Lithium effects on inositol phospholipids and inositol phosphates: evaluation of an in vivo model for assessing polyphosphoinositide turnover in brain

IP3 accumulation and/or inositol depletion: two downstream lithium's effects that may mediate its behavioral and cellular changes

Lithium is the prototype mood stabilizer but its mechanism is still unresolved. Two hypotheses dominate-the consequences of lithium's inhibition of inositol monophosphatase at therapeutically relevant concentrations (the 'inositol depletion' hypothesis), and of glycogen-synthase kinase-3. To further elaborate the inositol depletion hypothesis that did not decisively determine whether inositol depletion per se, or phosphoinositols accumulation induces the beneficial effects, we utilized knockout mice of either of two inositol metabolism-related genes-IMPA1 or SMIT1, both mimic several lithium's behavioral and biochemical effects. We assessed in vivo, under non-agonist-stimulated conditions, ³H-inositol incorporation into brain phosphoinositols and phosphoinositides in wild-type, lithium-treated, IMPA1 and SMIT1 knockout mice. Lithium treatment increased frontal cortex and hippocampal phosphoinositols labeling by several fold, but decreased phosphoinositides labeling in the frontal cortex of the wild-type mice of the IMPA1 colony strain by ~50%. Inositol metabolites were differently affected by IMPA1 and SMIT1 knockout. Inositoltrisphosphate administered intracerebroventricularly affected bipolar-related behaviors and autophagy markers in a lithium-like manner. Namely, IP₃ but not IP₁ reduced the immobility time of wild-type mice in the forced swim test model of antidepressant action by 30%, an effect that was reversed by an antagonist of all three IP₃ receptors; amphetamine-induced hyperlocomotion of wild-type mice (distance traveled) was 35% reduced by IP₃ administration; IP₃ administration increased hippocampal messenger RNA levels of Beclin-1 (required for autophagy execution) and hippocampal and frontal cortex protein levels ratio of Beclin-1/p62 by about threefold (p62 is degraded by autophagy). To conclude, lithium affects the phosphatidylinositol signaling system in two ways: depleting inositol, consequently decreasing phosphoinositides; elevating inositol monophosphate levels followed by phosphoinositols accumulation. Each or both may mediate lithium-induced behavior.

Phospholipase C γ1 regulates early secretory trafficking and cell migration via interaction with p115

The role of early secretory trafficking in the regulation of cell motility remains incompletely understood. Here we used a small interfering RNA screen to monitor the effects on structure of the Golgi apparatus and cell migration. Two major Golgi phenotypes were observed-fragmented and small Golgi. The latter exhibited a stronger correlation with a defect in cell migration. Among the small Golgi hits, we focused on phospholipase C γ1 (PLCγ1). We show that PLCγ1 regulates Golgi structure and cell migration independently of its catalytic activity but in a manner that depends on interaction with the tethering protein p115. PLCγ1 regulates the dynamics of p115 in the early secretory pathway, thereby controlling trafficking from the endoplasmic reticulum to the Golgi. Our results uncover a new function of PLCγ1 that is independent of its catalytic function and link early secretory trafficking to the regulation of cell migration.

Lithium and valproate and their possible effects on themyo-inositol second messenger system in healthy volunteers and bipolar patients

Over 25 years ago it was suggested that the mechanism by which lithium was clinically effective may be due to a stabilizing effect on the phosphoinositol second messenger system (PI-cycle), which has multiple effects within cells. It was proposed that lithium, which is an inhibitor of one of the key enzymes in the PI-cycle, acted to lower myo-inositol concentrations; termed the 'inositol-depletion hypothesis'. Initial animal evidence supported this hypothesis, and also suggested that it was possible that sodium valproate could affect the PI-cycle. Since the first magnetic resonance studies in this area in the early 1990s many studies have examined various aspects of this hypothesis in both healthy volunteers and patients utilizing magnetic resonance spectroscopy (MRS). The present review considers research in this area and concludes that, despite initial promise, current evidence suggests that it is unlikely that either lithium or valproate produce clinically relevant changes in myo-inositol concentrations or the PI-cycle. These findings do not suggest that lithium-induced changes in the PI-cycle are the primary mechanism by which lithium or valproate exert their beneficial clinical effects in bipolar disorder. Nonetheless, given the current technical and clinical limitations of the literature to date, this conclusion cannot be considered completely definitive.

