Acetylcholine as a mitogen: muscarinic receptor-mediated proliferation of rat astrocytes and human astrocytoma cells

Effect of ethanol on protein kinase Czeta and p70S6 kinase activation by carbachol: a possible mechanism for ethanol-induced inhibition of glial cell proliferation

The signal transduction pathways that mediate the mitogenic response of muscarinic acetylcholine receptors in astroglial cells have not been fully elucidated. In this study we investigated the activation of p70S6 kinase (p70S6K) by carbachol in 1321 N1 astroctyoma cells. Carbachol induced a dose- and time-dependent activation of p70S6K, as evidenced by increased phosphorylation at Thr-389, Thr-421 and Ser-424, by increased p70S6K activity, and by a shift in its molecular weight. Activation of p70S6K was mediated by M3 muscarinic acetylcholine receptors (mAChRs) and was inhibited by two phosphatidylinositol-3-kinase (PI3-K) inhibitors, by a pseudosubstrate to protein kinase C (PKC) zeta, and by the p70S6K inhibitor rapamycin. Carbachol-induced DNA synthesis was strongly inhibited by rapamycin, suggesting that p70S6K activation plays an important role in carbachol-induced cell proliferation. Ethanol (25-100 mm) has been shown to inhibit carbachol-induced proliferation of astroglial cells. In the same range of concentrations, ethanol also inhibits carbachol-induced activation of PKCzeta and of p70S6K. On the other hand, inhibition of PI3-kinase was only observed at higher ethanol concentrations. These results indicate that activation of the PKCzeta--> p70S6K pathway by M3 mAChRs may play a role in the increased DNA synthesis and may represent a target for ethanol-induced inhibition of astroglial cell proliferation.

Uranyl acetate-induced sensorimotor deficit and increased nitric oxide generation in the central nervous system in rats

We investigated the effects of uranyl acetate on sensorimotor behavior, generation of nitric oxide and the central cholinergic system of rats. Male Sprague-Dawley rats were treated with intramuscular injection of 0.1 and 1 mg/kg uranyl acetate in water, daily for 7 days. Control animals received equivalent amount of water. The treatment was stopped after the seventh injection because the animals in the 1-mg/kg group appeared lethargic. The animals were maintained for an additional observation period of 30 days. The study was initiated as a dose-finding study that covered doses of 10 and 100 mg/kg, as well. However, all the animals in the 100-mg/kg treatment group died after the third and fourth injections, and all animals given 10 mg/kg died after the fifth and sixth injections. On Day 30 following the cessation of treatment, the sensorimotor functions of the animals in the 0.1- and 1-mg/kg treatment groups were evaluated using a battery of tests that included measurements of postural reflexes, limb placing, orientation to vibrissae touch, grip time, beam walking and inclined plane performance. The animals were sacrificed the same day and the cerebral cortex, brainstem, cerebellum and midbrain were dissected. The levels of nitric oxide as marker for increased oxidative stress, and the integrity of the cholinergic system as reflected in acetylcholinesterase (AChE) activity and m2 muscarinic acetylcholine receptors ligand binding, were determined. The data from behavioral observations show that there was a dose-related deficit at the 0.1- and 1-mg/kg treatment groups for inclined plane performance. Both doses reduced grip time, but there was no significant difference between the two doses. Similarly, both beam-walk score and beam-walk time were impaired at both doses as compared with the controls. A significant increase in nitric oxide was seen at 0.1 mg/kg dose in cortex and midbrain, whereas brainstem and cerebellum showed an insignificant decrease at both the doses. Similarly, there was no significant change in nitric oxide levels in kidneys and liver of the treated animals as compared with the controls. There was a significant increase in AChE activity in the cortex of the animals treated with 1 mg/kg uranyl acetate, but not in other brain regions. Ligand binding densities for the m2 muscarinic receptor did not show any change. These results show that low-dose, multiple exposure to uranyl acetate caused prolonged neurobehavioral deficits after the initial exposure has ceased.

Recovery of deficient cholinergic calcium signaling by adenosine in cultured rat cortical astrocytes

The regulation of the cholinergic calcium signaling in astroglial cells is thought to play a crucial role in the pathogenesis of Alzheimer's disease. We investigated the action of the cell modulator adenosine on acetylcholine (Ach)-mediated intracellular calcium ([Ca(2+)](i)) transients in cultured rat cortical astrocytes using the Ca(2+) imaging technique. The stable adenosine analog 2-chloroadenosine (2ClA) potentiated the [Ca(2+)](i) rise induced by activation of muscarinic Ach receptors by shifting approximately 30-fold the half-effective Ach concentration. This 2ClA effect was maintained upon removal of extracellular Ca(2+), indicating that Ach-induced [Ca(2+)](i) elevation was due mainly to Ca(2+) mobilization from intracellular stores. Pharmacological studies demonstrated that the 2ClA action was mediated by A1 receptors. Incubation with pertussis toxin abrogated the 2ClA effect but left unchanged the [Ca(2+)](i) rise produced by Ach alone. The [Ca(2+)](i) response elicited by Ach alone was abolished upon blockade of muscarinic receptor subtypes that stimulate phospholipase C, whereas the [Ca(2+)](i) elevation generated by the combined action of subthreshold Ach and 2ClA was not affected. Collectively, these results suggest that the impaired cholinergic signaling, the cardinal symptom of Alzheimer's disease, can be reinforced at the second messenger level by an alternative intracellular Ca(2+) mobilizing path, which can be brought into play by the concomitant activation of A1 purinoceptors and muscarinic receptors negatively coupled to adenylyl cyclase.

