Potentiation by lithium of CMP-phosphatidate formation in carbachol-stimulated rat cerebral-cortical slices and its reversal by myo-inositol

Acyl chain selection couples the consumption and synthesis of phosphoinositides

Phosphoinositides (PIPn) in mammalian tissues are enriched in the stearoyl/arachidonoyl acyl chain species ("C38:4"), but its functional significance is unclear. We have used metabolic tracers (isotopologues of inositol, glucose and water) to study PIPn synthesis in cell lines in which this enrichment is preserved to differing relative extents. We show that PIs synthesised from glucose are initially enriched in shorter/more saturated acyl chains, but then rapidly remodelled towards the C38:4 species. PIs are also synthesised by a distinct 're-cycling pathway', which utilises existing precursors and exhibits substantial selectivity for the synthesis of C38:4-PA and -PI. This re-cycling pathway is rapidly stimulated during receptor activation of phospholipase-C, both allowing the retention of the C38:4 backbone and the close coupling of PIPn consumption to its resynthesis, thus maintaining pool sizes. These results suggest that one property of the specific acyl chain composition of PIPn is that of a molecular code, to facilitate 'metabolic channelling' from PIP2 to PI via pools of intermediates (DG, PA and CDP-DG) common to other lipid metabolic pathways.

Diverse antidepressants increase CDP-diacylglycerol production and phosphatidylinositide resynthesis in depression-relevant regions of the rat brain

Background

Major depression is a serious mood disorder affecting millions of adults and children worldwide. While the etiopathology of depression remains obscure, antidepressant medications increase synaptic levels of monoamine neurotransmitters in brain regions associated with the disease. Monoamine transmitters activate multiple signaling cascades some of which have been investigated as potential mediators of depression or antidepressant drug action. However, the diacylglycerol arm of phosphoinositide signaling cascades has not been systematically investigated, even though downstream targets of this cascade have been implicated in depression. With the ultimate goal of uncovering the primary postsynaptic actions that may initiate cellular antidepressive signaling, we have examined the antidepressant-induced production of CDP-diacylglycerol which is both a product of diacylglycerol phosphorylation and a precursor for the synthesis of physiologically critical glycerophospholipids such as the phosphatidylinositides. For this, drug effects on [³H]cytidine-labeled CDP-diacylglycerol and [³H]inositol-labeled phosphatidylinositides were measured in response to the tricyclics desipramine and imipramine, the selective serotonin reuptake inhibitors fluoxetine and paroxetine, the atypical antidepressants maprotiline and nomifensine, and several monoamine oxidase inhibitors.

Results

Multiple compounds from each antidepressant category significantly stimulated [³H]CDP-diacylglycerol accumulation in cerebrocortical, hippocampal, and striatal tissues, and also enhanced the resynthesis of inositol phospholipids. Conversely, various antipsychotics, anxiolytics, and non-antidepressant psychotropic agents failed to significantly induce CDP-diacylglycerol or phosphoinositide synthesis. Drug-induced CDP-diacylglycerol accumulation was independent of lithium and only partially dependent on phosphoinositide hydrolysis, thus indicating that antidepressants can mobilize CDP-diacylglycerol from additional pools lying outside of the inositol cycle. Further, unlike direct serotonergic, muscarinic, or α-adrenergic agonists that elicited comparable or lower effects on CDP-diacylglycerol versus inositol phosphates, the antidepressants dose-dependently induced significantly greater accumulations of CDP-diacylglycerol.

Conclusion

Chemically divergent antidepressant agents commonly and significantly enhanced the accumulation of CDP-diacylglycerol. The latter is not only a derived product of phosphoinositide hydrolysis but is also a crucial intermediate in the biosynthesis of several signaling substrates. Hence, altered CDP-diacylglycerol signaling might be implicated in the pathophysiology of depression or the mechanism of action of diverse antidepressant medications.

Developing skeletal muscle cells express functional muscarinic acetylcholine receptors coupled to different intracellular signaling systems

This study analyzed the expression of muscarinic acetylcholine receptors (mAChRs) in the rat cultured skeletal muscle cells and their coupling to G protein, phospholipase C and adenylyl cyclase (AC). Our results showed the presence of a homogeneous population of [(3)H]methyl-quinuclidinyl benzilate-binding sites in the membrane fraction from the rat cultured muscle (K(D) = 0.4 nM, B(max) = 8.9 fmol mg protein(-1)). Specific muscarinic binding sites were also detected in denervated diaphragm muscles from adult rats and in myoblasts isolated from newborn rats. Activation of mAChRs with carbachol induced specific [(35)S]GTPgammaS binding to cultured muscle membranes and potentiated the forskolin-dependent stimulation of AC. These effects were totally inhibited by 0.1-1 microM atropine. In addition, mAChRs were able to stimulate generation of diacylglycerol (DAG) in response to acetylcholine, carbachol or selective mAChR agonist oxotremorine-M. The carbachol-dependent increase in DAG was inhibited in a concentration-dependent manner by mAChR antagonists atropine, pirenzepine and 4-DAMP mustard. Finally, activation of these receptors was correlated with increased synthesis of acetylcholinesterase, via a PKC-dependent pathway. Taken together, these results indicate that expression of mAChRs, coupled to G protein and distinct intracellular signaling systems, is a characteristic of noninnervated skeletal muscle cells and may be responsible for trophic influences of acetylcholine during formation of the neuromuscular synapse.

Regulation of gene expression by lithium and depletion of inositol in slices of adult rat cortex

Lithium inhibits inositol monophosphatase at therapeutically effective concentrations, and it has been hypothesized that depletion of brain inositol levels is an important chemical alteration for lithium's therapeutic efficacy in bipolar disorder. We have employed adult rat cortical slices as a model to investigate the gene regulatory consequences of inositol depletion effected by lithium using cytidine diphosphoryl-diacylglycerol as a functionally relevant biochemical marker to define treatment conditions. Genes coding for the neuropeptide hormone pituitary adenylate cyclase activating polypeptide (PACAP) and the enzyme that processes PACAP's precursor to the mature form, peptidylglycine alpha-amidating monooxygenase, were upregulated by inositol depletion. Previous work has shown that PACAP can increase tyrosine hydroxylase (TH) activity and dopamine release, and we found that the gene for GTP cyclohydrolase, which effectively regulates TH through synthesis of tetrahydrobiopterin, was also upregulated by inositol depletion. We propose that modulation of brain PACAP signaling might represent a new opportunity in the treatment of bipolar disorder.

Peripheral inflammation increases phosphoinositide activity in the rat dorsal horn

Persistent pain leads to changes in the spinal cord that contribute to hyperalgesia and allodynia. The effort to characterize these changes has focused on neurotransmitters and receptors, while relatively little is known about pain-associated modulation of second-messenger responses. Nearly all neurotransmitters can activate the phosphoinositide (PI) second-messenger system which has been investigated using a method that localizes membrane-bound [(3)H]CDP-diacylglycerol (DAG) produced from the precursor [(3)H]cytidine [Science 249 (1990) 802]. The present study applied this method in spinal cord slices from rats injected with complete Freund's adjuvant in one hindpaw and from uninflamed control rats. Two days after the injection, slices were removed and maintained in vitro for pharmacological testing. Some slices were exposed to the acetylcholine agonist carbachol which is antinociceptive in the spinal cord. Inflammation resulted in increased baseline, unstimulated [(3)H]CDP-DAG accumulation, especially in superficial dorsal horn layers, as well as enhanced carbachol-stimulated labeling. These results suggest that persistent pain leads to neurochemical changes within the spinal cord that could potentially enhance responses to a spectrum of pain-modulating transmitters.

