Modulation of phosphoinositide hydrolysis by NaF and aluminum in rat cortical slices

Differential sensitivity of phosphoinositide metabolism to sodium fluoride and carbachol treatments in PC12 cells

Exposure to sodium fluoride (NaF) resulted in an increased accumulation (up to 10-fold) of total [(3)H]inositol phosphates (T-InsP) in rat PC 12 cells. The magnitude of the NaF effect was comparable to that for muscarinic acetylcholine receptor-mediated stimulation of T-InsP accumulation in the presence of saturating concentrations of carbachol, but effects of NaF and muscarinic agonists were additive at subsaturating concentrations. The NaF effect was atropine insensitive; was not mimicked by effects of NaCl (10 mM), aluminum fluoride (1 to 100 muM), forskolin (up to 100 muM), or dibutyryl cyclic AMP (1 mM); and was not altered by treatment with pertussis or cholera toxins (1 mug/ml for 24 h). By contrast, the carbachol response was fully sensitive to atropine and partly sensitive to pertussis toxin. Chelation of extracellular calcium ion following 10 min of pretreatment with EDTA or EGTA (3 mM) inhibited carbachol-stimulated T-InsP accumulation by 50%, but resulted in an enhancement of NaF-stimulated T-InsP accumulation. By contrast, inhibition of the mobilization of intracellular calcium ion with 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate inhibited NaF stimulation of T-InsP accumulation by more than 50% but inhibited carbachol-stimulated TInsP accumulation to a much lower extent. Enhanced calcium influx and cell depolarization stimulated by high extracellular concentrations of KCl markedly potentiated carbachol, but not NaF, stimulation of T-InsP accumulation. This differential sensitivity to muscarinic antagonists, cell depolarization, and manipulation of intra- and extracellular calcium ion indicates that different mechanisms underly NaF and carbachol stimulation of T-InsP accumulation. However, stimulation of T-InsP accumulation in the presence of carbachol alone, NaF alone, or carbachol plus NaF was inhibited to a similar extent in the presence of the phorbol ester, phorbol 12-myristate13-acetate. Taken together, these observations suggest that NaF and carbachol effects are mediated through distinct mechanisms but share a common target, perhaps a GTP-binding protein and/or phospholipase C, whose activity is known to be influenced by protein kinase C.

Neurotoxic effects of dietary aluminium

Neurochemical responses to chronic oral aluminium administration have been studied in rats. Aluminium (0.3%) was added to drinking water of adult rats for four weeks or longer and weanling rats were given aluminium for eight weeks. Selective cognitive impairment was demonstrated in the adult rats. Aluminium inhibited calcium flux and phosphoinositide metabolism, one product of which (inositol 1,4,5-trisphosphate) modulates intracellular calcium levels. In weanling rats aluminium decreased the in vivo concentration of inositol 1,4,5-trisphosphate in the hippocampus. An increase in cyclic AMP concentrations by 30-70% in various brain regions in adult and weanling rats was found. Aluminium enhanced agonist-stimulated but not basal cyclic AMP production in vitro. It was postulated that aluminium inhibits the GTPase activity of the stimulatory G protein, Gs, leading to prolonged activation of Gs after receptor stimulation and increased cyclic AMP production. Aluminium treatment also increased the phosphorylation of microtubule-associated protein 2 (MAP-2) and the 200 kDa neurofilament protein (NF-H) but several other phosphoproteins were unaffected. Concentrations of seven structural proteins--MAP-2, tau, NF-H, NF-M (150 kDa), NF-L (68 kDa), tubulin and spectrin--were measured in rat brain regions by immunoblot methods. MAP-2 was most consistently decreased. These studies show that chronic oral aluminium administration to rats has significant neurochemical consequences. Three sites of action are implicated: altered calcium homeostasis, enhanced cyclic AMP production, and changes in cytoskeletal protein phosphorylation states and concentrations.

Does neurotransmission impairment accompany aluminium neurotoxicity?

Neurobehavioral disorders, except their most overt form, tend to lie beyond the reach of clinicians. Presently, the use of molecular data in the decision-making processes is limited. However, as details of the mechanisms of neurotoxic action of aluminium become clearer, a more complete picture of possible molecular targets of aluminium can be anticipated, which promises better prediction of the neurotoxicological potential of aluminium exposure. In practical terms, a critical analysis of current data on the effects of aluminium on neurotransmission can be of great benefit due to the rapidly expanding knowledge of the neurotoxicological potential of aluminium. This review concludes that impairment of neurotransmission is a strong predictor of outcome in neurobehavioral disorders. Key questions and challenges for future research into aluminium neurotoxicity are also identified.

[Acute neurotoxic organophosphate poisoning: insecticides and chemical weapons]

OBJECTIVE

To review clinical and therapeutic bases of an organophosphate poisoning, either with insecticide or nerve agent.

DATA SOURCES

References were obtained from computerized bibliographic research (Medline), from personal data (academic memoir, documents under approbation of the National Defense Office), from Internet's data.

