Joël Prémont

PYRUVATE PROTECTS NEURONS AGAINST HYDROGEN PEROXIDE-INDUCED TOXICITY

Hydrogen peroxide (H2O2) is suspected to be involved in numerous brain pathologies such as neurodegenerative diseases or in acute injury such as ischemia or trauma. In this study, we examined the ability of pyruvate to improve the survival of cultured striatal neurons exposed for 30 min to H2O2, as estimated 24 hr later by the 3-[4,5-dimethylthiazol-2-yl]−2,5-diphenyltetrazoliumbromide assay. Pyruvate strongly protected neurons against both H2O2 added to the external medium and H2O2 endogenously produced through the redox cycling of the experimental quinone menadione. The neuroprotective effect of pyruvate appeared to result rather from the ability of α-ketoacids to undergo nonenzymatic decarboxylation in the presence of H2O2 than from an improvement of energy metabolism. Indeed, several other α-ketoacids, including α-ketobutyrate, which is not an energy substrate, reproduced the neuroprotective effect of pyruvate. In contrast, lactate, a neuronal energy substrate, did not protect neurons from H2O2. Optimal neuroprotection was achieved with relatively low concentrations of pyruvate (≤1 mm), whereas at high concentration (10 mm) pyruvate was ineffective. This paradox could result from the cytosolic acidification induced by the cotransport of pyruvate and protons into neurons. Indeed, cytosolic acidification both enhanced the H2O2-induced neurotoxicity and decreased the rate of pyruvate decarboxylation by H2O2. Together, these results indicate that pyruvate efficiently protects neurons against both exogenous and endogenous H2O2. Its low toxicity and its capacity to cross the blood–brain barrier open a new therapeutic perspective in brain pathologies in which H2O2is involved.

Sphingosine-1-phosphate induces proliferation of astrocytes: regulation by intracellular signalling cascades

Sphingosine‐1‐phosphate (S1P) is a potent lysophospholipid mediator mostly released by activated platelets. It is involved in several functions in peripheral tissues, but its effects in the central nervous system are poorly documented. Therefore, we have examined the effects of S1P on the proliferation of striatal astrocytes from the mouse embryo. These cells have been found to express mRNAs for the S1P receptors, Edg‐1 and Edg‐3. S1P stimulated thymidine incorporation and induced activation of extracellular signal‐regulated kinases (Erks). Both effects were prevented by U0126, an Erk kinase inhibitor. The S1P‐evoked activation of Erk1 was totally blocked in astrocytes pretreated with a combination of either phorbol ester (24 h) and LY294002, or phorbol ester (24 h) and pertussis toxin (PTX). Each individual treatment only partially inhibited Erk1 activation. This suggests that several separate mechanisms mediate this process, one involving protein kinase C and another involving Gi/Go proteins and phosphatidylinositol 3‐kinase. In contrast, the stimulatory effect of S1P on astrocyte proliferation was totally blocked by either PTX or LY294002, but not by a downregulation of protein kinase C. S1P dramatically inhibited the evoked production of cyclic AMP, a response that was impaired by PTX. Finally, S1P stimulated the production of inositol phosphates and increased intracellular calcium by mobilization from thapsigargin‐sensitive stores. These latter effects were mainly insensitive to PTX. Probably, Gi/Go protein activation and phosphoinositide hydrolysis are early events that regulate the activation of Erks by S1P. Altogether, these observations show that astrocytes are targets for S1P. Their proliferation in response to S1P could have physiopathological consequences at sites of brain lesions and alterations of the blood–brain barrier.

