Kovács

Publications and source records attributed to Z Kovács.

At least 19 recordsLinked to original sources
PubMed2002

Fluctuation formula for nonreversible dynamics in the thermostated Lorentz gas.

We investigate numerically the validity of the Gallavotti-Cohen fluctuation formula in the two- and three-dimensional periodic Lorentz gas subjected to constant electric and magnetic fields and thermostated by the Gaussian isokinetic thermostat. The magnetic field breaks the time reversal symmetry, and by choosing its orientation with respect to the lattice, one can have either a generalized reversing symmetry or no reversibility at all. Our results indicate that the scaling property described by the fluctuation formula may be approximately valid for large fluctuations even in the absence of reversibility.

Journal Article↗
PubMed2001

Topological aspects of chaotic scattering in higher dimensions.

We investigate the topological properties of the chaotic invariant set associated with the dynamics of scattering systems with three or more degrees of freedom. We show that the separation of one degree of freedom from the rest in the asymptotic regime, a common property in a large class of scattering models, defines a gate which is a dynamical object with phase space separating invariant manifolds. The manifolds form an invariant set causing singularities in the scattering process. The codimension one property of the manifolds ensures that the fractal structure of the invariant set can be studied by scattering functions defined over simple one-dimensional families of initial conditions as usually done in two-degree-of-freedom scattering problems. It is found that the fractal dimension of the invariant set is not due to the gates but to interior hyperbolic periodic orbits.

Journal Article↗
PubMed2001

[Cholesterol granuloma at the sella region: a new method of the differential diagnosis of craniopharyngioma].

Cholesterol-granuloma is a pseudotumoral mass that is believed to enlarge by a self-perpetuating sequence of repeated hemorrhages and reparative tissue reaction. Albeit an almost ubiquitous phenomenon throughout the body, cholesterol-granuloma has recently been appreciated as a distinctive lesion mimicking or associated with craniopharyngiomas. Upon review of a surgical series of 15 purported craniopharyngiomas, the authors identified 3 such occurrences. All were characterized by a predominance of slit-like cholesterol clefts with multi-nucleated giant cells embedded in a fibrotic stroma permeated with lipid laden macrophages, lymphocytes, as well as organizing hemorrhage. Non-craniopharyngioma specific cuboidal epithelium was present in one case. The mean age of patients--all males--with cholesterol-granuloma was 26 years, and all but one had an intrasellar tumor component. Clinical symptoms referrable to hypopituitarism predominated. At variance with the above, patients with adamantinomatous or papillary craniopharyngiomas were 23.5 and 46 years old, respectively, and presented with neurological deficits or ones due to hypothalamic involvement by their tumors. With marginal central nervous tissue present in 53 percent of the specimens, 75 percent of adamantinomatous craniopharyngiomas, but only 12 percent of cholesterol-granulomas showed invasive growth. At present cholesterol-granulomas are conceived as a clinicopathologically distinctive lesion of uncertain origin. They most probably represent a clinically relevant entity in the ontogenesis of adamantinomatous craniopharyngiomas with predisposing factors yet to be elucidated.

Adolescent↗
PubMed2000

[Oxidative stress and antioxidant defense in alcoholic liver disease and chronic hepatitis C].