DATA SYNTHESIS

Generally, organophosphate poisoning occurs during accidental exposure with agricultural insecticide or suicide. The effects of organophosphate compounds are due to the inhibition of the enzyme acetylcholinesterase. The intoxication symptoms can be divided into muscarine-like, nicotine-like effects, effects on the central nervous system and symptoms related to the dysfunction of the neuromuscular junction. The interest of biological acetylcholinesterase's measuring is minimal because it is weakly specific or sensitive. The immediate severity is due to hypoxia. Respiratory failure results from the lack of central drive inflated with excessive bronchial secretions, bronchospasm and respiratory muscles paralysis. The secondary complications are early myopathies whose gravity is correlated with the decrease of acetylcholinesterases, or later neuropathies induced by a different mechanism. Beside the symptomatic measures, atropine is the specific anticholinergic treatment. When promptly used, oximes can regenerate cholinesterases. The attempted effects of the treatment are mouth dryness, pupilar dilatation and flushing of the skin. Nerve agents are lethal toxics which have a short onset time and produce severe neurological pathology. In a terrorist incident, it is as important to identify rapidly the toxic agent and provide emergency decontamination as to manage medical care. An effective response must be multidisciplinary, involving clinicians, toxicologists, Emergency Medical Service and public's health personnel.

Fluoride-induced depletion of polyphosphoinositides in rat brain cortical slices: a rationale for the inhibitory effects on phospholipase C

Fluoride, which is used commonly as a pharmacological tool to activate phosphoinositide-phospholipase C coupled to the heterotrymeric Gq/11 proteins, inhibited the phosphorylation of phosphatidylinositol (PtdIns) to polyphosphoinositides (PtdIns4P and PtdIns4,5P2) in membranes from rat brain cortex. Fluoride enhanced basal production of 3H-inositol phosphates in membranes prepared from brain cortical slices that had been prelabeled with [3H]inositol, but inhibited the stimulation elicited by carbachol in the presence of GTPgammaS. However in both cases fluoride depleted [3H]PtdIns4P content by 95%. The inhibitory effects of fluoride on the release of 3H-inositol phosphates in slices were not apparent in a pulse [3H]inositol-labeling strategy, but became dramatic in a continuous labeling protocol, particularly at long incubation times. Prelabeling slices with [3H]inositol in the presence of fluoride precluded polyphosphoinositide labeling, and eliminated phospholipase C responsiveness to carbachol under normal or depolarizing conditions, and to the calcium ionophore ionomycin. The lack of response of 3H-polyphosphoinositide-depleted slices to phospholipase C stimuli was not due to fluoride poisoning, unaccessibility of the [3H]inositol label to phospholipase C or desensitization of Gq/11, as the effect of carbachol and GTPgammaS was restored, in the presence of ATP, in membranes prepared from slices that had been labeled in the presence of fluoride. In conclusion, our data show that fluoride, at a concentration similar to that used to stimulate directly Gq/11-coupled phospholipase C, effectively blocks the synthesis of phospholipase C substrates from PtdIns.

Pre- and postsynaptic alterations in the septohippocampal cholinergic innervations after prenatal exposure to drugs

The present study was designed to evaluate possible presynaptic and postsynaptic alterations in the hippocampal cholinergic innervations that account for the hippocampus-related behavioral deficits found after prenatal drug exposure. Mice were prenatally exposed to either phenobarbital or heroin. On postnatal day 50, the hippocampi were removed and protein kinase C (PkC) activity, the amounts of Gi, Go, and Gq guanosine 5'-triphosphate binding proteins (G-proteins), and choline transports were determined. Basal PkC activity was higher than control levels in both phenobarbital and heroin treated mice, by 41% and 35%, respectively. The increase of PkC activity in response to carbachol was impaired in both treatment groups: in control mice, membrane PkC activity in hippocampal slices increased by 40%-50%, while no such response, or even slight reduction in PkC activity, was observed in the drug-exposed offspring. A significant increase was found in Gi and Gq G-proteins (18%-21%) in mice exposed to phenobarbital or to heroin compared with control levels. The amount of choline transporters, determined by hemicholinium binding, increased by 70% compared with the control level in mice prenatally exposed to heroin, and increased by 71% in mice prenatally exposed to phenobarbital. The alterations in basal and carbachol-stimulated hippocampal PkC activity after prenatal drug exposure may be related to an impairment in long-term potentiation (LTP); which plays an important role in hippocampal related behavioral abilities, changes in which are caused by prenatal drug exposure.

Cholinergic stimulation of AP-1 and NF kappa B transcription factors is differentially sensitive to oxidative stress in SH-SY5Y neuroblastoma: relationship to phosphoinositide hydrolysis

Oxidative stress appears to contribute to neuronal dysfunction in a number of neurodegenerative conditions, notably including Alzheimer's disease, in which cholinergic receptor-linked signal transduction activity is severely impaired. To test whether oxidative stress could contribute to deficits in cholinergic signaling, responses to carbachol were measured in human neuroblastoma SH-SY5Y cells exposed to H2O2. DNA binding activities of two transcription factors that are respondent to oxidative conditions, AP-1 and NF kappa B, were measured in nuclear extracts. H2O2 and carbachol individually induced dose- and time-dependent increases in AP-1 and NF kappa B. In contrast, when given together, H2O2 concentration dependently (30-300 microM) inhibited the increase after carbachol in AP-1. Carbachol's stimulation of NF kappa B was not inhibited except with a high concentration (300 microM) of H2O2, which was associated with impaired activation of protein kinase C. Lower concentrations of H2O2 (30-300 microM) inhibited carbachol-induced [3H]phosphoinositide hydrolysis, and this inhibition correlated (r = 0.95) with the inhibition of carbachol-induced AP-1. Activation [3H]phosphoinositide hydrolysis by the calcium ionophore ionomycin was unaffected by H2O2, indicating that phospholipase C and phosphoinositides were impervious to this treatment. In contrast, activation with NaF of G-proteins coupled to phospholipase C was concentration dependently inhibited by H2O2, indicating impaired G-protein function. These effects of H2O2 are similar to signaling impairments reported in Alzheimer's disease brain, which involve deficits in receptor- and G-protein-stimulated phosphoinositide hydrolysis, but not phospholipase C activity. Thus, these findings indicate that oxidative stress may contribute to impaired phosphoinositide signaling in neurological disorders in which oxidative stress occurs, and that oxidative stress can differentially influence transcription factors activated by cholinergic stimulation.