Antagonism by riluzole of entry of calcium evoked by NMDA and veratridine in rat cultured granule cells: evidence for a dual mechanism of action

1 Intracellular calcium levels were measured in cultured cerebellar granule cells of the rat by use of the fluorescent dye, indo‐1/AM. 2 Intracellular calcium levels were increased by depolarizing stimuli such as N‐methyl‐d‐aspartate (NMDA) (100 μm), glutamic acid (20 μm), and veratridine (10 μm). This increase was essentially due to entry of external calcium. 3 Riluzole (10 μm) blocked responses to all the depolarizing agents. 4 Riluzole could still block the increase in intracellular calcium evoked by NMDA or glutamic acid when sodium channels were blocked by tetrodotoxin, suggesting that this effect is not mediated by a direct action of riluzole on the voltage‐dependent sodium channel. 5 Pretreatment of the cells with pertussis toxin (0.1 μg ml−1) did not modify the increases in intracellular calcium evoked by NMDA, glutamic acid or veratridine. 6 In pertussis toxin‐treated cells, riluzole could no longer block responses to excitatory amino acids, but still blocked responses to veratridine. 7 It is concluded that riluzole has a dual action on cerebellar granule cells, both blocking voltage‐dependent sodium channels and interfering with NMDA receptor‐mediated responses via a pertussis toxin‐sensitive mechanism. Furthermore, these two processes have been shown to be independent.

Nicotine protects cultured striatal neurones against N-methyl-D-aspartate receptor-mediated neurotoxicity.

The role of cholinergic mechanisms in N-methyl-D-aspartate (NMDA)-mediated neuronal death was investigated using mouse striatal neurones in primary culture. A 30 min exposure of striatal neurones to increasing concentrations of NMDA resulted 24 h later in dramatic neuronal degeneration as assessed by MTT staining, crystal violet incorporation and determination of microtubule-associated protein 2. The NMDA-induced neurodegeneration was strongly inhibited by the co-application of two non-selective cholinergic agonists, acetylcholine or carbachol. This protective effect appears to be mediated by nicotinic receptors since it was insensitive to the muscarinic antagonist atropine but mimicked by nicotine, nornicotine and 1,1-dimethyl-4-phenyl-piperazinium. Moreover, the nicotine-evoked neuroprotection was inhibited by the central nicotinic antagonist hexamethonium. Therefore, this study suggests that cholinergic interneurones play an important role in neuronal survival in the striatum.

Vasoactive Intestinal Polypeptide Receptors Linked to an Adenylate Cyclase, and Their Relationship with Biogenic Amine- and Somatostatin-Sensitive Adenylate Cyclases on Central Neuronal and Glial Cells in Primary Cultures

Abstract: The presence of vasoactive intestinal polypeptide (VIP) receptors coupled to an adenylate cyclase was demonstrated on membranes of neurons or glial cells grown in primary cultures originating from the cerebral cortex, striatum, and mesencephalon of mouse embryos. A biphasic pattern of activation was observed in all these cell types, involving distinct high‐ and low‐apparent‐affinity mechanisms. The absence of additive effects of VIP and 3,4‐dihydroxyphenylethylamine (DA, dopamine), isoproterenol (ISO), and 5‐hydroxytryptamine (5‐HT, serotonin) suggests that the peptide receptors are colocated with each of the corresponding amine receptors on neuronal membranes of the three structures studied. The nonadditivity between the VIP‐ and ISO‐induced responses on cortical and striatal glial membranes reveals as well a colocation of VIP and β‐adrenergic‐sensitive adenylate cyclases on the same cells. A subpopulation of mesencephalic glia could possess only one of the two types of receptors, as a partial additivity of the VIP and ISO responses was seen. In addition, VIP modified the characteristics of the somatostatin inhibitory effect on adenylate cyclase activity of neuronal membranes from the cerebral cortex and striatum but not from those of the mesencephalon. On striatal and mesencephalic glial membranes the somatostatin inhibitory effect was observed only in the presence of VIP. However, as previously seen with ISO, the presence of VIP did not allow the appearance of a somatostatin inhibitory response on cortical glial membranes. This suggests that cortical glia are devoid of somatostatin receptors.

Topographical distribution of dopaminergic innervation and of dopaminergic receptors in the rat striatum. II. Distribution and characteristics of dopamine adenylate cyclase--interaction of d-LSD with dopaminergic receptors.

