The interaction of lithium with thyrotropin-releasing hormone-stimulated lipid metabolism in GH3 pituitary tumour cells. Enhancement of stimulated 1,2-diacylglycerol formation

Diacylglycerol kinase (DGKA) regulates the effect of the epilepsy and bipolar disorder treatment valproic acid in Dictyostelium discoideum

Valproic acid (VPA) provides a common treatment for both epilepsy and bipolar disorder; however, common cellular mechanisms relating to both disorders have yet to be proposed. Here, we explore the possibility of a diacylglycerol kinase (DGK) playing a role in regulating the effect of VPA relating to the treatment of both disorders, using the biomedical model Dictyostelium discoideum. DGK enzymes provide the first step in the phosphoinositide recycling pathway, implicated in seizure activity. They also regulate levels of diacylglycerol (DAG), thereby regulating the protein kinase C (PKC) activity that is linked to bipolar disorder-related signalling. Here, we show that ablation of the single Dictyostelium dgkA gene results in reduced sensitivity to the acute effects of VPA on cell behaviour. Loss of dgkA also provides reduced sensitivity to VPA in extended exposure during development. To differentiate a potential role for this DGKA-dependent mechanism in epilepsy and bipolar disorder treatment, we further show that the dgkA null mutant is resistant to the developmental effects of a range of structurally distinct branched medium-chain fatty acids with seizure control activity and to the bipolar disorder treatment lithium. Finally, we show that VPA, lithium and novel epilepsy treatments function through DAG regulation, and the presence of DGKA is necessary for compound-specific increases in DAG levels following treatment. Thus, these experiments suggest that, in Dictyostelium, loss of DGKA attenuates a common cellular effect of VPA relating to both epilepsy and bipolar disorder treatments, and that a range of new compounds with this effect should be investigated as alternative therapeutic agents.

This article has an associated First Person interview with the first author of the paper.

Editor's choice: Here, using a tractable model system, Dictyostelium discoideum, we show that diacylglycerol kinase activity might contribute to the cellular mechanism of action of the epilepsy and bipolar disorder treatment, valproic acid.

New insights into negative effects of lithium on sea urchin Paracentrotus lividus embryos

The diffuse use of lithium in a number of industrial processes has produced a significant contamination of groundwater and surface water with it. The increased use of lithium has generated only scarce studies on its concentrations in ambient waters and on its effects on aquatic organisms. Only few contributions have focused on the toxicity of lithium in marine organisms (such as marine animals, algae and vegetables), showing that the toxic effect depends on the animal species. In the present study we describe the morphological and the molecular effects of lithium chloride (LiCl), using the sea urchin Paracentrotus lividus as a model organism. We show that LiCl, if added to the eggs before fertilization, induces malformations in the embryos in a dose-dependent manner. We have also followed by RT qPCR the expression levels of thirty seven genes (belonging to different classes of functional processes, such as stress, development, differentiation, skeletogenesis and detoxifications) to identify the molecular targets of LiCl. This study opens new perspectives for the understanding of the mechanism of action of lithium on marine organisms. The findings may also have relevance outside the world of marine organisms since lithium is widely prescribed for the treatment of human bipolar disorders.

Ca²⁺ influx-linked protein kinase C activity regulates the β-catenin localization, micromere induction signalling and the oral-aboral axis formation in early sea urchin embryos

Sea urchin embryos initiate cell specifications at the 16-cell stage by forming the mesomeres, macromeres and micromeres according to the relative position of the cells in the animal-vegetal axis. The most vegetal cells, micromeres, autonomously differentiate into skeletons and induce the neighbouring macromere cells to become mesoendoderm in the β-catenin-dependent Wnt8 signalling pathway. Although the underlying molecular mechanism for this progression is largely unknown, we have previously reported that the initial events might be triggered by the Ca2+ influxes through the egg-originated L-type Ca2+ channels distributed asymmetrically along the animal-vegetal axis and through the stretch-dependent Ca2+channels expressed specifically in the micromere at the 4th cleavage. In this communication, we have examined whether one of the earliest Ca2+ targets, protein kinase C (PKC), plays a role in cell specification upstream of β-catenin. To this end, we surveyed the expression pattern of β-catenin in early embryos in the presence or absence of the specific peptide inhibitor of Hemicentrotus pulcherrimus PKC (HpPKC-I). Unlike previous knowledge, we have found that the initial nuclear entrance of β-catenin does not take place in the micromeres, but in the macromeres at the 16-cell stage. Using the HpPKC-I, we have demonstrated further that PKC not only determines cell-specific nucleation of β-catenin, but also regulates a variety of cell specification events in the early sea urchin embryos by modulating the cell adhesion structures, actin dynamics, intracellular Ca2+ signalling, and the expression of key transcription factors.

Effect of chronic lithium administration on endothelium-dependent relaxation in rat aorta

1. The effects of chronic lithium administration on the relaxant responses of rat thoracic aortic rings in the presence of indomethacin (a cyclo-oxygenase inhibitor) and/or NG-nitro-L-arginine (L-NOARG; a nitric oxide synthase inhibitor) to acetylcholine (ACh) or sodium nitroprusside were investigated in the present study. 2. Acetylcholine produced a concentration-dependent relaxation in vessels precontracted by phenylephrine (PE), while in lithium-treated rats the maximal relaxation was significantly increased. 3. Indomethacin (20 mumol/L) significantly potentiated the ACh-induced relaxation in lithium-treated and control rats. 4. NG-Nitro-L-arginine (1 mumol/L) decreased the ACh-induced relaxation in both control and lithium-treated rats. In contrast, indomethacin (20 mumol/L) reversed the inhibitory effect of L-NOARG. 5. Sodium nitroprusside produced similar concentration-dependent relaxations of vessels from both control and lithium-treated rats, which was not affected by indomethacin. In endothelium-denuded rings, indomethacin (20 mumol/L) caused a rightward shift in the concentration-contraction curve to PE. 6. These data support evidence for a possible increase in endothelium-dependent relaxation induced by ACh during long-term administration of lithium in rat aortic rings.

Signalling pathways in the brain: cellular transduction of mood stabilisation in the treatment of manic-depressive illness

The long-term treatment of manic-depressive illness (MDI) likely involves the strategic regulation of signalling pathways and gene expression in critical neuronal circuits. Accumulated evidence has identified signalling pathways, in particular the family of protein kinase C (PKC) isozymes, as targets for the long-term action of lithium. Chronic lithium administration produces a reduction in the expression of PKC alpha and epsilon, as well as a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. More recently, studies have demonstrated robust effects of lithium on another kinase system, GSK-3beta, and on neuroprotective/neurotrophic proteins in the brain. Given the key roles of these signalling cascades in the amplification and integration of signals in the central nervous system, these findings have clear implications not only for research into the neurobiology of MDI, but also for the future development of novel and innovative treatment strategies.

