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Jan Albrecht

Publications and source records attributed to Jan Albrecht.

31 records · Page 2Linked to original sources
PubMed2004

Rabbit retinal organ culture as an in-vitro model of hepatic retinopathy.

BACKGROUND: In patients with liver insufficiency the blood ammonia concentration is elevated, which induces structural and functional retinal alterations referred to as "hepatic retinopathy". The fact that some of these alterations are reversible after timely liver transplantation rationalizes therapeutic/preventive approaches to maintain the retina before a donor organ is available. Here, we describe further elaboration and characterization of our retinal organ culture model. METHODS: Retinal pieces of neonatal rabbits were explanted and cultured for up to 22 days. After 7 days in vitro (DIV7), some of the cultures were exposed to 0.25, 0.5, 1.0, 3.0, or 7.0 mM ammonia for varying periods of time and then studied by light and electron microscopy. RESULTS: Untreated (control) cultures underwent several changes during prolonged culturing; these included increased expression of GFAP and of Bcl-2, and decreased levels of glutamine synthetase, in Müller (glial) cells. However, the retinal layering remained intact and even some retinal ganglion cells survived although their axons had been cut. Exposure to elevated levels of ammonia caused morphologic alterations in the explants compatible with those noted in hepatic retinopathy in vivo. Specifically, we observed neuronal cellular degeneration with destruction of retinal layers and formation of rosettes, as well as decreased expression of intermediate filament proteins, migration of cell nuclei, cell swelling, and up-regulation of glutamine synthetase immunoreactivity in (some) Müller cells. CONCLUSION: Our findings indicate that the organ culture model may be a useful tool for detailed studies on the cellular and subcellular mechanisms underlying hepatic retinopathy and for in vitro testing of measures to alleviate HR symptoms.

Ammonia↗
PubMed2004

Kynurenic acid synthesis in cerebral cortical slices of rats with progressing symptoms of thioacetamide-induced hepatic encephalopathy.

Increased ammonia is a major pathogenic factor in hepatic encephalopathy (HE), a neurologic syndrome associated with glutamatergic dysfunction. Previous studies have shown that in rat cerebral cortical slices or a glia-derived cell line, acute treatment with ammonia in vitro and in vivo inhibits the production of a broad-spectrum antagonist of excitatory amino acid receptors, kynurenic acid (KYNA). The present study analyzed KYNA synthesis in cerebral cortical slices obtained from rats with progressing HE symptoms accompanying acute liver failure induced by one, two, or three intraperitoneal administrations of thioacetamide (TAA) at 24-hr intervals. KYNA synthesis was found decreased to 83% of control 24 hr after one administration of TAA and unaffected after two TAA injections, when moderate hyperammonemia was associated by metabolic and bioelectric activation of the central nervous system, but was not accompanied by typical HE symptoms. KYNA synthesis was elevated to 155% of control after three TAA administrations, a period in which the rats showed advanced HE symptoms including stupor or coma. KYNA synthesis at the advanced HE stage was inhibited by glutamate in a degree comparable to that observed in control slices. The elevation of KYNA synthesis was associated with increased activity of a kynurenine aminotransferase (KAT) isomer, KAT-II. KYNA synthesis did not differ from control 21 days after the third TAA administration when HE symptoms receded. The results suggest that alterations of KYNA synthesis may contribute to the imbalance between neural excitation and inhibition at different stages of HE.

Animals↗
PubMed2004

Effects of methionine sulfoximine on the glutamine and glutamate content and cell volume in rat cerebral cortical slices: involvement of mechanisms not related to inhibition of glutamine synthesis.

Treatment of cerebral cortical slices with 5mM ammonium acetate (ammonia) elevated the glutamine (Gln) content and increased cell volume in the slices, in agreement with the postulated contribution of glutamine to hyperammonemic brain edema [Neurochem. Int. 43 (2003) 299]. In this study we show that, unexpectedly, treatment with a glutamine synthetase inhibitor, methionine sulfoximine (MSO) (0.1-5.0mM) in the absence of ammonia increases Gln content in the slices in a dose-independent manner, to levels higher than those recorded after ammonia treatment. MSO (>0.1mM) inhibited (>0.1mM) Gln uptake in crude cerebral cortical cell membranes (P2 fraction). Since Gln uptake in this preparation was largely facilitated by the Gln efflux-promoting systems ASC and N and less so by the uptake promoting system A, MSO-induced accumulation of Gln could result from inhibition of Gln efflux. MSO did not affect cell volume in the slices, showing that Gln retention is not as a rule a causative factor in cerebral edema. MSO at 5mM concentration increased cell swelling induced by ammonia, which is consistent with earlier observations pointing to the direct excitotoxic action of MSO in vivo and in vitro. The results emphasize the limits of applicability of MSO as an inhibitor of Gln synthesis in an in vitro system.