Predicting response to lithium in mood disorders: role of genetic polymorphisms

Lithium is considered to be the first choice mood stabilizer in recurrent mood disorders. Its widespread and large-scale use is the result of its proven efficacy. In spite of this fact, patients have been observed to show a variable response to lithium treatment: in some cases it is completely effective in preventing manic or depressive relapses, while in other cases it appears to show no influence on the disease course. The possible definition of a genetic liability profile for adverse effects and efficacy will be of great help, as lithium therapy needs at least 6 months to be effective in stabilizing mood disorders. During the last few years, a number of groups have reported possible liability genes. Lithium long-term prophylactic efficacy has been associated with serotonin transporter protein, tryptophan hydroxylase and inositol polyphosphate 1-phosphatase variants. A number of other candidate genes and anonymous markers did not yield positive associations. Therefore, even if some positive results have been reported, no unequivocal susceptibility gene for lithium efficacy has been identified. Although the available data may not currently allow a meaningful prediction of lithium response, future research is aimed at the development of individualized treament of mood disorders, including the possibility of 'pharmacological genetic counseling'.

Opposed effects of lithium on the MEK-ERK pathway in neural cells: inhibition in astrocytes and stimulation in neurons by GSK3 independent mechanisms

Lithium is widely used in the treatment of bipolar disorder, but despite its proven therapeutic efficacy, the molecular mechanisms of action are not fully understood. The present study was undertaken to explore lithium effects of the MEK/ERK cascade of protein kinases in astrocytes and neurons. In asynchronously proliferating rat cortical astrocytes, lithium decreased time- and dose-dependently the phosphorylation of MEK and ERK, with 1 mM concentrations achieving 60 and 50% inhibition of ERK and MEK, respectively, after a 7-day exposure. Lithium also inhibited [3H]thymidine incorporation into DNA and induced a G2/M cell cycle arrest. In serum-deprived, quiescent astrocytes, pre-exposure to lithium resulted in the inhibition of cell cycle re-entry as stimulated by the mitogen endothelin-1: under this experimental setting, lithium did not affect the rapid, peak phosphorylation of MEK taking place after 3-5 min, but was effective in inhibiting the long-term, sustained phosphorylation of MEK. Lithium inhibition of the astrocyte MEK/ERK pathway was independent of inositol depletion. Further, compound SB216763 inhibited Tau phosphorylation at Ser396 and stabilized cytosolic beta-catenin, consistent with the inhibition of glycogen synthase kinase-3 beta (GSK-3 beta), but failed to reproduce lithium effects on MEK and ERK phosphorylation and cell cycle arrest. In cerebellar granule neurons, millimolar concentrations of lithium enhanced MEK and ERK phosphorylation in a concentration-dependent manner, again through an inositol and GSK-3 beta independent mechanism. These opposing effects in astrocytes and neurons make lithium treatment a promising strategy to favour neural repair and reduce reactive gliosis after traumatic injury.

Signaling networks in the pathophysiology and treatment of mood disorders

Over the past decade, the focus of research into the pathophysiology of mood disorders (bipolar disorder and unipolar depression in particular) has shifted from an interest in the biogenic amines to an emphasis on second messenger systems within cells. Second messenger systems rely on cell membrane receptors to relay information from the extracellular environment to the interior of the cell. Within the cell, this information is processed and altered, eventually to the point where gene and protein expression patterns are changed. There is a preponderance of evidence implicating second messenger systems and their primary contact with the extracellular environment, G proteins, in the pathophysiology of mood disorders. After an introduction to G proteins and second messenger pathways, this review focuses on the evidence implicating G proteins and two second messenger systems-the adenylate cyclase (cyclic adenosine monophosphate, cAMP) and phosphoinositide (protein kinase C, PKC) intracellular signaling cascades-in the pathophysiology and treatment of bipolar disorder and unipolar depression. Emerging evidence implicates changes in cellular resiliency, neuroplasticity and additional cellular pathways in the pathophysiology of mood disorders. The systems discussed within this review have been implicated in neuroplastic processes and in modulation of many other cellular pathways, making them likely candidates for mediators of these findings.

Inositol and higher inositol phosphates in neural tissues: homeostasis, metabolism and functional significance

Inositol phospholipids and inositol phosphates mediate well-established functions in signal transduction and in Ca2+ homeostasis in the CNS and non-neural tissues. More recently, there has been renewed interest in other roles that both myo-inositol and its highly phosphorylated forms may play in neural function. We review evidence that myo-inositol serves as a clinically relevant osmolyte in the CNS, and that its hexakisphosphate and pyrophosphorylated derivatives may play roles in such diverse cellular functions as DNA repair, nuclear RNA export and synaptic membrane trafficking.

Visualization of muscarinic receptor-mediated phosphoinositide turnover in the hippocampus of young and aged, learning-impaired Long Evans rats

Hippocampal receptor-mediated phosphoinositide (PI) turnover is severely blunted in aged rats that demonstrate cognitive deficits in the Morris water maze. To further examine the anatomical localization of this deficit, we examined the topography of muscarinic receptor-mediated PI turnover in young and aged-learning impaired rats by taking advantage of an autoradiographic method that visualizes PI turnover by measuring the diacylglycerol (DAG) branch of the PI turnover signal transduction system. Using this method, muscarinic cholinergic receptors were stimulated in hippocampal slices with agonist, and the receptor-mediated incorporation of [3H] cytidine into [3H]CDP-DAG was subsequently quantified in subregions of the hippocampus using film autoradiography. Our results show a significant decrease in basal incorporation of [ 3H]CDP-DAG in the subiculum and in the dentate gyrus in the aged rats. The muscarinic receptor-mediated [3H]CDP-DAG response was significantly blunted in the aged rats in subiculum, CA3, and CA1. In contrast, the receptor-mediated response was maintained in the dentate gyrus and hilus. These results indicate that the age-associated impairment in receptor-mediated PI turnover differs regionally, with a reduction in the subiculum and hippocampus proper that is pronounced relative to the hilus and dentate gyrus.

Epi-inositol regulates expression of the yeast INO1 gene encoding inositol-1-P synthase

Myo-inositol exerts behavioral effects in animal models of psychiatric disorders and is effective in clinical trials in psychiatric patients. Interestingly, epi-inositol exerts behavioral effects similar to myo-inositol, even though epi-inositol is not a substrate for synthesis of phosphatidylinositol. We postulated that the behavioral effects of epi-inositol may be due to its effects on gene expression. Yeast INO1expression was measured in northern blots. INM1 was determined by beta-galactosidase activity in a strain containing the fusion gene INM1-lacZintegrated into the genome. Epi-inositol affects regulation of expression of the INO1 gene (encoding inositol-1-P synthase), even though it cannot support growth of an inositol auxotroph (suggesting that, as in mammalian cells, it is not incorporated into phosphatidylinositol). Like myo-inositol, although to a lesser extent, epi-inositol causes a significant reduction in INO1 expression, and reverses the lithium- or valproate-induced increase in INO1 expression. However, it does not affect regulation of INM1 (encoding inositol monophosphatase), the expression of which is up-regulated by myo-inositol. The observed regulatory effects of epi-inositol on expression of the most highly regulated gene in the inositol biosynthetic pathway may help to explain how this inositol isomer can exert behavioral effects without being incorporated into phosphatidylinositol.

