Effects of cerebral ischemia on [3H]inositol lipids and [3H]inositol phosphates of gerbil brain and subcellular fractions

Reduced mGluR5 Activity Modulates Mitochondrial Function

The metabotropic glutamate receptor 5 (mGluR5) is an essential modulator of synaptic plasticity, learning and memory; whereas in pathological conditions, it is an acknowledged therapeutic target that has been implicated in multiple brain disorders. Despite robust pre-clinical data, mGluR5 antagonists failed in several clinical trials, highlighting the need for a better understanding of the mechanisms underlying mGluR5 function. In this study, we dissected the molecular synaptic modulation mediated by mGluR5 using genetic and pharmacological mouse models to chronically and acutely reduce mGluR5 activity. We found that next to dysregulation of synaptic proteins, the major regulation in protein expression in both models concerned specific processes in mitochondria, such as oxidative phosphorylation. Second, we observed morphological alterations in shape and area of specifically postsynaptic mitochondria in mGluR5 KO synapses using electron microscopy. Third, computational and biochemical assays suggested an increase of mitochondrial function in neurons, with increased level of NADP/H and oxidative damage in mGluR5 KO. Altogether, our observations provide diverse lines of evidence of the modulation of synaptic mitochondrial function by mGluR5. This connection suggests a role for mGluR5 as a mediator between synaptic activity and mitochondrial function, a finding which might be relevant for the improvement of the clinical potential of mGluR5.

Alteration of phosphatidylinositol transfer protein during global brain ischemia-reperfusion in gerbils

Phosphatidylinositol transfer proteins (PI-TPs) are responsible for the transport of phosphatidylinositol and other phospholipids. Moreover, these proteins are involved in vesicle transport and in the function of cytoskeleton. Our previous data indicated that brain ischemia affected phosphoinositides metabolism and the level of lipid derived second messengers. In this study, the effect of ischemia-reperfusion injury on the level of PI-TPs and of the role of NMDA receptor stimulation on the alteration of these proteins was investigated during reperfusion after 5 min of forebrain ischemia in gerbils. Some groups of animals were injected intraperitoneally with MK-801, an antagonist of NMDA receptor 30 min before ischemia. The levels of both PI-TP isoforms alpha+beta and separately the alpha-isoform were determined in cytosol and membrane fraction from brain cortex and hippocampus using Western blot analysis. In the cytosolic fractions, the concentration of both isoforms of PI-TP was 2 times higher when compared to the membrane fraction. In brain cortex, PI-TP alpha isoform consist about 32-44% but in hippocampus 72-82% of both isoforms (PI-TP alpha+beta) in cytosolic and membrane fraction respectively. Ischemia-reperfusion had no effect on PI-TPs in brain cortex. However, in hippocampus after 5 min ischemia and during whole reperfusion time up till 7 days the level of PI-TP alpha+beta and PI-TP alpha was significantly higher by about 20-55%, respectively when compared to control. MK-801 eliminated ischemia-reperfusion evoked alteration of PI-TPs. To confirm the role of NMDA receptor in PI-TP alteration additional experiments were carried out on PC-12 cells in culture. The results indicated that activation of NMDA receptor enhances significantly the level of PI-TP alpha. The competitive antagonist of NMDA receptor inhibited this effect. These results indicated that activation of NMDA receptor is connected with PI-TPs alteration and plays an important role in modulation of PI-TPs during ischemia-reperfusion injury that may have important physiopathological consequence.

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 phosphatidylethanolamine degradation by enzyme(s) of subcellular fractions from cerebral cortex

