Phospholipids in synaptic function

Phosphoinositides: Roles in the Development of Microglial-Mediated Neuroinflammation and Neurodegeneration

Microglia are increasingly recognized as vital players in the pathology of a variety of neurodegenerative conditions including Alzheimer’s (AD) and Parkinson’s (PD) disease. While microglia have a protective role in the brain, their dysfunction can lead to neuroinflammation and contributes to disease progression. Also, a growing body of literature highlights the seven phosphoinositides, or PIPs, as key players in the regulation of microglial-mediated neuroinflammation. These small signaling lipids are phosphorylated derivates of phosphatidylinositol, are enriched in the brain, and have well-established roles in both homeostasis and disease.Disrupted PIP levels and signaling has been detected in a variety of dementias. Moreover, many known AD disease modifiers identified via genetic studies are expressed in microglia and are involved in phospholipid metabolism. One of these, the enzyme PLCγ2 that hydrolyzes the PIP species PI(4,5)P₂, displays altered expression in AD and PD and is currently being investigated as a potential therapeutic target.Perhaps unsurprisingly, neurodegenerative conditions exhibiting PIP dyshomeostasis also tend to show alterations in aspects of microglial function regulated by these lipids. In particular, phosphoinositides regulate the activities of proteins and enzymes required for endocytosis, toll-like receptor signaling, purinergic signaling, chemotaxis, and migration, all of which are affected in a variety of neurodegenerative conditions. These functions are crucial to allow microglia to adequately survey the brain and respond appropriately to invading pathogens and other abnormalities, including misfolded proteins. AD and PD therapies are being developed to target many of the above pathways, and although not yet investigated, simultaneous PIP manipulation might enhance the beneficial effects observed. Currently, only limited therapeutics are available for dementia, and although these show some benefits for symptom severity and progression, they are far from curative. Given the importance of microglia and PIPs in dementia development, this review summarizes current research and asks whether we can exploit this information to design more targeted, or perhaps combined, dementia therapeutics. More work is needed to fully characterize the pathways discussed in this review, but given the strength of the current literature, insights in this area could be invaluable for the future of neurodegenerative disease research.

Ultrastructural and molecular features of excitatory and glutamatergic synapses. The auditory nerve synapses

Glutamatergic synapses mediate fast synaptic transmission in the central nervous system. New developments highlight the importance of the synapse structural and molecular remodeling during development, aging and in neurological disorders. This chapter summarizes key structural and molecular aspects of the presynaptic and postsynaptic components of glutamatergic synapses in the brain. In addition, this chapter describes how the structure of the postsynaptic density and ionotropic glutamate content contribute to the function of auditory nerve synapses in the lower auditory brainstem.

Improving the hyperpolarization of (31)P nuclei by synthetic design

Traditional ³¹P NMR or MRI measurements suffer from low sensitivity relative to ¹H detection and consequently require longer scan times. We show here that hyperpolarization of ³¹P nuclei through reversible interactions with parahydrogen can deliver substantial signal enhancements in a range of regioisomeric phosphonate esters containing a heteroaromatic motif which were synthesized in order to identify the optimum molecular scaffold for polarization transfer. A 3588-fold ³¹P signal enhancement (2.34% polarization) was returned for a partially deuterated pyridyl substituted phosphonate ester. This hyperpolarization level is sufficient to allow single scan ³¹P MR images of a phantom to be recorded at a 9.4 T observation field in seconds that have signal-to-noise ratios of up to 94.4 when the analyte concentration is 10 mM. In contrast, a 12 h 2048 scan measurement under standard conditions yields a signal-to-noise ratio of just 11.4. ³¹P-hyperpolarized images are also reported from a 7 T preclinical scanner.

Phosphoinositides in membrane traffic at the synapse

Inositol phospholipids represent a minor fraction of membrane phospholipids; yet they play important regulatory functions in signaling pathways and membrane traffic. The phosphorylated inositol ring can act either as a precursor for soluble intracellular messengers or as a binding site for cytosolic or membrane proteins. Hence, phosphorylation-dephosphorylation of phosphoinositides represents a mechanism for regulation of recruitment to the membrane of coat proteins, cytoskeletal scaffolds or signaling complexes and for the regulation of membrane proteins. Recent work suggests that phosphoinositide metabolism has an important role in membrane traffic at the synapse. PtdIns(4,5)P(2) generation is implicated in the secretion of at least a subset of neurotransmitters. Furthermore, PtdIns(4,5)P(2) plays a role in the nucleation of clathrin coats and of an actin-based cytoskeletal scaffold at endocytic zones of synapses, and PtdIns(4,5)P(2) dephosphorylation accompanies the release of newly formed vesicles from these interactions. Thus, the reversible phosphorylation of inositol phospholipids may be one of the mechanisms governing the timing and vectorial progression of synaptic vesicle membranes during their exocytic-endocytic cycle.

Lipid metabolism and regulation of membrane trafficking

The past 20 years have witnessed tremendous progress in our understanding of the molecular machinery that controls protein and membrane transport between organelles (Scheckman R, Orci L. Coat proteins and vesicle budding. Science 1996;271: 1526-1533 and Rothman JE. Mechanisms of intracellular protein transport. Nature 1994;372: 55-63.) The research efforts responsible for these impressive advances have largely focused on the identification and characterization of protein factors that participate in membrane trafficking events. The role of membranes and their lipid constituents has received considerably less attention. Indeed, until rather recently, popular models for mechanisms of membrane trafficking had relegated membrane lipids to the status of a passive platform, subject to deformation by the action of coat proteins whose polymerization and depolymerization govern vesicle budding and fusion reactions. The 1990s, and particularly its last half, has brought fundamental reappraisals of the interface of lipids and lipid metabolism in regulating intracellular membrane trafficking events. Some of the emerging themes are reviewed here.

Essential role of phosphoinositide metabolism in synaptic vesicle recycling

Growing evidence suggests that phosphoinositides play an important role in membrane traffic. A polyphosphoinositide phosphatase, synaptojanin 1, was identified as a major presynaptic protein associated with endocytic coated intermediates. We report here that synaptojanin 1-deficient mice exhibit neurological defects and die shortly after birth. In neurons of mutant animals, PI(4,5)P2 levels are increased, and clathrin-coated vesicles accumulate in the cytomatrix-rich area that surrounds the synaptic vesicle cluster in nerve endings. In cell-free assays, reduced phosphoinositide phosphatase activity correlated with increased association of clathrin coats with liposomes. Intracellular recording in hippocampal slices revealed enhanced synaptic depression during prolonged high-frequency stimulation followed by delayed recovery. These results provide genetic evidence for a crucial role of phosphoinositide metabolism in synaptic vesicle recycling.