Cross-talk between the TGFβ and Wnt signaling pathways in murine embryonic maxillary mesenchymal cells

The transforming growth factor beta (TGFbeta) and Wnt signaling pathways play central roles regulating embryogenesis and maintaining adult tissue homeostasis. TGFbeta mediates its cellular effects through types I and II cell surface receptors coupled to the nucleocytoplasmic Smad proteins. Wnt signals via binding to a cell surface receptor, Frizzled, which in turn activates intracellular Dishevelled, ultimately leading to stabilization and nuclear translocation of beta-catenin. Previous studies have demonstrated several points of cross-talk between the TGFbeta and Wnt signaling pathways. In yeast two-hybrid and GST-pull down assays, Dishevelled-1 and Smad 3 have been shown to physically interact through the C-terminal one-half of Dishevelled-1 and the MH2 domain of Smad 3. The current study demonstrates that co-treatment of murine embryonic maxillary mesenchyme (MEMM) cells with Wnt-3a and TGFbeta leads to enhanced reporter activity from TOPflash, a Wnt-responsive reporter plasmid. Transcriptional cooperation between TGFbeta and Wnt did not require the presence of a Smad binding element, nor did it occur when a TGFbeta-responsive reporter plasmid (p3TP-lux) was transfected. Overexpression of Smad 3 further enhanced the cooperation between Wnt and TGFbeta while overexpression of dominant-negative Smads 2 and 3 inhibited this effect. Co-stimulation with TGFbeta led to greater nuclear translocation of beta-catenin, providing explanation for the effect of TGFbeta on Wnt-3a reporter activity. Wnt-3a exerted antiproliferative activity in MEMM cells, similar to that exerted by TGFbeta. In addition, Wnt-3a and TGFbeta in combination led to synergistic decreases in MEMM cell proliferation. These data demonstrate a functional interaction between the TGFbeta and Wnt signaling pathways and suggest that Wnt activation of the canonical pathway is an important mediator of MEMM cell growth.

Identification of PIK3C3 promoter variant associated with bipolar disorder and schizophrenia

BACKGROUND

Genes involved in phosphoinositide (PI) lipid metabolism are excellent candidates to consider in the pathogenesis of bipolar disorder (BD) and schizophrenia (SZ). One is PIK3C3, a member of the phosphatidylinositide 3-kinase family that maps closely to markers on 18q linked to both BD and SZ in a few studies.

METHODS

The promoter region of PIK3C3 was analyzed for mutations by single-strand conformation polymorphism analysis and sequencing. A case-control association study was conducted to determine the distribution of variant alleles in unrelated patients from three cohorts. Electromobility gel shift assays (EMSA) were performed to assess the functional significance of variants.

RESULTS

Two polymorphisms in complete linked disequilibrium with each other were identified, -432C- > T and a "C" insert at position -86. The -432T allele occurs within an octamer containing an ATTT motif resembling members of the POU family of transcription factors. In each population analyzed, an increase in -432T was found in patients. EMSAs showed that a -432T containing oligonucleotide binds to brain proteins that do not recognize -432C.

CONCLUSIONS

A promoter mutation in a PI regulator affecting the binding of a POU-type transcription factor may be involved in BD and SZ in a subset of patients.

