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

An endogenous Na+, K+-ATPase inhibitor enhances phosphoinositide hydrolysis in neonatal but not in adult rat brain cortex

The effect of an endogenous Na+, K+-ATPase inhibitor, termed endobain E, on phosphoinositide hydrolysis was studied in rat brain cortical prisms and compared with that of ouabain. As already shown for ouabain, a transient effect was obtained with endobain E; maximal accumulation of inositol phosphates induced by endobain E was 604 +/- 138% and 186 +/- 48% of basal values in neonatal and adult rats, respectively. The concentration-response plot for the interaction between endobain E and phosphoinositide turnover differed from that of ouabain, thus suggesting the involvement of distinct mechanisms. In the presence of endobain E plus ouabain at saturating concentrations, no additive effect was recorded, suggesting that both substances share at least a common step in their activation mechanism of inositol phosphates metabolism or that they enhance phosphatidylinositol 4,5-biphosphate breakdown from the same membrane precursor pool, until its exhaustion. Experiments with benzamil, a potent blocker of Na+/Ca2+ exchanger, showed that it partially and dose-dependently inhibited endobain E effect. These results indicate that the endogenous Na+, K+-ATPase inhibitor endobain E, like ouabain, is able to stimulate phosphoinositide turnover transiently during postnatal brain development.

Regulation of phospholipase Cbeta activity by muscarinic acetylcholine and 5-HT(2) receptors in crude and synaptosomal membranes from human cerebral cortex

Stimulation of phospholipase Cbeta by receptor agonists and G proteins has been characterized in crude cerebral membrane preparations, but little is known about their presynaptic localizations and little information is currently available for human brain tissue. The characteristics of phosphoplipase C transmembrane signaling were studied in crude and synaptosomal plasma membranes from postmortem human prefrontal cortex by measuring the hydrolysis of exogenous [(3)H]phosphatidylinositol4,5bisphosphate(PIP(2)) and the immunoreactive levels of phospholipase C (PLC) and G(alphaq/11) proteins. Regulation of PLC activity by Ca(2+) and the 5-HT(2) receptor agonist 5-methyltryptamine, but not by guanosine 5'-O-[3-thiotriphosphate] and the muscarinic acetylcholine receptor agonist carbachol were different between crude and synaptosomal membranes. KCl (20 mM) stimulation was absent in both preparations. Levels of G(alphaq/11)-protein subunits differed between preparations. The functional inhibition carried out with pirenzepine in crude membranes in order to reverse the carbachol-induced PLC stimulation indicates the existence of a component (53%) of the response that is activated by the M(1) muscarinic acetylcholine receptor subtype, and another component (47%) probably mediated by the M(3) muscarinic acetylcholine receptor subtype. In synaptosomal plasma membranes an increased inhibition of carbachol-induced PLC activation through M(1) was found. The PLC activation by 5-methyltryptamine (ketanserin-sensitive in crude membranes) was absent in synaptosomal plasma membranes suggesting the lack of activity mediated by 5-HT(2)-serotonin receptors.

Alteration of phosphoinositide degradation by cytosolic and membrane-bound phospholipases after forebrain ischemia-reperfusion in gerbil: effects of amyloid beta peptide

The reperfusion of previously ischemic brain is associated with exacerbation of cellular injury. Reperfusion occasionally potentates release of intracellular enzymes, influx of Ca2+, breakdown of membrane phospholipids, accumulation of amyloid precursor protein or amyloid beta-(like) proteins, and apolipoprotein E. In this study, the effect of reperfusion injury on the activity of cerebral cortex enzymes acting on phosphatidyl [3H] inositol (PI) and [14C-arachidonoyl] PI was investigated. Moreover the effect of amyloid beta25-35 on PI degradation by phospholipase(s) of normoxic brain and subjected to ischemia-reperfusion injury was determined. Brain ischemia in gerbils (Meriones unguiculatus) was induced by ligation of both common carotid arteries for 5 min and then brains were perfused for 15 min, 2 h and 7 days. Statistically significant activation of enzyme(s) involved in phosphatidylinositol degradation in gerbils subjected to ischemia-reperfusion injury was observed. Nearly all gerbils showed a higher activity of cytosolic PI phospholipase C (PLC) at 15 min after ischemia. Concomitantly, the significant enhancement of the level of DAG and AA radioactivity at this short reperfusion time confirmed the active PI degradation by phospholipase(s) in cerebral cortex and hippocampus. After a prolonged reperfusion time of 7 days after ischemia, both cytosolic and membrane-bound forms of PI-PLC were activated. The question arises if alteration of membranes by the degradation of phospholipids occurring after an ischemic episode potentiates the effect of Abeta on membrane-bound enzymes. A neurotoxic fragment of amyloid, Abeta 25-35, incubated in the presence of endogenous Ca2+, increased significantly the PI-PLC activity of normoxic brain. In its non-aggregated form, Abeta 25-35 activates PI-PLC but in the aggregated form the enzymatic activity decreased. Thus, Abeta 25-35 exerts a similar effect on the membrane-bound PI-PLC from normoxic brain or subjected to ischemia reperfusion injury. We conclude that the degradation of phosphatidylinositol by cytosolic phosphoinositide-phospholipase C may contribute to the pathophysiology of delayed neuronal death following cerebral ischemia. Thus, a specific inhibitor of this enzyme(s) may offer therapeutic strategies to protect the brain from damage triggered by ischemia. Ischemia-reperfusion injury had no effect on Abeta-evoked alterations of synaptic plasma membrane-bound PI-PLC.