Ziskind-Somerfeld Research Award. Protein kinase C signaling in the brain: molecular transduction of mood stabilization in the treatment of manic-depressive illness

Understanding the biology of the pharmacological stabilization of mood will undoubtedly serve to provide significant insight into the pathophysiology of manic-depressive illness (MDI). Accumulating evidence from our laboratories and those of other researchers has identified the family of protein kinase C isozymes as a shared target in the brain for the long-term action of both lithium and valproate. In rats chronically treated with lithium, there is a reduction in the hippocampus of the expression of two protein kinase isozymes, alpha and epsilon, as well as a reduction in the expression of a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. In addition, we have been investigating the down-stream impact of these mood stabilizers on another kinase system, GSK-3 beta and on the AP-1 family of transcription factors. Further studies have generated promising preliminary data in support of the antimanic action of tamoxifen, and antiestrogen that is also a PKC inhibitor. Future studies must address the therapeutic relevance of these protein targets in the brain using innovative strategies in both animal and clinical investigations to ultimately create opportunities for the discovery of the next generations of mood stabilizers for the treatment of MDI.

Differential expression of thyroid hormone receptor isoforms by thyroid hormone and lithium in rat GH3 and B103 cells

BACKGROUND

The interaction between lithium, a mood stabilizer, and the thyroid axis has been extensively studied; however, the regulation of thyroid hormone receptors by lithium is yet to be investigated.

METHODS

To test whether lithium affects thyroid hormones at the receptor level, we examined the effects of lithium in combination with triiodothyronine (T3) on gene expression of thyroid hormone receptor isoforms in GH3 and B103 cells.

RESULTS

The pattern of expression as well as the magnitude of regulation of the different thyroid hormone receptor isoforms appeared to be cell line specific. Whereas T3 regulated all four isoforms in GH3 cells at both time points, T3 did not alter thyroid hormone receptor TR alpha 1 and TR alpha 2 mRNA in B103 cells. Addition of lithium to thyroid hormone-deficient GH3 cells decreased TR alpha 1, alpha 2, and beta 2 expression without affecting TR beta 1 expression at 2 but not 5 days. Addition of lithium to T3-treated GH3 cells did not further modulate gene expression of TR alpha 1, alpha 2, beta 1, or beta 2 when compared to cells treated with T3 alone. The effects of lithium in B103 cells appeared to be isoform specific as well as time dependent, since TR alpha 1 expression was selectively decreased in B103 cells, when treated with T3 in the presence of lithium.

CONCLUSIONS

The present study provides direct evidence that T3 and/or lithium regulate TR gene expression in vitro in a both time-dependent and cell line-specific manner.

Action potential broadening induced by lithium may cause a presynaptic enhancement of excitatory synaptic transmission in neonatal rat hippocampus

Lithium enhances excitatory synaptic transmission in CA1 pyramidal cells, but the mechanisms remain unclear. The present study demonstrates that lithium enhances the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA) receptor-mediated components of the excitatory postsynaptic current (EPSC). Lithium decreased the magnitude of paired-pulse facilitation and presented an inverse correlation between the lithium-induced enhancement of synaptic transmission and initial paired-pulse facilitation, which is consistent with a presynaptic mode of action. The enhancement of synaptic strength is likely to act, at least in part, by increasing the amplitude of the presynaptic Ca2+ transient. One mechanism which could account for this change of the presynaptic Ca2+ transient is an increase in the duration of the action potential. We investigated action potential in hippocampal pyramidal neurons and found that lithium (0.5-6 mM) increased the half-amplitude duration and reduced the rate of repolarization, whereas the rate of depolarization remained similar. To find out whether the lithium synaptic effects might be explained by spike broadening, we investigated the field recording of the excitatory postsynaptic potential (EPSP) in hippocampal slices and found three lines of evidence. First, the prolongation of the presynaptic action potential with 4-aminopyridine and tetraethylammonium blocked or reduced the synaptic effects of lithium. Second, the lithium-induced synaptic enhancement was modulated when presynaptic Ca2+ influx was varied by changing the external Ca2+ concentration. Finally, both effects, the synaptic transmission increment and the action potential broadening, were independent of inositol depletion. These results suggest that lithium enhances synaptic transmission in the hippocampus via a presynaptic site of action: the mechanism underlying the potentiating effect may be attributable to an increased Ca2+ influx consequent to the broadening effect of lithium on the action potential.

Differential regulation of muscarinic acetylcholine receptor-sensitive polyphosphoinositide pools and consequences for signaling in human neuroblastoma cells

In this study we have quantitatively assessed the basal turnover of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and M3-muscarinic receptor-mediated changes in phosphoinositides in the human neuroblastoma cell line, SH-SY5Y. We demonstrate that the polyphosphoinositides represent a minor fraction of the total cellular phosphoinositide pool and that in addition to rapid, sustained increases in [3H]inositol phosphates dependent upon the extent of receptor activation by carbachol, there are equally rapid and sustained reductions in the levels of polyphosphoinositides. Compared with phosphatidylinositol 4-phosphate (PtdIns(4)P), PtdIns(4,5)P2 was reduced with less potency by carbachol and recovered faster following agonist removal suggesting protection of PtdIns(4,5)P2 at the expense of PtdIns(4)P and indicating specific regulatory mechanism(s). This does not involve a pertussis toxin-sensitive G-protein regulation of PtdIns(4)P 5-kinase. Using wortmannin to inhibit PtdIns 4-kinase activity, we demonstrate that the immediate consequence of blocking the supply of PtdIns(4)P (and therefore PtdIns(4,5)P2) is a failure of agonist-mediated phosphoinositide and Ca2+ signaling. The use of wortmannin also indicated that PtdIns is not a substrate for receptor-activated phospholipase C and that 15% of the basal level of PtdIns(4,5)P2 is in an agonist-insensitive pool. We estimate that the agonist-sensitive pool of PtdIns(4,5)P2 turns over every 5 s (0.23 fmol/cell/min) during sustained receptor activation by a maximally effective concentration of carbachol. Immediately following agonist addition, PtdIns(4,5)P2 is consumed >3 times faster (0.76 fmol/cell/min) than during sustained receptor activation which represents, therefore, utilization by a partially desensitized receptor. These data indicate that resynthesis of PtdIns(4,5)P2 is required to allow full early and sustained phases of receptor signaling. Despite the critical dependence of phosphoinositide and Ca2+ signaling on PtdIns(4,5)P2 resynthesis, we find no evidence that this rate resynthesis is limiting for agonist-mediated responses.

Diacylglycerol and phosphatidate generated by phospholipases C and D, respectively, have distinct fatty acid compositions and functions. Phospholipase D-derived diacylglycerol does not activate protein kinase C in porcine aortic endothelial cells

Stimulation of cells with certain agonists often activates both phospholipases C and D. These generate diacylglycerol and phosphatidate, respectively, although the two lipids are also apparently interconvertable through the actions of phosphatidate phosphohydrolase and diacylglycerol kinase. Diacylglycerol activates protein kinase C while one role for phosphatidate is the activation of actin stress fiber formation. Therefore, if the two lipids are interconvertable, it is theoretically possible that an uncontrolled signaling loop could arise. To address this issue structural analysis of diacylglycerol, phosphatidate, and phosphatidylbutanol (formed in the presence of butan-1-ol) from both Swiss 3T3 and porcine aortic endothelial cells was performed. This demonstrated that phospholipase C activation generates primarily polyunsaturated species while phospholipase D activation generates saturated/monounsaturated species. In the endothelial cells, where phospholipase D was activated by lysophosphatidic acid independently of phospholipase C, there was no activation of protein kinase C. Thus we propose that only polyunsaturated diacylglycerols and saturated/monounsaturated phosphatidates function as intracellular messengers and that their interconversion products are inactive.