Pharmacogenetics in affective disorders

Pharmacogenetics will be of substantial help in the field of affective disorders pharmacotherapy. The possible definition of a genetic liability profile for drug side-effects and efficacy will be of great help in treatments that need weeks to months to be effective. During the last few years, a number of groups have reported possible liability genes. The efficacy and time of onset of selective serotonin reuptake inhibitors have been associated with a polymorphism in the promoter region of the transporter (SERTPR) in many independent studies, while variants at the tryptophan hydroxylase gene, 5-HT2a receptor and G-protein beta3 have been associated with them in pilot studies. Lithium long-term prophylactic efficacy has been associated with SERTPR, TPH and inositol polyphosphate 1-phosphatase variants, though in unreplicated samples. A number of further candidate genes were not associated with these treatments. In conclusion, both acute and long-term treatments appear to be, at least to some extent, under genetic influence and preliminary data have identified possible liability genes.

Lithium long-term treatment in mood disorders: clinical and genetic predictors

Lithium is the most widely used long-term treatment for recurrent mood disorders. Despite its proven efficacy, patients show a variable response, ranging from complete efficacy to no influence at all. This paper reviews possible predictors of response focusing on molecular genetic studies. The functional polymorphism in the upstream regulatory region of the serotonin transporter gene (5-HTTLPR) has been associated with lithium long-term efficacy in two independent studies, marginal associations have been reported for tryptophan hydroxylase and inositol polyphosphate 1-phosphatase (INPP1). A number of other candidate genes and anonymous markers did not yield positive associations. Therefore, even though some positive results have been reported, no unequivocal susceptibility gene for lithium efficacy has been identified.

Temporal dissociation between lithium-induced changes in frontal lobe myo-inositol and clinical response in manic-depressive illness

OBJECTIVE

The most widely accepted hypothesis regarding the mechanism underlying lithium's therapeutic efficacy in manic-depressive illness (bipolar affective disorder) is the inositol depletion hypothesis, which posits that lithium produces a lowering of myo-inositol in critical areas of the brain and the effect is therapeutic. Lithium's effects on in vivo brain myo-inositol levels were investigated longitudinally in 12 adult depressed patients with manic-depressive illness.

METHOD

Medication washout (minimum 2 weeks) and lithium administration were conducted in a blinded manner. Regional brain myo-inositol levels were measured by means of quantitative proton magnetic resonance spectroscopy at three time points: at baseline and after acute (5-7 days) and chronic (3-4 weeks) lithium administration.

RESULTS

Significant decreases (approximately 30%) in myoinositol levels were observed in the right frontal lobe after short-term administration, and these decreases persisted with chronic treatment. The severity of depression measured by the Hamilton Depression Rating Scale also decreased significantly over the study.

CONCLUSIONS

This study demonstrates that lithium administration does reduce myo-inositol levels in the right frontal lobe of patients with manic-depressive illness. However, the acute myo-inositol reduction occurs at a time when the patient's clinical state is clearly unchanged. Thus, the short-term reduction of myo-inositol per se is not associated with therapeutic response and does not support the inositol depletion hypothesis as originally posited. The hypothesis that a short-term lowering of myo inositol results in a cascade of secondary signaling and gene expression changes in the CNS that are ultimately associated with lithium's therapeutic efficacy is under investigation.

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.

Muscarinic M1 receptors activate phosphoinositide turnover and Ca2+ mobilisation in rat sympathetic neurones, but this signalling pathway does not mediate M-current inhibition

1. The relationship between muscarinic receptor activation, phosphoinositide turnover, calcium mobilisation and M-current inhibition has been studied in rat superior cervical ganglion (SCG) neurones in primary culture. 2. Phosphoinositide-specific phospholipase C (PLC) stimulation was measured by the accumulation of [3H]-cytidine monophosphate phosphatidate (CMP-PA) after incubation with [3H]-cytidine in the presence of Li+. The muscarinic agonist oxotremorine methiodide (oxo-M) stimulated PLC in a dose-dependent manner with an EC50 of approximately 3.5 microM. 3. The concentration-response curve for oxo-M was shifted to the right by a factor of about 10 by pirenzepine (100 nM), suggesting a pKB (-log of the apparent dissociation constant) of 7.9 +/- 0.4, while himbacine (1 microM) shifted the curve by a factor of about 13 (pKB approximately 7.1 +/- 0.6). This indicates involvement of the M1 muscarinic receptor in this response. 4. The accumulation of CMP-PA was localised by in situ autoradiography to SCG principal neurones, with no detectable signal in glial cells present in the primary cultures. 5. The ability of oxo-M to release Ca2+ from inositol(1,4, 5)trisphosphate (InsP3)-sensitive stores was determined by fura-2 microfluorimetry of SCG neurones voltage clamped in perforated patch mode. Oxo-M failed to evoke intracellular Ca2+ (Ca2+i) mobilisation in SCG neurones voltage clamped at -60 mV, but produced a significant Ca2+i rise (67 +/- 15 nM, n = 9) in cells voltage clamped at -25 mV. 6. Thapsigargin (0.5-1 microM) caused a 70 % inhibition of the oxo-M-induced Ca2+i increase, indicating its intracellular origin, while oxo-M-induced inhibition of M-current in the same cells was unaffected by thapsigargin. 7. Our results do not support the involvement of InsP3-sensitive calcium mobilisation in M-current inhibition.

Differential coupling of G-protein-linked receptors to Ca2+ mobilization through inositol(1,4,5)trisphosphate or ryanodine receptors in cerebellar granule cells in primary culture

Rat cerebellar granule cells in primary culture possess muscarinic, metabotropic glutamatergic, histaminergic and alpha-adrenergic receptors which couple to phosphoinositide-specific phospholipase C. We have determined the ability of these receptors to elevate inositol(1,4,5)trisphosphate and to release intracellular calcium, in order to establish the correlation between these two responses. In resting cerebellar granule cells, only the muscarinic agonist carbachol evoked significant increases in both inositol(1,4, 5)trisphosphate and cytoplasmic free Ca2+. Mild depolarization (20 mM KCl) enhanced inositol(1,4,5)trisphosphate elevation by carbachol and histamine, but not by noradrenaline or the metabotropic glutamate agonist 1S,3R ACPD. In contrast, Ca2+-release responses were modified differently by 20 mM KCl-depolarization: the responses to carbachol, histamine and 1S,3R ACPD, but not the responses to noradrenaline, were markedly enhanced. The contribution of ryanodine-sensitive Ca2+-release channels (ryanodine receptors) to the calcium release signal in depolarized cells was determined. Ryanodine (10 microM) inhibited most effectively the cytoplasmic Ca2+ elevation evoked by 1S,3R ACPD (> 90%), while Ca2+ release upon stimulation by carbachol and histamine was only inhibited by approximately 60% and remained larger than in the absence of KCl. Our data are consistent with a specific coupling between metabotropic glutamate receptors and ryanodine-sensitive Ca2+-release channels which may not require generation of inositol(1, 4,5)trisphosphate.