Regulation of levels of 5-HT2A receptor mRNA

We have been able to identify two separate pathways present in P11 cells that can increase the levels and stability of 5-HT2A receptor mRNA. One pathway is activated following exposure of cells to serotonin and is dependent upon activation of a PKC isoform that is susceptible to downregulation by a 24-h pretreatment with PMA; another pathway is activated following treatment of cells with a calcium ionophore. This pathway is not dependent on activation of PKC isoforms that can be downregulated by a 24-h treatment with phorbol ester. Such heterologous regulation of levels of 5-HT2A receptor mRNA may be important in understanding in vivo events where multiple factors contribute to the modulation of levels of cell surface receptors.

Glutamate-stimulated production of inositol phosphates is mediated by Ca2+ influx in oligodendrocyte progenitors

The effect of glutamate on the accumulation of [3H]inositol phosphates was examined in oligodendrocyte progenitor cultures prepared from rat brains. Glutamate, and the analogues alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate, caused a concentration- and time-dependent increase in [3H]inositol trisphosphate (IP3) formation and the effect was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a competitive AMPA and kainate receptor antagonist. Similarly, the more selective, noncompetitive antagonist of AMPA receptors, 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), significantly reduced the effect of both AMPA and kainate. In contrast, antagonists of N-methyl-D-aspartate (NMDA) receptor, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5, 10-imine (MK-801) and R(-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), and antagonists of metabotropic receptors, L(+)-2-amino-3-phosphono-propanoic acid (L-AP3) and alpha-methyl-4-carboxyphenylglycine (MCPG), were ineffective. These results suggest that the effect of glutamate on [3H]IP3 accumulation is mediated through ionotropic AMPA receptors. Cyclothiazide, an inhibitor of AMPA receptor desensitization, strongly potentiated the AMPA and kainate-stimulated [3H]IP3 formation as well as the uptake of 45Ca2+ in line with the previous findings. 45Ca2+ uptake evoked by AMPA or kainate, in combination with cyclothiazide, was also prevented by both CNQX and GYKI 52466. Glutamate-stimulated [3H]IP3 accumulation was prevented by EGTA, suggesting a requirement for extracellular calcium. Pre-incubation with the voltage-gated Ca2+ channel blockers, diltiazem, nifedipine and CdCl2, partially prevented the glutamate-induced [3H]IP3 accumulation as well as 45Ca2+ uptake. Similarly, the Na+/Ca2+ exchanger blockers benzamil and 3,4-dichlorobenzamil reduced significantly kainate-stimulated 45Ca2+ uptake. These data indicate that glutamate-induced [3H]IP3 accumulation is triggered by calcium influx via AMPA receptors, voltage-gated calcium channels and the Na+/Ca2+ exchanger operating in reverse mode.

Nitric oxide synthase inhibitors do not attenuate diacylglycerol or monoacylglycerol lipase activities in synaptoneurosomes

Neuron-enriched cultures and synaptoneurosomal fractions from 10 day-old rat brain contain diacylglycerol and monoacylglycerol lipase activities. Glutamate and its analogs stimulate the activities of diacylglycerol and monoacylglycerol lipases in a time- and dose-dependent manner. Stimulation of diacylglycerol and monoacylglycerol lipases by glutamate or NMDA can be blocked by MK-801 (non-competitive antagonist). Nitro L-arginine methyl ester and L-methylarginine have no effect on glutamate stimulated activities of diacylglycerol and monoacylglycerol lipases. Our studies suggest that synaptoneurosomal preparations from young rat brain are useful for obtaining important information on signal transduction.

Functional characterisation of alpha 1-adrenoceptor subtypes mediating noradrenaline-induced inositol phosphate formation in rat thalamus slices

In cross-chopped slices from rat thalamus and in the presence of 10 mM LiCl, noradrenaline stimulated the accumulation of [3H]inositol phosphates with [3H]inositol monophosphates ([3H]IP1) being the major product detected (86 +/- 2% of total [3H]inositol phosphates). Noradrenaline-induced [3H]IP1 accumulation was concentration-dependent and yielded and EC50 of 4.6 +/- 0.2 microM, maximum effect of 272 +/- 3% of basal formation and Hill coefficient (nH) of 1.6 +/- 0.1. The effect of 100 microM noradrenaline was inhibited by the alpha 1-adrenoceptor antagonists prazosin, (+)-niguldipine, 5-methylurapidil and WB-4101 (2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane). The inhibition curve for prazosin best fit to a single-site model whereas curves for (+)-niguldipine, 5-methylurapidil and WB-4101 best fit to a two-site model. The putative alpha 1D-adrenoceptor-selective antagonist BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8- azaspiro[4.5]decane-7,9-dione) showed low potency and efficacy to inhibit the response to noradrenaline. Pre-treatment of the slices with chloroethylclonidine (100 microM; 30 min) decreased by 64 +/- 4% the maximum response. Noradrenaline-induced [3H]IP1 accumulation was significantly reduced by Ca2+ removal (by 64 +/- 2%) and by the Ca(2+)-channel blockers Ni2+, Co2+ and nimodipine (inhibition of 56 +/- 6%, 54 +/- 5% and 41 +/- 5%, respectively). Taken together these results indicate that noradrenaline-induced inositol phosphate formation in thalamus slices is mainly mediated by the activation of both alpha 1B and alpha 1A subtypes of alpha 1-adrenoceptors.