Acetates↗
PubMed2004

Glutamine uptake and expression of mRNA's of glutamine transporting proteins in mouse cerebellar and cerebral cortical astrocytes and neurons.

The relative roles of the three sodium-dependent transport systems: A, ASC and N in the uptake of [3H]Gln, and the compatibility of the uptake characteristics with the expression of mRNAs coding for the Gln transporting molecules, were examined in primary cultures of astrocytes and neurons derived from mouse cerebellum, a glutaminergic system-enriched structure, and in cerebral cortex. Gln uptake activity (Vmax) was higher in cerebellar astrocytes or neurons than in their cerebral cortical counterparts. The N-methylamino-isobutyric acid (MeAiB)- and pH-sensitive, system A-mediated component of the uptake, and the uptake of [14C]MeAiB itself, was much more active in neurons than in astrocytes derived from either region. Also, the expression of mRNA for GlnT (SAT1), a system A isoform specific for Gln, was only expressed in neurons derived from both structures, while an alanine (Ala)-preferring system A transporter, SAT2, was expressed in neurons and astrocytes from either region. System ASC-mediated Gln uptake and expression of ASCT2 mRNA were in both structures more pronounced in astrocytes than in neurons, consistent with the postulated role of ASCT2 in the efflux of de novo synthesized Gln from astrocytes. System N-mediated (threonine+MeAiB-inhibitable) Gln uptake showed comparable activities in all four types of cells, which is compatible with the ubiquitous expression of NAT2 mRNA-a mouse brain-specific N-system isoform.

Amino Acid Transport System A↗
PubMed2003

Role of N-methyl-D-aspartate receptors in the neuroprotective activation of extracellular signal-regulated kinase 1/2 by cisplatin.

Neurons are exposed to damaging stimuli that can trigger cell death and subsequently cause serious neurological disorders. Therefore, it is important to define defense mechanisms that can be activated in response to damage to reduce neuronal loss. Here we report that cisplatin (CPDD), a neurotoxic anticancer drug that damages DNA, triggered apoptosis and activated the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway in cultured rat cortical neurons. Inhibition of ERK1/2 activation using either pharmacological inhibitors or a dominant-negative mutant of the ERK1/2 activator, mitogen-activated protein kinase kinase 1, increased the toxicity of CPDD. Interestingly, N-methyl-d-aspartate (NMDA) receptor (NMDAR) antagonists reduced the ERK1/2 activation and exacerbated apoptosis in CPDD-treated neurons. Pre-treatment with CPDD increased ERK1/2 activation triggered by exogenous NMDA, suggesting that CPDD augmented NMDAR responsiveness. CPDD-enhanced response of NMDAR and CPDD-mediated ERK1/2 activation were both decreased by inhibition of poly(ADP-ribose) polymerase (PARP). Interestingly, PARP activation did not produce ATP depletion, suggesting involvement of a non-energetic mechanism in NMDAR regulation by PARP. Finally, CPDD toxicity was reduced by brain-derived neurotrophic factor, and this protection required ERK1/2. In summary, our data identify a novel compensatory circuit in central nervous system neurons that couples the DNA injury, through PARP and NMDAR, to the defensive ERK1/2 activation.

Adenosine Triphosphate↗
PubMed2003

Changes in the striatal extracellular levels of dopamine and dihydroxyphenylacetic acid evoked by ammonia and N-methyl-D-aspartate: modulation by taurine.

Acute hyperammonemia is associated with motor disturbances that are thought to involve striatal dopaminergic dysfunction. Discharge of striatal dopaminergic neurons is controlled by N-methyl-D-aspartate (NMDA) receptors, the excessive activation of which contributes to ammonia neurotoxicity. Here we show that ammonium chloride ("ammonia", extracellular concentration 5 mM) or NMDA (1 mM), when directly administered to the rat striatum via a microdialysis probe, evoke a prompt accumulation of dopamine (DA) in the microdialysates. However, while ammonia increases, NMDA decreases, the extracellular dihydroxyphenylacetate (DOPAC) level. The results point to the NMDA receptor-mediated enhancement of DA release and increased DA metabolism as two independent ways by which ammonia affects the striatal dopaminergic system. Taurine (extracellular concentration 10 mM) attenuated the NMDA- and ammonia-evoked DA release and ammonia-induced accumulation of DOPAC, reflecting two different neuroprotective mechanisms of this amino acid.

