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F Fonnum

Publications and source records attributed to F Fonnum.

At least 55 records · Page 3Linked to original sources
PubMed1996

Contributing mechanisms for cysteine excitotoxicity in cultured cerebellar granule cells.

Several possible mechanisms for cysteine toxicity on rat cerebellar granule cells were studied and compared with the excitotoxic effect of glutamate. It was shown that the excitotoxic potency of both cysteine and glutamate increased in the presence of elevated concentrations of bicarbonate or increased pH. Pharmacological studies showed that the cysteine toxicity was specifically coupled to the NMDA receptor, whereas the glutamate toxicity was mediated to a smaller extent also by non-NMDA receptors. Treatment of cerebellar granule cells with cysteine led to an increased extracellular level of glutamate. In addition, cysteine sensitized NMDA receptors by reducing disulfide bonds in the receptor to sulfhydryl groups. A mechanism for cysteine excitotoxicity may therefore be formation of cysteine-sensitized NMDA receptors that are stimulated either by cysteine and/or by endogenous glutamate. This mechanism may also be important for the effects observed during regulated physiological release of cysteine.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1996

Amino acid neurotransmission: dynamics of vesicular uptake.

Glutamate, GABA and glycine, the major neurotransmitters in CNS, are taken up and stored in synaptic vesicles by a Mg(2+)-ATP dependent process. The main driving force for vesicular glutamate uptake is the membrane potential, whereas both the membrane potential and the proton gradient contribute to the uptake of GABA and glycine. Glutamate is taken up by a specific transporter with no affinity for aspartate. Evans blue and related dyes are competitive inhibitors of the uptake of glutamate. GABA, beta-alanine, and glycine are taken up by the same family of transporter molecules. Aspartate, taurine, and proline are not taken up by any synaptic vesicle preparations. It is suggested that vesicular uptake and release are characteristics that identify these amino acids as neurotransmitters. We also discuss that "quanta" in the brain are not necessarily related the content of neurotransmitter in the synaptic vesicles, but rather to postsynaptic events.

Amino Acids↗
PubMed1996

Extract of the marine alga Prymnesium patelliferum induces release of acetylcholine from cholinergic nerves in the rat bronchial smooth muscle.

An extract of the marine algal flagellate Prymnesium patelliferum enhanced the spontaneous as well as the K+ evoked (51 mM K+) release of [3H]acetylcholine and endogenous acetylcholine from autonomic cholinergic nerves of rat bronchial smooth muscle. The effects were concentration-dependent and reversible. The enhancement of the K+ evoked release by the algal extract was partly dependent on extracellular Ca2+ and was significantly suppressed by the organic Ca2+ blockers omega-conotoxin GVIA (1 microM), diltiazem (100 microM), nifedipine (100 microM) and flunarizine (100 microM). The enhancement of the spontaneous release seemed Ca2+ independent and not sensitive to the Na+ channel blocker tetrodotoxin. Sphingosine (20 microM), a protein kinase C inhibitor, strongly potentiated the enhancement of spontaneous release of [3H]acetylcholine induced by the algal extract whereas another protein kinase C inhibitor. 1-(5-quinolinesulphonyl)-2-methylpiperazine (H-7) (20 microM), was without effect. A similar potentiation as seen with sphingosine was observed with procaine (100 microM) and flunarizine (100 microM). The results indicate that the enhancement of the K+ evoked release of [3H]acetylcholine by the toxic extract of P. patelliferum was partly caused by activation of voltage-dependent Ca2+ channels. The increase in the spontaneous release of [3H]acetylcholine and endogenous acetylcholine induced by the algal extract alone may be caused by an ionophore-like property of the algal extract. This effect of the algal extract may be enhanced by compounds that facilitate the interaction of the algal toxin with the plasma membrane such as the lipophilic compounds flunarizine, procaine and sphingosine.

Acetylcholine↗
PubMed1995

A study of the uptake of glutamate, gamma-aminobutyric acid (GABA), glycine and beta-alanine in synaptic brain vesicles from fish and avians.