Inositol phosphate formation in mice prenatally exposed to drugs: relation to muscarinic receptors and postreceptor effects

Mice were exposed to phenobarbital or heroin [diacetylmorphine (DAM)] prenatally by feeding the mother phenobarbital on gestation day 9-18; DAM was injected into the mother on gestation days 9-18. At the age of 50 days, mice exposed to phenobarbital or DAM prenatally were examined for long-term biochemical changes in the postsynaptic septohippocampal system as measured by alterations in formation of the second messenger inositol phosphate (i.p.). A significant increase in i.p. formation in response to carbachol was found after prenatal exposure to DAM. An increase in i.p. formation in response to 20 mM KCl alone or in the additional presence of 10 mM carbachol or 1mM physostigmine was found after prenatal exposure to phenobarbital or DAM. In addition, a significant increase in IP formation in response to sodium fluoride was found after prenatal exposure to phenobarbital or DAM. It is suggested that an increase in G-protein activation and in the second messenger formation accompanies the early drug-induced upregulation of the muscarinic receptors found in our previous studies.

Effects of aluminum on neuronal signal transduction: mechanisms underlying disruption of phosphoinositide hydrolysis

1. Aluminum is neurotoxic in humans and animals and alters formation of inositol phosphate (IP) second messengers following in vivo or in vitro exposure. 2. Several components of the IP signalling system including G-proteins, phosphatidylinositol-specific phospholipase C (PI-PLC), protein kinase C (PKC) and Ca2+ homeostasis are susceptible to inhibition/disruption by aluminum compounds. 3. Recent evidence suggests that, despite its effects on other components, competitive inhibition by aluminum of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis by PI-PLC underlies its effects on agonist-stimulated IP generation.

Mechanisms underlying AlCl3 inhibition of agonist-stimulated inositol phosphate accumulation. Role of calcium, G-proteins, phospholipase C and protein kinase C

Possible mechanisms of AlCl3-induced inhibition of agonist-stimulated inositol phosphate (IP) accumulation were investigated using rat brain cortex slices, synaptosomes or homogenates. Under conditions in which AlCl3 inhibits carbachol (CARB)-stimulated IP accumulation (Gp-mediated), AlCl3 did not affect CARB (100 microM)-induced decreases (Gi-mediated) in 30 microM forskolin-stimulated cAMP accumulation, suggesting that AlCl3 may be specific for Gp-mediated signal transduction. To determine whether AlCl3 interfered with Gp function and/or phosphatidylinositol-specific phospholipase C (PiPLC) activity, effects of AlCl3 on CARB- and Ca(2+)-stimulated IP accumulation were examined in cortical synaptosomes. AlCl3 (500 microM) decreased CARB (1 mM)- and Ca2+ (20 microM ionomycin)-stimulated IP accumulation to 77 and 75% of control, respectively, suggesting that AlCl3 may not directly affect Gp activity, but does inhibit PiPLC activity. In cortical homogenates, AlCl3 (10-500 microM) inhibited hydrolysis of [3H]phosphatidylinositol 4,5-bisphosphate (PIP2) by PiPLC in a concentration-dependent manner with an estimated IC50 of 100 microM. The effects of AlCl3 on modulation of IP accumulation by extracellular Ca2+ and PKC were also examined as potential mechanisms. Decreasing the extracellular Ca2+ concentration ([Ca2+]e) from 1.0 to 0.1 mM decreased CARB-stimulated IP accumulation in slices. AlCl3 (500 microM) decreased significantly 1 mM CARB-stimulated IP accumulation in 1.0 and 0.1 mM Ca2+ solutions; however, the effect of AlCl3 on IP accumulation did not depend on [Ca2+]e. In cortical slices, inhibition of 1 mM CARB-stimulated IP accumulation by 500 microM AlCl3 was not altered by the PKC activator phorbol 12,13-dibutyrate (PdBu, 1 microM), or the PKC inhibitor H-7 (10 microM), suggesting that AlCl3 does not interfere with IP accumulation by activation of PKC. Other studies found that AlCl3 (10-100 microM) inhibited PKC activity in a concentration-dependent manner in both cytosolic and membrane fractions of cortical homogenates with an estimated IC50 of 60 microM. These results support the hypothesis that AlCl3 inhibition of agonist-stimulated IP accumulation may be mediated by inhibition of PiPLC activity, rather than disruption of G-protein function or modulation of the IP signalling system by Ca2+ or PKC.

Phospholipase C and phospholipase D are independently activated in rat hippocampal slices

In order to investigate a possible G-protein-mediated activation of phospholipase D (PLD) and its relationship to the activation of phosphoinositide-specific phospholipase C (PI-PLC), we measured the effects of aluminium fluoride and carbachol on choline release, the PLD-specific transphosphatidylation reaction (generation of phosphatidylpropanol) and the formation of inositol phosphates in rat hippocampal slices. Aluminium fluoride markedly enhanced the formation of choline and phosphatidylpropanol but failed to increase the formation of inositol phosphates. In contrast, the muscarinic agonist carbachol strongly stimulated PI-PLC but failed to activate PLD. We conclude that PLD in hippocampal slices is activated by a G-protein independently of phosphoinositide hydrolysis.

