Nicotine prevents glutamate-induced proteolysis of the microtubule-associated protein MAP-2 and glutamate neurotoxicity in primary cultures of cerebellar neurons

The aim of this work was to assess whether nicotine prevents glutamate neurotoxicity in primary cultures of cerebellar neurons, to try to identify the receptor mediating the protective effect and to shed light on the step of the neurotoxic process which is prevented by nicotine. It is shown that nicotine prevents glutamate and NMDA neurotoxicity in primary cultures of cerebellar neurons. The protective effect of nicotine is not prevented by atropine, mecamylamine or dihydro-beta-erythroidine, but is slightly prevented by hexamethonium and completely prevented by tubocurarine and alpha-bungarotoxin, indicating that the protective effect is mediated by activation of alpha7 neuronal nicotinic receptors. Moreover, alpha-bungarotoxin potentiates glutamate neurotoxicity, suggesting a tonic prevention of glutamate neurotoxicity by basal activation of nicotinic receptors. Nicotine did not prevent glutamate-induced rise of free intracellular calcium nor depletion of ATP. Nicotine prevents glutamate-induced proteolysis of the microtubule-associated protein MAP-2 and disaggregation of the neuronal microtubular network. The possible mechanism responsible for this prevention is discussed.

Carbachol-induced hydrolysis of phospholipids in hippocampal slices may be mediated in part by subsequent activation of metabotropic glutamate receptors

We observed that AP-3, an antagonist of metabotropic glutamate receptors, reduced carbachol-induced hydrolysis of phospholipids in hippocampal slices. This inhibition could be explained in different ways, e.g.: 1) AP-3 acts also as antagonist of muscarinic receptors mediating the hydrolysis of phospholipids induced by carbachol, 2) Carbachol induces the release of glutamate which, by activating metabotropic glutamate receptors, leads to additional hydrolysis of phospholipids. The aim of this work was to test these possibilities. It is shown that AP-3 reduces carbachol-induced hydrolysis of phospholipids in hippocampal slices but not in cerebellar neurons at 10-14 days of culture, when these cells are not able to induce hydrolysis of phospholipids following activation of metabotropic glutamate receptors. It is also shown that carbachol induces a release of [3H]aspartate in hippocampal slices. The results reported suggest that the hydrolysis of phospholipids induced by carbachol in hippocampal slices would have two components. One part would be due to direct activation by carbachol of muscarinic receptors associated to activation of phospholipase C. This part would not be inhibited by AP-3. The second part would be due to subsequent release of glutamate and activation of metabotropic glutamate receptors. This part would be inhibited by AP-3.

Chronic exposure to aluminum impairs neuronal glutamate-nitric oxide-cyclic GMP pathway

Humans are exposed to aluminum from environmental sources and therapeutic treatments. However, aluminum is neurotoxic and is considered a possible etiologic factor in Alzheimer's disease and other neurological disorders. The molecular mechanism of aluminum neurotoxicity is not understood. We tested the effects of aluminum on the glutamate-nitric oxide-cyclic GMP pathway in cultured neurons. Neurons were exposed to 50 microM aluminum in culture medium for short-term (4 h) or long-term (8-14 days) periods, or rats were prenatally exposed, i.e., 3.7% aluminum sulfate in the drinking water, during gestation. Chronic (but not short-term) exposure of neurons to aluminum decreased glutamate-induced activation of nitric oxide synthase by 38% and the formation of cyclic GMP by 77%. The formation of cyclic GMP induced by the nitric oxide-generating agent S-nitroso-N-acetylpenicillamine was reduced by 33%. In neurons from rats prenatally exposed to aluminum but not exposed to it during culture, glutamate-induced formation of cyclic GMP was inhibited by 81%, and activation of nitric oxide synthase was decreased by 85%. The formation of cyclic GMP induced by S-nitroso-N-acetylpenicillamine was not affected. These results indicate that chronic exposure to aluminum impairs glutamate-induced activation of nitric oxide synthase and nitric oxide-induced activation of guanylate cyclase. Impairment of the glutamate-nitric oxide-cyclic GMP pathway in neurons may contribute to aluminum neurotoxicity.