The ATP-dependent uptake of amino acids into synaptic vesicles isolated from mammalian brain is well characterized. To determine whether these characteristics are fundamental to the vesicular uptake system, synaptic vesicles were isolated from brains of the vertebrate species, rainbow trout and chicken and assayed for glutamate, gamma-aminobutyric acid (GABA) and glycine uptake activity. Uptake was dependent upon temperature, Mg2+ and ATP and was also strongly inhibited by the alkylating agent N-ethylmaleimide which is known to inhibit the ATPase, confirming that this was an energy requiring process. Interestingly GABA and beta-alanine were inhibitors of vesicular uptake of glycine in both species. Likewise the uptake of GABA was inhibited by glycine and beta-alanine. Glutamate, GABA, glycine and beta-alanine were all taken up into vesicles from both trout and chicken, and the uptake ratios were similar to the corresponding uptake ratios in synaptic vesicles from rat. These results indicate that the synaptic vesicle uptake system for glutamate, GABA and glycine uptake system is conserved throughout the vertebrate class both in respect to ATP-dependency and substrate specificity.

Adenosine Triphosphate↗
PubMed1995

Metabolic differences between primary cultures of astrocytes and neurons from cerebellum and cerebral cortex. Effects of fluorocitrate.

Astrocytes and neurons cultured from mouse cerebellum and cerebral cortex were analyzed with respect to content and synthesis of amino acids as well as export of metabolites to the culture medium and the response to fluorocitrate, an inhibitor of aconitase. The intracellular levels of amino acids were similar in the two astrocytic populations. The release of citrate, lactate and glutamine, however, was markedly higher from cerebellar than from cortical astrocytes. Neurons contained higher levels of glutamate, aspartate and GABA than astrocytic cultures. Cortical neurons were especially high in GABA and aspartate, and the level of aspartate increased specifically when the extracellular level of glutamine was elevated. Fluorocitrate inhibited the TCA cycle in the astrocytes, but was less effective in cerebellar neurons. Whereas neurons responded to fluorocitrate with an increase in the formation of lactate, reflecting glycolysis, astrocytes decreased the formation of lactate in the presence of fluorocitrate, indicating that astrocytes to a high degree synthesize pyruvate and hence lactate from TCA cycle intermediates.

Amino Acids↗
PubMed1995

The effects of a purified toxic extract of Prymnesium patelliferum on transport of ions through the plasma membrane of synaptosomes.

Extract of the ichthyotoxic marine alga Prymnesium patelliferum has been shown to have several different effects on the transport of neurotransmitters across nerve membranes. It inhibits the sodium dependent uptake of L-glutamate and GABA and enhances the calcium-dependent release of acetylcholine. We have therefore investigated the effects of a purified toxic extract of P. patelliferum on some membrane properties using rat brain synaptosomes. We found that under conditions where the algal extract inhibited the uptake of L-glutamate, it increased the intracellular concentrations of Na+ and Ca2+, stimulated efflux of K+ determined as 86Rb efflux, and depolarized the synaptosomal membrane. There was no effect on Na+/K(+)-ATPase or ouabain-insensitive ATPase activities. Further, there was no leakage of the cytosolic marker LDH, indicating that the various effects of the algal extract were not due to nonspecific leakage or lysis of the synaptosomes. The rise in the cytosolic concentration of free Ca2+ induced by the algal extract was dependent on extracellular Ca2+, and was inhibited by flunarizine (1-100 microM) but not by the Ca2+ channel blockers omega-conotoxin GVIA (1 microM), diltiazem (100 microM), nifedipine (100 microM) or verapamil (100-500 microM). The increase in Na+ influx induced by the algal extract was insensitive to tetrodotoxin (3 microM) and procaine (100 microM), whereas both the Na+ influx and the membrane depolarization were inhibited by flunarizine (1-100 microM). The increase in K+ efflux was insensitive to flunarizine (5-100 microM). From these results it appears that the toxic extract of P. patelliferum increases the permeability of synaptosomes to Ca2+, Na+ and K+ and that these effects may be responsible for the plasma membrane depolarization and the disturbance of the neurotransmitter transport processes.