The characteristics of dopamine adenylate cyclase in the rat striatum were first studied on homogenates of fresh tissues. In the assay conditions used, dopamine (10(-4) M) stimulated the enzyme activity by 250%. This effect was completely blocked by fluphenazine (10(-5) M; Ki=9X10(-9) M) and by phentolamine (10(-5) M; Ki=3 X 10(-7) M). D-LSD stimulated the adenylate cyclase activity (Km=1.4 X 10(-7) M) by interacting with dopamine receptors; indeed the dopamine effect on the enzyme activity was competitively reduced in presence of D-LSD. L-Isoproterenol (Km=10(-6) M) activated an adenylate cyclase through a receptor distinct form the dopaminergic receptor; this stimulation was not affected by fluphenazine or phentolamine but suppressed by DL-propranolol (10(-4) M). The topographical distribution of the dopamine, D-LSD and L-isoproterenol adenylate cyclase activities were examined in homogenates prepared from discs punched out on serial frozed (--7C) slices of the striatum. Under this condition, tge dioanube naxunak stunykatuib was if 150%. A 4.8-fold progressive decrease in the amount of cyclic AMP produced in presence of dopamine (10(-4) M) was observed in the rostrocaudal plane of the structure; the decline of the basal activity was 3.6-fold. The topographical curves of maximal activation of adenylate cyclase by dopamine and D-LSD were superimposable confirming that D-LSD acts on dopaminergic receptors. This topographical distribution of dopamine sensitive adenylate cyclase is comparable on one hand to that of endogenous dopamine and on the other hand to that of the dopamine high affinity uptake activity measured in simultaneous experiments. In contrast to that observed with dopamine or D-LSD, the topographical distribution of the adenylate cyclase sensitive to L-isoproterenol was homogenous within the striatum.

Increase in External Glutamate and NMDA Receptor Activation Contribute to H2O2-Induced Neuronal Apoptosis

Abstract : The present study aims to investigate the role of extracellular glutamate and NMDA receptor stimulation in the neuronal death induced by a transient exposure to H2O2 of cultured neurons originating from mouse cerebral cortex. Most of the neuronal loss following a transient exposure to H2O2 of cortical neurons results from an apoptotic process involving a secondary stimulation of NMDA receptors, which occurs after H2O2 washout. Indeed, (a) the neurotoxic effect of H2O2 was strongly reduced by antagonists of NMDA receptors, (b) the neurotoxic effect of H2O2 was enhanced in the absence of Mg2+, (c) the protective effect of MK‐801 progressively decayed when it was applied with increasing delay time after H2O2 exposure, and (d), finally, the extracellular concentration of glutamate was increased after H2O2 exposure. The major part of H2O2‐induced neurotoxicity is mediated by the formation of hydroxyl radicals, which might be involved in (a) the delayed accumulation of extracellular glutamate and NMDA receptor activation and (b) the poly(ADP‐ribose) polymerase activation and the related NAD content decrease. The combination of these two mechanisms could lead to both an increase in ATP consumption and a decrease of ATP synthesis. The resulting large decrease in ATP content might be finally responsible for the neuronal death.

Modulation by monoamines of somatostatin-sensitive adenylate cyclase on neuronal and glial cells from the mouse brain in primary cultures.

Primary cultures of mouse embryonic neuronal or glial cells from the cerebral cortex, striatum, and mesencephalon were used to identify and determine the cellular localization of somatostatin receptors coupled to an adenylate cyclase. Somatostatin inhibited basal adenylate cyclase activity on neuronal but not on glial crude membranes in the three structures examined. The somatostatin‐inhibitory effect on neuronal crude membranes was still observed in the presence of (—)‐isoproterenol, 3,4‐dihydroxyphenylethylamine (dopamine, DA), or 5‐hydroxytryptamine (5‐HT, serotonin) used at a concentration (10−5M) inducing maximal adenylate cyclase activation. In addition, in most cases biogenic amines modified the pattern of the somatostatin‐inhibitory effect, triggering either an increase in the peptide apparent affinity for its receptors or an increase in the maximal reduction of adenylate cyclase activity or both. However, 5‐HT did not modify the somatostatin‐inhibitory response on striatal and cortical neuronal crude membranes. The changes in somatostatin‐inhibitory responses were interpreted as a colocalization of the amine and the peptide receptors on subtypes of neuronal cell populations. Finally, somatostatin was shown to inhibit adenylate cyclase activity following its activation by (—)‐isoproterenol on glial crude membranes of the striatum and the mesencephalon but not on those of the cerebral cortex.