Aluminum interaction with phosphoinositide-associated signal transduction

Concerning molecular and cellular mechanisms of aluminum toxicity, recent studies support the hypothesis that interactions of aluminum ions with elements of signal transduction pathways are apparently primary events in cells. In the case of the phosphoinositide-associated signalling pathway of neuroblastoma cells, guanine nucleotide-binding proteins (G proteins) and a phosphatidylinositol-4,5-diphosphate (PIP2)-specific phospholipase C are probable interaction sites for inhibitory actions of aluminum ions. Following interiorization of aluminum by the cell, metal interactions decrease the accumulation of inositol phosphates, especially that of inositol-1,4,5-triphosphate (IP3), concomitant with derangements of intracellular Ca2+ homeostasis. In the presence of high concentrations of Ca2+, formation of IP3 is also diminished in aluminum-pretreated cells, presumably involving a process not requiring Mg(2+)-dependent G proteins. At higher aluminum doses, metal-induced changes in the lipid milieu of the membrane-bound phospholipase may play a role. These types of primary interactions of aluminum ions with elements of cellular communication channels are probably crucial in the manifestation of the multifacetted aluminum toxicity syndrome. If present as a phosphate-like fluoro-aluminate, a stimulatory role of aluminum ions is displayed in G protein-coupled transmembrane signalling.

The potent free radical scavenger alpha-lipoic acid improves memory in aged mice: putative relationship to NMDA receptor deficits

alpha-Lipoic acid (alpha-LA) improved longer-term memory of aged female NMRI mice in the habituation in the open field test at a dose of 100 mg/kg body weight for 15 days. In a separate experiment, no such effect could be found for young mice. alpha-LA alleviated age-related NMDA receptor deficits (Bmax) without changing muscarinic, benzodiazepine, and alpha 2-adrenergic receptor deficits in aged mice. The carbachol-stimulated accumulation of inositol monophosphates was not changed by the treatment with alpha-LA. These results give tentative support to the hypothesis that alpha-LA improves memory in aged mice, probably by a partial compensation of NMDA receptor deficits. Possible modes of action of alpha-LA based on its free radical scavenger properties are discussed in relation to the membrane hypothesis of aging.

Aluminum decreases muscarinic, adrenergic, and metabotropic receptor-stimulated phosphoinositide hydrolysis in hippocampal and cortical slices from rat brain

Effects of aluminum chloride (AlCl3) (0.1 to 1000 microM) on inositol phosphate (IP) accumulation stimulated by carbachol (CARB), norepinephrine (NE) or quisqualate (QUIS) were examined in rat hippocampal and cortical slices. In the absence of agonist, only 1000 microM AlCl3 significantly reduced basal accumulation of IPs. For CARB-stimulated IP accumulation, 100 microM and greater AlCl3 significantly inhibited IP accumulation. In cortical slices, 1000 microM AlCl3 reduced CARB-stimulated IP accumulation by 55% and in hippocampal slices 1000 microM AlCl3 inhibited IP accumulation by 40%. Similar effects of AlCl3 were observed for NE-stimulated IP accumulation. In cortical slices, the concentration-response for AlCl3 effects on agonist-stimulated IP accumulation was significantly different from that in hippocampal slices. For QUIS-stimulated accumulation of IPs, 1000 microM AlCl3 significantly inhibited IP accumulation in hippocampal slices. However, in cortical slices a biphasic effect of AlCl3 was observed. 500 and 1000 microM AlCl3 significantly inhibited IP accumulation, whereas 10 and 50 microM AlCl3 significantly enhanced QUIS-stimulated IP accumulation. In both hippocampal and cortical slices, 500 microM AlCl3 significantly inhibited CARB-, NE- or QUIS-stimulated IP accumulation at all agonist concentrations (0.1 to 10000 microM) tested, indicating a post-receptor effect on agonist-mediated IP accumulation. Stimulation of G-proteins with NaF (5-30 mM) resulted in accumulation of IPs in hippocampal and cortical slices in the absence of added agonists. NaF (5-30 mM) plus 1 mM CARB produced increased accumulation of IPs over CARB or NaF alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Lithium modulation of phosphoinositide signaling system in rat cortex: selective effect on phorbol ester binding

Recent work indicates that the therapeutic action of lithium may be mediated through perturbation of postreceptor second messenger systems. To elucidate further the postreceptor cellular sites of action(s) of lithium, the effect of chronic lithium treatment on various components of the receptor-activated phosphoinositide pathway was investigated. We found that chronic administration of lithium (0.2% LiCl, 21 days) to adult male rats did not significantly affect phosphoinositide hydrolysis in cerebral cortical slices induced by carbachol (1 mM) or NaF (10 mM). Nor did the same treatment alter the carbachol (1 mM) potentiation of guanosine 5'-(gamma-thio)triphosphate (30 microM) stimulation of phosphoinositide hydrolysis (an index of receptor/G protein coupling) in cortical membranes. Immunoblotting studies revealed no changes in the levels of G alpha q/11 immunoreactivity in the cortex after chronic lithium treatment. The levels of protein kinase C, as revealed by specific binding of [3H]phorbol dibutyrate ([3H]PDBu), were significantly reduced in the cytosolic fraction and increased in the particulate fraction of rat cortex after chronic lithium, whereas the KD of [3H]PDBu binding remained relatively constant. A small and insignificant decrease in the density of [3H]inositol 1,4,5-trisphosphate binding was also found in the cortex. The above data suggest that chronic lithium treatment affects neither the muscarinic cholinergic-linked phosphoinositide turnover nor the putative G protein alpha subunit (G alpha q/11) responsible for phospholipase C activation. However, a possible translocation and activation of protein kinase C activity may be significant in the therapeutic effect of this mood-stabilizing agent.