Polymorphism screening of PIP5K2A: a candidate gene for chromosome 10p-linked psychiatric disorders

Lithium is a potent noncompetitive inhibitor of inositol monophosphatases, enzymes involved in phosphoinositide (PI) and inositol phosphate metabolism. A critical component of the PI pathway is phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), which is hydrolyzed to second messengers and has a direct role in synaptic vesicle function. Interestingly, a number of genes involved in the synthesis and dephosphorylation of PtdIns(4,5)P(2) are found in regions of the genome previously mapped in bipolar disorder (BD) including 10p12, 21q22, and 22q11, among others. Some of these regions overlap with loci mapped in schizophrenia (SZ). One gene involved in PI metabolism that maps to a region of interest is 10p12-linked PIP5K2A, a member of the phosphatidylinositol 4-phosphate 5-kinase family. Polymorphism screening revealed the existence of an imperfect CT repeat polymorphism located near the exon 9-intron 9 splice donor site. A modest difference was found in the distribution of alleles from this highly polymorphic variant when bipolar and schizophrenic subjects were compared with controls; relatively rare short repeat variants were found more commonly in patients and homozygosity for a common long repeat variant was found more commonly in controls. These data suggest that the imperfect CT repeat in PIP5K2A intron 9 should be further investigated as a possible candidate allele for 10p12-linked psychiatric disorders.

Chronic treatment with both lithium and sodium valproate may normalize phosphoinositol cycle activity in bipolar patients

BACKGROUND

It has been proposed that lithium may be clinically effective due to its actions on the phosphoinositol second messenger system (PI-cycle). Studies have also suggested that untreated manic patients may have raised myo-inositol and phosphomonoester (PME) concentrations and also that unmedicated euthymic bipolar patients may have lowered PME concentrations. The objective of the present study was to test the hypothesis that chronic treatment with either lithium or sodium valproate in patients with bipolar mood disorder leads to a normalization in the activity of the PI-cycle.

METHODS

This study had two parts each with different MRS methodology. The first part compared healthy controls (n = 19) with euthymic bipolar patients who were taking either lithium (n = 16) or sodium valproate (n = 11) using both (1)H-MRS and (31)P-MRS. In the second part we examined a separate group of euthymic bipolar disorder patients taking sodium valproate (n = 9) and compared these with age and sex-matched healthy controls (n = 11) using (1)H-MRS.

RESULTS

Both studies showed that there were no differences in either myo-inositol or phosphomonoester (PME) concentrations between controls and patients taking either medication.

CONCLUSIONS

These findings examine two key components of the PI-cycle in treated euthymic bipolar (myo-inositol and PME concentrations). The results from this study are consistent with the suggestion that chronic treatment with either lithium or sodium valproate in bipolar patients may normalize PI-cycle functioning.

Chronic lithium and sodium valproate both decrease the concentration of myo-inositol and increase the concentration of inositol monophosphates in rat brain

One of the mechanisms underlying lithium's efficacy as a mood stabilizer in bipolar disorder has been proposed to be via its effects on the phosphoinositol cycle (PI-cycle), where it is an inhibitor of the enzyme converting inositol monophosphates to myo-inositol. In contrast, sodium valproate, another commonly used mood stabilizer, appears to have no direct effects on this enzyme and was thus believed to have a different mechanism of action. In the present study, high resolution nuclear magnetic resonance (NMR) spectroscopy was used to study the chronic effects of both lithium and sodium valproate on the concentrations of myo-inositol and inositol monophosphates in rat brain. As predicted, lithium-treated rats exhibited a significant increase in the concentration of inositol monophosphates and a significant decrease in myo-inositol concentration compared to saline-treated controls. However, unexpectedly, sodium valproate administration produced exactly the same results as lithium administration. These novel findings suggest that both lithium and sodium valproate may share a common mechanism of action in the treatment of bipolar disorder via actions on the PI-cycle.

Effects of lithium and amphetamine on inositol metabolism in the human brain as measured by 1H and 31P MRS

BACKGROUND

The clinical effectiveness of lithium may be due to its decreasing the intracellular concentration of myo-inositol and increasing that of its inositol monophosphate precursors, which is known as the inositol depletion hypothesis.

METHODS

Magnetic resonance spectroscopy (MRS) was used to measure the concentration of both myo-inositol (1H MRS) and phosphomonoesters (PME) [31P MRS], in healthy volunteers in a double-blind placebo-controlled study. MRS measurements were made at baseline, again on the 7th day of lithium (1200 mg, n = 10) or placebo (n = 6) administration, and again on day 8, 2 hours following oral administration of 20 mg dextroamphetamine to stimulate the phosphoinositol (PI) cycle.