Regulation of signal transduction pathways and gene expression by mood stabilizers and antidepressants

OBJECTIVE

To determine whether the currently available evidence supports the hypothesis that antidepressants and mood stabilizers may bring about some of their long-term therapeutic effects by regulating signal transduction pathways and gene expression in the central nervous system.

METHODS

To address this question, we reviewed the evidence showing that chronic administration of antidepressants and mood stabilizers involves alterations in signaling pathways and gene expression in the central nervous system.

RESULTS

A large body of data has shown that lithium and valproate exert effects on the protein kinase C signaling pathway and the activator protein 1 family of transcription factors; in contrast, antidepressants affect the cyclic adenosine monophosphate pathway and may bring about their therapeutic effects by modulating cyclic adenosine monophosphate-regulated gene expression in the central nervous system.

CONCLUSIONS

Given the key roles of these signaling cascades in the amplification and integration of signals in the central nervous system, the findings have clear implications not only for research into the etiology and pathophysiology of the severe mood disorders but also for the development of novel and innovative treatment strategies.

Differentiated human NT2-N neurons possess a high intracellular content of myo-inositol

myo-Inositol plays a key role in signal transduction and osmotic regulation events in the CNS. Despite the known high concentrations of inositol in the human CNS, relatively little is known about its distribution within the different cell types. In this report, inositol homeostasis was studied in NT2-N cells, a unique cell culture model of human CNS neurons. Differentiation of precursor NT2 teratocarcinoma cells into NT2-N neurons by means of retinoic acid treatment resulted in an increase in inositol concentration from 24 to 195 nmol/mg of protein. After measurement of intracellular water spaces, inositol concentrations of 1.6 and 17.4 mM were calculated for NT2 and NT2-N cells, respectively. The high concentrations of inositol in NT2-N neurons could be explained by (1) an increased uptake of inositol (3.7 vs. 1.6 nmol/mg of protein/h, for NT2-N and NT2 cells, respectively) and (2) a decreased efflux of inositol (1.7%/h for NT2-N neurons vs. 9.0%/h for NT2 cells). Activity of inositol synthase, which mediates de novo synthesis of inositol, was not detected in either cell type. The observation that CNS neurons maintain a high intracellular concentration of inositol may be relevant to the regulation of both phosphoinositide signaling and osmotic stress events in the CNS.

Relationship between dopamine agonist stimulation of inositol phosphate formation and cytidine diphosphate-diacylglycerol accumulation in brain slices

Dopamine receptor-coupled stimulation of inositol phosphate formation has been characterized extensively, but little is known about the diacylglycerol arm of this dual-signaling pathway. This study examined several parameters of cytidine diphosphate-diacylglycerol (CDP-DG) accumulation as an index of agonist-stimulated DG formation. Rat brain slices pre-labeled with 5-[3H]cytidine were incubated with various test agents in the presence of LiCl and accumulated CDP-DG analyzed. Dopamine and SKF38393 significantly and dose-dependently stimulated CDP-DG accumulation. SKF38393 responses were inhibited by neomycin and reversed by myo-inositol or by exclusion of LiCl. Compared to inositol phosphate formation in 2-[3H]inositol-prelabeled slices, the CDP-DG responses were proportionately greater, while the agonist EC50 values were similar between the two assays. The D1-receptor antagonist SCH23390 inhibited SKF38393-mediated responses at 0.1-10 microM concentrations, whereas greater concentrations reversed the inhibition. SKF38393 effects were completely blocked by the DG kinase inhibitor R59022, thus precluding any role for phospholipase-D or de novo phosphatidate synthesis in the dopaminergic response. D609 which inhibits phosphatidylcholine-specific phospholipase-C (PLC), potently inhibited both CDP-DG accumulation and inositol phosphate formation. These findings demonstrate that the selective D1-receptor antagonist SCH23390 is a partial agonist at the D1-like dopamine receptor that couples to phosphoinositide signaling, that dopaminergic facilitation of phosphoinositide signaling is independent of de novo phosphatidate synthesis, and that the widely used enzyme inhibitor, D-609, is probably not selective for phosphatidylcholine-specific PLC in brain slice preparations. The greater sensitivity of the CDP-DG measurement presents this assay as a reliable and possibly superior index of dopamine receptor-coupled PLC activation in intact tissues.

Partial lithium-associated protection against apoptosis induced by C2-ceramide in cerebellar granule neurons

Primary cultures of cerebellar granule neurons, maintained in a serum-containing medium, underwent apoptosis when exposed to C2-ceramide, as assessed by mitochondrial reduction of MTT and intranucleosomal DNA fragmentation. After an 18 h exposure to 50 microM C2-ceramide, cell viability decreased by 25-40%. Addition of lithium together with C2-ceramide resulted in a partial protection of apoptosis, which was maximal at 5 mM lithium (37% protection). When lithium was added 5 h before the apoptotic stimulus the neuroprotective effect of the ion was clearly increased (66% protection). This effect was not due to intracellular inositol depletion or inhibition of NMDA receptors. Our data broaden the nature of apoptotic insults being reversed by lithium, stressing the neuroprotective effects of the ion.

1,25(OH)2D3 regulates protein kinase C activity through two phospholipid-dependent pathways involving phospholipase A2 and phospholipase C in growth zone chondrocytes

We have previously shown that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a major role in growth zone chondrocyte (GC) differentiation and that this effect is mediated by protein kinase C (PKC). The aim of the present study was to identify the signal transduction pathway used by 1,25(OH)2D3 to stimulate PKC activation. Confluent, fourth passage GC cells from costochondral cartilage were used to evaluate the mechanism of PKC activation. Treatment of GC cultures with 1,25(OH)2D3 elicited a dose-dependent increase in both inositol-1,4,5-trisphosphate and diacylglycerol (DAG) production, suggesting a role for phospholipase C and potentially for phospholipase D. Addition of dioctanoylglycerol to plasma membranes isolated from GCs increased PKC activity. Neither pertussis toxin nor choleratoxin had an inhibitory effect on PKC activity in control or 1,25(OH)2D3-treated GCs, indicating that neither Gi nor Gs proteins were involved. Phospholipase A2 inhibitors, quinacrine, OEPC (selective for secretory phospholipase A2), and AACOCF3 (selective for cytosolic phospholipase A2), and the cyclooxygenase inhibitor indomethacin decreased PKC activity, while the phospholipase A2 activators melittin and mastoparan increased PKC activity in GC cultures. Arachidonic acid and prostaglandin E2, two downstream products of phospholipase A2 action, also increased PKC activity. These results indicate that 1,25(OH)2D3-dependent stimulation of PKC activity is regulated by two distinct phospholipase-dependent mechanisms: production of DAG, primarily via phospholipase C and production of arachidonic acid via phospholipase A2.