Structure and functional properties of diacylglycerols in membranes

1. 1,2-Diacyl-sn-glycerols (DAG) are minor components of cell membranes (about 1 mole% of the lipids) and yet they are potent regulators of both the physical properties of the lipid bilayer and the catalytic behaviour of several membrane-related enzymes. 2. In the pure state DAG's present a considerable polymorphism, with several crystalline phases in addition to the neat fluid phase. The most stable crystalline phase is the so-called beta' phase, a monoclinic crystalline form with orthorhombic perpendicular subcell chain packing, in which both acyl chains lie parallel to each other in a hairpinlike configuration about the sn-1 and sn-2 glycerol carbon atoms. The molecules are organized in a bilayer, with the glycerol backbone roughly parallel to the plane of the bilayer, and the acyl chains tilted at approximately 60 degrees with respect to that plane. Acyl chain unsaturation, and particularly a single cis unsaturation, impairs chain packing in mixed-chain DAG's, and this results in an increased number of metastable crystalline phases. 3. DAG's mix with phospholipids in fluid bilayers when their melting temperature is below or close enough to the melting temperature of the bilayer system. When incorporated in phospholipid bilayers, the conformation of DAG is such that the glycerol backbone is nearly perpendicular to the bilayer, with the sn-1 chain extending from the glycerol Cl carbon into the hydrophobic matrix of the bilayer and the sn-2 chain first extending parallel to the bilayer surface, then making a 90 degrees bend at the position of the sn-1 carbonyl to become parallel to the sn-1 chain. DAG's are located in phospholipid bilayers about two CH2 units deeper than the adjacent phospholipids. DAG's mix nonideally with phospholipids, giving rise to in-plane separations of DAG-rich and -poor domains, even in the fluid state. DAG molecules also increase the separation between phospholipid headgroups, and decrease the hydration of the bilayer surface. Also, because the transversal section of the DAG headgroup is small when compared to that of the acyl chains, DAG favours the (negative) curvature of the lipid monolayers, and DAG-phospholipid mixtures tend to convert into inverted nonlamellar hexagonal or cubic phases. 4. A number of membrane enzyme activities are modulated (activated) by DAG, most notably protein kinase C, phospholipases and other enzymes of lipid metabolism. Protein kinase C activation (and perhaps that of other enzymes as well) occurs as the combined result of a number of DAG-induced modifications of lipid bilayers that include: changes in lipid headgroup conformation, interspacing and hydration, changes in the bilayer propensity to form inverted nonlamellar phases, and lateral phase separations of DAG-rich and -poor domains. Among the DAG-activated enzymes, phospholipases C show the peculiarity of yielding the activator DAG as their reaction product, and this allows the self-induced transition from a low- to a high-activity status. 5. DAG's induce or enhance membrane fusion in a number of ways, mainly through partial dehydration of the bilayer surface, increase in lipid monolayer curvature and perhaps lateral phase separation. DAG-increased fusion rates have been demonstrated in several instances of cation-induced fusion of model membranes, as well as in Ca(2+)-induced fusion of chromaffin granules with plasma membrane vesicles. Also phospholipase C has been shown to induce vesicle aggregation and fusion through the catalytic generation of DAG in the bilayers. A rather general property of DAG is that it promotes vesicular or interparticle aggregation. 6. In the living cell, DAG is often generated through phospholipid degradation in response to an extracellular agonist binding a specific receptor in the cell surface. DAG is said to act as an intracellular second messenger. (ABSTRACT TRUNCATED)

Characterization of phospholipase D activity in bovine photoreceptor membranes

Phospholipase D (E.C. 3.1.4.4.) was detected in isolated bovine rod outer segments (ROS) and its properties determined. The enzyme activity was assayed using either a sonicated microdispersion of 1,2-diacyl-sn-[2(3)H]glycerol-3-phosphocholine (PC), or [14C]ethanol. Using [3H]PC and ethanol as a substrate, we were able to detect the hydrolytic properties as well as the transphosphatidylation reaction catalyzed by phospholipase D (PLD): formation of [3H]phosphatidic acid and phosphatidylethanol [3H]PtdEt; whereas with [14C]ethanol or [3H]glycerol in the absence of exogenous PC, only transphosphatidylation reactions were detected (formation of [14C]PtdEt or [3H]phosphatidylglycerol, respectively). The use of varying concentrations of [3H]PC and 400 mM of ethanol gave an apparent Km value for PC of 0.51 mM and a Vmax value of 111 nmol x h(-1) x (mg protein)(-1). The activity was linear up to 60 min of incubation and up to 0.2 mg of protein. The optimal ethanol concentration was determined to be 400 mM, with an apparent Km of 202 mM and a Vmax value for ethanol of 125 nmol x h(-1) x (mg protein)(-1). A clear pH optimum was observed around 7. PLD activity was increased in the presence of 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate or sodium deoxycholate and inhibited with Triton X-100. The enzyme activity was also activated in the presence of Ca2+ or Mg2+ (1 mM) although these ions were not required for measuring PLD activity. The high specific activity of PLD found in purified ROS compared to the activity found in other subcellular fractions of the bovine retina suggests that this enzymatic activity is native to ROS. The present report is the first evidence of PLD activity associated with photoreceptor ROS.

Receptor-mediated diacylglycerol kinase translocation dependent on both transient increase in the intracellular calcium concentration and modification by protein kinase C

Diacylglycerol kinase (DG kinase) is activated by various stimuli in many types of cells. We reported earlier that carbachol (CCh) induced DG kinase translocation from the cytosolic fraction to the membrane fraction in guinea pig taenia coli (Biochem. Pharmacol., 50: 591-599, 1995). In this study, the regulation mechanisms of DG kinase translocation are reported, based on the following findings: 1) CCh sustained an increase in DG kinase in the membrane fraction and a decrease in the cytosolic fraction; 2) blocking calcium influx by removing extracellular calcium did not affect the CCh-induced sustained DG kinase translocation; 3) exposing purified protein kinase C (PKC) to DG kinase increased DG kinase affinity to octylglycoside micelles only with the enzyme extracted from the cytosolic fraction; and 4) CCh-induced DG kinase translocation was reversed by removing CCh, and the serine/threonine phosphatase inhibitor, okadaic acid, blocked the reversal of the translocation. These results suggest that CCh-induced DG kinase translocation is promoted by both a transient increase in intracellular calcium, which may be released from the intracellular store, and by DG kinase phosphorylation by PKC.