CCK-B receptor-mediated stimulation of polyphosphoinositide turnover in GH3 pituitary cells in response to cholecystokinin and pentagastrin

CCK-B receptor-mediated polyphosphoinositide (PPI) turnover in GH3 pituitary cells has been examined and comparisons are made with Ca2+ mobilisation and receptor binding data, previously described. Sulphated cholecystokinin octapeptide (CCK-8s) and the CCK-B-selective agonist pentagastrin dose-dependently stimulated PPI turnover in GH3 cells with similar maximal increases of 3.0 fold and 3.3 fold, respectively, in production of [3H]inositol phosphates over control. Responses, measured over 30min periods in the presence of 10mM LiCl, were generally maximal for both agonists at 100nM. Consistent with their [125]Bolton Hunter CCK-8s (BHCCK) binding affinities and with effects on Ca2+ mobilisation, CCK-8s was slightly more potent than pentagastrin in stimulating PPI turnover (EC50s 1.3nM and 3.9nM respectively). Both peptides showed higher potency in the PPI assay than in Ca2+ studies. 100nM pentagastrin-induced PPI turnover was dose-dependently inhibited by the CCK-B receptor-selective antagonist L-365,260 (IC50 470nM) whilst the CCK-A receptor antagonist, devazepide, only produced weak partial inhibition (18% at 10,000nM). Antagonists alone were observed to depress control activity in PPI turnover but not in Ca2+ mobilisation assays. The selectivity of L-365,260 compared to devazepide was similar in binding studies to that for both 100nM pentagastrin-induced functional responses. Schild analysis of antagonism of PPI turnover by L-365,260 yielded a line with slope close to unity (1.07) and a pKB of 8.27+/-0.05.

Inositol monophosphatase--a putative target for Li+ in the treatment of bipolar disorder

Attenuation of the phosphatidylinositol (PI) signal transduction pathway as a consequence of inhibition of inositol monophosphatase (IMPase) has been proposed as the mechanism for the efficacy of Li+ in the treatment of bipolar disorder. Nevertheless, Li+ also affects other aspects of PI signal transduction, and it is therefore not clear whether modulation of PI responses by Li+ can be attributed solely to inhibition of IMPase. However, inhibitors of IMPase mimic the effects of Li+ on some aspects of PI cell signalling, thus highlighting the potential of IMPase as a target for the treatment of bipolar disorder. The recent description of the three-dimensional structure of IMPase in conjunction with site-directed mutagenesis and kinetic studies has led to the elucidation of the enzyme mechanism. These structural and mechanistic data should prove useful in the development of novel inhibitors of IMPase that might ultimately prove useful clinically.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Effect of activation of protein kinase C on excitation-contraction coupling in frog twitch muscle fibres

Intracellular Ca2+ transients were recorded from frog twitch muscle fibres in response to voltage-clamp depolarizing pulses, using arsenazo III as an intracellular Ca2+ indicator. The effect of the activation of protein kinase C (PKC) on the Ca2+ transients was studied. With 1 microM phorbol 12,13-dibutyrate (PDBu), a PKC activator, the peak of the Ca2+ transients increased to about 120% of control during the first 0.5 h, and then decreased gradually to a plateau of 44% of control within the following 2 h. This effect of PDBu could be alleviated significantly by PKC inhibitors, 10 microM polymyxin B (PMB) or 30 microM 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7). Moreover, PDBu caused an upward shift of the strength/duration curve. In Li(+)-loaded muscle fibres the Ca2+ transients could not fully recover after 80 mM K+ exposure for 15 min, while the post-K+ Ca2- transients could be completely restored in the fibres not loaded with Li+. In the presence of 10 microM PMB or 30 microM H-7, a full restoration of the post-K+ Ca2+ transients was seen in Li(+)-loaded fibres. PMB supplemented after high-K+ exposure also could result in a complete recovery of the post-K+ Ca2+ transients in Li(+)-loaded fibres. The role of PKC in modulating excitation-contraction coupling in frog twitch muscle fibres is clearly indicated, but the mechanism(s) and physiological significance remain to be established.

The lymphoblast beta-adrenergic receptor in bipolar depressed patients: effect of chronic incubation with lithium chloride

We have recently reported a study of beta-adrenergic receptor binding characteristics in lymphoblast cell lines derived from patients with bipolar disorder (BD) and healthy, matched control subjects. In the present study we have investigated the effects of incubating cells from the same subjects with lithium chloride (1 mM) for 7 days prior to assay. There was no difference in beta-adrenergic receptor number between control and BD cell lines and incubation with lithium had no effect on receptor number in either group. Exposure of the cells to isoprenaline (1 nM) for 24 h immediately prior to assay caused significantly less down-regulation in BD cells (15 +/- 5%) than control cells (39 +/- 4%), as described previously. Incubation with lithium significantly increased the down-regulation response to isoprenaline in BD cells (39 +/- 6%) but not in control cells (30 +/- 7%). After lithium, the agonist-induced decrease in beta-AR number in BD cells was no longer significantly different from that in control cells. We conclude that lithium selectively enhanced the agonist down-regulation of beta-adrenergic receptors in cells derived from patients with bipolar disorder. The functional significance of this result and the potential biochemical mechanisms responsible for this effect are discussed.

Polyol pathway mediates high glucose-induced collagen synthesis in proximal tubule

The polyol pathway in diabetes is activated in tissues that are not dependent on insulin for glucose uptake. To examine the role of the polyol pathway in renal extracellular matrix accumulation, we incubated murine proximal tubule cells in either normal or high glucose concentration in the presence or absence of the aldose reductase inhibitor sorbinil. Rising medium glucose from 100 to 450 mg/dl for 72 hours increased cell sorbitol levels sevenfold. Addition of 0.4 mM sorbinil reduced sorbitol content to virtually undetectable levels as measured by gas chromatography. Sorbinil (0.1 to 0.2 mM) also reduced the secretion of collagens types IV and I in the high glucose concentration after 48 to 72 hours but had no appreciable effect in the normal glucose concentration. Concordantly, 0.1 mM sorbinil inhibited the high glucose-induced stimulation of alpha 1(IV) and alpha 2(I) mRNA levels without affecting levels in normal glucose concentration. To study the role of transcriptional activation of collagen genes, we transfected proximal tubule cells with a chloramphenicol acetyltransferase (CAT) reporter gene linked to the promoter and regulatory elements of alpha 1(IV) gene. CAT activity increased several-fold in the cells grown in the high versus normal glucose concentration; this transcriptional activation in culture media containing high glucose concentration was reduced by treatment of the cells with 0.1 mM sorbinil. Thus, high ambient glucose activates the polyol pathway in proximal tubule cells, and may mediate the high glucose-induced stimulation of gene expression for collagens types IV and I.(ABSTRACT TRUNCATED AT 250 WORDS)