Nitroarginine, an inhibitor of nitric oxide synthase, prevents changes in superoxide radical and antioxidant enzymes induced by ammonia intoxication

Injection of large doses of ammonium salts leads to the rapid death of animals. However, the molecular mechanisms involved in ammonia toxicity remain to be clarified. We reported that injecting ammonium acetate (7 mmol/kg) to rats increases the production of superoxide and reduces the activities of some antioxidant enzymes in rat liver and brain. We proposed that these effects induced by ammonia intoxication would be mediated by formation of nitric oxide. To test this possibility we tested whether injection of nitroarginine, an inhibitor of nitric oxide synthase, prevents the effects of ammonia intoxication on antioxidant enzymes and superoxide formation. Following injection of ammonia, glutathione peroxidase, superoxide dismutase and catalase activities were decreased in liver by 42%, 54% and 44%, respectively. In brain these activities were reduced by 35%, 46% and 65%, respectively. Glutathione reductase remained unchanged. Superoxide production in submitochondrial particles from liver and brain was increased by more than 100% in both tissues. Both reduction of activity of antioxidant enzymes and increased superoxide radical production were prevented by previous injection of 45 mg/kg of nitroarginine, indicating that ammonia induces increased formation of nitric oxide, which in turn reduces the activity of antioxidant enzymes, leading to increased formation of superoxide.

Superoxide production and antioxidant enzymes in ammonia intoxication in rats

Injection of large doses of ammonium salts lead to the rapid death of animals. However, the molecular mechanisms involved in ammonia toxicity remain to be clarified. We have tested the effect of injecting 7 mmol/kg of ammonium acetate on the production of superoxide and on the activities of some antioxidant enzymes in rat liver, brain, erythrocytes and plasma. Glutathione peroxidase, superoxide dismutase and catalase activities were decreased in liver and brain (both in cytosolic and mitochondrial fractions) and also in blood red cells, while glutathione reductase activity remained unchanged. Superoxide production in submitochondrial particles from liver and brain was increased by more than 100% in both tissues. Both diminished activity of antioxidant enzymes and increased superoxide radical production could lead to oxidative stress and cell damage, which could be involved in the mechanism of acute ammonia toxicity.

Diet induced hyperammonemia decreases neuronal nuclear size in rat entorhinal cortex

Hepatic encephalopathy is mainly caused by an excess of ammonium ions. Among other effects, glutamate transmission in the brain is impaired, and thereof, neuronal function in multiple systems is affected. We investigated in rats the effect of diet induced hyperammonemia in the entorhinal cortex, a well known glutamatergic pathway to the dentate gyrus, by measuring the neuronal nuclear area in two entorhinal cortex subfields (dorsolateral subfield (DLE) and dorsal intermediate subfield (DIE); [Insausti, R., Herrero, M.T. and Witter, M.P., Origin and distribution of cortical efferents from the entorhinal cortex in the rat, Hippocampus, 7 (1997) 146-183]) that project to separate septotemporal levels of the hippocampus. After 2, and more overtly, after 8 weeks of the ammonium enriched diet consumption, the neuronal nuclear size in layers II, III, V and VI of both entorhinal cortex subfields showed a significant reduction in size. We conclude that already at 2 weeks of treatment there is a decrease in neuronal nuclear size in all layers of the entorhinal cortex, which might have widespread functional effects on cortical and subcortical structures.

Modulation of glutamine synthesis in cultured astrocytes by nitric oxide

1. Previous results suggest that glutamine synthesis in brain could be modulated by nitric oxide. The aim of this work was to assess this possibility. 2. As glutamine synthetase in brain is located mainly in astrocytes, we used primary cultures of astrocytes to assess the effects of increasing or decreasing nitric oxide levels on glutamine synthesis in intact astrocytes. 3. Nitric oxide levels were decreased by adding nitroarginine, an inhibitor of nitric oxide synthase. To increase nitric oxide we used S-nitroso-N-acetylpenicillamine, a nitric oxide generating agent. 4. It is shown that S-nitroso-N-acetylpenicillamine decreases glutamine synthesis in intact astrocytes by approximately 40-50%. Nitroarginine increases glutamine synthesis slightly in intact astrocytes. 5. These results indicate that brain glutamine synthesis may be modulated in vivo by nitric oxide.