In the past decade it became accepted that free radicals, lipid peroxidation and antioxidant defense play a role in various tissues damages, thus in certain liver diseases as well. Since only limited data have been reported concerning the oxidative stress in viral hepatitis, a comparative study was performed in patients (pts) with chronic hepatitis C and alcoholic liver disease. In addition, the effects of a flavonolignan drug silymarin were assessed. 10 pts with chronic hepatitis C, 5 pts with alcoholic hepatitis and 13 pts with alcoholic cirrhosis have been investigated. Biochemical liver tests (serum bilirubin, aminotransferases, ALT, AST, lactate dehydrogenase (LDH), pseudocholinesterase, prothrombin), malandialdehyde (MDA) levels in plasma and red blood cell (RBC) hemolysate, superoxide radical generating capacity of stimulated polymorphonuclear granulocytes (PMN), plasma concentrations of reduced (GSH) and oxidized (GSSG) glutathione, vitamin A, luteine and beta carotene, furthermore RBC superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase activities were determined. The level of plasma MDA--as the marker of lipid peroxidation--was highest in alcoholic cirrhosis (five times of normal) (p < 0.05), the RBC hemolysate MDA was most elevated in chronic hepatitis C (p < 0.05). The mean PMNs' superoxide radical generating capacity was 116.6% of normal control in alcoholic hepatitis, where the mean GSH level was the lowest (89.8% of normal). Plasma vitamin A content was lowest in alcoholic cirrhosis (68% of control) (p < 0.05). SOD activity was elevated in both chronic hepatitis C and alcoholic cirrhosis, where GPx activity was decreased (p < 0.05). There was a correlation between LDH and SOD activities (r = 0.77, p = 0.015). Silymarin treatment of one month duration resulted in normalization of serum bilirubin in 55% of treated pts, AST became normal in 45%, and RBC hemolyzate MDA level normalized in similar rate. A significant increase in both GSH and retinoids was found. Alterations in oxidative stress and antioxidant defense system were shown in chronic hepatitis C, not only in alcoholic liver disease. The parameters of lipid peroxidation and antioxidant defense may be useful surrogate markers for monitoring pts with liver disease during hepatoprotective treatment.

Adult↗
PubMed1999

Uridine is released by depolarization and inhibits unit activity in the rat hippocampus.

Perfusion of 5 microM kainate through microdialysis probes induced >2-fold elevation of extracellular uridine and adenosine concentrations in the hippocampus and in the thalamus of anaesthetized rats. Administration of uridine via this route produced an estimated uridine concentration of 50-100 microM around the electrode surface. This markedly decreased the average firing rate of neurones in the hippocampus, but not in the thalamus. Activity of separated single hippocampal pyramidal cells was completely inhibited by uridine. The same amount of adenosine completely blocked neuronal activity in both hippocampus and thalamus. Uridine administration had no effect on extracellular adenosine concentration. These findings suggest an important neuromodulatory role for depolarization-released uridine in the CNS.

Adenosine↗
PubMed1999

Category learning and perceptual categorization in schizophrenia.

The aim of this study was to evaluate category learning in schizophrenia on tests of perceptual abstraction. Participants learned to categorize simple geometrical shapes. The categories were either well-defined (discrete categories, or DCs) or ill-defined (graded categories, or GCs). In DCs, the cues defining category membership can be verbalized in an all-or-none fashion, while in GCs they cannot be defined unambiguously. Three types of learning were used successively: serial presentation of category-exemplars, verbal description, and feedback. After the serial presentation, schizophrenia patients showed a deficit for GCs (p<0.005) but not for DCs (p = 0.98). After the verbal definition of GCs, the difference between schizophrenia patients and controls diminished (p = 0.09). Finally, after the feedback learning of GCs, a significant difference was observed again (p<0.0001), suggesting that schizophrenia patients were impaired in this learning paradigm. The GC-learning impairment after the serial presentation displayed a relationship with the score of the cognitive component assessed with the Positive and Negative Syndrome Scale (r = -0.66). In conclusion, these results suggest that the perceptual stage of abstraction is impaired in schizophrenia. This impairment can be partially compensated by instructions via top-down verbal processes.

Adult↗
PubMed1998

Abstraction is impaired at the perceptual level in schizophrenic patients.

This study investigates category learning in schizophrenia in order to evaluate abstraction abilities at the perceptual level. The participants learned about two categories of geometric shapes. The category exemplars were presented successively. In the case of schizophrenic patients, longer exposure time and more stimulus presentations were used to counterbalance attention impairments. In spite of this, the perceptual category learning was significantly impaired in the patient group. In contrast, when the training procedure involved verbal category definition and not perceptual learning, the performance of schizophrenics was similar to that of the healthy controls. These findings suggest that the perceptual learning of categories, but not free classification judgements are impaired in schizophrenia, and that this impairment is not due to pure attentional disturbances.