Stimulation of phospholipase C activity by norepinephrine, t-ACPD and bombesin in LA-N-2 cells

The release of [3H]inositol phosphates from myo-[3H]inositol-prelabeled LA-N-2 cells was measured in the presence of beta-adrenoceptor, metabotropic glutamate and bombesin agonists. Norepinephrine and isoproterenol increased the formation of [3H]inositol phosphates in a dose-dependent manner, with an EC50 of 100 microM for norepinephrine and an EC50 of 5 microM for isoproterenol. These stimulations were abolished by propranolol, a beta-adrenoceptor antagonist, with an IC50 in the range of 50-55 microM for both norepinephrine and isoproterenol. The stimulation of [3H]inositol phosphate appearance occurred with varying concentrations of trans-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD), a metabotropic glutamate receptor agonist. This release of [3H] inositol phosphates was blunted by its antagonist, 2-amino-3-phosphonopropionic acid (AP-3). Bombesin and neuromedin-B, a bombesin-like peptide, also increased the appearance of [3H]inositol phosphates. This was blunted by the antagonist [Tyr4, D-Phe12] bombesin. The appearance of [3H]inositol phosphates stimulated by t-ACPD was coupled through a cholera toxin-sensitive G-protein and the bombesin-stimulated appearance of [3H]inositol phosphates was coupled through a pertussis toxin-sensitive G-protein. The norepinephrine-stimulated appearance of [3H]inositol phosphates was toxin insensitive. The stimulation of the [3H]inositol phosphate appearance by these three agonists was protein kinase and Ca2+ independent.

Intra- vs extracellular calcium regulation of neurotransmitter-stimulated phosphoinositide breakdown

The dependence on Ca2+ of basal, glutamate- and carbachol-stimulated phosphoinositide (PI) turnover was studied on 8-day old rat brain synaptoneurosomes. For that purpose, intracellular and extracellular Ca2+ concentrations were buffered by bis-(alpha-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, in its tetra(acetoxymethyl)-ester form (BAPTA-AM) and in its free acid form (BAPTA), respectively. The effects of both forms of the calcium chelator intracellular and extracellular Ca2+ buffering on intracellular and extracellular Ca2+ concentration ([Ca2+]i and [Ca2+]e) were determined with fluorimetric assay using fura2, either in its acetoxymethyl ester form (fura2-AM) or in its free acid form. Intracellular chelation of Ca2+ ions with BAPTA-AM induced a dose-dependent reduction of the [Ca2+]i. Basal inositol phosphate (IP) formation was slightly affected by this [Ca2+]i buffering, while glutamate and carbachol stimulations of PI hydrolysis were similarly diminished. Chelation of extracellular Ca2+ ions with BAPTA produced a reduction of both [Ca2+]e and [Ca2+]i. Basal IP accumulation was maximally inhibited by 50%. The carbachol-induced PI hydrolysis was completely inhibited in the presence of 200 microM BAPTA, while a substantial residual glutamate-elicited IP response remained (40% of the control response). It is concluded that [Ca2+]i of synaptoneurosomes is not critical for basal and neurotransmitter-stimulated IP formation, whilst [Ca2+]e is critical. Glutamate may, in part, stimulate PI breakdown in a Ca(2+)-insensitive way.

Noradrenaline-induced inositol phosphate formation in rat striatum is mediated by alpha 1A-adrenoceptors

The aim of this study was to assess the contribution of alpha 1-adrenoceptor subtypes to noradrenaline (NA)-induced inositol phosphate formation in rat striatum. In cross-chopped slices and in the presence of 10 mM LiCl, NA stimulated the accumulation of [3H]inositol phosphates. After 60-min incubation with 100 microM NA, [3H]IP1 was the major product detected (82 +/- 3% of total [3H]inositol phosphates). Best-fit values for the concentration-response curve for NA-induced [3H]IP1 accumulation yielded an EC50 of 9.4 +/- 1.1 microM, maximum effect of 210 +/- 3% of basal, and Hill coefficient (nH) of 1.1 +/- 0.1. Pre-treatment of the slices for 30 min with the alkylating agent chloroethylclonidine (100 microM) failed to decrease significantly the response to 100 microM NA. Inhibition curves for four alpha 1-antagonists, (+)-niguldipine, prazosin, WB-4101 and 5-methylurapidil (5-MU), best-fit to a single-site model with pKi values of 9.4 ((+)-niguldipine), 9.2 (prazosin and WB-4101) and 8.8 (5-MU). The putative alpha 1 D-selective antagonist BMY 7378 reduced the response to NA only partially (30 +/- 3% inhibition at 1 microM: pKi 7.24). NA-induced [3H]IP1 accumulation was significantly reduced (to 20 +/- 9% of controls) by Ca2+ removal and increased as the extracellular Ca2+ concentration was raised from nominally zero (no added Ca2+) to 1 mM Ca2+. NA-induced [3H]IP1 accumulation was reduced by both the non-selective Ca2+ channel blocker Ni2+ (58 +/- 3% inhibition at 2 mM) and nimodipine, an antagonist of L-type voltage-operated Ca2+ channels (77 +/- 4% inhibition at 3 microM). Taken together these results indicate that NA-induced inositol phosphate formation in striatal slices is mediated by activation of alpha 1A-adrenoceptors coupled to Ca2+ entry and Ca2+ activation of phospholipase C.