The inhibition of phosphoinositide synthesis and muscarinic-receptor-mediated phospholipase C activity by Li+ as secondary, selective, consequences of inositol depletion in 1321N1 cells

Conditions are described for culture of 1321N1 cells under which cellular inositol is decreased from approximately 20 mM to < 0.5 mM but phosphoinositide concentrations are unaffected. The effects of the muscarinic-receptor agonist carbachol (1 mM) and/or LiCl (10 mM) on phosphoinositide turnover in these or in inositol-replete cells was examined after steady-state [3H]inositol labelling of phospholipid pools. In both inositol-replete and -depleted cells, carbachol stimulated similar initial (0-15 min) rates of phospholipase C (PLC) activity, in the presence of Li+. Subsequently (> 30-60 min) stimulated PLC activity and [3H]PtdIns concentrations declined dramatically only in depleted cells. In inositol-depleted cells, carbachol alone evoked increased concentrations of [3H]inositol, [3H]InsP1, [3H]InsP2, [3H]InsP3 and [3H]InsP4, which were largely sustained over 90 min, and concentrations of [3H]PtdIns, [3H]PtdInsP and [3H]PtdInsP2 were decreased only to approximately 82, 84 and 93% of control respectively. In the presence of Li+ in these cells, the stimulated rise in [3H]inositol was prevented and, although accumulation of [3H]InsP1, [3H]InsP2 and [3H]InsP3 was initially (0-30 min) potentiated, rates of accumulation of [3H]InsP1 and concentrations of [3H]polyphosphates later (> 30-60 min) declined, and concentrations of [3H]PtdIns, [3H]PtdInsP and [3H]PtdInsP2 were decreased respectively to approximately 39, 48 and 81% of control. After 60 min in the presence of both carbachol and Li+, stimulated PLC activity was decreased by approximately 70% compared with the initial rate in depleted cells. This decreased PLC activity was reflected by changes in the stimulated concentrations of [3H]Ins(1,3,4)P3 but not of [3H]Ins(1,4,5)P3, but effects of Li+ on the latter may have been obscured by the demonstrated, concomitant and equal stimulated accumulation of [3H]inositol 1:2cyclic,4,5-trisphosphate. These data suggest that receptor-mediated PLC activity is selectively impaired by Li+ as a secondary consequence of inositol monophosphatase inhibition in cells which are highly dependent on inositol re-cycling, but imply that, although Li+ attenuation of PLC activity correlates closely with parameters indicative of limiting inositol supply, it is not readily attributed to decreased PtdInsP2 availability. The potential for complex regulation of PLC and PtdIns synthase is discussed.

Long-term action of lithium: a role for transcriptional and posttranscriptional factors regulated by protein kinase C

Lithium, a simple monovalent cation, represents one of psychiatry's most important treatments and is the most effective treatment for reducing both the frequency and severity of recurrent affective episodes. Despite extensive research, the underlying biologic basis for the therapeutic efficacy this drug remains unknown, and in recent years, research has focused on signal transduction pathways to explain lithium's efficacy in treating both poles of manic-depressive illness. Critical to attributions of therapeutic relevance to any observed biochemical effect, however, is the observation that the characteristic prophylactic action of lithium in stabilizing the profound mood cycling of bipolar disorder requires a lag period for onset and is not immediately reversed upon discontinuation of treatment. Biochemical changes requiring such prolonged administration of a drug suggest alterations at the genomic level but, until recently, little has been known about the transcriptional and posttranscriptional factors regulated by chronic drug treatment, although long-term changes in neuronal synaptic function are known to be dependent upon the selective regulation of gene expression. In this paper, we will present evidence to show that chronic lithium exerts significant transcriptional and posttranscriptional effects, and that these actions of lithium may be mediated via protein kinase C (PKC)-induced alterations in nuclear transcription regulatory factors responsible for modulating the expression of proteins involved in long-term neural plasticity and cellular response. Such target sites for chronic lithium may help unravel the processes by which a simple monovalent cation can produce a long-term stabilization of mood in individuals vulnerable to bipolar illness.

Lithium decreases membrane-associated protein kinase C in hippocampus: selectivity for the alpha isozyme

We investigated the effects of lithium on alterations in the amount and distribution of protein kinase C (PKC) in discrete areas of rat brain by using [3H]phorbol 12,13-dibutyrate quantitative autoradiography as well as western blotting. Chronic administration of lithium resulted in a significant decrease in membrane-associated PKC in several hippocampal structures, most notably the subiculum and the CA1 region. In contrast, only modest changes in [3H]phorbol 12,13-dibutyrate binding were observed in the various other cortical and subcortical structures examined. Immunoblotting using monoclonal anti-PKC antibodies revealed an isozyme-specific 30% decrease in hippocampal membrane-associated PKC alpha, in the absence of any changes in the labeling of either the beta (I/II) or gamma isozymes. These changes were observed only after chronic (4 week) treatment with lithium, and not after acute (5 days) treatment, suggesting potential clinical relevance. Given the critical role of PKC in regulating neuronal signal transduction, lithium's effects on PKC in the limbic system represent an attractive molecular mechanism for its efficacy in treating both poles of manic-depressive illness. In addition, the decreased hippocampal membrane-associated PKC observed in the present study offers a possible explanation for lithium-induced memory impairment.

Diabetic rats show more resistance to neuromuscular blockade induced by aminoglycoside antibiotics

1. Neuromuscular blocking effect of gentamicin and neomycin evaluated in phrenic-nerve hemidiaphragm preparation in normal and diabetic rats and IC50 of both drugs determined from LDR curves. 2. IC50 of gentamicin was 0.91 +/- 0.05 and 1.67 +/- 0.08 mmol.l-1 for normal and diabetic rats respectively. 3. IC50 of neomycin was 0.42 +/- 0.02 and 0.61 +/- 0.06 mmol.l-1 for normal and diabetic rats respectively. 4. The results show diabetic rats are more resistant to neuromuscular blocking action of aminoglycoside antibiotics (AGs). 5. Replacement of sodium by lithium (1.5 and 5 mmol.l-1) in the media, reduced this effect of aminoglycosides (AGs) significantly. 6. It appears that the prevention of AGs induced-neuromuscular blockade by lithium is not significantly different in both groups.