RESULTS

Subjects who received lithium showed a greater increase in PME ratios in response to amphetamine administration than did placebo-treated subjects.

CONCLUSIONS

The present results support the hypothesis that lithium administration blocks the conversion of inositol monophosphates to myo-inositol, and that this effect is especially apparent following PI cycle stimulation. The effects of lithium treatment on myo-inositol in healthy volunteers in vivo are uncertain, and may have to await improvements in the ability to measure myo-inositol in the brain.

Alpha-1 noradrenergic receptor stimulation impairs prefrontal cortical cognitive function

BACKGROUND

Many neuropsychiatric disorders are associated with high levels of noradrenergic turnover, and most antipsychotic medications have alpha-1 adrenoceptor blocking properties, yet little is known about alpha-1 influences on higher cortical function.

METHODS

The alpha-1 adrenergic agonist, phenylephrine, was infused into the prefrontal cortex (PFC) of rats (0.1 microgram/0.5 microL) performing a spatial working memory task, delayed alternation. The phenylephrine response was challenged with coinfusion of the alpha-1 adrenergic antagonist, uripidil (0.01 microgram), or with a dose of lithium chloride (4 mEq/kg, i.p., 18 hours) known to suppress phosphotidylinositol (PI) turnover, the second messenger pathway coupled to alpha-1 adrenoceptors.

RESULTS

Phenylephrine infusions in PFC markedly impaired delayed alternation performance. The phenylephrine response was reversed by coinfusion of uripidil, or by pretreatment with lithium, consistent with actions at alpha-1 adrenoceptors coupled to a PI pathway.

CONCLUSIONS

These findings demonstrate that alpha-1 adrenoceptor stimulation in the PFC impairs cognitive function. Excessive stimulation of alpha-1 adrenoceptors may contribute to PFC deficits (e.g., distractibility, impulsivity) in disorders such as mania, dementia, and anxiety associated with high noradrenergic turnover.

Poly-phosphoinositide-mediated messengers in focal cerebral ischemia and reperfusion

The receptor-mediated poly-phosphoinositide (PI) signalling pathway is known to play an important role in maintaining intracellular calcium homeostasis, which in turn, is critical for mediating neuronal function. In this study, we examined the effects of focal cerebral ischemia induced in rats by temporary occlusion of the middle cerebral artery (MCA) and both common carotid arteries (CCAs) on this signal transduction pathway. Results indicate that several parts of the pathway are altered, both during the early phase of focal cerebral ischemic insult and after recirculation. Cerebral ischemia induced a decrease in levels of phosphatidylinositol 4,5-bisphosphate (PIP2) in the ischemic MCA cortex, due partly to stimulated poly-PI hydrolysis and partly to the depletion of ATP required for resynthesis of this substrate. ATP depletion during ischemia was also attributed to a sustained decrease in inositol 1,4,5-trisphosphate (IP3) levels. On the other hand, the decline in IP3 3-kinase activity after 30 min of ischemic insult was not related to ATP depletion. During reperfusion upon prolonged ischemic insult, neither IP3 level nor IP3 3-kinase activity were able to show recovery after reperfusion, despite that ATP levels recovered by 80%. In situ hybridization studies indicated a decrease in mRNA expression of IP3 receptor but not IP3 3-kinase during the initial 4 h of reperfusion after a 45 min ischemic insult. Under this same condition, insulted cortical neurons started to show morphological changes between 4 and 8 h after reperfusion and extensive cell death could be observed by 16 h. Taken together, these results demonstrated early and delayed changes in the poly-PI signalling pathway due to focal cerebral ischemia. These effects are likely to cause impairment in neuronal function and underline the process of cerebral ischemic damage.