Protein kinase C is involved in translocation of diacyglycerol kinase induced by carbachol in guinea pig taenia coli

The regulatory mechanisms of diacylglycerol (DG) kinase activity were studied in guinea pig taenia coli. In an octylglycoside mixed micellar assay system, DG kinase activities were distributed in both membrane and cytosolic fractions. Treatment of the tissue with carbachol (CCh) increased the activity in the membrane fraction and decreased the cytosolic fraction without affecting total DG kinase activity. The Km value of DG kinase in the membrane fraction was unchanged by treatment with CCh, although Vmax was increased. These findings suggest that DG kinase may be translocated from the cytosol to the membrane by CCh-stimulation. Increase in DG content by treatment of tissue with a cell-permeable species of DG, dioctanoyl-sn-glycerol, did not induce DG kinase translocation. Each treatment with protein kinase C (PKC) inhibitor and PKC-desensitization blocked CCh-induced DG kinase translocation; and phorbol ester induced the translocation only in intracellular calcium-accumulated tissues. Considering these results, CCh-induced DG kinase activation appears to involve DG kinase translocation from the cytosol to the membrane in association with both PKC and intracellular calcium concentration rather than cellular DG content.

Acylation of lysophosphatidylcholine by brain membranes

Brain microsomes catalyze the acylation of lysophosphatidylcholine (lysoPtdCho) in the presence and absence of added CoA derivatives. The catalytic activity is distributed widely in various subcellular fractions from rat or bovine cerebral cortex as measured by the conversion of 1-[14C]palmitoyl-sn-glycero-3-phosphocholine to [14C]PtdCho. Analysis of this latter compound revealed that the dipalmitoyl derivative is the predominant molecular species, which is formed in this reaction by transacylation between two [14C]lysoPtdCho molecules. This lysoPtdCho: lysoPtdCho transacylation reaction was enhanced several-fold by the addition of oleoyl-CoA, which also is an effective donor of acyl groups in the acyl-CoA: lysoPtdCho acyltransferase-catalyzed reaction. Measurements of the initial velocity of the transacylation reaction were used to determine kinetic constants. Apparent Km values for lysoPtdCho in the presence and absence of oleoyl-CoA were 29 microM and 104 microM, respectively, and the corresponding maximal velocities were 0.11 and 1.06 nmol.min-1.mg-1, respectively. Oleoyl-CoA at 4 microM produced half-maximal stimulation of the transacylation reaction. CoA also stimulated the rate of conversion of [14C]lysoPtdCho to [14C]PtdCho, either in the presence or absence of oleoyl-CoA, with a half-maximal effect of CoA at 80 microM. These results may be important in understanding the regulation of PtdCho synthesis and the mechanism by which acyl group composition of this compound is controlled.

Pulsed electric current enhances the phorbol ester induced oxidative burst in human neutrophils

Oxidative burst (OB) response in human neutrophils, measured with chemiluminescence (CL), has been used to determine whether pulsed electric current (PEC) might induce a functional response in these electrically nonexcitable cells, and also whether it might modify cellular response to tumor-promoting phorbol ester (PMA). Five minutes of PEC treatment caused no significant changes in neutrophil CL levels in HBSS (1.2 mM Ca2+ concentration) as well as in HBSS-EGTA, where the extracellular Ca2+ concentration was reduced to less than 30 nM. The CL level of PMA-activated neutrophils in HBSS was 52% higher than in HBSS-EGTA. In HBSS the CL level, after the combined PMA and PEC treatment, was 53% higher than in PMA-alone-treated neutrophils. Activation of the OB in HBSS-EGTA with PMA and PEC was 13% higher than in solely PMA treated neutrophils. The results suggest that in neutrophil OB response, the PEC effect is closely related with cellular calcium mobilization, since depletion of extracellular Ca2+ decreased the PEC effect.

Hepatic membranolytic stability alteration by metalloporphins in rats

The mothers of experimental neonates were administered excess bilirubin for a month, and the neonates were suffering from hyperbilirubinemia. The studies were conducted on the effect of excess bilirubin and metalloporphyrins on plasma membrane and mitochondrial membrane. We have isolated, separated, and estimated phospholipids, and also assayed the activity of phospholipase A2 from whole liver and mitochondrial and microsomal fractions. Excess of bilirubin administration decreased the total phospholipid level and inhibited the phospholipase A2 activity. Cr-PP (chromium protoporphyrin) induces the phospholipase A2 activity which is inhibited by simultaneous bilirubin administration. However, Zn-PP (zinc protoporphyrin) and Mn-PP (manganese protoporphyrin) showed a reverse pattern.

Sequential enzymatic synthesis and biodistribution of radiolabelled inositol and inositol analogs

The desired radiolabelled inositol (Ins) and inositol-1-phosphate (IP) were obtained from radiolabelled glucose by sequential enzyme reactions in a short time. Rapid separation and fractionation of enzymes and labelled products from the reaction mixture was achieved by HPLC with a gel-permeation chromatography column. Examination of the biodistribution of radiolabelled Ins, IP and their acetylated analogs suggested that intact Ins labelled with 11C would be more effective than 11C-labelled acetylated Ins as a brain diagnostic agent for positron emission tomographic studies of the metabolism of phosphatidylinositol and its role as a second messenger in the brain.