Hydrolysis of 1-acyl-2-[14C]arachidonoyl-sn-glycero-3-phosphoethanolamine was studied in cerebral cortex homogenate and subcellular fractions. The enzyme(s) confined to the synaptic plasma membrane (SPM) hydrolyze(s) [14C-arachidonoyl]phosphatidylethanolamine (PE) in the presence of EGTA to [14C-arachidonoyl]diacylglycerol (DAG) and a small amount of [14C]arachidonic acid (AA). Degradation of PE is time-, protein- and substrate-dependent with a pH optimum of 7.8. The highest activity of PE degradation was observed in the presence of 10 mM EGTA. Under this condition GTP gamma S has no effect on PE hydrolysis. In the presence of Ca2+ ions degradation of PE was significantly lower as compared to the conditions with EGTA. However, the percentage distribution of free AA in the sum of both products of PE hydrolysis (AA + DAG) increases from 16 and 20% observed in the presence of EGTA 2 mM and 10 mM to 34% and 43% in the presence of 0.5 mM CaCl2 alone and together with GTP gamma S, respectively. Cytosolic enzymes also degrade PE in the presence of 2 mM EGTA with the formation of DAG and AA. Radioactivity in the AA represents about 80% of the total radioactivity of the products of PE degradation. The hydrolysis of PE by cytosolic enzymes is almost completely inhibited by neomycin but the hydrolysis by the SPM-bound enzyme(s) is inhibited only 70%. Other studies with quinacrine indicated that only a small pool of PE is degraded by SPM-bound Ca(2+)-independent phospholipase A2 (PLA2). All of these data suggest that PE in cerebral cortex is mainly degraded by cytosolic and SPM-bound Ca(2+)-independent phospholipase C. Further studies towards a better understanding of the mechanisms of cerebral degradation and the physiological significance of Ca(2+)-independent pathways of PE hydrolysis are necessary.

A new method to measure cardiac inositol levels in intact animals

Inositol levels have been studied in cellular cultures and recently by perfusion of isolated hearts. The study was aimed to assess inositol turnover in rabbit hearts from intact animals. Thirty rabbits were injected i.v. three times (every 12 hr) with 25 microCi/kg of myo-3H-inositol. The rabbits 12 hr after the last injection were killed and the hearts perfused according to Langerdorff technique. Systolic and diastolic ventricular pressures (SVP, DVP), dp/dt, and coronary flow (CFl) were measured. The hearts (n = 14) were perfused under aerobic conditions and 16 hearts under ischemic conditions for 30 min. In addition, 5 hearts were perfused under aerobic conditions for 10 min, and 6 hearts were perfused under ischemic conditions for 10 min. Samples of myocardial tissue were taken from both groups at the end of 10-min and 30-min period of perfusion, and cAMP and inositol phosphates were assayed. The hearts subjected to ischaemia showed changes of cAMP and 3H-inositol. The cAMP was higher in the ischaemic (10 min and 30 min) than the control hearts, 0.22 +/- 0.09 and 0.21 +/- 0.08 versus 0.41 +/- 0.12 and 0.49 +/- 0.11 pmol 10(6) cells, respectively (p < .05, p < .001. The inositol trisphosphate was higher in control than ischemic hearts (10 min, 30 min), 0.42 +/- 0.02 and 0.39 +/- 0.01 versus 0.31 +/- 0.01 and 0.23 +/- 0.02 (percent of radioactivity) respectively, p < .001. Our data suggest that 3H-inositol may be studied by i.v. administration to intact animals. The ischemia was performed to verify the validity of this new technique.

Serotonin, a potent modulator of arachidonic acid turnover, interaction with glutamatergic receptor in brain cortex

Brain cortex synaptoneurosomes actively incorporated [14C]arachidonic acid (AA) into lipids. Serotonin (5-HT), at a concentration range of 10 microM-1 mM, significantly stimulates the incorporation of AA mainly into phosphatidylinositol (PI) of brain cortex synaptoneurosomes. The stimulation rate of AA incorporation by 5-HT was the same in the presence and absence of lysophosphatidylinositol (LPI). However, in the absence of LPI some stimulation of AA uptake was also observed into phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid. Buspirone, an agonist of 5-HT1A receptor, has a similar effect on AA incorporation into membrane lipids as serotonin itself. Moreover, ketanserin, an antagonist of 5-HT2 receptor, also induces activation of AA incorporation into membrane lipids. On the other hand, glutamate, in a concentration dependent manner, significantly inhibits AA uptake into PI and also has some inhibitory action on AA uptake into the other lipids. Serotonin itself and the agonist of 5-HT1A receptor through the activation of AA turnover counteract glutamate-induced inhibition of AA uptake into lipids of brain cortex. Our results indicated that serotonin directly, through the specific receptors, or indirectly, through the interaction with glutamatergic receptors, modulates turnover and the level of arachidonic acid in the brain.