Activation of the metabotropic glutamate receptor mGluR5 prevents glutamate toxicity in primary cultures of cerebellar neurons

1-Aminocyclopentane-trans-1,3-dicarboxylic acid, an agonist of the metabotropic glutamate receptors 1, 2, 3 and 5, prevents neurotoxicity of glutamate and of N-methyl-D-aspartate in primary cultures of cerebellar neurons. The aim of this work was to assess which of the metabotropic glutamate receptors (mGluRs) is responsible for the protective effect. We tested the protective effects of selective agonists for each type of receptor. It is shown that glutamate and N-methyl-D-aspartate neurotoxicity are prevented by the following compounds: 1-amino-cyclo-pentane-trans-1,3-dicarboxylic acid, agonist of mGluR1, 2, 3 and 5; 3,5-dihydroxyphenylglycine, agonist of mGluR1 and 5; S-4-carboxy-3-hydroxyphenylglycine, agonist of mGluR5 and antagonist of mGluR1; trans-azetidine-2,4-dicarboxylic acid, agonist of mGluR5. Glutamate neurotoxicity is not prevented by (2S,1'S,2'S)-2(2'-carboxycyclopropyl)glycine, an agonist of mGluR2 and mGluR3. Moreover, the protective effect of 1-aminocyclo-pentane-trans-1,3-dicarboxylic acid is prevented by alpha-methyl-4-carboxyphenylglycine, an antagonist of mGluR1 and 5, but not by alpha-methyl-4-tetrazoylphenylglycine, an antagonist of mGluR2 and 3. A protective effect of activation of mGluR1 can not be ruled out because of the limitations imposed by the lack of specificity of the agonists and antagonists currently available. The results shown clearly indicate that activation of mGluR5 prevents glutamate and N-methyl-D-aspartate neurotoxicity in primary cultures of cerebellar neurons.

Effects of acute hyperammonemia in vivo on oxidative metabolism in nonsynaptic rat brain mitochondria

The effects of hyperammonemia induced in vivo by injecting rats with ammonium acetate on oxidative phosphorylation, malate-aspartate shuttle, some related enzyme activities and metabolite levels in brain mitochondria were studied ex vivo. Rats were found to be either ammonia-sensitive (showing convulsions) or ammonia-resistant (without convulsions) after intraperitoneal injection of ammonium acetate (7 mmol/kg). Ammonium acetate administration to ammonia-sensitive rats led to inhibition of State 3 rates of brain mitochondria utilizing pyruvate, glutamate, isocitrate, and succinate as substrates and to decreased respiratory control index. In brain mitochondria isolated from ammonia-resistant animals, the ammonia-induced effect on such State 3 rates was not observed. In brain mitochondria from hyperammonemic rats without convulsions, a small increase in the activity of malate dehydrogenase was observed; glutamate dehydrogenase, succinate dehydrogenase, and aspartate aminotransferase were not affected. In brain mitochondria from rats with ammonia-induced convulsions, the activities of malate dehydrogenase and succinate dehydrogenase were reduced significantly. Ammonium acetate injection to rats was associated with a 5-fold increase in the brain mitochondrial ammonium ion content and a decrease (ca. 50%) in brain mitochondrial glutamate and aspartate; brain mitochondrial malate and 2-oxoglutarate levels remained unchanged. The rate of the malate-aspartate shuttle in brain mitochondria of hyperammonemic rats was decreased by 20% as compared to corresponding rate in control rats. We conclude that acute administration of ammonium acetate induces serious disturbances in the electron-transport chain, interferences of the malate-aspartate shuttle, alterations of the levels of shuttle intermediates and inhibition of the activities of malate and succinate dehydrogenases in brain mitochondria.

Determination of intracellular ATP in primary cultures of neurons

The aim of this work was to develop and characterize a quick and simple procedure to determine the intracellular content of ATP in monolayer primary cultures of neurons. The baseline was to use the minimum amount of cells which still provides reproducible results. The first step consists of releasing intracellular ATP from the cells. This is accomplished by treatment with a detergent solution, the somatic cell releasing reagent from Sigma. This reagent is claimed by the manufacturer to release ATP from a suspension of viable somatic cells. The procedure has been adapted to be used for attached cells (neurons or astrocytes growing in monolayer), thus avoiding the use of alternative time-consuming procedures to release ATP such as boiling buffers or trichloroacetic acid. After its release the free ATP was measured using the firefly luciferase reaction. We have used this protocol to assess the effect of neurotoxic concentrations of glutamate on the intracellular content of ATP in neurons. The same procedure has been used successfully to determine intracellular ATP in primary cultures of astrocytes.