Lithium modulation of phosphoinositide signaling system in rat cortex: selective effect on phorbol ester binding

Recent work indicates that the therapeutic action of lithium may be mediated through perturbation of postreceptor second messenger systems. To elucidate further the postreceptor cellular sites of action(s) of lithium, the effect of chronic lithium treatment on various components of the receptor-activated phosphoinositide pathway was investigated. We found that chronic administration of lithium (0.2% LiCl, 21 days) to adult male rats did not significantly affect phosphoinositide hydrolysis in cerebral cortical slices induced by carbachol (1 mM) or NaF (10 mM). Nor did the same treatment alter the carbachol (1 mM) potentiation of guanosine 5'-(gamma-thio)triphosphate (30 microM) stimulation of phosphoinositide hydrolysis (an index of receptor/G protein coupling) in cortical membranes. Immunoblotting studies revealed no changes in the levels of G alpha q/11 immunoreactivity in the cortex after chronic lithium treatment. The levels of protein kinase C, as revealed by specific binding of [3H]phorbol dibutyrate ([3H]PDBu), were significantly reduced in the cytosolic fraction and increased in the particulate fraction of rat cortex after chronic lithium, whereas the KD of [3H]PDBu binding remained relatively constant. A small and insignificant decrease in the density of [3H]inositol 1,4,5-trisphosphate binding was also found in the cortex. The above data suggest that chronic lithium treatment affects neither the muscarinic cholinergic-linked phosphoinositide turnover nor the putative G protein alpha subunit (G alpha q/11) responsible for phospholipase C activation. However, a possible translocation and activation of protein kinase C activity may be significant in the therapeutic effect of this mood-stabilizing agent.

Carbachol potentiates Q current and activates a calcium-dependent non-specific conductance in rat hippocampus in vitro

Intracellular recordings were made from CA1 neurons in rat hippocampal slices maintained in vitro. When Na+ currents were blocked with tetrodotoxin and K+ conductances known to be sensitive to suppression by muscarinic agonists were blocked by 2 mM Ba2+, CA1 cells were depolarized by carbachol (3-10 microM) with an attendant conductance increase, whereas prior to Ba2+ the agonist produced a decrease or no change in conductance. Under voltage clamp at approximately -60 mV and in the presence of tetrodotoxin and Ba2+, carbachol (3-10 microM) induced a variable-latency biphasic inward current of up to 380 pA associated with a conductance increase of approximately 50%. The first phase was associated with an increase (more than 2-fold) of the Cs(+)-sensitive, hyperpolarization-activated cationic current, IQ. Carbachol also accelerated the kinetics of IQ at -100 mV with an average 24% reduction in its activation time constant. The second phase reflected an additional inward current that was Cs(+)-resistant, displayed little apparent voltage sensitivity and had a mean extrapolated reversal potential, determined in the presence of external Cs+ (< or = 5 mM), of approximately -20 mV. In a small proportion of cells the second phase of inward current was followed (or overlapped) by an outward current, also associated with a conductance increase, which reversed at approximately -70 mV. These carbachol actions were prevented by extracellular 300 microM Cd2+ and 2 mM Mn2+, by high levels (> 5 mM) of extracellular Mg2+ or Ca2+, and by omission of Ca2+ or reduction of extracellular Na+ to 25 mM by substitution of NaCl with Tris or N-methyl-D-glucamine. Carbachol action was not mimicked by oxotremorine (< or = 60 microM), but was irreversibly blocked by this drug. Likewise, atropine (100 nM) irreversibly and gallamine (10 microM) reversibly antagonized carbachol's action. The action of carbachol was blocked shortly after prior exposure of slices to 2-5 mM caffeine. Chronic or acute incubation of slices with 2 mM Li+ potentiated (between 1- and 2-fold) carbachol responses. The data indicate that muscarinic activation increases cationic flux by a calcium-dependent potentiation of IQ and activation of a non-selective conductance. The probability that inositol phospholipid metabolism is involved in triggering these events is discussed.

Ionic mechanism of action of adenosine on the rat superior cervical ganglion

1. The ionic mechanism responsible for hyperpolarization of the rat superior cervical ganglion (SCG) and depression of the depolarizing response to muscarine by adenosine was studied using an extracellular grease-gap recording technique. 2. Both the hyperpolarizations to adenosine and 2-chloroadenosine and the depression of the response to muscarine by adenosine were potentiated in reduced external calcium (Ca2+). Hyperpolarizations to adenosine were either unaltered or potentiated in the presence of the dihydropyridine Ca2+ channel antagonists, nitrendipine or (+)PN200 110 respectively. Hyperpolarizations to adenosine were unaltered by inorganic Ca2+ channel antagonists except for cobalt, which also antagonized hyperpolarizations to carbachol and depolarizations to muscarine. 3. Hyperpolarizations to adenosine were unaltered in nominally magnesium (Mg2+)-free or in reduced external chloride (Cl-) media. When sodium ions (Na+) were replaced by lithium ions (Li+) maximal responses to adenosine were initially enhanced, returning to pretreatment levels and subsequently reduced in their duration. In contrast, responses to adenosine were significantly enhanced in nominally potassium (K+)-free medium and reduced upon doubling the extracellular K+. 4. Hyperpolarizations were enhanced in the presence of the K+ channel antagonists, 4-aminopyridine and 3,4-diaminopyridine, and reduced by a low concentration (2 mM) of tetraethylammonium (TEA), but not in 10 mM TEA. 5. The results support the hypothesis that adenosine-mediated hyperpolarization of postganglionic neurones of the rat SCG is by a Ca(2+)-independent mechanism and is probably mediated via an increase of a K+ current. The results also indicate that adenosine-induced hyperpolarizations of the rat SCG are independent of the presence of extracellular magnesium.

Effects of inositol starvation on the levels of inositol phosphates and inositol lipids in Neurospora crassa

An inositol-requiring strain of Neurospora crassa was labelled during growth in liquid medium with [3H]inositol, and the levels of inositol phosphates and phosphoinositides were determined under inositol-sufficient and inositol-starved conditions. Because the mutant has an absolute requirement for inositol, the total mass of inositol-containing compounds could be determined. Inositol-containing lipids were identified by deacylation and co-migration with standards on h.p.l.c.; PtdIns3P, PtdIns4P, and PtdIns(4,5)P2 were found in approximately equal amounts, in addition to large amounts of PtdIns. Inositol starvation decreased the level of PtdIns to 10% of the sufficient level, and decreased the levels of the other phosphoinositides to about 25%. A number of inositol phosphates were found, including several InsP3s, InsP4s and InsP5s and phytic acid. Ins(1,4,5)P3 was identified by co-migration with standards on h.p.l.c. and by digestion with inositol phosphomonoesterase. High concentrations of all inositol phosphates were found in the extracellular medium in inositol-starved cultures. Inositol starvation on both liquid and solid agar media decreased the intracellular levels of some inositol phosphates, but increased the levels of phytic acid and several other inositol phosphates which may be its precursors and/or breakdown products. These results may indicate that inositol starvation induces phytic acid synthesis as a protection against the free-radical production and lipid peroxidation characteristic of inositol-less death.