Aging diminishes serotonin-stimulated arachidonic acid uptake and cholinergic receptor-activated arachidonic acid release in rat brain cortex membrane

Synaptoneurosomal and synaptosomal fractions from the brain cortex of adult (4-month-old) and aged (27-month-old) rats were used for studies on the uptake and subsequent release of [14C]arachidonic acid ([14C]AA) from brain lipids. The incorporation of AA and the pattern of its uptake into lipids of the aged brain cortex synaptoneurosomes and synaptosomes were not significantly different when compared with those in the adult brain cortex fractions. Serotonin (5-HT), at 10 microM to 1 mM in the presence of pargyline and the agonist of the 5-HT1A receptor, buspirone, stimulated AA uptake into membrane lipids, mainly into phosphatidylinositol, by about 40% exclusively in adult brain synaptoneurosomes. Aging significantly diminished the effect of 5-HT on AA uptake. Synaptoneurosomal and synaptosomal fractions prelabeled with [14C]AA were used subsequently for investigation of voltage-dependent, muscarinic and 5-HT receptor-mediated AA release. Aging diminished markedly carbachol-stimulated Ca(2+)-dependent AA liberation from membrane lipids of synaptoneurosomes and synaptosomes. Moreover, aging decreased voltage-dependent and 5-HT2 receptor-mediated AA release. These results show that aging affects receptor-dependent AA uptake and pre- and postsynaptic receptor-mediated AA release. These modulations of AA incorporation and release in aged brain may be of pathophysiological significance, in view of the importance of these processes for signal transmission in the brain. The changes of receptor-dependent processes of deacylation and reacylation may be responsible for alteration in the function of neuronal cells and may affect learning and memory ability and brain plasticity during aging.

Protein kinase C as an early and sensitive marker of ischemia-induced progressive neuronal damage in gerbil hippocampus

In the model of transient brain ischemia of 6-min duration in gerbils we have estimated: 1. The concentration of brain gangliosides: A significant decrease to about 70% of control was observed selectively in the hippocampus at 3 and 7 d after ischemia. 2. The activity of Na+,K(+)-ATPase: The enzyme activity was not affected in either hippocampus nor in cerebral cortex. 3. The malonaldehyde (MDA) concentration: The levels of MDA had increased at 30 min after ischemia up to 123 and 129% of control in hippocampus and cerebral cortex, respectively. 4. Immunoreactivity of protein kinase C detected by Western blotting: In hippocampus the early translocation toward membranes was followed by a decrease in total enzyme content at 6, 24, 72, and 96 h of postischemic recovery. Also, a sharp increase of 50 kDa isoform (PKM) was noticed immediately and at the early recovery times. The behavior of these biochemical markers of ischemic brain injury in the hippocampus after the short (6 min) insult was contrasted with their reaction in the cerebral cortex as well as after prolongation of the ischemia to 15 min. These results taken together indicate that an early increase in PKC translocation followed by a decrease is the most symptomatic for selective, delayed, postischemic hippocampal injury, resulting from short duration (6 min) ischemia of the gerbil brain.

Effect of muscarinic cholinergic drugs on ischemia-induced decreases in glucose uptake and CA1 field potentials in rat hippocampus slices

To clarify the role of muscarinic acetylcholine receptors in the hypoxia/hypoglycemia (ischemia)-induced functional deficit in hippocampal neurons, we examined the effect of cholinergic drugs on ischemia-induced impairments of glucose uptake and CA1 field potentials in hippocampus slices. Muscarinic receptors were subdivided into M1 (high affinity for pirenzepine) and M2 (low affinity for pirenzepine) subtypes. The M1 receptor subtype is coupled to an increase in phosphoinositide hydrolysis and the M2 receptor subtype is associated with inhibition of adenylate cyclase. The greater potency of carbachol in stimulating phosphoinositide hydrolysis resulted in exacerbated ischemia-induced deficits. Treatment with the muscarinic receptor antagonists scopolamine and pirenzepine (M1 receptor-selective antagonist) had a strong dose-dependent protective effect against ischemia-induced deficits. Oxotremorine and McN-A-343, weak stimulators of phosphoinositide hydrolysis and strong inhibitors of adenylate cyclase, had a weak neuroprotective action against ischemia-induced deficits. These results suggest that stimulation of M1 muscarinic receptors coupled with an increase in phosphoinositide hydrolysis may play a facilitatory role in ischemia-induced deficits. Stimulation of M2 muscarinic receptors may play an inhibitory role in ischemia-induced neuronal deficits.