Animals↗
PubMed1995

Glial-neuronal interactions as studied by cerebral metabolism of [2-13C]acetate and [1-13C]glucose: an ex vivo 13C NMR spectroscopic study.

Mice were injected intravenously with [2-13C]-acetate or [1-13C]glucose and killed after 5, 15, or 30 min. Another group of animals was injected three times subcutaneously during 30 min with [2-13C]acetate to achieve a steady-state-like situation. Brain extracts were analyzed by 13C NMR spectroscopy, and the percent enrichment of various carbon positions was calculated for amino acids, lactate, and glucose. Results obtained with [2-13C]acetate, which is metabolized by glia and not by neurons, showed that glutamine originated from a glial tricarboxylic acid cycle (TCA cycle) that loses 65% of its intermediates per turn of the cycle. This TCA cycle was associated with pyruvate carboxylation, which may replenish virtually all of this loss, as seen from the labeling of glutamine from [1-13C]glucose. From the C-3/C-4 labeling ratios in glutamine and glutamate and from the corresponding C-3/C-2 labeling ratio in GABA obtained with [2-13C]acetate, it was concluded that the carbon skeleton of glutamine to some extent was passed through TCA cycles before glutamate and GABA were formed. Thus, astrocytically derived glutamine is not only a precursor for transmitter amino acids but is also an energy substrate for neurons in vivo. Furthermore, the neuronal TCA cycles may be control points in the synthesis of transmitter amino acids. Injection of [2-13C]acetate led to a higher 13C enrichment of the C-2 in glutamate and of the corresponding C-4 in GABA than in the C-3 of either compound. This could reflect cleavage of [2-13C]-citrate and formation of [3-13C]oxaloacetate and acetyl-Coa, i.e., the first step in fatty acid synthesis. [3-13C]-Oxaloacetate would, after entry into a TCA cycle, give the observed labeling of glutamate and GABA.

Acetates↗
PubMed1995

Uptake of L-glutamate into rat brain synaptic vesicles: effect of inhibitors that bind specifically to the glutamate transporter.

In this study we have described a series of new and potent inhibitors of the vesicular uptake of glutamate. The two most efficient inhibitors were the dyes Evans blue and Chicago Skye Blue 6B, which are structurally related to glutamate and were competitive inhibitors in the nanomolar range. The anion channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (SITS) and the diuretics furosemide and bumetanide are inhibitors of chloride transport in other organs but were competitive inhibitors of glutamate and noncompetitive with respect to chloride ions. Evans blue, Chicago Skye Blue 6B, SITS, furosemide, and bumetanide are all large organic acids with two centers of negative charge and an electron-donating group at close vicinity of the negative charge at physiological pH. The inhibition of the glutamate uptake with these inhibitors was noncompetitive with respect to Cl-. The inhibitors, therefore, probably interact directly with the glutamate carrier. Bafilomycin A1, which is a specific vacuolar ATPase inhibitor, was used as a control and inhibited the vesicular dopamine, glutamate, and GABA uptake to the same extent. None of the inhibitors had any effect on the plasma membrane carrier, which is therefore clearly different from the vesicular carrier.

ATP-Binding Cassette Transporters↗
PubMed1995

Toxin of the marine alga Prymnesium patelliferum enhances voltage dependent Ca(2+)-currents, elevates the cytosolic Ca(2+)-concentration and facilitates hormone release in clonal rat pituitary cells.