Age-related changes in receptor-mediated and depolarization-induced phosphatidylinositol turnover in mouse brain

The effect of aging on receptor- and G-protein-activated and on depolarization-induced phosphoinositide (PI) hydrolysis was examined in mechanically dissociated neurons from female NMRI mice. Additionally, age-dependent changes in Ca2+ homeostasis, i.e. changes in basal intracellular calcium ([Ca2+]i) and in depolarization-induced rise in [Ca2+]i were investigated. No age-related differences in PI hydrolysis were found after stimulation of muscarinic cholinergic, alpha 1, serotonin and quisqualate receptors coupled to the phosphoinositide-phospholipase C (PI-PLC) system. PI hydrolysis following stimulation with AMPA ((RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) revealed a significantly increased response in aged animals. Activation of G-proteins with NaF also induced a higher inositol monophosphate (InsP1) accumulation in aged mice. Moreover, InsP1 accumulation due to PLC activation by increased [Ca2+]i after depolarization with KCl was significantly increased in neurons from aged animals. Investigations about age-related changes in Ca2+ homeostasis revealed lower basal [Ca2+]i and lower rise in [Ca2+]i after depolarization with KCl. The data indicate that receptor-mediated and depolarization-induced PI hydrolysis are differentially affected by aging. Decreased availability of [Ca2+]i in aged animals may enhance the sensitivity of Ca(2+)-activated mechanisms. This may explain increased KCl- and AMPA-induced InsP1 accumulation whereas receptor-coupled PLC activation is less affected.

Contribution of G protein activation to fluoride stimulation of phosphoinositide hydrolysis in human neuroblastoma cells

To examine the possibility that NaF enhances phosphoinositide-specific phospholipase C (PIC) activity in neural tissues by a mechanism independent of a guanine nucleotide binding protein (Gp), we have evaluated the contribution of Gp activation to NaF-stimulated phosphoinositide hydrolysis in human SK-N-SH neuroblastoma cells. Addition of NaF to intact cells resulted in an increase in the release of inositol phosphates (450% of control values; EC50 of approximately 8 mM). Inclusion of U-73122, an aminosteroid inhibitor of guanine nucleotide-regulated PIC activity in these cells, resulted in a dose-dependent inhibition of NaF-stimulated inositol lipid hydrolysis (IC50 of approximately 3.5 microM). When added to digitonin-permeabilized cells, NaF or guanosine-5'-O-thiotriphosphate (GTP gamma S) resulted in a three- and sevenfold enhancement, respectively, of inositol phosphate release. In the combined presence of optimal concentrations of NaF and GTP gamma S, inositol phosphate release was less than additive, indicative of a common site of action. Inclusion of 2-5 mM concentrations of guanosine-5'-O-(2-thiodiphosphate) (GDP beta S) fully blocked phosphoinositide hydrolysis elicited by GTP gamma S, whereas that induced by NaF was partially inhibited (65%). However, preincubation of the cells with GDP beta S resulted in a greater reduction in the ability of NaF to stimulate inositol phosphate release (87% inhibition). Both GTP gamma S and NaF-stimulated inositol phosphate release were inhibited by inclusion of 10 microM U-73122 (54-71%). The presence of either NaF or GTP gamma S also resulted in a marked lowering of the Ca2+ requirement for activation of PIC in permeabilized cells. These results indicate that in SK-N-SH cells, little evidence exists for direct stimulation of PIC by NaF and that the majority of inositol phosphate release that occurs in the presence of NaF can be attributed to activation of Gp.

Aluminium impacts elements of the phosphoinositide signalling pathway in neuroblastoma cells

Inositol phosphate formation was examined in aluminium-treated murine neuroblastoma cells labelled with [3H]-myoinositol. Employing fluoride-stimulated intact cells, aluminium (0.2 microM to 1 mM) reduced inositol phosphate formation in a dose-dependent manner. In digitonin-permeabilized cells, stimulated with nonhydrolyzable GTP[S], inositol phosphate formation was also inhibited by increasing aluminium doses; the IC50 value was about 20 microM aluminium, while the inositol phosphate level was reduced 2.5 to 3 fold by 50 microM aluminium. The inhibitory effect of aluminium (50 microM) could not be reversed by increasing GTP[S] concentrations up to 500 microM. Prechelation of aluminium to citrate or EGTA completely abolished the aluminium-triggered inhibition of fluoride-stimulated inositol phosphate formation in intact cells, but had little effect on the inhibition of permeabilized cells stimulated with GTP[S]. In neuroblastoma cells phosphoinositide hydrolysis could be evoked either through a pathway involving the Mg2+/guanine nucleotide binding (Gp) protein, or via a pathway operative in the presence of high intracellular Ca2+ concentrations. In the Mg2+/Gp protein-mediated pathway, formation of inositol triphosphate, IP3, inositol diphosphate, IP2, and inositol monophosphate, IP, was apparently inhibited by aluminium in an interdependent manner. As to the Ca(2+)-mediated pathway, aluminium application mainly diminished the release of IP3. Following interiorization, aluminium thus acts upon elements critical for phosphoinositide-associated signal transduction. An aluminium target apparently resides on the Gp protein. Phosphatidylinositol-4,5-diphosphate-specific phospholipase C probably harbours a second aluminium target.