Chronic lithium does not alter human myo-inositol or phosphomonoester concentrations as measured by 1H and 31P MRS

Lithium may act by decreasing intracellular concentrations of myo-inositol. The present study measured the effects of chronic lithium on myo-inositol concentrations in volunteers. Eleven subjects received either lithium (n = 7) or placebo (n = 4) for 7 days in a double-blind study. Myo-inositol concentrations at baseline and day 8 were measured in vivo using 1H magnetic resonance spectroscopy (MRS). The results showed that lithium did not alter brain myo-inositol concentrations compared to placebo. In 5 other subjects we used 1H MRS and 31P MRS to measure changes in both myo-inositol and phosphomonoester concentrations. This second study showed that lithium did not alter myo-inositol or phosphomonoester concentrations. Thus, the present studies do not support the hypothesis that lithium significantly affects the brain concentrations of myo-inositol or phosphomonoesters; however, it is possible these findings represent an inability to detect the changes in myo-inositol and phosphomonoester concentrations that may have occurred following lithium administration.

In utero hypoxic ischemia decreases the cholinergic agonist-stimulated poly-phosphoinositide turnover in the developing rat brain

Perinatal hypoxic-ischemic (HI) insult is known to cause cellular and molecular disturbances leading to functional and behavioral abnormalities during brain development. In this study, we examined the effects of an in utero HI insult on poly-phosphoinositide turnover in vivo in the cerebrum and cerebellum as well as cholinergic-stimulated turnover in cortical slices from developing rat brain. In utero HI treatment was carried out by clamping the uterine blood vessels of near-term fetuses for 5, 10 and 15 min followed by resuscitation of the newborn pups. The in vivo protocol for examining poly-PI signaling activity in 2 week-old pup brain involved intracerebral injection of [3H]inositol for 16 hr and subsequent intraperitoneal injection with lithium (8 meq/kg) for 4 hr prior to decapitation. In the control pups, lithium elicited a 2.6 fold increase in labeled inositol phosphate (IP) in the cerebrum as compared to a 1.3 fold increase in the cerebellum. In utero HI insult (5 to 15 min) resulted in a small increase in labeled IP in the cerebrum but not in the cerebellum. Carbachol stimulation of poly-PI turnover was examined in brain slices prelabeled with [3H]inositol in vivo. Incubation of the prelabeled slices with carbachol in the presence of LiCl (10 mM) resulted in a time-, dose- and age-dependent increase in labeled IP. Brain slices from 2 week-old pups that experienced in utero HI-treatment for 10 and 15 min (but not 5 min) showed a significant decrease in carbachol-stimulation of labeled IP as compared with control pups. These results indicate the effects of in utero HI on the choninergic-stimulated poly-PI signaling pathway and its implication on related functional deficits in the developing brain.

Inositol trisphosphate, polyphosphoinositide turnover, and high-energy metabolites in focal cerebral ischemia and reperfusion

BACKGROUND AND PURPOSE

Although the signaling pathway involving polyphosphoinositide (poly-PI) hydrolysis and release of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] is an important mechanism for regulation of neuronal calcium homeostasis, the effect of cerebral ischemia-reperfusion on this calcium signaling pathway is not well understood. Because activity of this pathway is dependent on availability of ATP, this study is aimed at examining the poly-PI signaling pathway and high-energy metabolites in a rat stroke model.

METHODS

Focal cerebral ischemia in rats was induced by temporary occlusion of the right middle cerebral artery and both common carotid arteries. Levels of Ins(1,4,5)P3 were determined by use of the radioreceptor binding assay. Poly-PI turnover in rat cortex was assessed with an in vivo protocol involving intracerebral injection of [3H] inositol and systemic administration of lithium. High-energy metabolites (ATP, ADP, and AMP) were analyzed by high-performance liquid chromatography.

RESULTS

Ischemia induced an increase in poly-PI turnover in the right middle cerebral artery cortex, but reperfusion led to a decline in this signaling activity. However, Ins(1,4,5)P3 levels decreased during ischemia, and these levels were not restored if ischemic insults were longer than 30 minutes. ATP levels decreased to 26% of control during ischemia and recovered to 80% of control during the initial 4 hours of reperfusion; these changes were followed by a second phase of decline.