Down-regulation of brain muscarinic cholinergic receptor promoted by diacylglycerols and phorbol ester

Sustained agonist stimulation induces an asymmetric down-regulation of brain muscarinic acetylcholine receptor (mAChR): 43 +/- 2% in the right and 26 +/- 2% in the left cerebral hemisphere, respectively (Ref. 1). In order to determine the possible involvement of endogenous diacylglycerols produced under muscarinic stimulation in the down-regulation phenomenon, here we have studied the effects of synthetic diacylglycerols and a phorbol ester on cells dissociated from rat cerebral cortex. Oleylacetylglycerol decreased the amount of cell-surface mAChR by 37 +/- 2% and 25 +/- 2% in right and left cerebral cortex, respectively. Long-term treatment with phorbol dibutyrate also produced internalization of the mAChR (25 +/- 1.5% and 33 +/- 2% in right and left cortical cells, respectively). These changes occurred without modification of the Kdapp for the selective antagonist pirenzepine. The action of calcium ions was also studied using incubation of cells with the ionophore A23187. No changes were observed in the amount of mAChR detected at the plasma membrane with the ionophore alone, but when used in combination with phorbol dibutyrate and the agonist carbamylcholine a sinergistic decrease in mAChR was apparent. It is concluded that long-term exposure to exogenously added diacyglycerols and phorbol ester significantly reduces the amount of mAChR detected at the plasma membrane and abolishes the asymmetry of the down-regulation phenomenon observed under specific muscarinic stimulation, suggesting that diacylglycerols may be one of the factors responsible for such asymmetry.

Activation of phospholipase C in SH-SY5Y neuroblastoma cells by potassium-induced calcium entry

1. We used SH-SY5Y human neuroblastoma cells to investigate whether depolarization with high K+ could stimulate inositol (1,4,5)trisphosphate (Ins(1,4,5)P3) formation and, if so, the mechanism involved. 2. Ins(1,4,5)P3 was measured by a specific radioreceptor mass assay, whilst [Ca2+]i was measured fluorimetrically with the Ca2+ indicator dye, Fura-2. 3. Depolarization with K+ caused a time- and dose-dependent increase in [Ca2+]i (peak at 27 s, EC50 of 50.0 +/- 9.0 mM) and Ins(1,4,5)P3 formation (peak at 30 s, EC50 of 47.4 +/- 1.1 mM). 4. Both the K(+)-induced Ins(1,4,5)P3 formation and increase in [Ca2+]i were inhibited dose-dependently by the L-type voltage-sensitive Ca2+ channel closer, (R+)-BayK8644, with IC50 values of 53.4 nM and 87.9 nM respectively. 5. These data show a close temporal and dose-response relationship between Ca2+ entry via L-type voltage-sensitive Ca2+ channels and Ins(1,4,5)P3 formation following depolarization with K+, indicating that Ca2+ influx can activate phospholipase C in SH-SY5Y cells.

Modulation of signal transduction in rat synaptoneurosomes by platelet activating factor

The potential involvement of platelet activating factor (PAF, 1-O-alkyl 2-O-acetyl-sn-glycero-3-phosphocholine) in aggravation of ischemic brain injury has been recently postulated. Reported evidences in support of this thesis include increases of brain PAF concentration during ischemia and the neuroprotective effect exerted by PAF antagonists. In this article, we demonstrate that several PAF-mediated biochemical responses in synaptoneurosomes in vitro resemble these observed previously in ischemic brain and are widely acknowledged as the potentially causal factors in this pathology. In synaptoneurosomes prepared from rat hippocampus, 10 nM PAF caused an observable elevation of intracellular calcium as measured by fluorescence Fura-2A probe. A similar elevation of synaptoneurosomal [Ca2+]i was evoked by 1 mM glutamate treatment. As an effect of calcium entry after PAF application, a translocation of protein kinase C (PKC) toward plasma membranes was demonstrated by 3H-labeled phorbol-binding method. It was followed by an increase of 50 kDa proteolytic fragment of the enzyme (PKM) recognized on Western blots with anti-PKC antibody. Incubation of synaptoneurosomes in the presence of calcium chelators abolished these effects of PAF and significantly decreased the content of PKC in the membranes. Furthermore, PAF treatment markedly attenuated the receptor- and postreceptor-activated cAMP accumulation in synaptoneurosomes. The decrease of cAMP level seems to be secondary to the PAF-induced calcium entry with subsequent activation of cAMP-specific phosphodiesterase, since it was completely blocked by IBMX, a potent inhibitor of this enzyme. Our observations indicate that PAF in a concentration found in ischemic brain can elevate [Ca2+]i and potentiate calcium-dependent intracellular signalling in synaptoneurosomes in vitro, including PKC translocation/activation and proteolysis, followed by IBMX-sensitive inhibition of cAMP production. The relative contribution of these events to ischemic brain injury is currently under extensive investigation.

Differential effects of polychlorinated biphenyl congeners on phosphoinositide hydrolysis and protein kinase C translocation in rat cerebellar granule cells

Previous reports from our laboratory have suggested that the neuroactivity of some polychlorinated biphenyl (PCB) congeners is associated with perturbations in cellular Ca(2+)-homeostasis. We have characterized further the neurochemical effects of PCBs on signal transduction in primary cultures of cerebellar granule cells. The present experiments found that neither 2,2'-dichlorobiphenyl (DCBP), an ortho-substituted congener, nor 3,3',4,4',5-pentachlorobiphenyl (PCBP), a non-ortho-substituted congener, affected basal phosphoinositide (PI) hydrolysis in cerebellar granule cells. However, at concentrations up to 50 microM, DCBP potentiated carbachol-stimulated PI hydrolysis, while decreasing it at 100 microM. PCBP, on the other hand, had no effect on carbachol-stimulated PI hydrolysis in concentrations up to 100 microM. [3H]Phorbol ester ([3H]PDBu) binding was used to determine protein kinase C (PKC) translocation. DCBP increased [3H]PDBu binding in a concentration-dependent manner and a twofold increase was observed at 100 microM in cerebellar granule cells. PCBP had no effect on [3H]PDBu binding at concentrations up to 100 microM. The effect of DCBP on [3H]PDBu binding was time-dependent and was also dependent on the presence of external Ca2+ in the medium. To test the hypothesis that DCBP increases [3H]PDBu binding by acting on receptor-activated calcium channels, the effects of DCBP were compared to those of L-glutamate. The effects of DCBP (50 microM) and glutamate (20 microM) were additive.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca(2+)-dependent inactivation of P-type calcium channels in nerve terminals