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)

Subcellular distribution of agonist-stimulated phosphatidylinositol synthesis in 1321 N1 astrocytoma cells

In an inositol-depleted 1321 N1 astrocytoma cell line, propranolol at 0.5 mM concentration and carbachol in the presence of Li+ induce a large increase (30-60-fold) in the amount of CMP-phosphatidate, the lipid substrate of PtdIns synthase. The actions of both agents on CMP-phosphatidate accumulation were reversed by co-incubation with 1 mM inositol. In cells grown in the presence of 40 microM inositol the propranolol- and carbachol-mediated CMP-phosphatidate accumulation was much smaller (2-4-fold). Propranolol- and carbachol-mediated increases in CMP-phosphatidate accumulation were at least additive in both inositol-replete and -depleted cells. The subcellular distribution of accumulated CMP-phosphatidate was investigated by sucrose-density-gradient centrifugation of a lysate of inositol-depleted cells. There were two coincident peaks of carbachol-stimulated [3H]CMP-phosphatidate and PtdIns synthase activity, respectively. The first peak of accumulated [3H]CMP-phosphatidate and PtdIns synthase activity is characteristic of a 'light vesicle' fraction, since it sediments at sucrose densities similar to that of endocytosed 125I-transferrin. The later peak, containing both carbachol-stimulated [3H]CMP-phosphatidate and PtdIns synthase activity, has a distribution in the gradient that is similar to NADPH-cytochrome c reductase activity, an endoplasmic-reticulum marker. By contrast, propranolol-stimulated [3H]CMP-phosphatidate accumulates in membranes which sediment as a single peak corresponding to the endoplasmic-reticulum marker. These observations suggest that agonist-stimulated PtdIns synthesis occurs in the endoplasmic reticulum and in at least one additional membrane compartment which is insensitive to propranolol, an inhibitor of endoplasmic-reticulum phosphatidate phosphohydrolase.

Elevated phosphatidyl-CMP is not the source of diacylglycerol accumulation induced by lithium in NG108-15 cells

Previous studies have shown that in the neuroblastoma x glioma hybrid cell line NG108-15 lithium is able to induce an increase in diacylglycerol levels. This effect was shown to be enhanced by the presence of bradykinin. Another striking effect of lithium was a marked gain in the level of the liponucleotide phosphatidyl-CMP. Increased phosphatidyl-CMP levels were detected in the presence of lithium alone but were considerably more pronounced in the presence of both lithium and bradykinin. These results are consistent with the inhibitory action of lithium on key enzymes of the degradation pathway of inositol phosphates, resulting in a decrease in cellular inositol content and in an elevation in levels of phosphorylated inositols. Comparison of the mass of the inositol phosphates and diacylglycerol showed that the lithium-induced diacylglycerol levels were substantially greater than would be expected from phosphoinositide hydrolysis alone. One possible reason for the increase in the level of diacylglycerol through the action of lithium is the reversal of the reaction for the formation of phosphatidyl-CMP. The resulting phosphatidic acid would then need to be further dephosphorylated to diacylglycerol. The lithium-induced elevation of phosphatidyl-CMP was prevented by addition of myo-inositol (10-30 mM), suggesting that the increase in liponucleotide level was due to depletion of cellular inositol. Under the same conditions the elevated diacylglycerol concentration remained unchanged. Consequently, phosphatidyl-CMP is not its source, and diacylglycerol may arise through an effect of lithium on the degradation of phospholipids other than phosphoinositides. The action of phospholipase C or D on phosphatidylcholine is the most likely mechanism.

Restoration of brain myo-inositol levels in rats increases latency to lithium-pilocarpine seizures

Lithium pretreatment in rats potentiates the epileptogenic effects of pilocarpine and other cholinergic agonists. In order to determine if this effect of lithium could be reversed by myo-inositol, rats were pretreated with intracerebroventricular (ICV) injections of myoinositol, artificial CSF or L-chiro-inositol. Lithium chloride, 3 meq/kg was administered intraperitoneally 20-24 h prior to the subcutaneous injection of pilocarpine, 20 or 30 mg/kg. In both experiments, myo-inositol significantly prolonged the latency to the appearance of clonic seizures and lowered the pilocarpine seizure score. myo-Inositol prevented the development of clonic seizures in 50% of the rats receiving pilocarpine, 20 mg/kg. The levels of cortical myo-inositol in rats injected with myo-inositol were approximately double those of the CSF and L-chiro-inositol groups.

Phorbol esters alter cell fate during development of sea urchin embryos

Protein kinase C (PKC) has been implicated as important in controlling cell differentiation during embryonic development. We have examined the ability of 12-O-tetradecanoyl phorbol-13-acetate (TPA), an activator of PKC, to alter the differentiation of cells during sea urchin development. Addition of TPA to embryos for 10-15 min during early cleavage caused dramatic changes in their development during gastrulation. Using tissue-specific antibodies, we have shown that TPA causes the number of cells that differentiate as endoderm and mesoderm to increase relative to the number that differentiate as ectoderm. cDNA probes show that treatment with TPA causes an increase in accumulation of RNAs specific to endoderm and mesoderm with a concomitant decrease in RNAs specific to ectoderm. Treatment of isolated prospective ectodermal cells with TPA causes them to differentiate into endoderm and mesoderm. The critical period for TPA to alter development is during early to mid cleavage, and treatment of embryos with TPA after that time has little effect. These results indicate that PKC may play a key role in determining the fate of cells during sea urchin development.

Eclosion Hormone Stimulates Cyclic GMP Levels in Manduca sexta Nervous Tissue via Arachidonic Acid Metabolism with Little or No Contribution from the Production of Nitric Oxide

The neuropeptide eclosion hormone acts directly on the nervous system of the tobacco hornworm, Manduca sexta, to trigger ecdysis behavior at the end of each molt. Previous studies have shown that the action of eclosion hormone is mediated via the intracellular messenger cyclic GMP. In the present study we have investigated the mechanisms involved in the eclosion hormone-stimulated increases in cyclic GMP. No stimulation of guanylate cyclase was seen in homogenized nervous tissue, suggesting that eclosion hormone does not directly stimulate a membrane-bound form of guanylate cyclase. Nitric oxide synthase inhibitors, N-methylarginine and nitroarginine, had no effect on eclosion hormone-stimulated cyclic GMP levels. By contrast, 4-bromophenacyl bromide, an inhibitor of arachidonic acid release, and nordihydroguaiaretic acid, an inhibitor of arachidonic acid metabolism, almost completely abolished the eclosion hormone-stimulated cyclic GMP increase. We hypothesize that eclosion hormone receptors are coupled to a lipase, activation of which causes the release of arachidonic acid. Either the arachidonic acid directly stimulates the soluble guanylate cyclase or further metabolism of arachidonic acid yields compounds that activate guanylate cyclase.