The marine flagellate Prymnesium patelliferum produces toxins lethal to fish. The toxin extracted from the alga has haemolytic, cytotoxic and neurotoxic effects, but the action mechanisms of the toxin are not known in detail. We have examined the toxin effects on the voltage sensitive Ca(2+)-currents, the cytosolic Ca(2+)-level ([Ca2+]i) and the prolactin release in clonal rat anterior pituitary GH4C1 cells, which possess T- and L-type Ca(2+)-channels. The trans-membrane Ca(2+)-current was recorded using whole-cell voltage clamp. After 5-15 min exposure to the algal toxin at a final concentration of 50,000-100,000 cells mL-1, the Ca(2+)-currents through both the T- and L-channels showed a 2-3-fold enhancement. The voltage sensitivity of the Ca(2+)-currents was not affected by the algal toxin, and the toxin-induced currents were inhibited by 100 microM of the Ca(2+)-channel blocker D-600. In toxin-exposed cells microfluorometric measurements based on fura-2 revealed an increase of [Ca2+]i from 100-150 to 300-500 nM. This elevation was delayed and partially inhibited by 100 microM D-600. The algal toxin induced prolactin release in a dose-dependent manner, and this effect was inhibited by the Ca(2+)-channel blocker verapamil. We therefore conclude that the toxin of P. patelliferum affects the Ca2+ homeostasis of the pituitary cells by increasing the leak through voltage sensitive Ca(2+)-channels, resulting in increased [Ca2+]i and secretion of prolactin.

Animals↗
PubMed1994

Neurotoxicity of albumin in vivo.

The neurotoxicity of albumin was studied in the rat. Solutions of rat albumin (3, 10 and 30 mg/ml) essentially free of fatty acids and globulins were injected into one neostriatum, physiological saline into the other. Injections were also performed with sodium glutamate (10 and 30 mM). Both albumin and glutamate produced lesions in a concentration-dependent manner. Thus 3, 10 and 30 mg/ml albumin produced lesions in excess of saline of 22 +/- 24 microns3, 67 +/- 25 microns3 and 170 +/- 44 microns3, (P = 0.82, 0.03 and 0.0005, respectively). 10 and 30 mM sodium glutamate caused lesions of 45 +/- 14 microns3 and 315 +/- 56 microns3 in excess of saline (P = 0.04 and 0.0004, respectively). Injection of 10 mg/ml albumin together with 10 mM sodium glutamate caused lesions of 70 +/- 11 microns3 in excess of saline (P = 0.005). This was not significantly different from the lesions caused by any of the two substances alone. Thus no potentiating effect of one substance on the toxicity of the other was seen in this study. The neurotoxicity of albumin could be of importance in disease states which are accompanied by leakiness of the blood-brain barrier.

Animals↗
PubMed1994

The effect of trichlorfon and other organophosphates on prenatal brain development in the guinea pig.

The organophosphates trichlorfon, dichlorvos, dimethoate, soman, triortho-cresyl phosphate (TOCP), and the diethoxy-analogue of trichlorfon (O,O-diethyl 2,2,2-trichloro-1-hydroxyethylphosphonate, ethyl-trichlorfon), were administered to guinea pigs between day 42 and 46 of gestation. When the offsprings were examined at birth, there was a severe reduction in brain weight in the case of trichlorfon and dichlorvos, but not after treatment with the other organophosphates. The reduction in weight was most pronounced for cerebellum, medulla oblongata, thalamus/hypothalamus and quadrigemina. The effect was less marked for cerebral cortex and hippocampus. Since soman, a potent anticholinesterase, and TOCP, an inhibitor of neuropathy target esterase, did not show any effects, this excludes that the brain hypoplasia can be caused by inhibition of these two enzymes. Further, the lack of effect with ethyl-trichlorfon has shed some light on the part of the trichlorfon molecule which could be involved in the formation of the hypoplasia. It is suggested that alkylation of DNA may be involved in the development of the lesion. The possible consequences for a teratogenic effect of trichlorfon and dichlorvos on humans are discussed.

Abnormalities, Drug-Induced↗
PubMed1994

NMR spectroscopy of cultured astrocytes: effects of glutamine and the gliotoxin fluorocitrate.