Rapid Ca2+ signals evoked by K+ depolarization of rat cerebral cortical synaptosomes were measured by dual-channel Ca2+ spectrofluorometry coupled to a stopped-flow device. Kinetic analysis of the signal rise phase at various extracellular Ca2+ concentrations revealed that the responsible voltage-dependent Ca2+ channels, previously identified as P-type Ca2+ channels, inactivate owing to the rise in intracellular Ca2+ levels. At millimolar extracellular Ca2+ concentrations the channels were inactivated very rapidly and the rate was dependent on the high influx rate of Ca2+, thus limiting the Ca2+ signal amplitudes to 500-600 nM. A slower, probably voltage-dependent regulation appears to be effective at lower Ca2+ influx rates, leading to submaximal Ca2+ signal amplitudes. The functional feedback regulation of calcium channels via a sensor for intracellular Ca2+ levels appears to be responsible for the different inhibition characteristics of Cd2+ versus omega-agatoxin IVa.

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.

Sulphur-containing excitatory amino acid-stimulated inositol phosphate formation in primary cultures of cerebellar granule cells is mediated predominantly by N-methyl-D-aspartate receptors

The stimulatory effect of excitatory sulphur-containing amino acids on inositol phosphate formation was investigated in primary cultures of cerebellar granule cells. L-Cysteine sulphinate (CSA), L-cysteate (CA), L-homocysteine sulphinate (HCSA), L-homocysteate (HCA) and S-sulpho-L-cysteine (SSC) dose-dependently stimulated the formation of [3H]inositol phosphates exhibiting EC50 values in the range 60-200 microM and maximal effects of six- to 17-fold that of basal [3H]inositol phosphate levels. Endogenous L-glutamate spontaneously released into the extracellular medium or following exposure of cells to HCSA, HCA or SSC did not contribute significantly to formation of [3H]inositol phosphates, whereas 10% of the total [3H]inositol phosphates accumulated following exposure to CSA and CA was due to released L-glutamate. The selective N-methyl-D-aspartate receptor antagonist, D,L-2-amino-5-phosphonopentanoic acid (APV, 500 microM) attenuated by 20% (HCSA) to between 80 and 100% (CSA, CA, SSC, HCA) the formation of [3H]inositol phosphates induced by 1 mM sulphur-containing amino acids. When, however, HCSA was used at 100 microM (a concentration near to its EC50 for phosphoinositide hydrolysis), APV inhibited induced responses by 70%. Sulphur-containing amino acid-stimulated [3H]inositol phosphate formation was unaffected by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). Inhibition of sulphur-containing amino acid-stimulated [3H]inositol phosphate formation by co-administration of APV and CNQX was similar to that obtained in the presence of APV alone. CSA-, CA-, SSC- and HCA-stimulated [3H]inositol phosphate formation was markedly reduced by removal of Ca2+ from the extracellular medium whereas that stimulated by HCSA was less affected. A similar inhibitory profile was observed when the levels of sulphur-containing amino acid-induced increases in intracellular free calcium ([Ca2+]i) were measured in the presence of 500 microM APV; 1 mM HCSA-induced responses being inhibited by only 30% whereas responses to the remaining sulphur-containing amino acid (also at 1 mM) were inhibited by > 45%. When the sulphur-containing amino acids were used at concentrations approximating their EC50 values for phosphoinositide hydrolysis, APV inhibited the induced increases in [Ca2+]i by 70-100%. HCA and SSC co-administered with the less efficacious but selective metabotropic glutamate receptor agonist, (+-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) at maximally effective concentrations (1 mM) of each agonist stimulated [3H]inositol phosphate formation in an additive manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging modulates calcium-dependent phosphatidylinositol degradation by cerebral cortex synaptic plasma membrane phospholipases

The synaptic plasma membrane (SPM) and cytosol fractions from cerebral cortex of adult (4-mo-old) and aged (27-mo-old) rats were used as a source of phospholipase A2 (PLA2) and phospholipase C (PLC). The activity of PLC acting on [3H-inositol]phosphatidylinositol ([3H]PtdIns) was investigated in the presence of endogenous and 2 mM Ca2+. Arachidonic acid (AA) release was studied in the same conditions, using 1-stearoyl-[2-14C]arachidonyl-sn-glycerophosphoinositol ([14C]PtdIns) as a substrate. In the presence of endogenous Ca2+ (i.e., no added Ca2+) SPM-bound PLC and PLA2 or diacylglycerol (DAG) lipase of aged brain exert significantly higher activity in degradation of PtdIns as compared to their activities in adult brain. Moreover, these enzymes of aged brain are less or not further activated by 2 mM Ca2+, contrary to the enzymes isolated from adult brain. The activity of cytosolic enzymes involved in degradation [3H]PtdIns and [14C]PtdIns and their regulation by Ca2+ ions are not significantly changed in senescent cerebral cortex as compared to the adult. The intracellular calcium concentration ([Ca2+]i), measured with fura-2, is lower in aged brain compared to adult brain, which may suggest the modification in Ca2+ ion redistribution in aged brain and probably its higher concentration in membranes. These results indicate that aging modifies significantly the activity of membrane-bound, Ca(2+)-dependent phospholipase(s) degrading PtdIns, which may be connected with alteration of Ca2+ ion redistribution and may influence the formation and accumulation of very potent lipid messengers as diacylglycerol, lysophospholipid, and arachidonic acid, known to be involved in neurotransmission processes.