Adult↗
PubMed1998

[Testing basic visual functions in the evaluation of extrapyramidal side effects of antipsychotic agents].

One of the cornerstones of antipsychotic therapy is the evaluation of the effective dose that cause minimal extrapyramidal side effects. It is known that the visual system (e. g. the retina) contains dopaminergic cells similarly to the striatum where dopaminergic blockade is crucial in the induction of extrapyramidal symptoms. This rises the possibility that striatal functions can be tested by the evaluation of certain visual functions. This study compared the visual contrast-sensitivity of 20 schizophrenic patients on antipsychotic medication (9 patients were on risperidone, 9 on haloperidol, and 1-1 on zuclopenthixol and fluanxol treatment) to that of normal control subjects, and found significant impairment in the former group. The contrast-sensitivity impairments correlated with the severity of extrapyramidal symptoms. These results suggest that contrast-sensitivity measurements can be useful in the evaluation of neuroleptic threshold in the clinical practice.

Adult↗
PubMed1998

Uneven regional distribution of nucleotide metabolism in human brain.

Adenine and uridine nucleotides and adenosine are proposed to act as neuromodulators and other nucleotides and nucleosides are also suggested to be involved in brain function. A following major step towards the verification of the functional role of nucleotides and nucleosides in the brain would be the examination of regional distribution of purines, pyrimidines and the enzymes involved in their metabolism. Using our recently developed chromatography-based assay for nucleosides from tissue homogenates, we analysed nucleosides in microdissected samples derived from various regions of human brain. Marked differences in the levels of nucleosides were measured in the cerebral cortex, cerebellar cortex, thalamus and white matter. The greatest levels of most nucleosides were found in the cerebral cortex, followed by the cerebellar cortex and the white matter while the smallest concentrations were found in the thalamus, although adenosine and xanthine showed a different distribution pattern in these brain areas. Within the cerebral cortex, the measured substances showed little variations except certain high levels in the cingulate and low levels in the frontal cortex. Even distribution of nucleosides was found in the thalamic nuclei while relative high values were measured in the medial geniculate body. Since a dramatic change in nucleoside concentrations occurs after death, the measured nucleoside concentrations are an interplay of original nucleotide and nucleoside concentrations and enzyme reactions following death. Thus our results suggest regional differences in nucleotide and nucleoside composition and nucleotide metabolising enzyme activities between brain areas.

Adenosine↗
PubMed1997

[11C-methionine: an effective radio-tracer for PET scan in the detection of tumors of low proliferating capacity].

11C-methyl-methionine is available at the PET Center of University Medical School Debrecen since June, 1996. The first 5 oncological examinations were indicated for clinically suspected recurrent/residual tumorous tissue of low-grade/low proliferative capacity, following the negative or inconclusive results of previous 18F-deoxyglucose (FDG) examinations. In these situations, the methionine examinations provided conclusive results in 4 cases (out of the total of 5 examinations). On the basis of published data and own experience, the authors recommend methionine PET investigations for diagnosis, differential diagnosis and therapy monitoring of tumours of low-grade/low proliferative capacity following inconclusive results of previous FDG examination.

Adult↗
PubMed1997

A chronological evaluation of experimental brain infarct by diffusion-mapping and magnetization transfer contrast imaging.

BACKGROUND AND PURPOSE: There is a complex system of evolving physiochemical processes in the ischemic brain. The evaluation of this chain of processes is a major challenge of recent stroke studies. Two magnetic resonance imaging techniques: diffusion-weighted (DW) and magnetization transfer contrast (MTC) imaging were introduced in experimental studies and were shown to have sensitivity for different stages of brain infarct. MATERIALS AND METHODS: We used a reproducible middle cerebral artery (MCA) occlusion model in rat to examine infarcts of different time courses. Magnetic resonance T2-weighted (T2). DW, and MTC imaging were performed 3 h, 1 d, 3 d, 5 d, 2 w, 3 w, and 4 w after MCA occlusion. Haematoxylin/eosin (HE) stained sections, which revealed sub regions within infarct lesions, were compared to T2, DW, diffusion-mapping and MTC-mapping images. RESULTS: On DW images 3 hours post occlusion lesions were detected as an area with high signal intensity, while T2 imaging does not raise significant contrast of the lesion at this early stage. Three sub regions of the lesion having different ADCs, are discernible on diffusion-mapping images from 1 day to 7 days. The decrease of MTC effect was measured within the infarct lesion from core to marginal zone, from 7 days to 4 weeks. CONCLUSIONS: Diffusion-mapping imaging may help to examine brain infarction from 3 hours to 5 days. MTC imaging recognizes infarcts 7 days after the onset. Following this stage, MTC-mapping images may provide a quantitative method to assess infarct size.