The synergistic effects of vitamin D metabolites and transforming growth factor-beta on costochondral chondrocytes are mediated by increases in protein kinase C activity involving two separate pathways

Transforming growth factor-beta (TGFbeta), as well as the vitamin D3 metabolites 1,25-dihydroxyvitamin D3 (1,25) and 24,25-dihydroxyvitamin D3 (24,25), regulate chondrocyte differentiation and maturation during endochondral bone formation. Both the growth factor and secosteroids also affect protein kinase C (PKC) activity, although each has its own unique time course of enzyme activation. Vitamin D3 metabolite effects are detected soon after addition to the media, whereas TGFbeta effects occur over a longer term. The present study examines the interrelation between the effects of 1,25, 24,25, and TGFbeta on chondrocyte differentiation, matrix production, and proliferation. We also examined whether the effect is hormone-specific and maturation-dependent and whether the effect of combining hormone and growth factor is mediated by PKC. This study used a chondrocyte culture model developed in our laboratory that allows comparison of chondrocytes at two stages of differentiation: the more mature growth zone (GC) cells and the less mature resting zone chondrocyte (RC) cells. Only the addition of 24,25 with TGFbeta showed synergistic effects on RC alkaline phosphatase-specific activity (ALPase). No similar effect was found when 24,25 plus TGFbeta was added to GC cells or when 1,25 plus TGFbeta were added to GC or RC cells. The addition of 1,25 plus TGFbeta and 24,25 plus TGFbeta to GC and RC cells, respectively, produced a synergistic increase in [35S]sulfate incorporation and had an additive effect on [3H]thymidine incorporation. To examine the signal transduction pathway involved in producing the synergistic effect of 24,25 and TGFbeta on RC cells, the level of PKC activity was examined. Addition of 24,25 and TGFbeta for 12 h produced a synergistic increase in PKC activity. Moreover, a similar effect was found when 24,25 was added for only the last 90 min of a 12-h incubation. However, a synergistic effect could not be found when 24,25 was added for the last 9 min or the first 90 min of incubation. To further understand how 24,25 and TGFbeta may mediate the observed synergistic increase in PKC activity, the pathways potentially leading to activation of PKC were examined. It was found that 24,25 affects PKC activity through production of diacylglycerol, not through activation of G protein, whereas TGFbeta only affected PKC activity through G protein. The results of the present study indicate that vitamin D metabolites and TGFbeta produced a synergistic effect that is maturation-dependent and hormone-specific. Moreover, the synergistic effect between 24,25 and TGFbeta was mediated by activation of PKC through two parallel pathways: 24,25 through diacylglycerol production and TGFbeta through G protein activation.

Selective effects of long-term lithium and carbamazepine administration on G-protein subunit expression in rat brain

The efficacy of lithium and carbamazepine in treatment of bipolar affective disorder is well established. Although a number of biochemical effects have been found the exact molecular mechanisms underlying their therapeutic actions have not been elucidated. Nor have the target regions in the brain been located. The objectives of the present investigation were to identify the selective effects and target regions of long-term treatment, with either lithium or carbamazepine, on G-protein subunit expression in rat brain. Effects were measured in hippocampus, hypothalamus, amygdala, frontal cortex, neostriatum, thalamus, raphe nuclei and cerebellum. At the protein level amounts of Galphao decreased significantly (P < 0.01) in neostriatum and Gbeta increased in frontal cortex in response to both drug treatments. At the mRNA level amounts of Galphai1 increased significantly (P < 0.01) in neostriatum. Galphas messenger amounts decreased in frontal cortex and increased in thalamus. These effects were common for both drugs, however, in addition also some differential effects, specific for either of the two drugs, were observed. We conclude frontal cortex and neostriatum are important target regions of long-term treatment with either lithium or carbamazepine and suggest Galphao, Galphas, Galphai1 and Gbeta to be selective target molecules.

CDP-diacylglycerol synthase from mammalian tissues

CDP-diacylglycerol resides at the branch point of glycerolipid biosynthesis as precursor of both the phosphoinositides and phosphatidylglycerol. The discovery of the phosphoinositide signal transduction pathway and the recognition of its prominent role in intracellular communication has focused new attention on CDP-diacylglycerol synthase. As a rate-limiting step in this pathway, it is a likely target for regulation. Exploration of this possibility will be facilitated by the recent cloning of mammalian CDP-DAG synthase.

Effects of NMDA on carbachol-stimulated phosphatidylinositol resynthesis in rat brain cortical slices

N-methyl-D-aspartate (NMDA) inhibits carbachol-stimulated phosphoinositide breakdown in rat brain cortical slices but not in isolated membranes (1). To gain insight into the mechanisms, we examined the effects of NMDA on carbachol-stimulated [3H]inositol phosphate and intermediates of phosphatidylinositol cycle accumulation in rat cortical slices. The inhibition is primarily on the synthesis of inositol phospholipids subsequent to activation of muscarinic cholinergic receptors. In the absence of lithium, NMDA inhibited carbachol-stimulated [32P]PtdIns but not [32P]PtdOH synthesis. Carbachol-stimulated CDP-DAG formation required trace amount of Ca2+ and the response was inhibited by NMDA at low but not high extracellular Ca2+ concentrations. The inhibition due to NMDA was only seen at millimolar extracellular Mg2+. The inhibition of carbachol-stimulated CDP-DAG formation was not affected by adding tetrodotoxin or cobalt chloride suggesting the inhibitory effect was not due to releasing of neurotransmitters. The inhibitory effects of NMDA could be abolished by MK-801, the specific NMDA receptor associated channel antagonist. When cortical slices were preincubated with ligands and lithium to allow the build up of CDP-DAG, carbachol stimulated the incorporation of [3H]PtdIns. However, this response was not inhibited by NMDA. These results suggest that CDP-DAG synthesis is the primary site of regulation by NMDA. Because CDP-DAG cytidyltransferase requires Mg2+ as cofactor and is sensitive to Ca2+ it is possible that NMDA inhibits ligand-stimulated PtdIns breakdown by blocking the replenish of agonist-sensitive PtdIns pool through changes of divalent cation homeostasis.

Lithium blocks cell cycle transitions in the first cell cycles of sea urchin embryos, an effect rescued by myo-inositol

Lithium is a classical inhibitor of the phosphoinositide pathway and is teratogenic. We report the effects of lithium on the first cell cycles of sea urchin (Lytechinus pictus) embryos. Embryos cultured in 400 mM lithium chloride sea water showed marked delay to the cell cycle and a tendency to arrest prior to nuclear envelope breakdown, at metaphase and at cytokinesis. After removal of lithium, the block was reversed and embryos developed to form normal late blastulae. The lithium-induced block was also reversed by myo- but not epi-inositol, indicating that lithium was acting via the phosphoinositide pathway. Lithium microinjection before fertilization caused arrest prior to nuclear envelope breakdown at much lower concentrations (3-5 mM). Co-injection of myo-inositol prevented the block. Microinjection of 1–2 mM lithium led to block at the cleavage stage. This was also reversed by coinjection of myo-inositol. Embryos blocked by lithium microinjection proceeded rapidly into mitosis after photolysis of caged inositol 1,4,5-trisphosphate. These data demonstrate that a patent phosphoinositide signalling pathway is essential for the proper timing of cell cycle transitions and offer a possible explanation for lithium's teratogenic effects.