Glial synthesis of glutamine, citrate, and other carbon skeletons, as well as metabolic effects of the gliotoxin fluorocitrate, were studied in cultured astrocytes with 13C and 31P NMR spectroscopy. [2-13C]Acetate and [1-13C]glucose were used as labeled precursors. In some experiments glutamine (2.5 mM) was added to the culture medium. Fluorocitrate (20 microM) inhibited the tricarboxylic acid (TCA) cycle without affecting the level of ATP. The net export of glutamine was reduced significantly, and that of citrate increased similarly, consistent with an inhibition of aconitase. Fluorocitrate (100 microM) inhibited TCA cycle activity even more and (without addition of glutamine) caused a 40% reduction in the level of ATP. In the presence of 2.5 mM glutamine, 100 microM fluorocitrate did not affect ATP levels, although glutamine synthesis was nearly fully blocked. The consumption of the added glutamine increased with increasing concentrations of fluorocitrate, whereas the consumption of glucose decreased. This shows that glutamine fed into the TCA cycle, substituting for glucose as an energy substrate. These findings may explain how fluorocitrate selectively lowers the level of glutamine and inhibits glutamine formation in the brain in vivo, viz., not by depleting glial cells of ATP, but by causing a rerouting of 2-oxoglutarate from glutamine synthesis into the TCA cycle during inhibition of aconitase. Analysis of the 13C labeling of the C-2 versus the C-4 positions in glutamine obtained with [2-13C]acetate revealed that 57% of the TCA cycle intermediates were lost per turn of the cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetates↗
PubMed1994

Toxicity of four potentially ichthyotoxic marine phytoflagellates determined by four different test methods.

The toxicity of the marine phytoflagellates Prymnesium parvum. Prymnesium patelliferum, Chrysochromulina polylepis, and Chrysochromulina leadbeateri isolated from ichthyotoxic blooms in Norwegian coastal waters was compared using four different test methods developed for the detection of toxins produced by these species. The test methods were (1) lethality to the crustacean Artemia salina exposed to living algae, (2) hemolytic activity (lysis of human erythrocytes) by crude algal lipid extracts, and inhibition of the uptake of the neurotransmitters L-glutamate and gamma-aminobutyric acid (GABA) into (3) synaptosomes and (4) synaptic vesicles of rat brain by crude algal lipid extracts. All test methods indicated different levels of toxicity among the algal species. Prymnesium parvum, P. patelliferum, and C. polylepis were toxic as determined by all four test methods. The cultured strain of C. leadbeateri, although isolated from a toxic algal bloom, appeared nontoxic by the methods used here, and served as a negative control. The hemolytic activity of P. parvum extract was about nine times higher than that of P. patelliferum extract, whereas the activity was only two to three times higher using the other three methods. Chrysochromulina polylepis had higher toxic activity than P. patelliferum except for less inhibitory effect on synaptosomes. The inhibition of synaptosomal and vesicular neurotransmitter uptake systems by extracts of P. parvum, P. patelliferum, and C. polylepis appeared to be due to similar mechanisms of action, since similar inhibition ratios between the uptake of L-glutamate and GABA were obtained in both synaptosomes and synaptic vesicles. We suggest that P. parvum, P. patelliferum, and C. polylepis produce a set of similar toxins and that the relative amounts of each toxin vary among the three species.

Animals↗
PubMed1994

Early entry of plasma proteins into damaged neurons in brain infarcts. An immunohistochemical study on experimental animals.

Entry of plasma proteins into damaged neurons has previously been demonstrated in various pathological conditions, but little is known about brain infarcts in this respect. In the present study, focal ischemic lesions were produced in rats by permanent occlusion of the middle cerebral artery (MCA). The animals were killed from 1 to 48 h postlesion. Leakage of plasma proteins across the blood-brain barrier into the infarcted area was visualized with immunostaining 2-3 h after the occlusion. This is earlier than in most previous reports. Entry of plasma proteins into ischemic neurons was seen 3 h after permanent occlusion of the MCA, while reliable changes were not seen until 12-24 h in sections stained with hematoxylin and eosin (H & E). Ischemic neurons stained for plasma proteins irrespective of their morphological appearance. Even cells that appeared normal with H & E staining were positively labeled. The technique may be used to diagnose very early ischemic lesions.