Cholinergic and serotonergic stimulation of phosphoinositide hydrolysis is decreased in Alzheimer's disease

Agonist-stimulated phosphoinositide (PPI) hydrolysis is a major signal transduction pathway in brain. These studies investigated neurotransmitter stimulated PPI hydrolysis in postmortem human brain. Preliminary studies using rat brain suggested that moderate postmortem delay has little effect on PPI hydrolysis and that human tissue might be reliably studied for differences in receptor-PLC coupling. Studies in human brain membranes (frontal cortex) indicated that the time course for GTP gamma S and carbachol/GTP gamma S-stimulated PPI hydrolysis was linear for at least 20 min. GTP gamma S-stimulated [3H]inositol phosphate (InsP) formation was enhanced by carbachol (232%) and 5-Hydroxytryptamine (5HT-147%). SAX-HPLC separation of [3H]inositol polyphosphates indicated that the major isomer of inositol trisphosphate (InsP3) was Ins(1.4.5)P3, the expected product of PtdIns(4,5)P2 hydrolysis. Ca2+ increased PPI hydrolysis progressively from 100 nM through 50 microM and synergistically enhanced carbachol/GTP gamma S stimulation. Comparisons of age-matched controls with Alzheimer's patients indicated that GTP gamma S, carbachol/GTP gamma S, and 5HT/GTP gamma S-stimulation of PPI hydrolysis is reduced approximately 50% in membranes prepared from Alzheimer's patients. Ca2+ of PPI hydrolysis was not different between controls and Alzheimer's patients suggesting that muscarinic cholinergic and serotonergic receptors are uncoupled from PLC in Alzheimer's disease. These studies indicate that there are changes in cholinergic and serotonergic signal transduction in Alzheimer's disease. Further, this method can be used to study signal transduction events in postmortem human brain.

Analysis of rapid calcium signals in synaptosomes

A combination of the stopped-flow technology with dual channel spectrofluorometry of Ca(2+)-indicators was utilized for the measurement of rapid Ca(2+)-signals in rat cerebral cortical synaptosomes evoked by K(+)-depolarization. There was no observable contribution of Ca(2+)-ions from intracellular stores to the rise in [Ca2+]i. The kinetics of the fast increase in intracellular Ca2+ concentration was analysed in relation to the depolarization strength. The maximal increase in [Ca2+]i and the time course of Ca(2+)-channel inactivation were determined for depolarizations obtained by different extracellular K(+)-concentrations ([K+]o). An apparent threshold was observed at about 18 mM [K+]o; a maximal Ca(2+)-signal amplitude was estimated at about 40 mM [K+]o. Pharmacological properties of the involved Ca(2+)-channels were determined using selective Ca(2+)-channel blockers (Dihydropyridines, omega-Conotoxin, omega-Agatoxins); the results suggest that a P-type voltage-dependent Ca(2+)-channel is the relevant channel type, generating the evoked Ca(2+)-signals in rat cerebral cortical synaptosomes.

Further characterization of 5-HT- and 5-HT3 receptor agonists'-stimulated phosphoinositol phosphates accumulation

The calcium requirement for serotonin (5-HT)- and the 5-HT3 receptor agonists, 2-Me-5-HT- and PBG-dependent breakdown of phosphatidyl inositol has been examined in the rat fronto-cingulate cortex. The omission of added Ca2+ from the Kreb's incubation medium reduced the [3H]inositol phosphate accumulation from pre-labelled phospholipids. Removal of Ca2+ by pre-incubation with EGTA (0.5 mM), as well as the addition of the calcium channel blocker, lanthanum (10 microM), abolished the 5-HT- and the 5-HT3 receptor agonists'-stimulated phosphoinositide (PI) response. By contrast, the calcium ionophores, A 23187 and Ionomycin (both at 30 microM) stimulated PI hydrolysis, and this effect was additive to the increased PI turnover induced by 5-HT, 2-Me-5-HT and PBG. The increase in phosphoinositide hydrolysis induced by 5-HT and 2-Me-5-HT was significantly inhibited by phorbol dibutyrate (PDBu) and phorbol myristate acetate, indicating that the activation of protein kinase C (PKC) may provide negative feedback to the PI response induced by 5-HT and 2-Me-5-HT-stimulated PI metabolism was reversed by the PKC inhibitors, staurosporine, calphostin C and chelerythrine (all at 10 microM), however, Pertussis toxin (0.5 and 1 microgram) had no effect on either 5-HT's or 2-Me-5-HT's increased stimulation of PI hydrolysis, suggesting that this response is not associated to a Gi GTP binding protein.