TGF-beta inhibits the platelet-derived growth factor-induced formation of inositol trisphosphate in MG-63 human osteosarcoma cells

Epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and heparin-binding growth factor-1 (HBGF-1) stimulated the proliferation of a variant of the human osteosarcoma cell line, MG-63-LS (LS = low serum). Transforming growth factor beta (TGF-beta) completely inhibited cell growth in basal medium supplemented with 2% fetal calf serum (FCS), blocked PDGF- and EGF-stimulated cell proliferation, and modulated that of HBGF-1. PDGF, but not EGF or HBGF-1, activated the inositol trisphosphate/diacylglycerol (IP3/DAG) second message system in a dose-dependent manner. EGF inhibited phosphoinositol lipid turnover and HBGF-1 and TGF-beta stimulated phosphatidylinositol hydrolysis to produce inositol phosphate (IP) but not IP3. Preincubation of quiescent cells with TGF-beta for 30-40 minutes prior to the addition of PDGF resulted in an inhibition of PDGF-induced production of IP3. This suggested that TGF-beta was an indirect inhibitor and blocked PDGF-stimulated cell growth in part by interfering with the generation of the second messenger, IP3.

Phenytoin affects metabolism of free fatty acids and nucleotides in rat cerebral ischemia

We investigated the effects of phenytoin on the rate of enzymatic release of free fatty acids and on the levels of energy metabolites and nucleoside phosphates in ischemic brain. Phenytoin (10 mg/kg i.v.) was administered 30 minutes before the onset of ischemia induced in 30 male Wistar rats by occluding the basilar and both common carotid arteries. The rats' brains were frozen in situ after 0, 5, or 30 minutes of ischemia or 10, 30, or 60 minutes of recirculation following 30 minutes of ischemia (n = 5 at each time). Nucleoside triphosphate levels were higher in the phenytoin-treated rats than in corresponding untreated rats at each time during and after ischemia. Phenytoin significantly attenuated the accumulation of lactate and free fatty acids (arachidonic acid and stearic acid) during ischemia and accelerated their recovery during recirculation. These results suggest that phenytoin has favorable protective effects on ischemic brain and that phenytoin may inhibit calcium-mediated phenomena, especially the inositol cycle, in cerebral ischemia.

Modulation of arachidonate turnover in cerebral phospholipids

The effects of the neurotransmitters NE and 5HT on the turnover of AA in cerebral PL were investigated in slices of rat brain cortex. Incorporation of 3[H]-AA into individual PL was first analyzed in accordance with a closed two-compartmental model. Apparent rates of deacylation and reacylation as well as sizes of the metabolically active PL-bound AA pools were calculated. It was found that rates of reacylation of individual PL varied markedly, while deacylation rates remained within a relatively narrow range. The rate of PI acylation was found to be the most rapid, while the rate of PS acylation was the slowest observed. The pool of PL-bound AA that is readily accessible to deacylation-reacylation processes was distributed differentially among the various PL, with more than 50% of this pool in PI; but only 0.75% of the PI content was associated with this pool. Both NE and 5HT enhanced the incorporation of 3[H]-AA into PI in a dose-related manner, while they attenuated its incorporation into other PL. Pharmacological studies indicated that the neurotransmitter effects were not mediated by known NE or 5HT receptors and that a functional presynaptic reuptake system was not required for these effects. The observed effects did not appear to be related to the formation of hydrogen peroxide by the action of MAO on the neurotransmitter. Examination of the structure-activity relationships indicated that the presence of two hydroxyl groups in the aromatic ring was needed for attenuating 3[H]-AA incorporation into PC, whereas an active catechol nucleus with an additional hydroxyl group in the beta position of the side chain appeared to enhance 3[H]-AA incorporation into PI. Results obtained with the phospholipase A-2 inhibitor mepacrine and the acyltransferase inhibitor THC suggest that NE attenuates PL acylation by activating phospholipase A-2, but it concomitantly enhances PI acylation by selectively stimulating a PI-specific arachidonyl transferase via mechanisms that have not yet been elucidated.

Effects of idebenone on the levels of acetylcholine, choline, free fatty acids, and energy metabolites in the brains of rats with cerebral ischemia

Cerebral ischemia was induced by a 200-s occlusion of both common carotid arteries in rats in which both vertebral arteries had been permanently cauterized. In the ischemic rats, a significant decrease in acetylcholine (ACh) and a marked increase in choline were observed in the cerebral cortex, hippocampus, striatum, and diencephalon. A slight increase in choline was also observed in the cerebellum and brain stem. Pretreatment with idebenone (10 mg/kg, i.p.) inhibited the decrease in ACh and the increase in choline in the forebrain regions. In addition, the same dose of idebenone inhibited the increments of lactate and free fatty acid contents and tended to inhibit the decrement of the ATP content in the cerebral cortex of the cerebral ischemic rats. These results indicate that idebenone inhibits the alteration of the ACh level and the disruption of membrane phospholipids in the brain of ischemic rats; these effects may be mediated by improved cerebral energy metabolism.

Modulation of phospholipase A2 activity by neutral and anionic glycosphingolipids in monolayers

The effect of neutral (galactocerebroside and asialo-ganglioside GM1) or anionic (sulphatide and gangliosides GM1, GD1a and GT1b) glycosphingolipids on the activity of phospholipase A2 from pig pancreas was studied in mixed monolayers of dilauroyl phosphatidylcholine with the glycosphingolipids in different molar fractions at various constant surface pressures. The activity of the enzyme depends on the proportion and type of glycosphingolipid in the interface. Sulphatide activates the enzyme at all proportions, whereas galactocerebroside shows inhibition or activation depending on its proportion in the film. Asialo-ganglioside GM1 and gangliosides GM1, GD1a and GT1b can strongly inhibit the enzyme at relatively low molar fractions in the film in the following order: asialo-ganglioside GM1 less than ganglioside GM1 less than ganglioside GT1b less than ganglioside GD1a. The changes of activity are not due to a direct action of the lipids on the active centre or interfacial recognition region of the enzyme.

Studies of blocking mechanisms of carbachol-induced polyphosphoinositide turnover in rat cortical synaptosomes by neuroactive drugs

1. The incorporation of 32Pi into 4 phospholipids of rat cortical synaptosomes was altered in the presence of carbachol (1 mM), viz. a decrease of phosphatidylinositol-4,5-bisphosphate and phosphatidyl-4-phosphate by 34 and 21%, and an increase of phosphatidylinositol and phosphatidic acid by 52 and 96% of basal controls respectively. 2. The IC30 values calculated from the dose-response curves for drugs affecting carbachol-induced 32Pi incorporation into these phospholipids, and [3H]QNB binding to the cortical synaptosomes were similar for the typical antimuscarinic agents (i.e. atropine, pirenzepine and trihexyphenidyl), and tricyclic antidepressants (i.e. amitriptyline, doxepin and imipramine) studied. 3. The IC30 values obtained for drugs affecting carbachol-induced 32Pi incorporation into these phospholipids, and high potassium-induced 45Ca2+-uptake by this preparation were similar for neuroselective calcium channel blockers (i.e. cinnarizine and flunarizine) studied. 4. Our results suggest that the neuroactive drugs studied can either act at, or beyond the receptor level, perhaps on the availability of calcium ion, to block carbachol-induced polyphosphoinositide turnover in rat cortical synaptosomes.