Animals↗
PubMed1993

Structural, neurochemical and behavioural consequences of neonatal blockade of NMDA receptor through chronic treatment with CGP 39551 or MK-801.

Recent evidence suggests that NMDA receptors may be involved in survival of neurons and establishment of correct connectivity during development. We have treated rat pups from postnatal day 1 to 22 with daily s.c. injections of a competitive (CGP 39551) and a non-competitive (MK-801) antagonist of the NMDA receptor. Body weight of treated rats was decreased by 50-65% at postnatal day 24 and by 25-32% at 70 days of age. Brain weight was decreased by 16-24% at both ages. Among the different brain regions, the cerebellum and striatum appeared more decreased in size than the cortex and hippocampus. Only few minor, and in some cases transient, differences were measured in the cerebellum, the hippocampus and the cortex for a battery of neurochemical markers related to cholinergic, GABAergic and glutamatergic transmission as well as to astrocyte and oligodendrocyte activity. When tested in actometric cages from postnatal days 28 to 60, treated rats exhibited a dramatic increase of spontaneous locomotor activity which was maximal in 28-day-old animals (380% and 250% of control values in CGP 39551 and MK-801 groups, respectively) and was still significant at 60 days of age. Therefore, long-lasting alteration of motor behaviour is obtained by the schedule of chronic treatment adopted for the present experiments. Our results suggest that blockade of NMDA receptors during the critical period of brain maturation may result in permanent alteration of neural circuits.

2-Amino-5-phosphonovalerate↗
PubMed1993

Changes in evoked potentials and amino acid content during fluorocitrate action studied in rat hippocampal cortex.

Fluorocitrate inhibits the glial tricarboxylic acid cycle and thereby the synthesis of glutamine, which is the main precursor for transmitter glutamate. We investigated the possibility that there is a functional correlate to fluorocitrate action by recording evoked field potentials in rat hippocampal slices. The excitatory postsynaptic potential (field-EPSP) was markedly depressed after 7-8 h of fluorocitrate action. The population spike was also reduced, but a major part of the reduction may be the result of weaker synaptic activation rather than reduced excitability of the postsynaptic cells. The activity of thin unmyelinated fibres was only slightly affected. Preceding the changes in the field-EPSP there was a decrease in the glutamine content in the fluorocitrate treated slices relative to controls. Only a small decrease in tissue glutamate was seen concomitantly with the synaptic failure, probably because the transmitter pool of glutamate in those fibres stimulated makes little contribution to the total tissue glutamate.

Adenosine Triphosphate↗
PubMed1993

Evidence using in vivo microdialysis that aminotransferase activities are important in the regulation of the pools of transmitter amino acids.

The effect of aminooxyacetic acid (AOAA), an inhibitor of pyridoxal phosphate-dependent enzymes (including the aminotransferases), on the K(+)-evoked release of amino acids was studied during microdialysis of neostriatum in anesthetized rats. K(+)-evoked (100 mM) release of aspartate, glutamate, and GABA was inhibited by 74%, 70%, and 63%, respectively, by 20 mM Mg2+ and are therefore reflecting release from the transmitter pools of these amino acids. Treatment with AOAA decreased the K(+)-evoked release of aspartate, glutamate, and GABA instantly, with a delayed decrease in the efflux of glutamine and alanine, arguing that the synthesis of transmitter amino acids in particular is sensitive to the activity of pyridoxal phosphate-dependent enzymes. Interestingly, GABA release increased severalfold following the initial decrease, probably reflecting inhibition by AOAA on GABA aminotransferase, the enzyme most sensitive to inhibition by AOAA, and responsible for enzymatic inactivation of transmitter GABA.

Alanine↗