NMDA receptor antagonists prevent acute ammonia toxicity in mice

We proposed that acute ammonia toxicity is mediated by activation of NMDA receptors. To confirm this hypothesis we have tested whether different NMDA receptor antagonists, acting on different sites of NMDA receptors, prevent death of mice induced by injection of 14 mmol/Kg of ammonium acetate, a dose that induces death of 95% of mice. MK-801, phencyclidine and ketamine, which block the ion channel of NMDA receptors, prevent death of at least 75% of mice. CPP, AP-5, CGS 19755, and CGP 40116, competitive antagonists acting on the binding site for NMDA, also prevent death of at least 75% of mice. Butanol, ethanol and methanol which block NMDA receptors, also prevent death of mice. There is an excellent correlation between the EC50 for preventing ammonia-induced death and the IC50 for inhibiting NMDA-induced currents. Acute ammonia toxicity is not prevented by antagonists of kainate/AMPA receptors, of muscarinic or nicotinic acetylcholine receptors or of GABA receptors. Inhibitors of nitric oxide synthase afford partial protection against ammonia toxicity while inhibitors of calcineurin, of glutamine synthetase or antioxidants did not prevent ammonia-induced death of mice. These results strongly support the idea that acute ammonia toxicity is mediated by activation of NMDA receptors.

Carnitine and choline derivatives containing a trimethylamine group prevent ammonia toxicity in mice and glutamate toxicity in primary cultures of neurons

Carnitine prevents acute ammonia toxicity in animals. We propose that acute ammonia toxicity is mediated by activation of N-methyl-D-aspartate receptors and have shown that carnitine prevents glutamate neurotoxicity. The aim of this work was to assess whether other compounds containing a trimethylamine group are able to prevent ammonia toxicity in mice and/or glutamate toxicity in primary neuronal cultures. It is shown that betaine, trimethylamine-N-oxide, choline, acetylcholine, carbachol and acetylcarnitine prevent ammonia toxicity in mice. They also prevent glutamate but not N-methyl-D-aspartate neurotoxicity. Choline, acetylcholine and acetylcarnitine afford partial (approximately 50%) protection at nanomolar concentrations and nearly complete protection at micromolar concentrations. Trimethylamine-N-oxide, carbachol and betaine afford nearly complete protection at approximately 0.2 mM. The protective effect against glutamate neurotoxicity is prevented by 2-amino-3-phosphonopropionic acid, an antagonist of metabotropic glutamate receptors. Atropine, an antagonist of muscarinic receptors, prevents the protective effect of most of the above compounds against ammonia toxicity in mice and against glutamate toxicity in cultured neurons. These results support the idea that acute ammonia toxicity is mediated by activation of N-methyl-D-aspartate receptors and that glutamate neurotoxicity could be prevented by activating metabotropic glutamate receptors and/or muscarinic receptors.

H7, an inhibitor of protein kinase C, prevents serum-induced phosphorylation of Raf and MAP kinase in neuroblastoma cells

We have previously shown that 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), an inhibitor of protein kinase C, inhibits proliferation of neuroblastoma cells in culture. We have now tested whether the effect of H7 is mediated by MAP kinase and Raf. It is shown that, in Neuro 2a cells, activation of protein kinase C by addition of 4 beta-phorbol-12 beta-myristate-13 alpha-acetate (PMA), leads to phosphorylation of Raf and Mitogen-activated protein kinase (MAP kinase). PMA-induced phosphorylation of these proteins is prevented by H7. When quiescent Neuro 2a were stimulated to proliferate by addition of serum, Raf and MAP kinase were rapidly phosphorylated. Serum-induced phosphorylation of Raf and MAP kinase is prevented by H7. These results suggest that, in Neuro 2a cells, the control of proliferation by protein kinase C could be mediated by phosphorylation (and concomitant activation) of Raf and MAP kinase.