The activation of phosphatidylinositol turnover is not directly involved in the modulation of neurotransmitter release mediated by presynaptic muscarinic receptors

In the rat cerebral cortex, the comparative effects of various muscarinic agonists on the release of [3H]dopamine ([3H]DA), [3H]acetylcholine ([3H]ACh), and [3H]5-hydroxytryptamine ([3H]5-HT) from superfused nerve endings and on phosphatidylinositol (PI) turnover were studied. Acetylcholine (ACh) was found to be the most potent among the agonists tested on all three release systems examined, and also on the activation of PI turnover. Oxotremorine and bethanechol were very weak agonists when tested as stimulators of PI turnover. However, oxotremorine was very effective as a release modulator, while bethanechol was completely ineffective. Our data suggest that the activation of PI turnover is not directly involved in the modulation of neurotransmitter release mediated by presynaptic muscarinic receptors.

A physicochemical study of the interaction of phosphatidylinositol with buprenorphine and naloxone

The interactions of two opioid molecules (buprenorphine and naloxone) with phosphatidylinositol and phosphatidylcholine were studied in lipid monolayers at the air-water interface. The influence of Na+, Ca2+, and Mn2+ ions in these interactions has also been determined. Neither buprenorphine nor naloxone influence the ordered state of phosphatidylcholine monolayers. On the contrary, both opioid molecules interact specifically with phosphatidylinositol monolayers. The area/molecule of phosphatidylinositol spread on buprenorphine containing subphases is highly affected by this molecule and also by ions. The phosphatidylinositol/naloxone interactions are rather weak and less affected by ions.

Formation of second messengers in response to activation of ion channels in excitable cells

1. Depolarization of excitable cells of the central nervous system results in the formation of the second messengers cyclic AMP, cyclic GMP, inositol phosphates, and diacylglycerides. 2. Depolarization-evoked accumulation of cyclic AMP in brain preparations can be accounted for mainly by the release of adenosine, which subsequently interacts with stimulatory adenosine receptor linked to adenylate cyclase. 3. Depolarization-evoked formation of cyclic GMP in brain preparations is linked to activation of voltage-dependent calcium channels, presumably leading to activation of guanylate cyclase by calcium ions. 4. In brain slices depolarization-evoked stimulation of phosphoinositide breakdown and subsequent formation of inositol phosphates and diacylglycerides are linked to activation of voltage-dependent calcium channels, which are sensitive to dihydropyridines, presumably leading to activation of phospholipase(s) C by calcium ions. 5. In the synaptoneurosome preparation depolarization-evoked stimulation of phosphoinositide breakdown does not involve activation of dihydropyridine-sensitive calcium channels and, instead, appears to be regulated primarily by the intracellular concentration of sodium ions. Thus, agents that induce increases in intracellular sodium--such as toxins that open or delay inactivation of voltage-dependent sodium channels; ouabain, an inhibitor of Na+/K+ ATPase that transports sodium outward and a sodium ionophore--all stimulate phosphoinositide breakdown. Mechanistically, increases in intracellular sodium either might directly affect phospholipase(s) C or might lead to influx of calcium ions through Na+/Ca2+ transporters. 6. Depolarization-evoked stimulation of cyclic AMP formation and phosphoinositide breakdown can exhibit potentiative interactions with responses to receptor agonists, thereby providing mechanisms for modulation of receptor responses by neuronal activity. 7. Since all these second messengers can induce phosphorylation of ion channels through the activation of specific kinases, it is proposed that depolarization-evoked formation of second messengers represents a putative feedback mechanism to regulate ion fluxes in excitable cells.

Modulation of carbachol-stimulated inositol phospholipid hydrolysis in rat cerebral cortex

Cortical slices from rat brain were used to study carbachol-stimulated inositol phospholipid hydrolysis. Omission of calcium during incubation of slices with [3H]inositol increased its incorporation into receptor-coupled phospholipids. Carbachol-stimulated hydrolysis of [3H]inositol phospholipids in slices was dose-dependent, was affected by the concentrations of calcium and lithium present and resulted in the accumulation of mostly [3H]inositol-1-phosphate. Incubation of slices with N-ethylmaleimide or a phorbol ester reduced the response to carbachol. Membranes prepared from cortical slices labeled with [3H]inositol retained the receptor-stimulated inositol phospholipid hydrolysis reaction. The basal rate of inositol phospholipid hydrolysis was higher than in slices and addition of carbachol further stimulated the process. Addition of GTP stimulated inositol phospholipid hydrolysis, suggesting the presence of a guanine nucleotide-binding protein coupled to phospholipase C. Carbachol and GTP-stimulated inositol phospholipid hydrolysis in membranes was detectable following a 3 min assay period. In contrast to slices, increased levels of inositol bisphosphate and inositol trisphosphate were detected following incubation of membranes with carbachol. These results demonstrate that agonist-responsive receptors are present in cortical membranes, that the receptors may be coupled to phosphatidylinositol 4, 5-bisphosphate, rather than phosphatidylinositol, hydrolysis and that a guanine nucleotide-binding protein may mediate the coupling of receptor activation to inositol phospholipid hydrolysis in brain.