Early postanoxic changes of polyphosphoinositides and bound Ca2+ content in relation to neuronal activity in brain cortex

We studied the changes in the content of membrane-bound calcium (Cab) and the polyphosphoinositides (poly-PI): bis- and trisphosphoinositide (PIP and PIP2) in the cat brain cortex during the early period (up to 30 min) of reoxygenation after 2.5 min and 5 min of anoxia. In vivo experiments were performed on a living cat cortical preparation. Studies included Cab estimation with clortetracycline, a calcium fluorescent chelate probe, and simultaneous registration of neuronal activity. Anoxia resulted in a significant drop of Cab and PIP2 in the cortex along with an absence of neuronal activity. During reoxygenation after 2.5 min of anoxia we observed an increase of Cab, however the Cab did not recover to the preanoxic level. An elevation of PIP and PIP2 content to 20% above the preanoxic level and recovery of neuronal activity with symptoms of hyperactivation were also observed. After 5 min of anoxia two qualitatively different types of changes were disclosed for the 30 min period of reoxygenation. In one half of the animals only slight symptoms of recovery in some of the indices were found. In the other group Cab and PIP2 content increased to a level significantly exceedingly the preanoxic one and abnormal spike activity appeared. Based on these results we suggest that disturbances in Ca- and poly-PI-related second messenger systems may significantly affect the recovery of neuronal function after anoxia.

NMDA receptor-mediated arachidonic acid release in neurons: role in signal transduction and pathological aspects

The N-methyl-D-aspartate (NMDA)-sensitive subtype of glutamate receptor, which gates Ca(2+)-permeable ion channels, is known for its role in learning and memory formation, in the induction of long-term potentiation, and also in seizure activity and neurotoxicity. In primary cultures of cerebellar neurons, agonists of NMDA receptors induce a dose-dependent release of [3H]arachidonic acid ([3H]AA), which is potentiated by activation of the glycine-positive modulatory site and inhibited by NMDA receptor antagonists. NMDA receptor-induced [3H]AA release is inhibited by quinacrine and partially depends on the presence of extracellular calcium. The [3H]AA release is not sensitive, however, to pretreatment with pertussis or cholera toxin, which suggests a Ca(2+)-dependent activation of phospholipase A2 not employing G proteins. Pretreatment of cultures with the natural and semisynthetic sphingolipids GT1b and PKS 3, respectively, inhibits NMDA receptor-mediated [3H]AA release. We also demonstrated glutamate-evoked [3H]AA release from rat hippocampal slices, which is NMDA receptor mediated, calcium dependent and sensitive to quinacrine. Arachidonic acid and its metabolites have been shown to play a role as second messengers and to modulate neuronal activity. Moreover, they are thought to act as transsynaptic modulators in the mechanism of NMDA receptor-induced long-term potentiation in the hippocampus. Their role in ischemic brain pathology has also been postulated. Our experiments on cultured cerebellar granule cells, incubated in a Mg(2+)-free medium deprived of glucose and oxygen, demonstrated a time-dependent stimulation of [3H]AA release. This release was inhibited by antagonists of NMDA receptors and by quinacrine. Stimulation of NMDA-sensitive glutamate receptors and the subsequent calcium-mediated activation of phospholipase A2 may play a role in the in vivo release of arachidonic acid during brain ischemia. This hypothesis is supported by the observation that the enhanced level of thromboxane B2 in the gerbil brain after 5 min of global ischemia is reduced by the systemic application of either the NMDA antagonist MK-801 or the ganglioside GM1.