Pyruvate and lactate protect striatal neurons against N-methyl-D-aspartate-induced neurotoxicity.

A sustained release of glutamate contributes to neuronal loss during cerebral ischaemia. Using cultured mouse striatal neurons, we observed that glucose deprivation, which occurs in this pathological process, enhanced the N‐Methyl‐d‐aspartate (NMDA)‐ or α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate (AMPA)‐induced neurotoxicity. The end products of glycolysis, lactate and pyruvate, strongly protected neurons from these neurotoxic effects. The neuroprotective effect of pyruvate (which is more prominent in the absence of glucose) was not related to its ability to react with H2O2 by a decarboxylation process. Pyruvate and l‐lactate strongly counteracted the deep decrease in the neuronal ATP content induced by NMDA, indicating that they might protect striatal neurons by rescuing cellular energy charge. Addition of MK‐801 after the NMDA withdrawal completely protected neurons, suggesting that NMDA neurotoxicity resulted from a delayed NMDA receptor activation probably linked to a delayed release of an endogenous agonist in the extracellular medium. The strong accumulation of extracellular glutamate which was found in both sham and NMDA‐treated cultures was markedly decreased by pyruvate. Thus, pyruvate might also exert its protecting activity by decreasing the delayed accumulation of glutamate which seemed to be neurotoxic only after a preexposure of neurons to NMDA.

Gap junctional communication and pharmacological heterogeneity in astrocytes cultured from the rat striatum

1 Indo‐1 and fluo‐3 imaging techniques were used to investigate the role of gap junctions in the changes in cytosolic calcium concentrations ([Ca2+]i) induced by several receptor agonists. Subpopulations of confluent cultured astrocytes from the rat striatum were superfused with submaximal concentrations of endothelin‐1 (Et1) and the α1‐adrenergic and muscarinic receptor agonists, methoxamine and carbachol, respectively. 2 Combined binding and autoradiographic studies indicated that all striatal astrocytes possess binding sites for Et1. In contrast, α1‐adrenergic and muscarinic binding sites were found to be heterogeneously distributed. In agreement with these findings, Et1 induced fast calcium responses in all cells while only subsets of striatal astrocytes responded to the application of either methoxamine or carbachol. 3 Halothane, heptanol and octanol, which are commonly used as gap junction inhibitors, drastically reduced the amplitude of Et1‐induced calcium responses. In contrast, 18‐α‐glycyrrhetinic acid (αGA) used at a concentration known to block gap junction permeability in astrocytes had no significant effect on the amplitude of these calcium responses. 4 As demonstrated by quantitative and topological analysis, Et1 application similarly increased [Ca2+]i levels in all astrocytes in both the absence and presence of αGA. 5 In control conditions, subpopulations of cells responding to methoxamine or carbachol exhibited two main types of calcium responses which differed in their shape and kinetic characteristics. In the presence of αGA the number of cells responding to these receptor agonists was significantly reduced. Indeed, responses characterized by their long latency, slow rise time and weak amplitude disappeared in the presence of αGA while responses with short latency and fast rise time were preserved. 6 These results indicate that permeable gap junction channels tend to attenuate the pharmacological and functional heterogeneity of populations of astrocytes, while their inhibition restricts calcium responses in astrocytes expressing high densities of transmitter receptors coupled to phospholipase C.