CONCLUSIONS

Results show an important relationship between ischemia-induced depletion of high-energy metabolites and poly-PI signaling activity. However, the uncoupling between Ins(1,4,5)P3 and ATP during reperfusion after severe ischemia suggests that metabolism of Ins(1,4,5)P3 is more stringently regulated than ATP.

The muscarinic receptor agonist L-658,903 modulates the in vivo accumulation of inositol monophosphates in mouse brain

In the present study we examined the effects of lithium chloride and the muscarinic receptor agonists pilocarpine hydrochloride and L-658,903 (3-(3-methyl-1,2,4-oxadiazol-5-yl) quinuclidine hydrochloride) upon the accumulation of inositol monophosphates in mouse brain using a radiometric technique. Lithium was able to stimulate dose dependently the accumulation of inositol monophosphates with a minimal effective dose (MED) of 3 mEq/kg s.c. and maximal effect seen at 20 mEq/kg. This corresponded to an increase in the radioactivity in the inositol monophosphate fraction from 1.4 +/- 0.06% to 4.6 +/- 0.60%. The response was time-dependent, with a peak effect observed at 4 h post administration and returning to basal levels by 48 h. The muscarinic receptor agonist pilocarpine (MED 10 mg/kg i.p.) was able to enhance dose dependently the response to 10 mEq/kg lithium, with a maximum response seen at 30 mg/kg (9.3% of the total brain radioactivity present in the inositol monophosphate fraction). The efficacious oxadiazole muscarinic receptor agonist L-658,903 also enhanced the response to lithium, producing a maximal effect of 10.4% of the total brain radioactivity present in the inositol monophosphate fraction at 1 mg/kg i.p. This stimulation was blocked by 1 mg/kg scopolamine i.p. but not by 1 mg/kg N-methylscopolamine. These results demonstrate the linkage of muscarinic receptors to the accumulation of inositol monophosphates in vivo, and confirm that following peripheral administration L-658,903 is a potent efficacious at muscarinic receptors within the central nervous system.

The cholinergic receptor-linked phosphoinositide metabolism in mouse cerebrum and cerebellum in vivo

The cholinergic receptor-linked poly-phosphoinositide hydrolysis was studied in mouse cerebrum and cerebellum after prelabeling the brain with [3H]inositol. I.p. injection of Li (8 meq/kg) to C57Bl/6J mice for 4 h resulted in 14- and five-fold increases in [3H]inositol-labeled inositol monophosphate (IP1) in cerebrum and cerebellum, respectively. The labeled inositol bisphosphate (IP2) was also increased 83 and 19% in cerebrum and cerebellum, respectively. Prior injection of atropine (100 mg/kg) resulted in inhibition of Li-induced increases in labeled IP1 by 74 and 56% in cerebrum and cerebellum, respectively. Administration of pilocarpine (20 mg/kg) to the Li-treated mice for 30 min resulted in further increases in labeled IP1 and IP2 and a concomitant decrease in labeled inositol in cerebrum but not in cerebellum. Mass measurements of IP1 and IP2 isomers by HPLC revealed that inositol 1-monophosphate (Ins(1)P), inositol 4-monophosphate (Ins(4)P) and inositol 1,4-bisphosphate (Ins(1,4)P2) were all increased by pilocarpine administration in the Li-treated mouse cerebrum. The effects of pilocarpine administration in mouse cerebrum (increases in IP1 and IP2) could be completely inhibited by preinjection of atropine. Atropine injection also decreased the levels of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. Surprisingly, a decrease in Ins(1,4,5)P3 level was also found in non-Li-treated mice after pilocarpine administration (30 mg/kg, 10-40 min). Except for the increase (20%) in [32P]-labeled PIP in the cerebrum, Li or Li together with pilocarpine administration did not alter the levels of [3H]inositol or [32P]phosphate-labeled phosphoinositides.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathways of dephosphorylation of 1-D-myo-inositol 1,4,5-trisphosphate in GH3 pituitary tumor cells