Carbachol-stimulated release of arachidonic acid and eicosanoids from brain cortex synaptoneurosome lipids of adult and aged rats

Synaptoneurosomes from the brain cortex of adult rats (4 months old) and aged rats (27 months old), prelabeled with [14C]arachidonic acid (AA), were used as the source of enzyme(s) and substrates to study the effect of a cholinergic agonist on the release of AA and eicosanoids. In synaptoneurosomes from adult brains, carbachol, the nonhydrolyzable analog of acetylcholine, increased AA release by 16% in the presence of 2 mM calcium. This agonist-mediated AA release occurred specifically from phosphatidylinositol (PI). Concomitantly, carbachol in the presence of 2 mM Ca2+ significantly activated the formation of 15-HETE and PGF2 alpha. This effect of carbachol on the level of eicosanoids was also observed in the presence of endogenous calcium. In synaptoneurosomes from aged brains, carbachol had no effect on the release of AA and eicosanoids. The results of studies involving inhibitors of phospholipase A2 (PLA2) and phospholipase C (PLC) suggested that PLA2 is almost completely responsible for the Ca(2+)-dependent, carbachol-mediated AA liberation. The distribution of labeled AA in the lipids after incubation of synaptoneurosomes in the presence of 2 Mm Ca2+ and carbachol indicated that in aged synaptoneurosomes, the muscarinic receptor-mediated degradation of phosphoinositides through phospholipase C is preserved, but the turnover of the phosphoinositide cycle is probably suppressed. These results indicate that aging significantly affects the population of cholinergic-muscarinic receptors coupled to PLA2.

Ca(2+)-independent, Ca(2+)-dependent, and carbachol-mediated arachidonic acid release from rat brain cortex membrane

Synaptoneurosomes obtained from the cortex of rat brain prelabeled with [14C]arachidonic acid [( 14C]AA) were used as a source of substrate and enzyme in studies on the regulation of AA release. A significant amount of AA is liberated in the presence of 2 mM EGTA, independently of Ca2+, primarily from phosphatidic acid and polyphosphoinositides (poly-PI). Quinacrine, an inhibitor of phospholipase A2 (PLA2), suppressed AA release by about 60% and neomycin, a putative inhibitor of phospholipase C (PLC), reduced AA release by about 30%. An additive effect was exhibited when both inhibitors were given together. Ca2+ activated AA release. The level of Ca2+ present in the synaptoneurosomal preparation (endogenous level) and 5 microM CaCl2 enhance AA liberation by approximately 25%, whereas 2 mM CaCl2 resulted in a 50% increase in AA release relative to EGTA. The source for Ca(2+)-dependent AA release is predominantly phosphatidylinositol (PI); however, a small pool may also be liberated from neutral lipids. Carbachol, an agonist of the cholinergic receptor, stimulated Ca(2+)-dependent AA release by about 17%. Bradykinin enhanced the effect of carbachol by about 10-15%. This agonist-mediated AA release occurs specifically from phosphoinositides (PI + poly-PI). Quinacrine almost completely suppresses calcium-and carbachol-mediated AA release. Neomycin inhibits this process by about 30% and totally suppresses the effect of bradykinin. Our results indicate that both phospholipases PLA2 and PLC with subsequent action of DAG lipase are responsible for Ca(2+)-independent AA release. Ca(2+)-dependent and carbachol-mediated AA liberation occurs mainly as the result of PLA2 action. A small pool of AA is probably also released by PLC, which seems to be exclusively responsible for the effect of bradykinin.

Interaction of aluminum ions with phosphoinositide metabolism in rat cerebral cortical membranes

Aluminum (Al) is believed to exert a primary role in the neurotoxicity associated with dialysis encephalopathy and has been suggested to be involved in a number of other neurological disorders, including Alzheimer's disease. Al, complexed with fluoride to form fluoroaluminate (AlF4-), can activate the GTP-binding (G) proteins of the adenylate cyclase and retinal cyclic GMP phosphodiesterase systems. Since an involvement of G-proteins with cerebral phosphoinositide (PtdIns) metabolism has also been suggested, in this study we investigated the interaction of the stable GTP analogue GTP(S), Al salts and NaF with this system. In rat cerebral cortical membranes, GTP(S) dose-dependently stimulated [3H]inositol phosphates ([3H]InsPs) accumulation. This effect was potentiated by carbachol and was partially prevented by the GTP-binding antagonist GDP(S), indicating that CNS muscarinic receptor activation is coupled to PtdIns hydrolysis via putative G-protein(s). GTP(S) stimulation was also inhibited by phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, which is known to exert a negative feedback control on agonist-stimulated PtdIns metabolism. Both Al salts and NaF mimicked the action of GTP(S) in stimulating PtdIns turnover. Their actions were highly synergistic, suggesting that AlF4- could be the active stimulatory species. However, the stimulatory effects of AlCl3 and/or NaF were not potentiated by carbachol and were not inhibited by GDP(S) and PMA, suggesting that separate sites of action might exist for GTP(S) and AlF4-. In the nervous tissue, activation of PtdIns hydrolysis by Al (probably as AlF4-) may be mediated by activating a regulatory G-protein at a location distinct from the GTP-binding site or by a direct stimulation of phospholipase C.