Animals↗
PubMed1997

Apparent diffusion coefficient (ADC) and magnetization transfer contrast (MTC) mapping of experimental brain tumor.

Brain tumor tissue contains different pathological areas, such as tumor cell rich parts, necrotic tissues, and cyst. Furthermore, both neovascularization and edema formation progress along with the tumor progression. In this study we employed diffusion weighted (DW) and magnetization transfer contrast (MTC) imaging to chronologically investigate the biological characteristics of a rat glioma. RG-2 glioma cells were implanted stereotactically into the right hemisphere of male Wistar rats. MR images were taken 1, 2 and 3 weeks after inoculation. Apparent diffusion coefficient (ADC) and MTC values were calculated as follows; ADC = -ln (SI-DW/SI-T2)/1096, MTC = 1-SI-MTon/SI-MToff. Each mapping image was made based on the calculated average values of four pixels. The spatial signal changes and the real values were compared to the histological findings. The apparent increase of ADC was noted in the parenchyma adjacent to tumor suggesting the progression of edema. The tumor itself had similar or slightly increased ADC. Cystic and necrotic components appeared 2 weeks after implantation and they showed significantly higher ADC than those calculated in the contralateral putamen. On the other hand, MTC was slightly decreased in the parenchyma adjacent to the tumor, markedly within the tumor, and maximally in the cystic and necrotic area suggesting accumulation of macromolecules such as growth factors, cytokines, and serum albumin.

Animals↗
PubMed1996

VEGF and flt. Expression time kinetics in rat brain infarct.

BACKGROUND AND PURPOSE: Vascular endothelial growth/vascular permeability factor (VEGF) is a candidate for an angiogenic and hyperpermeability inducing factor in an infarct because it is a secretable mitogen specific for endothelial cells and is upregulated by hypoxia. Our study attempts to clarify the chronological expression of VEGF and its receptor (flt) system in experimental cerebral infarction. METHODS: With the use of a reproducible middle cerebral artery occlusion model in rats, VEGF expression was identified by Western blotting with anti-VEGF antibody. The chronological expression of the VEGF/flt system was analyzed semiquantitatively by immunohistochemical means in infarcts with different time courses from 3 hours to 3 weeks. RESULTS: VEGF and flt were expressed exclusively in the ischemic brain. The bands obtained on the immunoblot at 38 and 45 kD are related to those of VEGF121 and VEGF165 isoforms. Macrophages, neurons, and glial cells chronologically expressed VEGF immunoreactivity in a different fashion. Both VEGF (bound) and flt were detected in endothelial cells along with the development of angiogenesis. CONCLUSIONS: In the ischemic brain the macrophages, neurons, and glial cells appear to contain VEGF. The VEGF receptor flt was induced in endothelial cells along with the progression of angiogenesis in infarct. The VEGF/flt system is thus considered to be involved in the healing process of brain infarct.

Animals↗
PubMed1996

Low dehydroepiandrosterone sulfate (DHEA-S) level is not a good predictor of hormonal activity in nonselected patients with incidentally detected adrenal tumors.