Ca-dependent K channels in smooth muscle cells permeabilized by beta-escin recorded using the cell-attached patch-clamp technique

Using the cell-attached patch-clamp technique, the activity of single, Ca-dependent K channels was recorded in single smooth muscle cells permeabilized by beta-escin. The conductance and the relationship between the open probability of the channels and pCa recorded in permeabilized cells were very similar to those obtained in excised inside-out patches. At pCa 7, application of 30 microM acetylcholine (ACh) or 0.1 microM substance P (SP) together with 1 mM guanosine 5'-trisphosphate to permeabilized cells elicited transient bursts of channel openings similar to those which occur in intact cells. Transient activation was also observed when 2-30 microM inositol trisphosphate (IP3) was applied to permeabilized cells. This single channel activity was inhibited by pretreatment with low-molecular-weight heparin at 50-100 micrograms/ml. Channel activity at pCa 7.0 was greatly enhanced by 200 microM cyclic adenosine monophosphate. These results provide direct evidence that single Ca-dependent K channel activity is regulated by the transmitters ACh and SP, as well as a second messenger, IP3, via the release of intracellular Ca from intracellular sites which are blocked by heparin. This novel approach is valuable in elucidating second messenger mechanisms involved in the regulation of single channel activity by transmitters and autocoids, since permeabilization by beta-escin preserves the entire system of receptor-operated signal transduction and allows intracellular application of second messengers at fixed concentrations.

Diabetes-induced glomerular dysfunction: links to a more reduced cytosolic ratio of NADH/NAD+

These studies were undertaken to examine effects of elevated glucose levels on glycolysis, sorbitol pathway activity, and the cytosolic redox state of NADH/NAD+ in isolated glomeruli. Blood-free glomeruli were isolated from kidneys of male, Sprague-Dawley rats using standard sieving techniques, then incubated for one hour at 37 degrees C, pH 7.4, pO2 approximately 500 torr, in Krebs bicarbonate/Hepes buffer containing 5 or 30 mM glucose. Elevated glucose levels increased glucose 6-phosphate, fructose 6-phosphate, total triose phosphates, lactate, the lactate/pyruvate ratio, sorbitol, and fructose, but did not affect sn-glycerol 3-phosphate, pyruvate, or myo-inositol levels. The more reduced glomerular cytosolic redox state (manifested by the tissue lactate/pyruvate ratio) induced by 30 mM glucose was completely abrogated by aldose reductase inhibitors added to the diet two to seven days prior to glomerular isolation. These observations, coupled with evidence linking glucose- and diabetes-induced glomerular dysfunction to increased sorbitol pathway metabolism, support the hypothesis that metabolic imbalances associated with a more reduced ratio of cytosolic NADH/NAD+ (resulting from increased glucose metabolism via the sorbitol pathway) play an important role in mediating glucose- and diabetes-induced glomerular dysfunction.

Chronic lithium administration alters a prominent PKC substrate in rat hippocampus

The therapeutic effect of lithium in the treatment of bipolar disorder exhibits a significant delay in the onset of action and a persistence of efficacy beyond abrupt discontinuation of treatment. Lithium is known to alter receptor-coupled phosphoinositide second messenger pathway in brain, resulting in indirect changes in an endogenous activator of protein kinase C (PKC). Such evidence has suggested that PKC may be involved in the mechanism of action of lithium in the brain. PKC represents a site wherein long-term regulatory changes in cell function occur through the phosphorylation of specific phosphoproteins involved in processes including neurotransmitter release and receptor activation. In studies of rats exposed to lithium, however, we have found no significant effects of chronic administration on the relative activity, subcellular distribution, or activation of PKC in hippocampus. We did find a major reduction in the in vitro PKC mediated phosphorylation of two major substrates, 83 kDa and 45 kDa, in hippocampus of rats exposed to chronic lithium and maintaining clinically relevant therapeutic levels in brain. Using immunoblot analysis we have identified a known myristoylated alanine-rich C kinase substrate (MARCKS) at 83 kDa. In vivo levels of MARCKS in hippocampus were found to be significantly reduced after chronic lithium exposure. These findings persist in animals withdrawn from lithium, but are not apparent following acute treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for coupling of resynthesis to hydrolysis in the phosphoinositide cycle

Previous data suggest that agonist-induced hydrolysis of phosphatidylinositol bisphosphate is accompanied by resynthesis through phosphatidylinositol such that these metabolic events function in a cyclic manner. However, it is not known whether resynthesis depends on the presence of agonist or is a direct result of agonist-induced breakdown. In the present study we demonstrate that: (1) increasing the intracellular free inositol concentration will not stimulate phosphatidylinositol synthesis, as measured by assessing the amount of [32P]Pi incorporation; (2) regeneration of free inositol is required for resynthesis; however, addition of exogenous inositol can sustain resynthesis under conditions which inhibit the regeneration of endogenous inositol; (3) resynthesis can take place in the absence of agonist provided that cells have been previously incubated under conditions which prevent resynthesis; and (4) the presence of agonist does not increase the rate of resynthesis. Thus the resynthetic phase of the phosphoinositide cycle is a compensatory event triggered either by the decrease in the level of phosphatidylinositol or by an increase in precursor substrates. The agonist itself appears to have no direct effect on the resynthesis process.

Increases in Na/K pump numbers in isolated human lymphocytes exposed to lithium in vitro. Reversal by myo-inositol and by inhibitors of protein kinase C and the Na/H antiport

Lithium (1-8 mM) caused a dose-dependent increase in the number of [3H]ouabain binding sites and in sodium/potassium (Na/K) pump activity in normal lymphocytes after incubation for 72 h. The increase in Na/K pump activity was due to an increase in the Vmax of the pump, with no change in the apparent affinity (Km) for potassium (rubidium). There was no change in the turnover number of the pump and the intracellular sodium concentration fell. The increase in [3H]ouabain binding sites was prevented by the addition of myo-inositol (10 mM), by inhibition of the protein kinase C with staurosporine (100 nM) and by inhibition of the Na/H antiport with dimethylamiloride (50 microM). These results suggest that the increase in Na/K pump activity caused by lithium is due to an increase in pump numbers and not due to increased activity of individual pumps or to an alteration in the affinity of the pumps for potassium. The increase in Na/K pump numbers and activity in lymphocytes exposed to lithium for 72 h may be related to altered Na/H antiport activity secondary to inhibition of phosphoinositol breakdown by lithium.

Influence of lithium on second messenger accumulation in NG108-15 cells

Treatment of NG108-15 cells with bradykinin for 10 min resulted in a reduction of phosphatidylinositol levels with no change in the other inositol lipids. Accompanying this was a small increase in diacylglycerol but no modification in the level of inositol phosphates. Addition of bradykinin to cells incubated for 30 min in buffer containing lithium did not cause a further decrease in the labeling of phosphatidylinositol, but was accompanied by an increase in mass of the three inositol phosphates. The increase was about 150% for inositol monophosphate, 40% for inositol bisphosphate and 80% for inositol trisphosphate. Under these conditions there was a marked increase in the ability of bradykinin to elevate the content of diacylglycerol. Moreover lithium by itself induced an increase in diacylglycerol six times as great as the increase in total inositol phosphates. The excess diacylglycerol may have its origin in phospholipids other than phosphoinositides.