Lithium ion does not protect brain against transient ischemia in gerbils

It has been proposed that lithium ion desensitizes neuronal receptors that function via the inositol phospholipid signaling mechanism. We examined the effects of lithium chloride on the morphologic outcome after 5 minutes of cerebral ischemia induced in gerbils by occluding both common carotid arteries under brief halothane anesthesia. In three treated groups of 10 gerbils each, 5 meq/kg i.p. lithium chloride was given 2 days, 1 day, and 2 hours before ischemia; 2 hours before ischemia; or immediately after the end of ischemia. Corresponding control groups of nine or 10 gerbils each received equivalent volumes of saline injected at comparable times. All gerbils were perfusion-fixed 1 week later, and neuronal density of the hippocampal CA1 pyramidal cells was determined. Lithium induced very mild intraischemic systemic hypothermia, but postischemic hyperthermia developed in both treated and control groups. Neuronal densities were equal in corresponding groups. The results indicate that our regimen of lithium administration provides no benefit in survival of hippocampal neurons, and intraischemic hypothermia of less than 0.8 degrees C is not protective. Other strategies to inactivate the signal transduction system that is specific for excitatory neurotransmission should be evaluated.

Phosphoinositide breakdown in rat hippocampal slices: sensitivity to glutamate induced by in vitro anoxia

We examined the effects of in vitro anoxia on phosphoinositide (PI) breakdown in rat hippocampal slices stimulated by glutamate and quisqualate. In addition to assays of accumulations of 3H-inositol phosphates (3H-IPs) degraded from prelabeled PI, we adopted direct assay procedures of inositol 1,4,5-triphosphate (1,4,5-IP3) using 1,4,5-IP3-specific binding protein to determine the formation of 1,4,5-IP3. The first effect, observed with anoxic incubation by itself, was the diminished quisqualate (10(-5) M)-stimulated accumulation of 3H-IPs degraded from prelabeled PI under prolonged anoxia. Quisqualate caused a transient increase in 1,4,5-IP3 formation in the early phase of anoxia, similar to that under oxygenated conditions. Glutamate (10(-5) M), under normal conditions, influenced neither the accumulation of 3H-IPs nor the formation of 1,4,5-IP3. Also, the accumulation of 3H-IPs under prolonged anoxia was unaffected. The same concentration of glutamate, however, gave rise to a transient increase in 1,4,5-IP3 content in the early phase of anoxia, similar to that caused by quisqualate. The second effect, observed by oxygenation following anoxia, was the induction of glutamate-stimulated accumulation of 3H-IPs. When the hippocampal slices were oxygenated following a sufficiently long (greater than 30-min) exposure to anoxia, glutamate (10(-5) M) caused a significant increase in accumulation of 3H-IPs degraded from prelabeled PI. Quisqualate-stimulated accumulation of 3H-IPs under oxygenated incubations was also increased by prior exposure of slices to anoxia. These results support the hypothesis that an exposure of hippocampal slices to anoxia induces a sensitivity of the PI breakdown pathway to glutamate and that, given an oxygen supply following sufficiently long exposure to anoxia, the slices maintain their sensitivity to glutamate with an apparent increase in the accumulation of 3H-IPs.

Effect of partial ischemia on phospholipids and postischemic lipid peroxidation in rabbit spinal cord

Rabbit spinal cord, subjected to severe partial ischemia induced by abdominal aorta ligation tightly below the renal arteries, was analyzed for phospholipid composition and levels of lipid peroxidation products after 10, 20, and 40 min of the insult. Under conditions when spinal cord blood flow was decreased below 5% of control, concentrations of inositol and ethanolamine phospholipids were decreased by 30% and 10%, respectively. Phosphatidic acid concentration was also altered during ischemia. No accumulation of thiobarbituric acid reactive substances (TBA-RS), conjugated dienes and fluorescent lipid soluble material was found throughout the ischemic period. Pattern of TBA-RS, conjugated diene, and fluorophore formation during postischemic in vitro incubation without and with a peroxidation couple (Fe2+, ascorbic acid) showed increased susceptibility to postischemic lipid peroxidation in tissues after 20 and 40 min of ischemia.