Extraction of peripheral proteins is accompanied by selective depletion of certain glycerophospholipid classes and changes in the phosphorylation pattern of acetylcholine-receptor-rich-membrane proteins

The widely used alkaline treatment of acetylcholine-receptor (AChR)-rich membranes from Torpedo marmorata (electric fish) and Discopyge tschudii (a marine ray) results not only in the extraction of non-receptor peripheral proteins but also in that of glycerophospholipids (approximately 13%). Minor acidic phospholipids, notably phosphatidic acid and polyphosphoinositides, are particularly enriched in the NaOH extracts. When electrocytes or receptor-rich membranes are incubated with [32P]Pi or [gamma-32P]ATP, polyphosphoinositides accumulate most of the label (approximately 45% in D. tschudii; 96% in T. marmorata) and exhibit the highest specific radioactivity. Furthermore, more than 50% of these phosphorylated lipids are extracted by NaOH together with the peripheral membrane proteins. NaOH treatment also results in modification of the phosphorylation pattern of AChR membrane proteins. Phosphorylation decreases in the Mr-43,000 group of peripheral proteins and in the gamma-subunit of the receptor. The results indicate that polyphosphoinositides constitute a metabolically very active lipid pool in the postsynaptic membrane, and that a substantial proportion of these phospholipids are preferentially released from the membrane together with other acidic phospholipids upon peripheral-protein extraction. The conclusion is drawn that membranes submitted to the above treatments can no longer be considered equivalent to native ones in terms of their phospholipid composition and phosphorylation characteristics.

Cerebral phosphoinositide and energy metabolism during and after insulin-induced hypoglycemia

During and after insulin-induced hypoglycemia, changes in levels of cerebral phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidic acid (PA), triacylglycerol (TAG), diacylglycerol (DAG), and free fatty acids (FFAs) as well as the cerebral energy state were studied in relation to the EEG. In hypoglycemic rats with an EEG pattern of quasiperiodic sharp or slow sharp waves, which preceded the development of an isoelectric EEG, PIP2 levels increased significantly, together with a slight decrease in PI content. Levels of the other lipids did not change during this period. The cerebral energy state was affected only slightly in spite of profound decreases in plasma and tissue glucose levels. With 30 min of an isoelectric EEG, levels of all phosphoinositides and PA decreased significantly; total FFA and DAG contents increased seven- and twofold, respectively; the TAG-palmitate level decreased, and that of TAG-arachidonate increased. Plasma and tissue glucose were nearly depleted, and the cerebral energy state deteriorated severely. The increment in fatty acids in the DAG and FFA pools was less than their loss from phosphoinositides and PA, an observation suggesting vascular washout or oxidation of a portion of the FFAs produced. Following 90 min of glucose infusion, PIP and PA levels recovered to control values; however, the PIP2 content exceeded control levels, and that of PI remained below control levels. DAG and FFA contents returned to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral phosphoinositide, triacylglycerol and energy metabolism during sustained seizures induced by bicuculline

In ventilated rats, levels of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2), diacylglycerol (DAG), triacylglycerol (TAG), free fatty acids (FFA) and phosphatidic acid, as well as their fatty acid contents, were measured in forebrain tissue after 1, 20 and 60 min of seizures induced by bicuculline. Cerebral energy state was also measured. PI decreased progressively throughout 60 min of seizures, whereas the levels of PIP and PIP2 did not change. DAG increased modestly and persistently. FFA increased markedly during the early seizure period, but decreased later. Following an initial drop, TAG rose above control. Phosphatidic acid did not change. The levels of ATP and energy charge potential decreased slightly and lactate accumulated. Stearic acid (18:0) and arachidonic acid (20:4) primarily accounted for the changes in the levels of the lipids. At the onset of seizures, the decrease of 18.0 and 20:4 in PI occurred in parallel with an enrichment of these fatty acids in FFA and DAG. Despite the fact that the losses of 18:0 and 20:4 from PI were quantitatively similar to each other at all times examined, the increase in free 18:0 was much larger than the increase in free 20:4 at 20 min of seizures. Concurrently there was a rise of 20:4 in TAG. As the FFA levels declined thereafter, 20:4 and docosahexaenoate (22:6) in TAG continued to increase. The results are consistent with the view that seizure activity stimulates the hydrolytic breakdown of brain phosphoinositides--the pathway catalyzed by phosphodiesterase of the phospholipase C type followed by lipases, and probably the pathway catabolized by phospholipases A as well. Preferential incorporation of polyunsaturated fatty acids into TAG-acyl residues may represent a mechanism to reduce the level of their free forms when the latter are produced in large amounts.

Guanine nucleotide and NaF stimulation of phospholipase C activity in rat cerebral-cortical membranes. Studies on substrate specificity

Guanyl-5'-yl imidodiphosphate (p[NH]ppG) stimulated a rapid phospholipase C-mediated breakdown of exogenously added phosphatidylinositol 4,5-bisphosphate (PIP2) in rat cerebral-cortical membranes, with half-maximal activation at approx. 33 microM. NaF stimulated phospholipase C activity, with half-maximal activation at 0.5 mM. Stimulation of phospholipase C activity by NaF exhibited pH optima at approx. 5.5 and 7.0, with the stimulatory activity at pH 7.0 greater than that at pH 5.5. With p[NH]ppG, only stimulation at pH 7.0 was observed. Neither p[NH]ppG nor NaF stimulated hydrolysis of added phosphatidylinositol (PI) or phosphatidylinositol 4-phosphate (PIP). Mg2+ (0.5 mM) potentiated p[NH]ppG-stimulated breakdown of PIP2. Ca2+ increased basal and p[NH]ppG-stimulated breakdown of PIP2. PI breakdown was stimulated only by high Ca2+ concentrations and was unaffected by p[NH]ppG at any Ca2+ concentration examined. These results indicate that, in cerebral-cortical membranes, activation of phospholipase C by guanine nucleotides or fluoride directly increases a phospholipase C activity which specifically hydrolyses PIP2.

How calcium may cause exocytosis in sea urchin eggs

The process of secretory granule-plasma membrane fusion can be studied in sea urchin eggs. Micromolar calcium concentrations are all that is required to bring about exocytosis in vitro. I discuss recent experiments with sea urchin eggs that concentrate on the biophysical aspects of granule-membrane fusion. The backbone of biological membranes is the lipid bilayer. Sea urchin egg membrane lipids have negatively charged head groups that give rise to an electrical potential at the bilayer-water interface. We have found that this surface potential can affect the calcium required for exocytosis. Effects on the surface potential may also explain why drugs like trifluoperazine and tetracaine inhibit exocytosis: they absorb to the bilayer and reduce the surface potential. The membrane lipids may also be crucial to the formation of the exocytotic pore through which the secretory granule contents are released. We have measured calcium-induced production of the lipid, diacylglycerol. This lipid can induce a phase transition that will promote fusion of apposed lipid bilayers. The process of exocytosis involves the secretory granule core as well as the lipids of the membrane. The osmotic properties of the granule contents lead to swelling of the granule during exocytosis. Swelling promotes the dispersal of the contents as they are extruded through the exocytotic pore. The movements of water and ions during exocytosis may also stabilize the transient fusion intermediate and consolidate the exocytotic pore as fusion occurs.