Inhibition by oxytocin of voltage-activated calcium influx in cultured PC12 pheochromocytoma cells

1. Voltage-activated dihydropyridine-sensitive Ca2+ influx was measured in PC12 pheochromocytoma cells using 45Ca. 2. It has been found that oxytocin inhibits voltage-activated dihydropyridine-sensitive Ca2+ influx with ED50 about 0.30 x 10(-6) M. 3. Tolbutamide (1.3 x 10(-3) M) has no visible effect on both Ca2+ influx itself and on the inhibitory oxytocin effect. 4. External application of Li+ (10 mM) causes a slight shift of ED-curve to lower oxytocin concentrations. 5. It is suggested that the hydrolysis of phosphoinositides may play a role in oxytocin action on Ca2+ influx in PC12 cells.

Reduced inositol polyphosphate accumulation and inositol supply induced by lithium in stimulated cerebral cortex slices

The ability of lithium to interfere with phosphoinositide metabolism in rat cerebral cortex slices has been examined by monitoring the accumulation of CMP-phosphatidate (CMP-PtdOH) and the reduction in Ins(1,4,5)P3 and Ins(1,3,4,5)P4 levels. A small accumulation of [14C]CMP-PtdOH was seen in slices prelabelled with [14C]cytidine and stimulated with carbachol (1 mM) or Li+ (1 mM). However, simultaneous addition of both agents for 30 min produced a 22-fold accumulation, with Li+ producing a half-maximal effect at a concentration of 0.61 +/- 0.19 mM. Kinetic studies revealed that the effects of carbachol and Li+ on CMP-PtdOH accumulation occurred with no initial lag apparent under these conditions and that preincubation with myo-inositol (10 or 30 mM) dramatically attenuated CMP-PtdOH accumulation. myo-Inositol could also attenuate the rate of accumulation of CMP-PtdOH when added 20 min after carbachol and Li+; these effects were not observed when equimolar concentrations of scyllo-inositol were added. Use of specific radioreceptor assays allowed the mass accumulations of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 to be monitored. Following a lag of 5-10 min, Li+ resulted in a marked reduction in the accumulation of both inositol polyphosphates resulting from muscarinic-cholinergic stimulation. Preincubation of cerebral cortex slices with myo- (but not scyllo-) inositol delayed, but did not prevent, the reduction in the accumulation of Ins(1,4,5)P3 or Ins(1,3,4,5)P4. The results suggest that cerebral cortex, at least in vitro, is very sensitive to myo-inositol depletion under conditions of muscarinic receptor stimulation. The relationship of such depletion to the generation of inositol polyphosphate second messengers is discussed.

Chronic Li+ attenuates agonist- and phorbol ester-mediated Na+/H+ antiporter activity in HL-60 cells

The effects of Li+ on signal transduction in dibutyryl cAMP-differentiated HL-60 cells were studied. Upon differentiation, these human promyelocytic leukemia cells express a chemotactic formyl peptide receptor, which is coupled through a guanine nucleotide-binding protein to phospholipase C. Stimulation with fMet-Leu-Phe results in changes in intracellular pH which are thought to be mediated by protein kinase C regulation of Na+/H+ antiporter function. Acute LiCl treatment (10 mM) was without any effect on Na+/H+ activity. However, pretreatment of HL-60 cells with 1 or 10 mM LiCl for at least 5 days resulted in a marked attenuation of fMet-Leu-Phe effects on Na+/H+ activity. In undifferentiated HL-60 cells, which lack fMet-Leu-Phe receptors, intracellular acidification induced by the proton ionophore nigericin generates an alkalinization response. Chronic (but not acute) Li+ treatment also resulted in an inhibition of the nigericin-mediated response. Furthermore, stimulation of the Na+/H+ antiporter by the phorbol ester, phorbol-12-myristate-13-acetate, was also markedly attenuated by chronic LiCl treatment, suggesting an impairment of protein kinase C activity. In contrast, fMet-Leu-Phe-induced increases in intracellular Ca2+ and phospho-inositide breakdown were unchanged in cells treated with Li+ for 5 days. These results indicate that chronic but not acute Li+ treatment alters intracellular pH regulation possibly at a site distal to the fMet-Leu-Phe receptor.

Lithium potentiates agonist formation of [3H]CDP-diacylglycerol in human platelets

Thrombin stimulated a rapid formation of [3H]CDP-diacylglycerol in platelets prelabelled with [3H]cytidine. This response was increased in the presence of LiCl after a delay of 5 min; potentiation could be prevented by myo-inositol. Since Li+ inhibits the liberation of inositol from inositol phosphates, the conversion of diacylglycerol to phosphatidylinositol via CDP-diacylglycerol may be dependent on the regeneration of inositol from this pathway.

Lithium enhances neuronal muscarinic excitation by presynaptic facilitation

The mechanisms underlying the psychotropic actions of lithium are not established, but modulation of endogenous brain neurotransmitter systems is likely to be important. Several interactions of lithium with muscarinic responses have been reported, including a marked potentiation of seizures produced by muscarinic agonists. Because the mechanism by which lithium augments muscarinic seizures may be related to the mechanism by which it produces its psychotropic effects, we have studied the interaction of lithium and muscarinic agonists in vitro. Using rat hippocampal slices, we found that a muscarinic agonist, pilocarpine, increased postsynaptic neuronal excitability, but simultaneously decreased synaptic transmission because of presynaptic inhibition. Lithium did not alter pilocarpine's postsynaptic excitatory actions, but reversed its presynaptic inhibitory action, leading to markedly increased action potential firing. These presynaptic effects are not caused by alterations in presynaptic action potential shape or reliability of conduction, and do not involve pertussis toxin-sensitive G proteins. Activation of protein kinase C with phorbol-12,13-dibutyrate, or inhibition with H-7 and sphingosine, did not affect muscarinic presynaptic inhibition, but abolished lithium's ability to enhance synaptic transmission, suggesting that this effect of lithium involves protein kinase C. We propose that presynaptic facilitation accounts for lithium's potentiation of muscarinic seizures. Since these effects occur with concentrations of lithium used clinically, similar presynaptic effects in endogenous brain neurotransmitter systems may be important for lithium's psychotropic actions.

Effect of prolonged in vitro lithium treatment on calcium transients in frog twitch muscle fibres and its reversal by exogenous myo-inositol

Using arsenazo III as an intracellular indicator to monitor the calcium transients elicited by voltage-clamp depolarizing pulse, the effect of prolonged in vitro lithium treatment on excitation--contraction coupling in frog twitch muscle fibres was investigated. Incubation in 10 mM Li+ Ringer's solution for 2 days caused a 46% increase in the amplitude of the calcium transients, while treatment with 30 mM Li+ for 2 days produced a depression of 44%. Shortening the bathing time to 1 day, the decrease of calcium transients caused by 30 mM Li+ was reversed to a small increase. For the 2-day incubation, both the increase in the amplitude with 10 mM and the decrease with 30 mM Li+ were abolished by the presence of 1 mM myo-inositol in the bathing medium. These results imply that the turnover of inositol phospholipids is involved in regulating excitation-contraction coupling in skeletal muscle fibres.