Noradrenaline stimulation unbalances the phosphoinositide cycle in rat cerebral cortical slices

Muscarinic cholinergic and alpha 1-adrenoceptor-mediated stimulation of phosphoinositide hydrolysis in rat cerebral cortex were compared by measuring carbachol- and noradrenaline-induced accumulation of various intermediates of the phosphoinositide cycle. Unlike carbachol, noradrenaline in the presence of guanosine 5'-O-(3-thiotriphosphate) did not stimulate phospholipase C activity in brain cortical membranes. In cortical slices, the efficacy of noradrenaline to stimulate accumulation of 3H-inositol phosphates and [32P]phosphatidic acid was 2.5 to threefold that of carbachol. However, noradrenaline was less effective than carbachol in stimulating accumulation of [3H]CDP-diacylglycerol and resynthesis of phosphatidylinositol. This was not due to calcium inhibition of CTP:phosphatidate cytidyltransferase or to different lithium requirements for carbachol- and noradrenaline-stimulated accumulation of [3H]CDP-diacylglycerol. The noradrenaline-induced unbalance of the phosphoinositide cycle, which was most apparent at relatively high concentrations of calcium (2.5 mM) in the incubation buffer, was qualitatively reproduced with ionomycin. The use of the alpha 1a-subtype-selective adrenoceptor antagonists WB4101 and 5-methylurapidil revealed a single alpha 1a-like component mediating the effects of noradrenaline. Our results suggest that the primary mechanism for phospholipase C activation by brain alpha 1 adrenoceptors involves an increase in intracellular calcium concentration.

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.

The control of intracellular calcium and neurotransmitter release in guinea pig-derived cerebral cortical synaptoneurosomes

Synaptoneurosomes are a simply derived brain vesicular preparation which are believed to contain elements of both presynaptic and postsynaptic material. Inositol phosphates production and neurotransmitter release in the synaptoneurosome have previously been shown to be under the control of a number of receptor agonists. However, there have been few investigations of the role of intracellular calcium ([Ca2+]i) in these events. In this study we report that potassium (K+; 50 mM) was able to increase [Ca2+]i and subsequently release [3H]noradrenaline in guinea pig cerebral cortical synaptoneurosomes via activation of dihydropyridine-insensitive, voltage-sensitive calcium channels. Veratridine (30 microM) produced similar effects but these involved activation of sodium channels which could be blocked by pre-incubation with tetrodotoxin (0.15 microM). A number of agonists were used to investigate possible modulation of these events and to look for agonist-stimulated mobilization of [Ca2+]i. No evidence was found for either receptor-mediated release of calcium from intracellular stores or for modulation of K(+)-induced neurotransmitter release. This might be related to the observed passive entry of calcium through the synaptoneurosomal membrane and the subsequently high levels of [Ca2+]i.

Synergistic activation of phosphoinositide hydrolysis induced in brain slices by norepinephrine and the excitatory amino acid agonist, trans-ACPD

Norepinephrine and trans-1-aminocyclopentyl-1,3-dicarboxylic acid (ACPD) each individually stimulated hydrolysis of phosphoinositides and when tested in combination caused a stimulation that was 50-90% greater than additive in hippocampal and cortical slices of the rat but not in striatal slices. This synergistic augmentation of hydrolysis of phosphoinositide was evident with all stimulatory concentrations of norepinephrine and of ACPD up to 1 mM, at which point ACPD was inhibitory. A time-course study revealed no lag in the synergistic interaction and no down-regulation through 60 min of incubation of the augmented response to the combined agonists. The synergistic reaction was mediated by alpha 1-adrenergic receptors and by metabotropic excitatory amino acid receptors. Increased intracellular calcium, but not activation of protein kinase C, may play a role in mediating the synergistic interaction. Thus, a unique synergistic stimulatory interaction was found between two receptors coupled with phosphoinositide metabolism, a finding which also supports the suggestion that these two systems are co-localized in some cells.

Guanine nucleotide- and muscarinic agonist-dependent phosphoinositide metabolism in synaptoneurosomes from cerebral cortex of immature rats

Guanine nucleotide-, neurotransmitter-, and fluoride-stimulated accumulation of [3H]inositol phosphates ([3H]InsPs) was measured in [3H]inositol-labeled synaptoneurosomes from cerebral cortex of immature (7-day-old) and adult rats, in order to clarify the role of GTP-binding proteins (G-proteins) in modulating phosphoinositide (PtdIns) metabolism during brain development. GTP(S) [Guanosine 5'-O-(3-thio)triphosphate] time- and concentration-dependently stimulated PtdIns hydrolysis. Its effect was potentiated by full (carbachol, metacholine) and partial (oxotremorine) cholinergic agonists through activation of muscarinic receptors. The presence of deoxycholate was required to demonstrate agonist potentiation of the guanine nucleotide effect. The response to GTP(S) was higher in adult than in immature rats, while the effect of cholinergic agonists was similar at the two ages examined. At both ages, histamine potentiated the effect of GTP(S), while norepinephrine was ineffective. At both ages, guanosine 5'-O-(2-thio)diphosphate [GDP(S)] and pertussis toxin significantly decreased GTP(S)-induced [3H]InsPs formation. The phorbol ester phorbol 12-myristate 13-acetate (PMA), on the other hand, did not inhibit the guanine nucleotide response in synaptoneurosomes from immature rats. NaF mimicked the action of GTP(S) in stimulating PtdIns hydrolysis. Its effect was not affected by carbachol and was highly synergistic with that of AlCl3, according to the concept that fluoroaluminate (AlF4-) is the active stimulatory species. No quantitative differences were found in the response to these salts between immature and adult animals. These results provide evidence that, in both the immature and adult rat brain, neuroreceptor activation is coupled to PtdIns hydrolysis through modulatory G-proteins.