Regional differences in arachidonic acid release in rat hippocampal CA1 and CA3 regions during cerebral ischemia

Changes in the levels of arachidonic acid during ischemia in selectively vulnerable areas of the hippocampus were studied in the rat brain. Since neurons in the CA1 region are more vulnerable to ischemia than neurons in the adjacent CA3 region, the release of arachidonic acid in these two regions was measured during decapitation ischemia of 4- to 12-min duration. The concentration of free arachidonic acid increased with the duration of ischemia in both regions. However, the level was significantly higher in CA1 than in CA3 after 8 and 12 min of ischemia. This difference in arachidonic acid accumulation may reflect differences between the regions in agonist-dependent phospholipid breakdown as well as calcium-dependent phospholipase activity. The importance for the development of neuronal necrosis is discussed.

(Na+,K+)-ATPase and noradrenergic function: effects of chronic ethanol

The experiments in this paper examined interactions between ethanol and repeated noradrenergic stimulation in vivo on regulation of (Na+,K+)-ATPase. The increase in ouabain binding and K+-phosphatase activity associated with (Na+,K+)-ATPase in rats treated with repeated yohimbine injections was prevented by chronic ethanol. Ethanol did not affect the yohimbine-induced alterations in noradrenergic receptor binding or in content of the norepinephrine metabolite 3-methoxy-4-hydroxyphenylglycol, showing that prevention of noradrenergic stimulation of (Na+,K+)-ATPase was not caused by a decrease in availability of norepinephrine. In addition, norepinephrine depletion with the neurotoxin DSP4 did not prevent the increases in (Na+,K+)-ATPase indices during chronic ethanol treatment, showing that they did not result from increased norepinephrine exposure. These results suggest that chronic ethanol reduces sensitivity of (Na+,K+)-ATPase to norepinephrine in vivo, possibly as a consequence of membrane effects of ethanol tolerance.

Agonist-stimulation of cerebral phosphoinositide turnover following long-term treatment with antidepressants

Receptor-mediated stimulation of the formation of inositol phosphates (IP) in cerebral tissue may serve as a useful tool for studying long-term changes in the function of serotonin-2 (5-HT2), alpha-1-adrenergic (al), and muscarinic-cholinergic (musc) receptors. In this study we have evaluated the effects of chronic treatment with various antidepressants on receptor-mediated formation of IP in rat brain. Imipramine (IMI: 10 mg/kg/day; 14 days), Bupropion (BUPR: 40 mg/kg/day; 14 days), Lithium (Li: 0.5% in diet; 7 days) and electroshock treatment (EST: 20-30 mA/day; 7 days) were investigated. Cross-chopped slices of cerebral cortex from control and treated rats were prelabelled with myo-3H-inositol in HEPES buffer containing 11.1 mM LiCl. Accumulation of IP was measured in the presence and absence of serotonin (5-HT, 10 uM), norepinepherine (NE, 5 uM), and carbamylcholine (CCH, 100 uM). Values for agonist-stimulated IP formation in control rats were: 5-HT = 123 +/- 5%; NE = 268 +/- 16%; CCh = 205 +/- 21% of the basal level. The IP response to 5-HT was significantly lower following BUPR and higher following EST. Responses to NE and CCH were significantly lower following BUPR treatment but were not affected by the other antidepressant treatments. These observations are consistent with results of receptor-binding studies indicating up-regulation of 5-HT2 receptors by EST but are not consistent with studies showing down-regulation of 5-HT2 receptors by IMI and a lack of effect on 5-HT2 receptors by BUPR. Our results are not supportive of the notion, based mainly on [3H]prazosin binding studies, that al receptors are up-regulated by EST as well as by different antidepressant drugs.

Effects of anticonvulsant drugs on calcium transport and polyphosphoinositide metabolism in rat cortical synaptosomes

1. Anticonvulsants: phenytoin, phenobarbital, carbamazepine and valium at concentrations of 10-100 microM had a significantly inhibitory effect on both K+-stimulated Ca2+-uptake and 32Pi incorporation into phospholipids of rat cortical synaptosomes. 2. Other anticonvulsant, valproic acid, at concentration upto 100 microM had no effect on these two events. 3. Our results suggest that there is a link between Ca2+-influx and polyphosphoinositide turnover in synaptosomes, and this link may relate to the inhibitory effect of these drugs on neurotransmitter release mechanisms of this preparation.

Neurotransmitter-stimulated inositol phosphate accumulation in hippocampal slices

The agonist-induced accumulation of [3H]IP in the presence of lithium represents a sensitive method for studying a number of receptor-mediated events in brain. In the presence of lithium, receptor-mediated [3H]IP3 formation in brain is difficult to demonstrate. It is possible that lithium itself may prevent the formation if IP3 and, thus, the presence of lithium in the incubation may account for such a discrepancy. It is likely that the effects of lithium on the PI cascade are more complex then previously envisioned. The recent identification of the inositol polyphosphates, myo-inositol 1,3,4-trisphosphate and myo-inositol 1,3,4,5-tetrakisphosphate, indicates that other inositol phosphates need to be considered in future studies of receptor-mediated PI metabolism.

Inositol 1,4,5-trisphosphate induced mobilization of Ca2+ from rat brain synaptosomes

Inositol 1,4,5-trisphosphate (IP3) was found to release Ca2+ from presynaptic nerve endings (synaptosomes) made permeable with saponin. ATP-dependent Ca2+ uptake was carried out until equilibrium was reached. Addition of IP3 produced a rapid release of Ca2+, which was complete within 60 sec, followed by Ca2+ reaccumulation to the original level in 5-7 min. Cholinergic receptor stimulation with muscarine also produced a similar Ca2+ release from synaptic endoplasmic reticulum. Ca2+ release by IP3 was not detectable in the absence of the mitochondrial inhibitors oligomycin or sodium azide. Reaccumulation of Ca2+ was prevented by the presence of vanadate, a potent inhibitor of Ca2+/Mg2+ ATPase. Half maximal and near complete release of Ca2+ took place at 0.4 microM and 3 microM IP3 concentrations, respectively. These studies demonstrate for the first time IP3 mobilization of Ca2+ from endoplasmic reticulum within synaptic plasma membranes.