Protein kinase C isoforms and cell proliferation in neuroblastoma cells

The expression of protein kinase C isoforms in the neuroblastoma cell line Neuro 2a has been studied. It is shown that Neuro 2a cells express alpha, delta, epsilon and zeta PKCs. Inhibition of cell proliferation by using protein kinase C inhibitors (H7 or calphostin C) or medium without glutamine affects markedly the pattern of PKC isoforms. All treatments reduced significantly (50-70%) the content of PKC alpha. None of the treatments altered PKC zeta or epsilon. The content of PKC delta was increased (88-120%) in cells treated with PKC inhibitors but was slightly reduced in cells incubated in medium without glutamine. However, none of the treatments affected the content of the corresponding mRNAs. Long-term treatment of synchronized cells with the phorbol ester PMA depletes PKC alpha but not PKC delta or zeta and only partially PKC epsilon. This treatment with PMA did not affect DNA synthesis, indicating that PKC alpha does not play a significant role in the control of proliferation of these cells.

Glutamate induces a calcineurin-mediated dephosphorylation of Na+,K(+)-ATPase that results in its activation in cerebellar neurons in culture

In primary cultures of cerebellar neurons glutamate neurotoxicity is mainly mediated by activation of the NMDA receptor, which allows the entry of Ca2+ and Na+ into the neuron. To maintain Na+ homeostasis, the excess Na+ entering through the ion channel should be removed by Na+,K(+)-ATPase. It is shown that incubation of primary cultured cerebellar neurons with glutamate resulted in activation of the Na+,K(+)-ATPase. The effect was rapid, peaking between 5 and 15 min (85% activation), and was maintained for at least 2 h. Glutamate-induced activation of Na+,K(+)-ATPase was dose dependent: It was appreciable (37%) at 0.1 microM and peaked (85%) at 100 microM. The increase in Na+,K(+)-ATPase activity by glutamate was prevented by MK-801, indicating that it is mediated by activation of the NMDA receptor. Activation of the ATPase was reversed by phorbol 12-myristate 13-acetate, an activator of protein kinase C, indicating that activation of Na+,K(+)-ATPase is due to decreased phosphorylation by protein kinase C. W-7 or cyclosporin, both inhibitors of calcineurin, prevented the activation of Na+,K(+)-ATPase by glutamate. These results suggest that activation of NMDA receptors leads to activation of calcineurin, which dephosphorylates an amino acid residue of the Na+,K(+)-ATPase that was previously phosphorylated by protein kinase C. This dephosphorylation leads to activation of Na+,K(+)-ATPase.

Ammonia prevents activation of NMDA receptors by glutamate in rat cerebellar neuronal cultures

Acute ammonia toxicity is mediated by activation of NMDA receptors and is prevented by chronic moderate hyperammonaemia. The aim of this work was to assess whether the protective effect of chronic hyperammonaemia is due to impaired activation of the NMDA receptor. It is shown that chronic hyperammonaemia in rats decreases the binding of [3H]MK-801 to synaptosomal membranes from the hippocampus but not the amount of NMDAR1 receptor protein as determined by immunoblotting. In primary cultures of cerebellar neurons, long-term treatment with 1 mM ammonia also decreased significantly the binding of [3H]MK-801. These results suggest that ammonia impairs NMDA receptor activation. To confirm this possibility we tested the effect of long-term treatment of the cultured neurons with 1 mM ammonia on three well known events evoked by activation of the NMDA receptor: neuronal death induced by glutamate, increase in aspartate aminotransferase activity and increase in free intracellular [Ca2+]. Long-term treatment with ammonia prevented noticeably the effects of glutamate or NMDA on all these parameters. These results indicate that long-term treatment of neurons with 1 mM ammonia leads to impaired function of the NMDA receptor, which cannot be activated by glutamate or NMDA. Activation of protein kinase C by a phorbol ester restored the ability of the NMDA receptor to be activated in neurons treated with ammonia. This suggests that ammonia impairs NMDA receptor function by decreasing protein kinase C-dependent phosphorylation.