24,25-(OH)2D3 regulates protein kinase C through two distinct phospholipid-dependent mechanisms

We have previously shown that 24,25-(OH)2D3 plays a major role in resting zone (RC) chondrocyte differentiation and that this vitamin D metabolite regulates protein kinase C (PKC). The aim of the present study was to identify the signal transduction pathway used by 24,25-(OH)2D3 to stimulate PKC activation. Confluent, fourth passage RC cells from rat costochondral cartilage were used to evaluate the mechanism of PKC activation. Treatment of RC cultures with 24,25-(OH)2D3 for 90 min produced a dose-dependent increase in diacylglycerol (DAG). Addition of R59022, a diacylglycerol kinase inhibitor, significantly increased PKC activity in cultures treated with 24,25-(OH)2D3. Addition of dioctanoylglycerol (DOG) to plasma membranes isolated from RC increased PKC activity 447-fold. Addition of pertussis toxin or cholera toxin to control cultures elevated basal PKC activity. When added together with 10(-9) M 24,25-(OH)2D3, there was an additive effect on PKC activity but in cultures treated with 10(-8) M 24,25-(OH)2D3, only the hormone-dependent stimulation of PKC was observed. The phospholipase C inhibitor, U73-122, had no effect on PKC activity, indicating that the DAG produced in response to 24,25-(OH)2D3 is not derived from phosphatidylinositol. Addition of the tyrosine kinase inhibitor, genistein, also had no effect on 24,25-(OH)2D3-stimulated PKC, further supporting the hypothesis that phospholipase C is not involved in the mechanism and that phospholipase D is responsible for the increase in DAG production. Phospholipase A2 inhibitors, quinacrine and AACOCF3, and the cyclooxygenase inhibitor indomethacin increased PKC activity in the RC cultures. Exogenous PGE2, one of the downstream products of phospholipase A2 action, inhibited PKC activity. These results suggest that 24,25-(OH)2D3 regulates PKC activity by two distinct phospholipid-dependent mechanisms: production of DAG via phospholipase D and inhibition of the production of PGE2 via inhibition of phospholipase A2 and cyclooxygenase.

Receptor-mediated phosphoinositide metabolism in peripheral nerve and cultured Schwann cells

Peripheral nerve possesses muscarinic cholinergic receptors, predominantly of the M3 subtype, that stimulate phosphoinositide metabolism. Evidence suggests that one site of this response is the myelin sheath. Purified peripheral nerve myelin contains several heterotrimeric GTP-binding proteins. Furthermore, carbachol and guanosine-5'-(3-O-thio)triphosphate-stimulated hydrolysis of exogenous phosphatidylinositol-4,5-bis-phosphate that is blocked by atropine can be reconstituted in a purified peripheral myelin-rich fraction. Nerve phosphoinositide turnover is also stimulated by adenosine analogs and blocked by adenosine receptor antagonists in a pattern consistent with the presence of adenosine A2 receptors in the tissue. Receptor-mediated phosphoinositide metabolism has also been studied in a human tumor-derived Schwann cell line (NF1T) derived from a neurofibromatosis-1 patient. By the same experimental criteria, NF1T cells also appear to contain adenosine A2 receptors which upon activation stimulate phosphoinositide turnover. However, phosphoinositide metabolism in these cells is not increased by either carbachol or ATP. Our findings taken together with other reports suggest that Schwann cells may possess a variety of receptors which regulate phosphoinositide metabolism.

Epi-inositol is biochemically active in reversing lithium effects on cytidine monophosphorylphosphatidate (CMP-PA). Short communication

In CHOm3 cells and rat cerebral cortex slices, epi-inositol was less potent but as effective as myo-inositol in reversing carbachol/lithium-stimulated CMP-PA accumulation whereas L-chiro- and scyllo-inositol were less active or inactive. These results with the four inositol isomers in two tissues correlate exactly with their effects on lithium-pilocarpine induced seizures and suggest a common mechanism of action for biochemical and behavioural effects.

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.

Myo-inositol attenuates the enhancement of the serotonin syndrome by lithium

Lithium elicits opposite effects on two behavioural syndromes in rats: enhancement of the 5-HT1A-linked serotonin syndrome and attenuation of the 5-HT2-linked wet dog shakes. The ability of intracerebroventricular (ICV) myo-inositol or forskolin to reverse the enhancement of the serotonin syndrome by lithium was tested in rats that were fed chronic dietary lithium or control diet and injected with the serotonin agonist 5-MeODMT (5-methoxy-N, N-dimethyltryptamine). Lithium enhanced the total serotonin syndrome score and particularly flat posture and tremor. Inositol, but not forskolin, mitigated the effects of lithium. Inositol was also injected in the lateral ventricle of rats pretreated with chronic dietary lithium or regular rat chow for 3 weeks and injected with carbidopa and L-5-hydroxytryptophan (5-HTP). Lithium attenuated wet dog shakes, but inositol had no significant effect on lithium-treated or control rats. These findings suggest that the enhancement of the serotonin syndrome by lithium may be related to lithium-induced inositol depletion.

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.

Lithium stimulates accumulation of second-messenger inositol 1,4,5-trisphosphate and other inositol phosphates in mouse pancreatic minilobules without inositol supplementation

Previous studies showed that lithium, beginning at therapeutic plasma concentrations in the treatment of manic depression, increased the accumulation of second-messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in cerebral cortex slices of guinea pig and rhesus monkey [Lee, Dixon, Reichman, Moummi, Los and Hokin (1992) Biochem. J. 282, 377-385; Dixon, Lee, Los and Hokin (1992) J. Neurochem. 59, 2332-2335; Dixon, Los and Hokin (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 8358-8362]. These studies have now been extended to a peripheral tissue, mouse pancreatic minilobules. In the presence of carbachol, concentrations of lithium from 1 to 20 mM sharply and progressively increased the accumulation of Ins(1,4,5)P3 and inositol 1,3,4,5-tetrakisphosphate, followed by a decrease. Assay of these inositol polyphosphates by either the prelabelling technique or mass assay gave similar results. Atropine quenching of cholinergically stimulated pancreatic minilobules led to a rapid disappearance of Ins(1,4,5)P3. This disappearance was impeded by lithium. This suggested that the lithium-induced elevation in Ins(1,4,5)P3 was due to inhibition of the 5-phosphatase and, on the basis of the markedly elevated concentrations of inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] and inositol 1,4-bisphosphate in the presence of lithium, probably by feedback inhibition by these latter two compounds. An additional mechanism, i.e. a stimulatory effect of lithium on phospholipase C, cannot, however, be ruled out. The other reaction product of phospholipase C, inositol cyclic 1:2,4,5-trisphosphate, also increased in the presence of lithium. This may also be due to inhibition of the 5-phosphatase, which is the exclusive mechanism for removal of this compound. The effects of lithium on the accumulation of other inositol phosphates paralleled that of Ins(1,4,5)P3, with the exception of inositol 3,4-bisphosphate, which decreased. This was presumably due to the inhibition of Ins(1,3,4)P3 1-phosphatase by lithium. Unlike mouse cerebral cortex slices [Lee, Dixon, Reichman, Moummi, Los and Hokin (1992) Biochem. J. 282, 377-385], inositol supplementation was not required to demonstrate lithium-stimulated Ins(1,4,5)P3 accumulation in mouse pancreatic minilobules. This indicates that inositol depletion sufficient to impair lithium-stimulated Ins(1,4,5)P3 accumulation does not occur in mouse pancreatic minilobules, even though an elevation of cytidine diphosphodiacylglycerol occurred, indicating some inositol depletion due to lithium. Elevation of Ins(1,4,5)P3 by lithium may be a general phenomenon in the central nervous system and peripheral tissues under non-rate-limiting concentrations of inositol.