Previous work in [3H]inositol-labelled GH3 pituitary tumor cells stimulated with thyrotropin-releasing hormone (TRH) reported the existence of at least ten distinct [3H]inositol-containing substances which were identified as different inositol mono-, bis- and tris-phosphate isomers [1]. Here a complete kinetic study of the dephosphorylation pathways of the second messenger Ins(1,4,5)P3 is reported in GH3 cell homogenates, identifying a new intermediate, Ins(4,5)P2, in the metabolism of the second messenger. in vitro results obtained with exogenous substrates are compared with in vivo results obtained measuring levels of the endogenous [3H]inositol-labelled isomers that participate in the dephosphorylation pathways of Ins(1,4,5)P3 in resting and TRH-stimulated GH3 cells. The effect of Li+ on the activity of the different phosphatases involved in these pathways is studied as well.

Lithium enhances muscarinic receptor-stimulated CDP-diacylglycerol formation in inositol-depleted SK-N-SH neuroblastoma cells

The psychotherapeutic action of Li+ in brain has been proposed to result from the depletion of cellular inositol secondary to its block of inositol monophosphatase. This action is thought to slow phosphoinositide resynthesis, thereby attenuating stimulated phosphoinositidase-mediated signal transduction in affected cells. In the present study, the effect of Li+ on muscarinic receptor-stimulated formation of the immediate precursor of phosphatidylinositol, CDP-diacylglycerol (CDP-DAG), has been examined in human SK-N-SH neuroblastoma cells that have been cultured under conditions that alter the cellular content of myo-inositol. Resting neuroblastoma cells, like brain cells in vivo, were found to concentrate inositol from the culture medium, achieving an intracellular level of 60.0 +/- 4 nmol/mg of protein. The addition of carbachol to [3H]cytidine-prelabeled cells elicited a four- to fivefold increase in the accumulation of labeled CDP-DAG. This stimulated formation of [3H]CDP-DAG was completely blocked by the addition of 10 microM atropine, was not dependent on the presence of Li+, nor was it affected by co-incubation with myo-inositol. This result was in sharp contrast to findings in rat brain slices, in which carbachol-stimulated formation of [3H]CDP-DAG was potentiated approximately 10-fold by Li+ and substantially reduced by coincubation with inositol. The formation of [3H]CDP-DAG in labeled SK-N-SH cells by carbachol was both concentration and time dependent. The order of efficacy of muscarinic ligands in stimulating [3H]-CDP-DAG accumulation paralleled that established in these cells for inositol phosphate accumulation, i.e., carbachol > or = oxotremorine-M > bethanecol > or = arecoline > oxotremorine > pilocarpine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of acute ethanol administration on polyphosphoinositide turnover and levels of inositol 1,4,5-trisphosphate in mouse cerebrum and cerebellum

Although ethanol is known for its central depressant action, its effect on the polyphosphoinositide (poly-PI) signal transduction activity in brain has not been examined in detail. In this study, C57Bl/6J mice were injected intracerebrally with [3H]inositol, and poly-PI turnover in brain was assessed by determining the levels of labeled inositol monophosphates (IP1) accumulated after intraperitoneal injection of LiCl (6 meq/kg body weight) 4 hr before killing. Using this experimental protocol, acute ethanol administration (by gavage) resulted in time- and dose-dependent decreases in the levels of labeled IP1 in both cerebrum and cerebellum as compared with controls. The ethanol-induced decrease in labeled IP1 correlated well with the decrease in levels of inositol 1,4,5-triphosphate (as measured by the radioreceptor assay) and the increase in blood ethanol concentration. Despite a 4-fold higher accumulation of labeled IP1 in the cerebrum compared with the cerebellum, there were no major differences in the steady-state levels of inositol 1,4,5-triphosphate (based on tissue weight) in either brain region. Intraperitoneal injection of atropine (50 mg/kg) (a muscarinic cholinergic receptor antagonist) to the lithium-treated mice resulted in a 34% decrease in labeled IP1 as compared with controls. This result suggests that a substantial proportion of the signals transduced were due to activation of the muscarinic cholinergic receptor. Administration of ethanol (5 g/kg) to the atropine-treated mice resulted in a further decrease in labeled IP1 and longer sleep time as compared with those given ethanol alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of inositol 1,4,5-trisphosphate in mouse brain due to decapitation ischemic insult: effects of acute lithium administration and temporal relationship to diacylglycerols, free fatty acids and energy metabolites