Effects of subchronic administration of pyritinol on receptor deficits and phosphatidylinositol metabolism in the brain of the aged mouse

The effect of pyritinol, a commonly used nootropic drug, on receptor properties and function was investigated in different neuronal systems, possibly associated with age-related decline in brain function. Chronic treatment (15 days) of aged (22 months) female NMRI mice with pyritinol (200 mg/kg) restored the reduced density of N-methyl-D-aspartate receptors in the aged mouse brain. Furthermore, the total number of binding sites of the alpha 2-receptor ([3H]yohimbine binding) decreased after treatment with drug, while the number of high-affinity agonist binding sites ([3H]UK 14304 binding) was not changed. In both systems, receptor affinity was not influenced. The densities of other receptors investigated (muscarinic-cholinergic, benzodiazepine and beta-adrenergic) were not altered by treatment with pyritinol. Additionally, the effect of pyritinol on phosphatidylinositol (PI) metabolism was investigated in dissociated neurones from young and aged mice. Muscarinic-cholinergic induced accumulation of phosphatidylinositol and the inositol phosphate response due to activation of G-protein by fluoride was increased in aged animals, treated with drug. The inositolphosphate response after stimulation with pilocarpine was slightly but not significantly increased. The metabolism of phosphatidylinositol in young animals was not altered by treatment with drug. These results support the hypothesis of a nootropic-mediated restoration of age-related brain deficits. Changes caused by pyritinol may be due to beneficial effects on age-related alterations of the properties of the neuronal membrane.

Aluminium interferes with signal transduction in neuroblastoma cells

The effects of aluminium on inositol phosphate formation were examined in murine neuroblastoma cells labelled with [3H]-myo-inositol. In aluminium-pretreated cells, the bradykinin-triggered inositol triphosphate, IP3, release and the change in intracellular [Ca2+] were appreciably less compared with the control group. Stimulating digitonin-permeabilized cells with non-hydrolyzable guanosine 5'-[gamma-thio]-triphosphate, GTP[S], inositol phosphate formation decreased in the presence of aluminium. A primary target of aluminium toxicity may reside on the guanine nucleotide-binding protein(Gp)/phospholipase C system, at a site different from that of the GTP[S] binding site.

G proteins coupled to phospholipase C: molecular targets of long-term ethanol exposure

Long-term ethanol exposure is known to inhibit bradykinin-stimulated phosphoinositide hydrolysis in cultures of neuroblastoma x glioma 108-15 cells. In the present study, [3H]bradykinin binding, GTP-binding protein function, and phospholipase C activity were assayed in cells grown for 4 days in 100 mM ethanol with the aim of elucidating the molecular target of ethanol on signal transduction coupled to inositol trisphosphate and diacylglycerol formation. Ethanol exposure reduced guanosine 5'-O-(3-thiotriphosphate) [GTP(S)]- and, to a lesser extent, NaF/AlCl3-stimulated phosphoinositide hydrolysis, whereas it had no effect on the enzymatic activity of a phosphatidylinositol 4,5-bisphosphate-specific phospholipase C. [3H]Bradykinin binding in the absence of GTP(S) was not influenced by ethanol exposure. However, the reduction in [3H]bradykinin binding seen in control cells after addition of GTP analogue was inhibited in cells grown in ethanol-containing medium. The results indicate that long-term ethanol exposure exerts its effects on receptor-stimulated phosphoinositide hydrolysis primarily at the level of the GTP-binding protein.

Multiple actions of fluoride ions upon the phosphoinositide cycle in the rat brain

The effects of sodium fluoride upon basal and agonist-stimulated inositol phospholipid breakdown have been investigated in rat brain miniprisms. NaF concentration dependently increased basal inositol phospholipid breakdown, with a maximum effect being seen at 20 mM. NaF reduced the inositol phospholipid breakdown responses to stimulation by carbachol, noradrenaline, serotonin and quisqualate, but not to the stimulation produced by raising the assay [K+] from 6 to 18 mM. More detailed study demonstrated NaF to have a 'levelling' effect, reducing all InsP/(Lipid + InsP) values greater than 0.15 (i.e. produced by carbachol at raised [K+], noradrenaline and by 50 mM K+) to about this value. Time-course experiments indicated that NaF treatment reduced the rate of carbachol-stimulated inositol phospholipid breakdown up to this InsP/(Lipid + InsP) level and thereafter blocked further breakdown. Inhibitory effects upon carbachol-stimulated inositol phospholipid breakdown were not seen with forskolin, sodium nitroprusside or 8BrcGMP. Under conditions where there is no de novo synthesis of phosphoinositides from [3H]myo-inositol, NaF reduced the total Lipid + InsP labelling by about 20%. NaF in addition inhibits the activity of Ins(1,4)P2-phosphatase in cerebral cortical homogenates. It is concluded that fluoride ions inhibit agonist-stimulated inositol phospholipid breakdown via actions not only on G-proteins but also on phosphoinositide-specific phospholipase C substrate availability.