To assess its differential diagnostic value, dehydroepiandrosterone sulfate (DHEA-S) was measured in a nonselected cohort of 84 patients with incidentally detected adrenal tumors (incidentaloma). Of the 38 histologically confirmed cases, 6 of 12 patients with primary or metastatic malignant tumor of the adrenals and 7 of 14 patients with benign cortical adenoma had low DHEA-S levels. Thus, the sensitivity, specificity, and predictive value of a low DHEA-S level to indicate a benign adrenal tumor were 0.35, 0.50, and 0.60, and the values to indicate a cortical adenoma were 0.50, 0.67, and 0.47, respectively. Of the 14 cases of histologically confirmed benign cortical adenoma, 10 had signs of hormonal activity, but DHEA-S was suppressed in only 7 cases. Thus, the sensitivity, specificity, and predictive value of a low DHEA-S level to indicate clinically significant hormonal activity of a benign cortical adenoma were 0.60, 0.75, and 0.86, respectively. For comparison, 5 of 5 males and 2 of 5 females with metastatic carcinomatosis, but without involvement of the adrenals, also had low DHEA-S levels. The data clearly show that in nonselected cases of incidentaloma a suppressed DHEA-S level is not a good predictor of hormonal activity and that DHEA-S measurement may be valuable only after having ascertained the cortical origin and benign feature of the tumor.

Adenoma↗
PubMed1995

[Anterior pituitary responsiveness in central Cushing disease and in Cushing syndrome caused by adrenal cortex tumors, as well as in simple obesity].

To evaluate whether the PRL, TSH and gonadotropin secretion is altered in conditions with elevated body mass index, 7 patients with central Cushing's disease before and after transsphenoidal surgery, 7 untreated patients with Cushing's syndrome caused by adrenal adenoma, 17 simplex obese (obese) women and 9 non-obese controls (all females, aged 18-45 years) were tested with TRH (200 micrograms i.v. bolus) and GnRH (100 micrograms i.v. bolus) and the hormone responses were measured. There were no differences in the basal pituitary hormone secretion among the groups. In obese subjects the PRL response was reduced as compared to untreated patients with corticotrop pituitary adenoma. No significant differences of TSH release could be observed among the groups, whereas serum total T4 levels were higher in obesity than in patients with hypercorticism either caused by pituitary or adrenal Cushing's syndrome. No differences were found in the LH response, but the stimulated FSH release was lower in obesity, in patients with central Cushing's disease after transsphenoidal surgery and in patients with primary Cushing's syndrome as compared to the normal controls.

Adenoidectomy↗
PubMed1995

Prevalence of thyroid dysfunction in different geriatric subpopulations from a moderately iodine-deficient Hungarian region. Comparative clinical and hormonal screening.

The aim of this study was to investigate the prevalence of thyroid dysfunction in different geriatric subpopulations from a moderately iodine-deficient Hungarian region and to compare the efficacy of clinical versus hormonal screening. A screening study was done on 279 chronically ill geriatric patients (Group I) and 256 consecutive hospital admissions over 60 years of age (Group II). The method of clinical screening was different from those used so far: the object was not to search for symptoms of hypo- or hyperthyroidism but to find any sign justifying a further thyrotrophin-based biochemical evaluation, i.e. history of thyroid disease or goitre or any clinical sign of hormonal dysfunction. The rates of overt hypothyroidism, overt hyperthyroidism, subclinical hypothyroidism and subclinical hyperthyroidism discovered by the hormonal screening were 2.9, 1.1, 3.6 and 5.7% in Group I and 3.5, 2.3, 3.9 and 2.0% in Group II. The sensitivities of the clinical screening to suspect overt or overt+subclinical dysfunctions were, respectively, 0.82 and 0.64 in Group I and 1.0 and 0.7 in Group II (or 0.67 and 0.4 if the clinical investigation was done not by an endocrinologist but by the medical attendants). A primarily clinical investigation-based screening would have spared 171/279 thyrotrophin estimation in Group I and 161/256 in Group II, but would have missed 2/11 overt and 11/26 subclinical dysfunctions in Group I. In Group II, no overt but 9/15 subclinical dysfunctions would have been lost in this way. Our approach of a clinical investigation-based screening was rather efficient in suspicion of overt thyroid dysfunction but failed to detect many cases with subclinical dysfunction.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