Lack of correlation between glutamate-induced depletion of ATP and neuronal death in primary cultures of cerebellum

The aim of this work was to identify, using primary cultures of cerebellar neurons, the receptors involved in glutamate-induced depletion of ATP and to assess whether there is a correlation between glutamate-induced ATP depletion and neuronal death. Glutamate induced a rapid depletion of ATP (40% decrease at 5 min). After 60 min incubation with 1 M glutamate ATP content decreased by 60-70%. Similar effects were induced by glutamate, NMDA and kainate while quisqualate, AMPA or trans-ACPD did not affect significantly ATP content. The EC50 were approximately 6, 25 and 30 microM for glutamate, NMDA and kainate, respectively. DNQX and AP-5, competitive antagonists of kainate and NMDA receptors, respectively, prevented in a dose-dependent manner the glutamate-induced depletion of ATP. These results indicate that glutamate-induced depletion of ATP is mediated by activation of kainate and NMDA receptors. Glutamate-induced neuronal death was prevented by MK-801, calphostin C, H7, carnitine, nitroarginine and W7. However, only MK-801 and W7 prevented glutamate-induced depletion of ATP, while calphostin C, H7, carnitine and nitroarginine did not. This indicates that there is not a direct correlation between ATP depletion and neuronal death.

Prenatal exposure of rats to ammonia impairs NMDA receptor function and affords delayed protection against ammonia toxicity and glutamate neurotoxicity

The aim of this work was to assess whether perinatal hyperammonemia impairs the function of NMDA receptors and whether this impairment affords protection against acute ammonia toxicity and glutamate and NMDA neurotoxicity. Rats were exposed to ammonia during the prenatal and lactation periods by feeding the female rats an ammonium-containing diet since day 1 of pregnancy. After weaning (at postnatal day 21), the pups were fed a normal diet with no ammonia added. This treatment resulted in a marked decrease of the growth rate of the animals, which was maintained even 1 month after normalization of ammonia levels. Rats exposed to ammonia were more resistant than controls to acute ammonia toxicity 13 days after feeding a normal diet but not at 3 months. Primary cultures of cerebellar neurons from hyperammonemic rats showed decreased binding of [3H]MK-801 and were remarkably more resistant than controls to glutamate and NMDA toxicities. Also, the increase in aspartate aminotransferase activity induced by low concentrations of NMDA was not produced in such cultures. These results indicate that exposure to ammonia during the prenatal and lactation periods results in long-lasting impairment of NMDA receptor function. This would be the reason for the delayed protection afforded by exposure to low ammonia levels against acute ammonia toxicity in animals and against glutamate and NMDA toxicity in neuronal cultures.

Nitroarginine, an inhibitor of nitric oxide synthetase, attenuates ammonia toxicity and ammonia-induced alterations in brain metabolism

We have proposed that acute ammonia toxicity is mediated by activation of the N-methyl-D-aspartate type of glutamate receptors. MK-801, a selective antagonist of these receptors, prevents death of animals induced by acute ammonia intoxication as well as ammonia-induced depletion of ATP. It seems therefore that, following activation of the N-methyl-D-aspartate receptors, the subsequent events in ammonia toxicity should be similar to those involved in glutamate neurotoxicity. As it has been shown that inhibitors of nitric oxide synthetase such as nitroarginine prevent glutamate toxicity, we have tested whether nitroarginine prevents ammonia toxicity and ammonia-induced alterations in brain energy and ammonia metabolites. It is shown that nitroarginine prevents partially (approximately 50%), but significantly death of mice induced by acute ammonia intoxication. Nitroarginine also prevents partially ammonia-induced depletion of brain ATP. It also prevents completely the rise in glucose and pyruvate and partially that in lactate. Injection of nitroarginine alone, in the absence of ammonia, induces a remarkable accumulation of glutamine and a decrease in glutamate. The results reported indicate that nitroarginine attenuates acute ammonia toxicity and ammonia-induced alterations in brain energy metabolites. The effects of MK-801 and of nitroarginine are different, suggesting that ammonia can induce nitric oxide synthetase by mechanisms other than activation of N-methyl-D-aspartate receptors.