Prolonged ischemia differently affects phospholipase C acting against phosphatidylinositol and phosphatidylinositol 4,5-bisphosphate in brain subsynaptosomal fraction

The effect of 10 min ischemia on the activity of phospholipase C acting against [3H]inositol-phosphatidylinositol (PI) and [3H]inositol-phosphatidylinositol 4,5-bisphosphate (PIP2) in the brain subsynaptosomal fractions was investigated. In the presence of endogenous CaCl2, specific activity of phospholipase C acting on phosphatidylinositol was as follows: synaptic cytosol (SC) greater than synaptic vesicles (SV) greater than synaptic plasma membrane SPM). Brain ischemia activated phospholipase C acting on PI by about 60% and 40% in SV and SPM, respectively. The enzyme of synaptic cytosol was not affected by ischemic insult. Phospholipase C acting against PIP2 in the presence of endogenous calcium expressed the specific activity in the following order: SV greater than SPM greater than SC. After 10 min of brain ischemia, activity of phospholipase C acting on PIP2 was significantly suppressed in all subsynaptosomal fractions by about 50-60%. These results indicate that prolonged ischemia produced activation exclusively of phospholipase C acting against phosphatidylinositol.

Stimulation of phosphoinositide degradation and phosphatidylinositol-4-phosphate phosphorylation by GTP exclusively in plasma membrane of rat brain

The effect of GTP on the hydrolysis of [3H]phosphatidylinositol (PI), [3H]phosphatidylinositol-4-phosphate (PIP) and [3H]phosphatidylinositol-4,5-bisphosphate (PIP2) by phospholipase C of rat brain plasma membrane, microsomes and cytosol was determined. Moreover the regulation of PI and PIP phosphorylation by GTP in brain plasma membrane was investigated. In the presence of EGTA PIP2 was actively degraded, opposite to PI and PIP which require Ca2+ for their hydrolysis. Addition of calcium ions in each case caused stimulation of inositide phosphodiesterase(s). GTP independently of calcium ions activates by about 3 times phospholipase C acting on PIP and PIP2 exclusively in the plasma membrane. PI degradation was unaffected by GTP. In the presence of Ca2+ guanine nucleotides have synergistic stimulatory effect on plasma membrane bound phospholipase C acting on PIP2. PIP kinase of brain plasma membrane was stimulated by GTP by about 20-100% in the presence of exogenous and endogenous substrate respectively. PI kinase was negligible activated by about 20% exclusively in the presence of endogenous substrate. These results indicated that guanine nucleotide modulates the level of second messengers as diacylglycerol and IP3 through the activation of phospholipase C acting on PIP2 exclusively in brain plasma membrane. The stimulation of phospholipase C by GTP may occur directly or through the enhancement of substrate level PIP2 due to stimulation of PIP kinase.

Effects of oxygen depletion on norepinephrine- and carbachol-stimulated phosphoinositide turnover in rat brain slices

We examined the effects of in vitro anoxia and in vivo hypoxia (8% O2/92% N2) on norepinephrine (NE)- and carbachol-stimulated phosphoinositide (PI) turnover in rat brain slices. The formation of 3H-labeled polyPI in cortical slices was impaired by in vitro anoxia and fully restored by reoxygenation. Accumulation of 3H-labeled myo-inositol phosphates (3H-IPs) stimulated by 10(-5) M NE was significantly reduced by anoxia (control at 60 min, 1,217 +/- 86 cpm/mg of protein; anoxia for 60 min, 651 +/- 82 cpm/mg; mean +/- SEM; n = 5; p less than 0.01), and reoxygenation following anoxia resulted in overshooting of the accumulation (control at 120 min, 1,302 +/- 70 cpm/mg; anoxia for 50 min plus oxygenation for 70 min, 1,790 +/- 126 cpm/mg; n = 5; p less than 0.01). The underlying mechanisms for the two phenomena--the decrease caused by anoxia and the overshooting caused by reoxygenation following anoxia--seemed to be completely different because of the following observations. (a) Although the suppression of NE-stimulated accumulation at low O2 tensions was also observed in Ca2+-free medium, the overshooting in response to reoxygenation was not. (b) Carbachol-stimulated accumulation was significantly reduced by anoxia and was restored by reoxygenation only to control levels. Thus, the postanoxic overshooting in accumulation of 3H-IPs seems to be a specific response to NE. (c) The decrease observed at low O2 tensions was due to a decrease in Emax value, whereas the postanoxic overshooting was due to a decrease in EC50 value.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of in vivo exposure to hypoxia on muscarinic cholinergic receptor-coupled phosphoinositide turnover in the rat brain