Previous studies have shown that global cerebral ischemia induced by decapitation leads to the stimulated hydrolysis of poly-phosphoinositides. In this study, the decapitation model was used to further examine the temporal events related to metabolism of Ins(1,4,5)P3 and the release of diacylglycerols (DGs) and free fatty acids (FFAs) in the mouse brain. Since lithium administration is known to inhibit inositol monophosphatase activity in brain, the effects of acute lithium injection on Ins(1,4,5)P3 metabolism were also examined. Cerebral ischemia induced by decapitation of C57 Bl/6J mice resulted in transient increases of Ins(1,4,5)P3, Ins(1,4)P2 and Ins(4)P which peaked at 35, 65 and 125 s, respectively. The level of Ins(1)P, however, was not altered. Mice administered lithium by intraperitoneal injection (8 meq/kg for 4 h) gave rise to a 40- and 4-fold increase in levels of Ins(1)P, Ins(4)P, respectively, a 20% increase in levels of Ins(1,4)P2 but no apparent changes in the levels of Ins(1,4,5)P3. Decapitation also induced an increase in the levels of DGs and FFAs. Unlike the transient appearance of Ins(1,4,5)P3, however, DG levels increased steadily for 2 min and then reached a plateau whereas the FFAs showed a lag time of 35 s prior to a biphasic increase. During the initial 2 min after decapitation, there was a preferential increase in the DG species containing 18:0 and 20:4. Lithium administration did not alter the decapitation-induced release of DG and FFA. As expected, decapitation gave rise to a rapid decrease in the levels of phosphocreatine and ATP and the decline in ATP was marked by a transient appearance of ADP and a concomitant increase in AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Contributions to arachidonic acid release in mouse cerebrum by the phosphoinositide-phospholipase C and phospholipase A2 pathways

Recent studies have indicated two major mechanisms for the release of arachidonic acid (20:4) from membrane phospholipids: 1) activation of phospholipase A2 and 2) stimulated hydrolysis of poly-phosphoinositides (PI) and diacylglycerols (DG) through phospholipase C and diacylglycerol lipase, respectively. In mammalian brain both mechanisms seem to be operable, although the relative contributions by these two pathways have not been carefully assessed. In this study three experimental protocols were used to examine 20:4 release in brain due to ischemia and agonist stimulation, as well as the metabolic relationship between this release and the increase in diacylglycerols, lysophospholipids, and inositol phosphates. The preferential release of arachidonic acid during the initial phase after decapitation was attributed mainly to the sequential hydrolysis of poly-PI to DG. During the second phase, the release of 20:4 along with other free fatty acids (FFA) correlated well with the increase in labeled lysophospholipids, suggesting the involvement of phospholipase A2. Diacylglycerols in brain are enriched in 18:0 and 20:4. Decapitation induced a rapid increase in the level of DG, which remained elevated during the 30 min period under study. Between 5 sec and 5 min, the increase in FFA lagged behind that of DG. The parallel increases in 18:0 and 20:4 in the FFA pool further support the notion that, during the early phase, 20:4 could be derived from the sequential hydrolysis of poly-PI and DG. Decapitation also induced a sequential appearance of Ins(1,4,5)P3, Ins(1,4)P2, and Ins(4)P, which peaked at 30 sec, 1 min, and 2 min, respectively. The level of 20:4 in brain was also examined with respect to poly-PI turnover due to stimulation by cholinergic agonists. Administration of pilocarpine to lithium-treated mice resulted in increased accumulation of labeled inositol monophosphate (IP1) compared to the amount in controls receiving lithium alone, as well as a less obvious increase in 20:4. Both pilocarpine-mediated increases (IP1 and 20:4) could be blocked by atropine. These results point to the presence of an active mechanism for poly-PI turnover and for the recycling of 20:4 in brain.