Hydrolysis of cell surface inositol phospholipid leads to the delayed stimulation of phosphatidylinositol synthesis in bovine aortic endothelial cells

In order to address the issue of how inositol phospholipid synthesis is controlled in a resting cell we looked for enhanced [3H]phosphatidylinositol (PtdIns) labelling in response to the hydrolysis of cell surface PtdIns. Bacillus thuringiensis PtdIns-PLC when added to intact bovine aortic endothelial (BAE) cells rapidly hydrolysed 9.1 +/- 1% of the total cellular PtdIns. This result suggests that BAE cells have a cell surface pool of PTdIns. Hydrolysis of cell surface PtdIns, in contrast to the agonist-stimulated hydrolysis of inner leaflet PtdIns, did not lead to a rapid (minutes) stimulation of PtdIns resynthesis. Prolonged incubation of BAE cells with PtdIns-PLC led to further hydrolysis of PtdIns (up to 20% of total cellular PtdIns). This second phase of PtdIns-PLC induced hydrolysis was inhibited by the addition of brefeldin A suggesting that it was dependent on vesicular traffic to the plasma membrane from the endoplasmic reticulum. Furthermore, the above result suggests that prolonged incubation of intact cells with PtdIns-PLC leads to the slow depeletion of intracellular PtdIns stores. This second phase of PtdIns-PLC induced hydrolysis was associated with PtdIns resynthesis since prolonged incubation with PtdIns-PLC, but not B. cereus PtdCho-PLC (which does not hydrolyse PtdIns), led to enhanced PtdIns labelling. The results indicate that extracellular PtdIns-PLC induced PtdIns resynthesis may occur due to PtdIns-PLC induced intracellular PtdIns depletion.

Palmitoylation but not the extreme amino-terminus of Gq alpha is required for coupling to the NK2 receptor

Gq alpha and G11 alpha differ from other G protein alpha subunits in that they have unique, conserved 6 residue amino-terminal extensions. Wild-type and amino-terminal mutants of Gq alpha expressed in COS cells were analyzed for their ability to functionally couple with co-expressed neurokinin NK2 receptor. Wild-type, T2A and delta 2-7 Gq alpha were able to stimulate agonist driven phospholipase C (PLC) activity in identical manners. Other activities of these two amino-terminal mutants including aluminum fluoride stimulated PLC activity, palmitoylation, interaction with G beta gamma subunits and GTP gamma S-induced trypsin resistance are also similar to the wild-type alpha subunit. This demonstrates that the NK2 receptor is able to functionally interact with the alpha subunit of Gq and that the first seven amino-acids of Gq alpha are not required for any of the alpha subunit functions tested. In contrast to the T2A and delta 2-7 mutants, a C9,10A Gq alpha mutant was not able to couple to either the NK2 receptor or PLC, as assessed by high-affinity agonist binding and activation of PLC either in intact cells or in vitro. The C9,10A protein was able to assume a GTP gamma S-induced trypsin-resistant conformation and partitioned primarily to the pelletable fraction in a manner similar to the wild-type protein. However, it was not labeled with [3H]palmitic acid. This suggests that blocking palmitoylation at the amino-terminus of Gq alpha results in a loss of functional activity which reflects an inability to interact with both the receptor and downstream signaling targets.

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.

Muscarinic receptor-dependent activation of phospholipase C in the developing human fetal central nervous system

The coupling of muscarinic-cholinergic receptors (mAChR) to adenylate cyclase and phospholipase C (PLC) second messenger systems has been demonstrated in many animal species. However, little is known about this association in the developing human central nervous system. Because of the proposed role of acetylcholine in the regulation of development and differentiation of neural cells, an understanding of these relationships during human fetal development gains importance. We report, in this communication, the coupling of mAChR with PLC in the human fetal brain. This coupling was determined using two independent approaches that relied upon estimating the accumulation of inositol phosphates (IPs) and cytidine diphosphate diacylglycerol (CDP-DAG). Carbachol treatment of brain slices, in the presence of lithium, resulted in the accumulation of IPs. Analysis of the kinetics of this accumulation showed that IP3 and IP2 increased rapidly, reaching a peak or plateau before IP. The results also showed that agonist-stimulated PLC produced two second messengers, IP3 and DAG. The production of DAG was strongly supported by the carbachol-dependent increase of CDP-DAG. The accumulation of IP and CDP-DAG was dependent on agonist concentration. The obtained EC50 values were approximately: carbachol 47 microM; acetylcholine 6 microM; and oxotremorine 25 microM. Unexpectedly, all three agonists demonstrated a similar efficacy. The cholinergic stimulation of inositide hydrolysis appears to be the result of activation of the m1 muscarinic receptor.

Visualization of agonist-stimulated inositol phospholipid turnover in individual neurons of the rat cerebral cortex and hippocampus

A novel autoradiographic method to identify individual neurons responding to neurotransmitter stimulation with increased phosphoinositide turnover is described. When phosphoinositide-coupled receptors are activated, phosphatidylinositol 4,5-bisphosphate is hydrolysed by phospholipase C generating the two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. During prolonged receptor stimulation, both second messengers are actively recycled to maintain the effective intracellular levels of agonist-sensitive phosphoinositides. Lithium ions inhibit this recycling pathway by blocking the recovery of free inositol from inositol 1,4,5-trisphosphate thus leading to the accumulation of phosphatidyl cytidine monophosphate, a membrane bound molecule which is the activated precursor of the synthesis of phosphoinositides. Therefore, addition of excess myo-inositol reverts the effects of lithium inhibition. Thus, taking advantage of this fact and using [3H]cytidine as precursor, phosphatidyl [3H]cytidine monophosphate accumulation was induced in rat neocortical and hippocampal slices after muscarinic or metabotropic glutamate receptor stimulation. The labelled slices were then fixed, dehydrated and embedded in Durcupan resin. Semithin sections (1 micron thick) were cut and exposed to autoradiographic emulsion for several weeks. Biochemical analysis of the incorporation of [3H]cytidine into the chloroform extracted (containing lipids) and the alkali-solubilized (containing nucleic acids and proteins) fractions were carried out in parallel with morphological studies. The stimulation of both receptor types induced labelling of neurons in neocortex and hippocampus. In labelled cells silver grains were characteristically observed over the cytoplasm surrounding the nucleus and main dendritic processes. The anatomical location and distribution of labelled cells as well as the levels of response obtained in both brain regions studied, was found to be receptor specific. Inclusion of 30 mM myo-inositol in the incubation media reversed completely both the accumulation of phosphatidyl [3H]cytidine monophosphate and the labelling of cells, thus demonstrating that the label detected autoradiographically corresponds to phosphatidyl [3H]cytidine monophosphate. It is concluded that the method is sensitive and specific, allowing identification of individual neurons in both neocortical and hippocampal slices and after stimulation of both muscarinic and metabotropic glutamate receptor subtypes. The method may open a new means to study the phosphoinositide second messenger signalling pathway and the cells in which it takes place.