Calcium- versus G protein-mediated phosphoinositide. Hydrolysis in rat cerebral cortical synaptoneurosomes

The role of calcium and sodium in stimulating phosphoinositide hydrolysis in brain was investigated in rat cerebral cortical synaptoneurosomes. In buffer containing 136 mM sodium and various concentrations of added calcium (0-1.0 mM), basal, potassium-stimulated, and norepinephrine-stimulated formation of 3H-inositol phosphates decreased with decreasing extracellular calcium. Potassium- and norepinephrine-stimulated formation of 3H-inositol phosphates was reduced to basal levels by addition of EGTA. Isosmotically replacing sodium with choline chloride or N-methyl-D-glucamine to disrupt Na+/Ca2+ exchange resulted in a large increase in the formation of 3H-inositol phosphates. Measurement of cytosolic calcium with fura-2 revealed that the cytosolic calcium concentration was sensitive to changes in the extracellular calcium concentration and increased on resuspension of synaptoneurosomes in sodium-free rather than sodium-containing medium. In the absence of sodium, potassium-stimulated formation of 3H-inositol phosphates was reduced or eliminated, depending on the extracellular calcium concentration. Subtraction of basal formation of 3H-inositol phosphates from that in the presence of 1 mM carbachol or 100 microM norepinephrine revealed that the carbachol-stimulated component was the same in the presence and absence of sodium, whereas the norepinephrine-stimulated component was reduced in the absence of sodium. Addition of the protein kinase C activator 12-O-tetradecanoylphorbol 13-acetate inhibited norepinephrine- and, to a lesser extent, carbachol but not basal or aluminum fluoride-stimulated formation of 3H-inositol phosphates in sodium-free medium. These results suggest that an increase in intracellular calcium, via disruption of Na+/Ca2+ exchange or depolarization-induced calcium influx, may explain previous demonstrations that agents that stimulate Na+ influx can also stimulate phosphoinositide hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum uptake by neuroblastoma cells

Aluminum uptake studies in viable neuroblastoma cells were performed. Aluminum uptake was largely dependent on the pH of the suspension medium. At physiological pH values, cells were apparently unable to incorporate detectable amounts of aluminum in the absence of proper mediators. Aluminum uptake was enhanced as the pH decreased, attaining a plateau at about pH 6.0. In experiments with 2 x 10(6) cells/ml, pH 6.0, and 25 microM aluminum in the medium, aluminum incorporation reached saturation at 5 nmol of aluminum/mg of cellular protein, accounting for 60-70% of aluminum added. At pH 6.0, cells showed a large capacity for accumulating aluminum; about 70% of intracellular aluminum was associated with the postmitochondrial fraction. At neutral pH, application of apotransferrin seemed to facilitate aluminum translocation into cells via membrane receptors. Fatty acids were also capable of mediating aluminum uptake at neutral pH, probably by forming aluminum-fatty acid complexes. Low molecular weight aluminum chelators, e.g., citrate, inhibited aluminum uptake. Treatment of cells with energy metabolism blockers had virtually no influence on aluminum uptake, indicative of passive mechanisms. The results suggest that aluminum uptake occurs via different modes dependent on growth conditions, such as medium pH.

Dephosphorylation of tau factor by protein phosphatase 2A in synaptosomal cytosol fractions, and inhibition by aluminum

When the synaptosomal cytosol fraction from rat brain was chromatographed on a DEAE-cellulose column and assayed for protein phosphatases for tau factor and histone H1, two peaks of activities, termed peak 1 (major) and peak 2 (minor), were separated. Each peak was in a single form 2 (minor), were separated. Each peak was in a single form on Sephacryl S-300 column chromatography. Both peaks 1 and 2 dephosphorylated tau factor phosphorylated by Ca2+/calmodulin-dependent protein kinase II and the catalytic subunit of cyclic AMP-dependent protein kinase. The Km values were in the range of 0.42-0.84 microM for tau factor. There were no differences in kinetic properties of dephosphorylation between the substrates phosphorylated by the two kinases. The phosphatase activities did not depend on Ca2+, Mn2+ and Mg2+. Immunoprecipitation and immunoblotting analysis using polyclonal antibodies to the catalytic subunit of brain protein phosphatase 2A revealed that both protein phosphatases are the holoenzymic forms of protein phosphatase 2A. Aluminum chloride inhibited the activities of both peaks 1 and 2 with IC50 values of 40-60 microM. These results suggest that dephosphorylation of tau factor in presynaptic nerve terminals is controlled mainly by protein phosphatase 2A and that the neurotoxic effect of aluminum seems to be related mostly to inhibition of dephosphorylation of tau factor.

Potentiation of malaoxon-induced convulsions by lithium: early neuronal injury, phosphoinositide signaling, and calcium

Convulsions, neuronal morphology, brain phosphoinositide (PI) signaling, and calcium levels were studied in rats 1, 4, and 72 hr after malaoxon (MO; 26.2 or 39.2 mg/kg ip) subsequent to pretreatment with saline or LiCl (10 meq/kg ip). The high dose of MO induced convulsions in 60% of the rats whereas the low dose was ineffective. In nonconvulsing rats, MO transiently increased cerebral inositol 1-phosphate (Ins1P), an intermediate in PI cycle, but consistently elevated brain Ins1P in convulsing rats. LiCl did not induce convulsions, but elevated the resting level of Ins1P and decreased that of inositol. Lithium also increased the potential of MO to cause convulsions but attenuated MO-induced elevations of Ins1P. Moreover, total Ca2+ in cortex increased in LiCl-pretreated convulsing and nonconvulsing rats after MO. Astrocytic edema and cytoplasmic vacuolation and/or shrinkage of neurons in cortical layers 2-3 and in the hippocampus as well as in some subcortical structures occurred only in convulsing rats. These results suggest that PI signaling may be involved in convulsions and contribute to the early neuronal injury. Cerebral Ca2+ elevations seemed to precede permanent neuronal injury. A target other than the inhibition of the hydrolysis of inositol phosphates may be the site of lithium's action in cholinergic convulsions.