Effect of ethanol on receptor-stimulated phosphatidic acid and polyphosphoinositide metabolism in mouse brain

The effects of chronic ethanol consumption as well as the effects of ethanol added in vitro on phosphoinositide metabolism were determined in mouse forebrain. [32P] incorporation into synaptosomal phosphatidic acid (PhA) was stimulated through both M1 muscarinic cholinergic and alpha 1 adrenergic receptor activation. Similarly, [3H]inositol 1-PO4 accumulation in brain slices was stimulated through these same receptors, but could also be stimulated by histamine1 receptor activation. In mice made physically dependent to ethanol, the magnitude of receptor-mediated [32P] incorporation in PhA did not differ from that of control animals. However, ethanol (100mM) added in vitro to synaptosomes from control mice significantly inhibited the carbamylcholine stimulated PhA response, but had no effect on the response to norepinephrine. Carbamylcholine stimulated [32P] incorporation into PhA, however, was no longer significantly inhibited by the addition of 100mM ethanol to synaptosomes from physically dependent-tolerant animals indicating that a cellular tolerance had developed. In contrast, receptor mediated [3H]inositol 1-PO4 accumulation in brain slices was not significantly affected by either chronic ethanol treatment or the in vitro addition of ethanol as high as 200mM. It is concluded that the muscarinic cholinergic stimulation of [32P] incorporation into PhA, but not [3H]inositol 1-PO4 accumulation is relatively more sensitive to the direct effects of ethanol than are the other receptor mediated phospholipid responses examined in the present investigation and that this sensitivity is lost in animals made behaviorally tolerant and physically dependent to ethanol.

Phospholipid requirement of Ca2+-stimulated, Mg2+-dependent ATP hydrolysis in rat brain synaptic membranes

The phospholipid requirement for Ca2+-stimulated, Mg2+-dependent ATP hydrolysis (Ca2+/Mg2+-ATPase) and Mg2+-stimulated ATP hydrolysis (Mg2+-ATPase) in rat brain synaptosomal membranes was studied employing partial delipidation of the membranes with phospholipase A2 (Hog pancreas), phospholipase C (Bacillus cereus) and phospholipase D (cabbage). Treatment with phospholipase A2 caused an increase in the activities of both Ca2+/Mg2+-ATPase and Mg2+-ATPase whereas with phospholipase C treatment both the enzyme activities were inhibited. Phospholipase D treatment had no effect on Ca2+/Mg2+-ATPase but Mg2+-ATPase activity was inhibited. Inhibition of Mg2+-ATPase activity after phospholipase C treatment was relieved with the addition of phosphatidylinositol-4,5-bisphosphate (PIP2) and to a lesser extent with phosphatidylinositol-4-phosphate (PIP) and phosphatidylcholine (PC). Phosphatidylserine (PS), phosphatidic acid (PA), PIP and PIP2 brought about the reactivation of Ca2+/Mg2+-ATPase. Phosphatidylinositol (PI) and PA inhibited Mg2+-ATPase activity. Kms for Ca2+ (0.47 microM) and Mg2+ (60 microM) of the enzyme were found to be unaffected after treatment with the phospholipases.

Effects of cannabinoids on the activities of mouse brain lipases

Cannabinoids were found to augment phospholipase activities and modify lipid levels of mouse brain synaptosomes, myelin and mitochondria. Delta-1-tetra-hydrocannabinol (delta 1-THC) and several of its metabolites induced a dose-dependent (0.32-16 microM) stimulation of phospholipase A2 (PLA2) activity resulting in the increased release of free arachidonic acid from exogenous [1-14C]phosphatidylcholine (PC). The potencies of the cannabinoids in modulating PLA2 activity were approximately of the order: 7-OH-delta 1-THC greater than delta 1-THC greater than 7-oxo-delta 1-THC greater than delta 1-THC-7-oic acid = 6 alpha OH-delta 1-THC much greater than 6 beta-OH-delta 1-THC. The hydrolysis of phosphatidylinositol (PI) by synaptosomal phospholipase C (PLC) was enhanced significantly by delta 1-THC and promoted diacylglyceride levels by greater than 100 percent compared to control values. In contrast, arachidonate was the major product resulting from phospholipase activities of a 20,000 g pellet. Synaptosomal diacylglyceride lipase activity was inhibited by delta 1-THC. [1-14C]Arachidonic acid was readily incorporated into subcellular membrane phospholipids and after exposure to cannabinoids led to diminished phosphoglyceride levels and concomitant increases in released neutral lipid products. These data suggest that cannabinoids control phospholipid turnover and metabolism in mouse brain preparations by the activation of phospholipases and, through this mechanism, may exert some of their effects.

Observations on the muscarinic activation of catecholamine secretion in the chicken adrenal

Cells were isolated by collagenase digestion of chicken adrenal glands. Catecholamine secretion could be stimulated by acetylcholine, carbamylcholine, potassium or veratridine. Methacholine, muscarine and oxotremorine were also effective secretagogues whereas nicotine was not. Secretion evoked by acetylcholine was blocked by low concentrations of atropine but was relatively insensitive to hexamethonium. Atropine-sensitive secretion required both external sodium and calcium, was unaffected by tetrodotoxin, blocked by methoxy verapamil and nifedipine, and potentiated by BAY-K-8644. These data suggest that muscarinic activation of these cells facilitates tetrodotoxin insensitive depolarization, thereby opening conventional voltage-sensitive calcium channels. The mechanism by which calcium activates catecholamine secretion was investigated in cells that had been made permeable by exposure to brief intense electric fields. Catecholamine release required Mg-adenosine 5' triphosphate, was half-maximally activated by 1 microM Ca2+ and could be inhibited by high concentrations of Mg2+. At low Ca2+ concentrations, release was potentiated by 12-O-tetradecanoylphorbol 13-acetate, dioctanoylglycerol, guanosine 5'-O-(3-thiotriphosphate) and 5'-guanylylimidodiphosphate, all of which increased the apparent affinity of exocytosis for Ca2+.