Effect of mood stabilizing agents on agonist-induced calcium mobilization in human platelets

The effect of mood stabilizing agents such as lithium, carbamazepine, valproic acid and clonazepam on serotonin(5-HT)- or thrombin-induced intracellular calcium (Ca) mobilization was studied in the platelets of healthy subjects using the fluorescent Ca indicator fura-2. After incubating platelet-rich plasma with these drugs for one or four hours, there was no significant difference in either basal Ca2+ concentration or 5-HT-stimulated Ca response between each agent treatment and control. 5-HT- or thrombin-induced Ca mobilization was not altered by four weeks of lithium carbonate administration in healthy volunteers. These results indicate that these mood stabilizers fail to affect the agonist-stimulated intracellular Ca mobilizing pathway either in vitro or ex vivo in the platelets of healthy subjects.

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.

Ziskind-Somerfeld Research Award 1993. Biochemical, behavioral, and clinical studies of the role of inositol in lithium treatment and depression

Lithium (Li) reduces brain inositol levels by inhibiting the enzyme inositol monophosphatase. The enzyme inositol-1-phosphatase was measured in human red blood cells of controls, Li-free bipolar patients, and Li-treated bipolar patients and was found to be reduced by 80% in Li-treated bipolars, thus supporting the concept that chronic Li at therapeutic concentrations inhibits this enzyme. Two behaviors in rats caused by Li, reduction of rearing, and Li-pilocarpine seizures, are reversed by intracerebroventricular replenishment of inositol. The reversal is stereospecific to the naturally occurring myo-inositol; whereas the stereoisomer L-chiro-inositol is ineffective. The reversal is dose-dependent, requiring a dose consistent with known quantities of brain inositol depletion; and is time-dependent, as inositol must be given 1-8 h before stimulation. High-dose peripheral inositol also reverses the limbic seizures induced by Li-pilocarpine, and using gas chromatography was shown to increase brain inositol levels that had been reduced by Li treatment. Low-dose inositol could be shown to reverse a peripheral Li-induced side effect, polyuria/polydipsia, in rats and in patients treated with Li. A higher dose of inositol markedly reduced Hamilton Depression Ratings in 9 of 11 unipolar major depressive disorder patients previously unresponsive to tricyclics, in an open design, but had no effect on chronic schizophrenics in a controlled double-blind randomized crossover trial. A new inositol monophosphatase inhibitor, a fungal product originally discovered as a complement inhibitor, was found to act like Li and lower the seizure threshold for subconvulsant doses of pilocarpine. These data suggest that inositol monophosphatase inhibition is a key mechanism of Li's therapeutic action and that design of new inositol monophosphatase inhibitors may be a practical strategy to create new compounds with Li-like therapeutic effects.

Effects of chronic lithium treatment on platelet PKC isozymes in Alzheimer's and elderly control subjects

Patients with Alzheimer's disease (AD) have been reported to have abnormalities in peripheral cells similar to some of those found in the brain, including decreased levels of protein kinase C (PKC) in fibroblasts. Since increasing evidence suggests that lithium affects PKC function, we investigated the effects of 3 weeks of lithium administration on the immunolabeling of 4 PKC isozymes (alpha, beta, epsilon, and zeta) in particulate and soluble fractions from platelets of 7 patients with probable AD and 6 age-matched controls. AD patients had significantly less particulate or membrane-associated PKC zeta than normals during the placebo phase (P < 0.003). After 3 weeks of lithium treatment, AD patients had significantly less membrane-associated PKC alpha (P < 0.002), epsilon (P < 0.003), and zeta (P < 0.001) than normals. This is the first report of a difference in PKC in blood cells between AD and control subjects. These findings appear to indicate that some PKC isozymes may be differentially regulated in AD versus elderly controls, at least as evidenced in this peripheral cellular system.

Characterization of the effects of lithium on phosphatidylinositol (PI) cycle activity in human muscarinic m1 receptor-transfected CHO cells

1. The effects of lithium on [3H]-inositol and [3H]-cytidine incorporation into [3H]-inositol monophosphates ([3H]-InsP1) and [3H]-cytidine monophosphorylphosphatidate ([3H]-CMP-PA), respectively, and inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) mass were studied in carbachol-stimulated human m1 muscarinic receptor-transfected Chinese hamster ovary cells (m1 CHO cells). 2. Lithium alone (10 mM) had no appreciable effects on any of the four parameters measured; it was only in carbachol-stimulated cells that the effects of lithium became apparent. 3. In the presence of carbachol (1 mM), lithium (10 mM) caused a relatively rapid (within 5 min) accumulation of [3H]-InsP1 and [3H]-CMP-PA which continued up to about 20-30 min, after which accumulation slowed down. On the other hand, the elevation in InsP3 and InsP4 levels produced by carbachol was not altered by lithium in the short-term and only at later times (> 20-30 min) was the response attenuated, with InsP3 and InsP4 levels approaching basal. 4. The effects of lithium on carbachol-stimulated [3H]-InsP1 and [3H]-CMP-PA accumulation and the attenuation of the carbachol-induced elevation of InsP3 and InsP4 were all dose-dependent, with EC50s in the region of 1 mM. 5. The lithium-induced effects on [3H]-CMP-PA and InsP3 and InsP4 in carbachol-stimulated cells could be reversed, in a dose-dependent manner, by preincubation with exogenous myo-inositol (EC50 = 2-3 mM) but not by the inactive analogue scyllo-inositol, indicating that these effects occur as a consequence of depletion of inositol. 6. The temporal effects of lithium are consistent with lithium inhibiting inositol monophosphatase,causing accumulation of InsP1, resulting in lower free inositol levels. This leads to accumulation of CMP-PA and reduced PI synthesis which, once agonist-linked membrane inositol phospholipids are depleted, produces attenuated InsP3 and InsP4 responses.7. These results in ml CHO cells support the hypothesis that lithium affects the PI cycle cell signalling pathway by depletion of inositol due to inhibition of inositol monophosphatase.

Comparative effects of lithium on the phosphoinositide cycle in rat cerebral cortex, hippocampus, and striatum

The effects of lithium on muscarinic cholinoceptor-stimulated phosphoinositide turnover have been investigated in rat hippocampal, striatal, and cerebral cortical slices using [3H]inositol or [3H]cytidine prelabelling and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] mass determination methods. Carbachol addition resulted in maintained increases in Ins(1,4,5)P3 and Ins(1,3,4,5)P4 mass levels in hippocampus and cerebral cortex, whereas in striatal slices these responses declined significantly over a 30-min incubation period. Carbachol-stimulated Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulations were inhibited by lithium in all brain regions studied in a time- and concentration-dependent manner. For example, in hippocampal slices significant inhibitory effects of LiCl were observed at times > 10 min after agonist challenge; IC50 values for inhibition of agonist-stimulated Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulations by lithium were 0.22 +/- 0.09 and 0.33 +/- 0.13 mM, respectively. [3H]CMP-phosphatidate accumulation increased in all brain regions when slices were stimulated by agonist and lithium. The ability of myo-inositol to reverse these effects, as well as lithium-suppressed Ins(1,4,5)P3 accumulation, implicates myo-inositol depletion in the action of lithium in the hippocampus and cortex at least. The results of this study suggest that although significant differences in the magnitude and time courses of changes in inositol (poly)phosphate metabolites occur in different brain regions, lithium evokes qualitatively similar enhancements of [3H]inositol monophosphate and [3H]-CMP-phosphatidate levels and inhibitions of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulations. However, the inability of striatal slices to sustain carbachol-stimulated inositol polyphosphate accumulation in the absence of lithium and the inability to reverse effects with myo-inositol may indicate differences in phosphoinositide signalling in this brain region.