A specific transduction mechanism for the glutamate action on phosphoinositide metabolism via the quisqualate metabotropic receptor in rat brain synaptoneurosomes: II. Calcium dependency, cadmium inhibition

In this article, we demonstrate that an increase in intracellular Ca2+ concentration may represent a specific common step(s) in the mechanism(s) of action of glutamate (Glu) and depolarizing agents on formation of inositol phosphates (IPs) in 8-day-old rat forebrain synaptoneurosomes. In fact, A23187, a Ca2+ ionophore, induces a dose-dependent accumulation of IPs, which is not additive with that evoked by Glu and K+ but is slightly synergistic with that induced by carbachol. In addition, Glu and K+ augment the intracellular Ca2+ concentration in synaptoneurosome preparations as measured by the fura-2 assay. The absence of external Ca2+ decreases basal and Glu-, and K(+)-stimulated formation of IPs. Cd2+ (100 microM) fully inhibits both Glu- and K(+)-evoked formation of IPs without affecting the carbachol-elicited response of IPs. Zn2+ inhibits Glu- and K(+)-stimulated accumulation of IPs (IC50 approximately 0.4 mM) but with a lower affinity than Cd2+ (IC50 approximately 0.035 mM). The organic Ca2+ channel blockers verapamil (10 microM), nifedipine (10 microM), omega-conotoxin (2 microM), and amiloride (10 microM) as well as the inorganic blockers Co2+ (100 microM) and La3+ (100 microM) block neither Glu- nor K(+)-evoked formation of IPs, a result suggesting that the opening of the L-, T-, N-, or P-type Ca2+ channels does not participate in these responses. All these data suggest that an increase in intracellular Ca2+ concentration resulting from an influx of Ca2+, sensitive to Cd2+ but not to other classical Ca2+ antagonists, may play a key role in the transduction mechanism activated by Glu or depolarizing agents.

Sodium inhibits hormone release and stimulates calcium efflux from isolated nerve endings of the rat neurohypophysis

1. We studied the effects of extracellular sodium on the secretion of vasopressin (VP) and oxytocin (OT) and the efflux of 45Ca from isolated, perfused nerve endings of the rat neurohypophysis (neurosecretosomes). 2. Upon removal of sodium from the perfusing medium, basal release of VP and OT increased by 3.95 +/- 0.23- and 3.71 +/- 0.22-fold, respectively, followed by a decline to about double the levels in normal (150 mM) sodium (P less than or equal to 0.1). 3. Compared to neurosecretosomes perfused in normal (150 mM) sodium, omission of sodium from the medium augmented ionomycin-induced VP and OT secretion by 66 +/- 5- and 20 +/- 3-fold, respectively, and A23187-induced secretion was increased 1.3 +/- 0.4- and 1.3 +/- 0.1-fold (P less than or equal to 0.01 for both ionophores). 4. The inhibition of ionomycin-induced secretion by sodium was concentration dependent (P less than or equal to 0.01 for sodium greater than or equal to 5 mM); the IC50 was about 10 mM sodium for both hormones, and the Hill slope was close to -1. 5. The rate of 45Ca efflux from neurosecretosomes showed 2.7 +/- 0.1-fold stimulation upon increasing sodium from 4.5 to 150 mM (P less than or equal to 0.01). 6. Our results suggest that sodium inhibits basal and stimulated secretion at the nerve terminal, possibly by reducing intraterminal calcium through sodium/calcium exchange.

Pretreatment of cerebellar granule cells with concanavalin A potentiates quisqualate-stimulated phosphoinositide hydrolysis

The hydrolysis of phosphoinositides (PI) elicited in cerebellar granule cell cultures by agonists of metabolotropic glutamate receptors, glutmate and quisqualate, was enhanced when the cells were pretreated with concanavalin A (Con-A). A similar effect was produced by wheat germ agglutinin, but not by several other lectins tested. Con-A produced a dose-dependent effect (EC50 = 3 microM) and increased the efficacy but not the potency of the agonists. In contrast, Con-A failed to enhance PI hydrolysis evoked by N-methyl-D-aspartate, kainate, carbachol, the calcium ionophore A23187, or 50 mM K+. The Con-A stimulatory effect was prevented by simultaneous pretreatment with the agonists of ionotropic quisqualate receptors quisqualate, kainate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, but not by the antagonist 6-cyano-7-nitroquioxaline-2,3-dione (CNQX). CNQX, which did not inhibit quisqualate-stimulated PI hydrolysis in untreated cells, abolished the component of quisqualate response enhanced by Con-A pretreatment. The pretreatment with Con-A also increased the influx of 45Ca2+ in granule cells stimulated by quisqualate. This increase was inhibited by CNQX. Moreover, the potentiation of PI hydrolysis by Con-A, but not the response to quisqualate alone, was abolished in the absence of Ca2+ and Na+. Pretreatment of granule cells with pertussis toxin inhibited PI hydrolysis stimulated by the metabolotropic quisqualate receptor and the Con-A-potentiated response by the same percentage, but Ca2+ influx induced by quisqualate was not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytosolic Ca2+ oscillations in REF52 fibroblasts: Ca(2+)-stimulated IP3 production or voltage-dependent Ca2+ channels as key positive feedback elements

Oscillations in cytosolic free calcium concentrations ([Ca2+]i) can be elicited in REF52 fibroblasts by three different modes of stimulation. We have previously demonstrated that [Ca2+]i oscillations result when these cells are simultaneously depolarized and stimulated with a hormone linked to phosphoinositide breakdown. Further evidence is now presented that such oscillations are linked to fluctuations in the concentration of IP3 and the Ca2+ content of an IP3-sensitive Ca2+ store. [Ca2+]i oscillations can also be generated in REF52 cells either by direct stimulation of G-proteins with GTP gamma S or AlF4- or by destabilizing the membrane potential and opening voltage-dependent calcium channels. This report compares the different types of oscillations and their mechanisms.