Alcohol exposure during brain development reduces 3H-MK-801 binding and enhances metabotropic-glutamate receptor-stimulated phosphoinositide hydrolysis in rat hippocampus

Glutamate receptors play important roles during brain development. We have investigated the effect of chronic maternal alcohol intake on the ontogenic profile of hippocampal glutamate receptor subtypes in their offspring. Binding of 3H-MK-801 to N-methyl-D-aspartate (NMDA) receptor was measured in isolated membranes from the hippocampus of the offspring of pair-fed control and alcohol-fed rats at different times during the postnatal life. Phosphatidylinositol triphosphate (PIP2) hydrolysis was also assayed to provide a measure of the possible effect of ethanol on the metabotropic glutamate receptor (mGluR). In pair-fed control rats, at postnatal day (PND) 3, the 3H-MK-801 binding represents 60% of adult values. Binding then rises to 170% at PND 11, and gradually decreases to adult levels. A transient overshoot in the mGluR-coupled PIP2 hydrolysis was also observed during postnatal development in rat hippocampus. Alcohol-exposed rats showed a similar pattern, but a significant decrease in the specific binding for NMDA receptor was observed on all the postnatal days analyzed. In addition, alcohol exposure significantly decreases the number of specific 3H-MK-801 binding sites, with no change in the affinity of the sites for 3H-MK-801. Moreover, this treatment enhanced the mGluR-activated PIP2 hydrolysis in hippocampus of alcohol-exposed rats. These results may contribute to an understanding of the toxic effects of ethanol on the developing central nervous system (CNS) and help explain the cognitive deficits associated with prenatal alcohol exposure.

Protein kinase C inhibitors, H7 and calphostin C, inhibit induction of DNA synthesis by cytosolic extracts of exponentially growing neuroblastoma cells in isolated nuclei

Cytoplasmic extracts from proliferating Neuro-2a cells contain a protein factor, ADR (activator of DNA replication) that induces DNA synthesis in isolated quiescent nuclei. Cytoplasmic extracts derived from quiescent-made Neuro-2a cells contain none or very little ADR activity, but this activity can be generated after a brief exposure of cytosolic extracts to a membrane-enriched fraction derived from exponentially growing Neuro-2a cells. ADR activity appears at the beginning of the S phase of the cell cycle. Moreover it appears to be a protease, because aprotinin inhibits ADR activity. ADR activity can be also inhibited by the protein kinase C inhibitors, 1-(5-isoquinoline-sulfonyl)-2- methylpiperazine (H7) and calphostin C.

Brain ATP depletion induced by acute ammonia intoxication in rats is mediated by activation of the NMDA receptor and Na+,K(+)-ATPase

Injection of large doses of ammonia into rats leads to depletion of brain ATP. However, the molecular mechanism leading to ATP depletion is not clear. The aim of the present work was to assess whether ammonium-induced depletion of ATP is mediated by activation of the NMDA receptor. It is shown that injection of MK-801, an antagonist of the NMDA receptor, prevented ammonia-induced ATP depletion but did not prevent changes in glutamine, glutamate, glycogen, glucose, and ketone bodies. Ammonia injection increased Na+,K(+)-ATPase activity by 76%. This increase was also prevented by previous injection of MK-801. The molecular mechanism leading to activation of the ATPase was further studied. Na+,K(+)-ATPase activity in samples from ammonia-injected rats was normalized by "in vitro" incubation with phorbol 12-myristate 13-acetate, an activator of protein kinase C. The results obtained suggest that ammonia-induced ATP depletion is mediated by activation of the NMDA receptor, which results in decreased protein kinase C-mediated phosphorylation of Na+,K(+)-ATPase and, therefore, increased activity of the ATPase and increased consumption of ATP.

High ammonia levels decrease brain acetylcholinesterase activity both in vivo and in vitro

We have tested the effect of ammonium injection on the activity of acetylcholinesterase in rat brain. Fifteen minutes after ip injection of 7 mmol/kg of ammonium acetate, the activity of acetylcholinesterase in brain was reduced significantly. The inhibitory effect varied in a wide range, with a maximum decrease of 60%, and was proportional to the concentration of ammonia reached in the brain. It is also shown that ammonium salts added in vitro to the assay mixture inhibit acetylcholinesterase in brain homogenates competitively. The Ki values for inhibition of the enzyme in vitro were 7.2 and 8.5 mM for ammonium acetate and ammonium chloride, respectively, when acetylcholinesterase was assayed in rat brain homogenates, and 7.6 and 8.3 mM when assayed in mice brain homogenates. These results suggest that at least part of the neurologic effects of ammonia could be mediated by an increase of acetylcholine as a consequence of the inhibition of acetylcholinesterase.