3,4-Dihydroxyphenylacetic Acid↗
PubMed2003

Taurine reduces ammonia- and N-methyl-D-aspartate-induced accumulation of cyclic GMP and hydroxyl radicals in microdialysates of the rat striatum.

Acute ammonia neurotoxicity caused by intraperitoneal administration of ammonium salts is mediated by overactivation of N-methyl-D-aspartate (NMDA) receptors, with ensuing generation of free radicals and extracellular accumulation of cyclic GMP (cGMP) arising from stimulation of nitric oxide (NO) synthesis. In this study, infusion of ammonium chloride or NMDA into the striata of rats via microdialysis probes increased the contents of cyclic GMP and hydroxyl radicals in the microdialysates. Co-infusion of taurine virtually abolished both the ammonia- and NMDA-induced accumulation of cGMP. Taurine also attenuated accumulation of hydroxyl radicals evoked by either treatment. This result is the first evidence of a potential of taurine to attenuate the effects of NMDA receptor overactivation by ammonia in vivo and points to the inhibition of the NMDA receptor-mediated NO synthesis as a possible mechanism of its neuroprotective action. Taurine or its blood-brain barrier penetrating analogues may be applicable in treatment of ammonia-induced neurological deficits.

Ammonium Chloride↗
PubMed2003

Effect of ammonia on taurine transport in C6 glioma cells.

Both short-term and prolonged treatment with ammonia stimulate Tau efflux from C6 glioma cells, which confirms earlier observations with different CNS preparations. In addition, prolonged, but not short-term treatment reversibly decreases Tau uptake. The results are consistent with in vivo observations showing robust extracellular accumulation of Tau in the CNS under hyperammonemic conditions. Enhancement of Tau efflux by short term ammonia treatment is associated with its passage via a channel (or channels) which in contrast to the release from control cells is (are) not inhibited by NA. However, the increased Tau efflux does not appear to involve active Tau transport. Insensitivity to NA distinguishes ammonia-dependent Tau efflux from C6 cells from that reported in other CNS-derived preparations. Tau release evoked by prolonged ammonia treatment likewise shows a NA-insensitive component, but also a component associated with activation of Tau transport in a reverse mode. Increased outward transport of Tau may be associated with transiently increased expression of TauT mRNA. Mutual relation of the two components, but also the identity of the NA-insensitive efflux route with the anion channels so far described remains to be analyzed.

Ammonia↗
PubMed2002

Regulation of kynurenic acid synthesis in C6 glioma cells.

Studies with brain slices have provided evidence that synthesis of kynurenic acid (KYNA) from kynurenine (KYN), which occurs in astrocytes, is modulated by changes in the ionic composition of the medium and the presence of depolarizing agents or the excitatory amino acid glutamate (Glu). The present study analyzed the effects of changes in incubation medium on KYNA synthesis in cultured C6 glioma cells. The synthesis was not affected by omission of Na(+) and raising K(+) concentration to 50 mM, conditions that in brain slices stimulate or inhibit KYNA formation, respectively. KYNA synthesis in C6 cells was inhibited by the absence of Ca(2+), which contrasts with its Ca(2+) independence in brain slices. Also, lack of Mg(2+) and addition of a chloride channel blocker, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonate (SITS), did not affect the synthesis. KYNA synthesis in C6 cells was dose dependently inhibited by Glu. The inhibitory effect of Glu was not affected by GDPbetaS, an antagonist of metabotropic Glu receptors, the receptor class prevailing in C6 cells, suggesting that Glu acted intracellularly. NH(4)Cl and veratridine decreased KYNA production, mirroring the effects noted in brain slices. KYNA synthesis was strongly reduced in the presence of leucine (Leu), and the uptake of [(14)C]Leu was inhibited by the KYNA precursor KYN, which points to Leu as a potential endogenous modulator of KYNA formation in CNS cells.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed2002

The role of inhibitory amino acidergic neurotransmission in hepatic encephalopathy: a critical overview.