We examined the effects of a transient exposure of rats to severe (5% O2, 95% N2 for 30 min) or mild (10% O2, 90% N2 for 30 min) hypoxia and a chronic exposure to mild hypoxia (10% O2, 90% N2 up to 48 h) on carbachol-stimulated phosphoinositide (PI) turnover and [3H]quinuclidinyl benzilate (QNB) binding in 4 regions of the brain (cerebral cortex, striatum, hippocampus and cerebellum). When the rats were exposed to hypoxia transiently, significant changes were caused only by severe hypoxia. In all 4 regions examined, basal incorporation of [3H]inositol into inositol phosphates (IPs) in the absence of carbachol was significantly increased following transient severe hypoxia and remained high for 48 h. Carbachol stimulation of PI turnover (% of basal) was significantly enhanced only in the hippocampus. The increase in carbachol stimulation in the hippocampus was observed only after a 6-h exposure to room air following the severe hypoxia and lasted for at least 48 h. In Ca2+-free incubation medium, the increase in basal incorporation of [3H]inositol into IPs elicited by severe hypoxia was not observed but carbachol still had an enhanced stimulatory effect. Binding studies showed that maximum binding capacity (Bmax values) of [3H]QNB binding was significantly increased only in the hippocampus and only after a 6-h exposure to room air following the severe hypoxic exposure and remained increased for 48 h. Kd values showed no significant change. While a transient exposure of rats to mild hypoxia caused no significant change either in carbachol-stimulated PI turnover or in [3H]QNB binding parameters, a chronic exposure to mild hypoxia for more than 6 h caused a significant increase in [3H]QNB binding capacity localized in the hippocampus, which was accompanied by an enhanced stimulatory effect of carbachol on PI turnover. These changes observed with mild hypoxia, however, were transient and tended to return to control levels following a 48-h exposure. These results suggest an up-regulation of muscarinic cholinergic receptor-coupled PI turnover in the rat brain caused either by transient severe hypoxia or by chronic mild hypoxia.

Effect of atropine and gammahydroxybutyrate on ischemically induced changes in the level of radioactivity in [3H]inositol phosphates in gerbil brain in vivo

Brain ischemia in gerbils was induced by ligation of both common carotid arteries for 1 min or 10 min. Sham-operated animals served as controls. Intracerebral injection of [3H]inositol into gerbil brain 16 hr before ischemic insult resulted in equilibration of the label between inositol lipids and water-soluble inositol phosphate. A short ischemic period (1 min) resulted in a statistically significant increase in the radioactivity of inositol triphosphate (IP3) and inositol monophosphate (IP), by about 48% and 79%, respectively, with little change in that of the intermediate inositol biphosphate (IP2), which increased by about 16%. When the ischemic period was prolonged (10 min), an increase in the radioactivity of inositol monophosphate exclusively, by about 84%, was observed. The level of radioactivity in inositol phosphates IP2 and IP3 decreased by about 50%, probably as a consequence of phosphatase activation by the ischemic insult. The agonist of the cholinergic receptor, carbachol, injected intracerebrally (40 micrograms per animal) increased accumulation of radioactivity in all inositol phosphates. The level of radioactivity in IP3, IP2, and IP was elevated by about 40, 23, and 147%, respectively. The muscarinic cholinergic antagonist, atropine, injected intraperitoneally in doses of 100 mg/kg body wt. depressed phosphoinositide metabolism in control animals. The level of radioactivity in water-soluble inositol metabolites in the brain of animals pretreated with atropine was evidently about 32% lower than in untreated animals. Pretreatment with atropine decreased the radioactivity of all inositol phosphates in the brain of animals subjected to 1-min ischemia and the radioactivity of IP in the case of 10-min brain ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)