Changes in cerebral inositol-1-phosphate concentrations in LiCl-treated rats: regional and strain differences

LiCl-induced (5 mEq/kg) regional differences in the cerebral phosphoinositide (PI) cycle were studied by measuring inositol-1-phosphate (Ins-1-P), an intermediate in the PI cycle, in male Sprague Dawley and Han/Wistar rats by gas chromatography/mass spectrometry. Control Ins-1-P levels were higher frontally than caudally in both rat strains. LiCl increased Ins-1-P levels 1.8 to 7.4 fold in different regions of brain of Sprague Dawley rats but only 1.2 to 1.8 fold in Han/Wistar rats. This strain difference offers a way to compare the effects of lithium on PI metabolism versus receptor-G protein-phospholipase C coupling mechanisms.

Guanosine 5'-O-thiotriphosphate and sodium fluoride activate polyphosphoinositide hydrolysis in rat cortical membranes by distinct mechanisms

NaF and guanosine 5'-O-thiotriphosphate [GTP(S)] stimulated the accumulation of [3H]inositol monophosphate ([3H]InsP) in rat brain cortical membranes, with half-maximal stimulation at 2 mM and 1 microM, respectively. Calcium also increased basal [3H]InsP formation over a range of concentrations from 10(-7) to 10(-4) M. The stimulatory effect of GTP(S) (30 microM) on [3H]InsP production was insensitive to Ca2+, whereas NaF-evoked [3H]InsP formation was dependent on Ca2+ concentrations. Guanosine 5'-O-thiodiphosphate significantly attenuated GTP(S)- but not NaF-stimulated [3H]InsP production. Coincubation of GTP(S) (30 microM) and submaximal concentrations of NaF (1 or 3 mM) stimulated [3H]InsP formation to a degree that was nearly additive with that produced by either drug alone. However, the resultant accumulation of [3H]InsP in the presence of maximally effective concentrations of GTP(S) and NaF was not different from that produced by NaF alone. Incubation of cortical membranes with GTP(S) and NaF for 1 min stimulated the accumulation of [3H]inositol bisphosphate (InsP2) but not [3H]InsP. [3H]InsP2 production elicited by GTP(S) was markedly enhanced by the muscarinic cholinergic agonist carbachol. In contrast, NaF-stimulated [3H]InsP2 formation was not potentiated by carbachol. Our findings of different characteristics of GTP(S) and fluoride activation of polyphosphoinositide (PPI) hydrolysis suggest that separate regulatory mechanisms are involved in these two modes of stimulation in brain membranes. Activation of PPI hydrolysis by fluoride may be mediated by a direct stimulation of PPI phosphodiesterase or by activating a putative guanine nucleotide regulatory protein at a location distinct from the GTP-binding site.

Effect of monovalent ions upon G proteins coupling muscarinic receptors to phosphoinositide hydrolysis in the rat cerebral cortex

It has been suggested that K+, Li+ and Fl- affect the function of G proteins coupled to signal transducing enzymes. Lithium, at concentrations which were found to reduce forskolin-stimulated adenylate cyclase activity, was without effect on either membrane [3H]phosphatidylinositol-4,5-bisphosphate ([3H]PIP2) hydrolysis measured in the absence or presence of 5'-guanylyl-imidodiphosphate (Gpp(NH)p), or (at greater than or equal to 2.3 mM Li+) upon the stimulation of rat cerebral cortical inositol phospholipid breakdown by either carbachol, noradrenaline or NaF measured at either 6 or 18 mM K+. The increase in assay [K+] greatly enhanced the inositol phospholipid response to carbachol but not to NaF. The inhibitory effect of carbachol upon forskolin-stimulated adenylate cyclase was not affected by raising the [K+] from 6 to 18 mM. At 6 mM K+ (both in the absence and presence of 15 microM AlCl3), the effects of carbachol and NaF upon inositol phospholipid breakdown were essentially additive, whereas at 18 mM K+, the breakdown response to carbachol (antagonised by pirenzepine with a pA2 value of 7.6) was similar in the absence and presence of NaF. It is concluded that in the rat cerebral cortex: (a) Li+ does not affect the function of either the phosphoinositide-specific phospholipase C enzyme itself or the Gp coupled to this enzyme; (b) the difference between the additivity between NaF and carbachol seen at different assay [K+] may reflect the K(+)-dependent changes in the tetrodotoxin-resistant and tetrodotoxin-sensitive pathways of carbachol stimulation of inositol phospholipid breakdown reported by Gurwitz and Sokolovsky (1987, Biochemistry 26, 633); and (c) the effect of K+ on muscarinic receptor-coupled inositol phospholipid breakdown is not found for muscarinic receptors inhibitorily coupled to adenylate cyclase. Evidence is also presented to suggest that NaF affects the dephosphorylation of the formed [3H]inositol polyphosphates.

Ethanol effects on bradykinin-stimulated phosphoinositide hydrolysis in NG 108-15 neuroblastoma-glioma cells

The effect of short-and long-term ethanol exposure on bradykinin-stimulated hydrolysis of phosphatidylinositol 4.5-bisphosphate (PIP2) was investigated in neuroblastoma X glioma hybrid cells (NG 108-15). Acute exposure of 50-150 mM ethanol neither influenced the bradykinin-stimulated accumulation of [3H]-inositol phosphates (IP1, IP2, IP3) nor the hydrolysis of PIP2 in cells labelled with [3H]-inositol. Furthermore, ethanol (100 mM) added in the absence of agonist did not influence these parameters. However, in cells cultivated for 4 days in 100 mM ethanol, PIP2 hydrolysis and IP1, IP2 and IP3 formation after stimulation by 10(-6)-10(-5) M bradykinin was markedly inhibited while there was no effect on the basal levels or on the levels found after stimulation with low concentrations of bradykinin. The inhibitory effect of ethanol on IP accumulation became significant after 2-3 days of ethanol.