Gamma-Aminobutyric acid (GABA) is the main inhibitory amino acid in the central nervous system (CNS). Experiments with animal models of HE, and with brain slices or cultured CNS cells treated with ammonia, have documented changes in GABA distribution and transport, and modulation of the responses of both the GABA(A)-benzodiazepine receptor complex and GABA(B) receptors. Although many of the data point to an enhancement of GABAergic transmission probably contributing to HE, the evidence is not unequivocal. The major weaknesses of the GABA theory are (1) in a vast majority of HE models, there were no alterations of GABA content in the brain tissue and/or extracellular space, indicating that exposure of neurons to GABA may not have been altered, (2) changes in the affinity and capacity of GABA receptor binding were either absent or qualitatively different in HE models of comparable severity and duration, and (3) no sound changes in the GABAergic system parameters were noted in clinical cases of HE. Taurine (Tau) is an amino acid that is thought to mimic GABA function because of its agonistic properties towards GABA(A) receptors, and to contribute to neuroprotection and osmoregulation. These effects require Tau redistribution between the different cell compartments and the extracellular space. Acute treatment with ammonia evokes massive release of radiolabeled or endogenous Tau from CNS tissues in vivo and in vitro, and the underlying mechanism of Tau release differs from the release evoked by depolarizing conditions or hypoosmotic treatment. Subacute or chronic HE, and also long-term treatment of cultured CNS cells in vitro with ammonia, increase spontaneous Tau "leakage" from the tissue. This is accompanied by a decreased potassium- or hypoosmolarity-induced release of Tau and often by cell swelling, indicating impaired osmoregulation. In in vivo models of HE, Tau leakage is manifested by its increased accumulation in the extrasynaptic space, which may promote inhibitory neurotransmission and/or cell membrane protection. In chronic HE in humans, decreased Tau content in CNS is thought to be one of the causes of cerebral edema. However, understanding of the impact of the changes in Tau content and transport on the pathogenic mechanisms of HE is hampered by the lack of clear-cut evidence regarding the various roles of Tau in the normal CNS.

Amino Acids↗
PubMedDate not supplied

Excitotoxic mechanism of cell swelling in rat cerebral cortical slices treated acutely with ammonia.

Swelling of CNS cells due to endogenous ammonia is a major cause of cerebral oedema in hyperammonaemic encephalopathies. In the present study, incubation in the presence of 5mM ammonium acetate ("ammonia") decreased steady-state distribution of [14C]inulin within incubated rat cerebrocortical minislices, indicating cell swelling. NMDA receptor antagonists, MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cycloheptene-5,10-imine maleate,10 microM) and DL-AP-5 (DL-2-amino-5-phosphonovaleric acid, 250 microM), a nitric oxide synthase inhibitor, L-nitroarginine (L-NNA, 500 microM), and an antioxidant, taurine (Tau, 10 mM), markedly attenuated the cell volume-increasing effect of ammonia. The effect of Tau (10mM) was abolished by the GABA(A) receptor antagonist bicuculline (100 microM), but was unaffected by the Tau transport inhibitor guanidynoethyl-sulfonate (GES, 500 microM). Ammonia increased the slice content of Gln, an amino acid whose excess accumulation has been implicated in hyperammonemic oedema. However, treatments that reduced the cell volume did not affect Gln content. These results indicate that ammonia-induced cell swelling is in a large degree mediated by overactivation of NMDA receptors and the ensuing generation of NO and free radicals.

2-Amino-5-phosphonovalerate↗
PubMedDate not supplied

Glutamine transport in C6 glioma cells shows ASCT2 system characteristics.

Previous studies from this laboratory have shown that glutamine (Gln) uptake in a rat astrocytoma-derived C6 cell line shows characteristics similar with the uptake of a model ASC system substrate, threonine, whose pH-dependence and partial tolerance of Li(+) substitution for Na(+) resemble the ASCT2 variant of the system. In support of the previous findings, RT-PCR analysis revealed that C6 cells strongly express ASCT2 mRNA, but not at all GlnT mRNA or NAT2 mRNA, the A and N system variants specifically engaged in Gln transport in normal CNS. Other features of Gln transport in C6 cells indicating the involvement of ASCT2 system included its resistance to ouabain and stimulation of Gln efflux from the cells in the presence of excess Gln or cysteine (Cys), demonstrating that the system operates in the exchange mode. Replacement of NaCl in the incubation medium with isoosmotic sucrose did neither significantly affect the kinetics, nor any other major characteristics of Gln or Thr transport, including its pH-dependence, inhibition by ASCT system substrates or resistance to the model system A substrate-N-methylamino-isobutyric acid (MeAiB).

Amino Acid Transport System ASC↗