Inorganic lead activates the mitogen-activated protein kinase kinase-mitogen-activated protein kinase-p90(RSK) signaling pathway in human astrocytoma cells via a protein kinase C-dependent mechanism

We have previously reported that lead acetate activates protein kinase Calpha (PKCalpha) and induces DNA synthesis in human 1321N1 astrocytoma cells. In this study, we investigated the ability of lead to activate the mitogen-activated protein kinase (MAPK) cascade. We found that exposure to lead acetate (1-50 microM) resulted in concentration- and time-dependent activation of MAPK (extracellular signal responsive kinase 1/2), as shown by increased phosphorylation and increased kinase activity. This effect was significantly reduced by the PKC-specific inhibitor bisindolylmaleimide (GF109203X), by down-regulation of PKC with 12-O-tetradecanoyl-phorbol 13-acetate, by a pseudosubstrate to PKCalpha, and by selective down-regulation of PKCalpha by prior lead exposure. Lead was also shown to activate MAPK kinase (MEK1/2), and this effect was mediated by PKC. Two MEK inhibitors, 2-(2'-amino-3'-methoxyphenol)-oxanaphthalen-4-one (PD98059) and 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (UO126), blocked lead-induced MAPK activation and inhibited lead-induced DNA synthesis, as measured by incorporation of [methyl-3H]thymidine into cell DNA. The 90 kDa ribosomal S6 protein kinase, p90(RSK), a substrate of MAPK, was also found to be activated by lead in a PKC- and MAPK-dependent manner. Stimulation of DNA synthesis by lead in astrocytoma cells may be of interest in light of the observed association between exposure to lead and an increased risk of astrocytomas.

Vessel-specific stimulation of protein synthesis by nitric oxide synthase inhibition: role of extracellular signal-regulated kinases 1/2

Although conduit arteries develop hypertrophy after chronic NO synthesis blockade, resistance arteries remodel without hypertrophy under the same conditions. Similar findings have been described in essential hypertension. We postulated that this regional difference may be related to a heterogeneous effect of endogenous NO on proliferation along the vascular tree. Newly synthesized proteins were radiolabeled in vivo with [(3)H]L-leucine in basal conditions and during NO synthase inhibition, with or without PD98059 (inhibitor of the extracellular signal-regulated kinases [ERK] 1/2). Blocking the generation of NO by 3 different L-arginine analogues increased protein synthesis by an average of 75% in the aorta, in association with enhanced ERK 1/2 phosphorylation. PD98059 significantly reduced L-arginine analogue-induced protein synthesis and ERK 1/2 phosphorylation, confirming the involvement of ERK 1/2 as an important signaling element. In small arteries, L-arginine analogues did not influence the extent of protein synthesis, although phosphorylation of ERK 1/2 was also enhanced. To determine the role of NO in a condition of enhanced protein synthesis, angiotensin II was infused for 24 hours. Angiotensin II augmented protein synthesis in mesenteric arteries and the aorta, and was additive to NO synthase blockade in the aorta. In conclusion, endogenous NO exerts a tonic inhibitory influence on aortic growth, with limited impact on small arteries in basal and hypertrophic conditions. This heterogeneous role of NO on vascular growth may explain the heterogeneity of vascular remodeling observed in essential hypertension, a condition associated with endothelial dysfunction.

Histamine H1 receptor activation stimulates mitogenesis in human astrocytoma U373 MG cells

In human astrocytoma U373 MG cells that express histamine H1 receptors (180 +/- 6 fmol/mg protein) but not H2 or H3 receptors, histamine stimulated mitogenesis as assessed by [3H]-thymidine incorporation (173 +/- 2% of basal; EC50, 2.5 +/- 0.4 microM). The effect of 100 microM histamine was fully blocked by the selective H1 antagonist mepyramine (1 microM) and was markedly reduced (93 +/- 4% inhibition) by the phospholipase C inhibitor U73122 (10 microM). The activator of protein kinase C (PKC) phorbol 12-tetradecanoyl-13-acetate (TPA, 100nM) stimulated [3H]-thymidine incorporation (270 +/- 8% of basal), and this response was not additive with that to 100 microM histamine. The incorporation of [3H]-thymidine induced by 100 microM histamine was partially reduced by the PKC inhibitor Ro 31-8220 (57 +/- 7% inhibition at 300 nM) and by the compound PD 098,059 (30 microM, 62 +/- 14% inhibition), an inhibitor of the mitogen-activated kinase (MAPK) kinases MEK1/MEK2. These results show that histamine H1 receptor activation stimulates the proliferation of human astrocytoma U373 MG cells. The action of histamine appears to be partially mediated by PKC stimulation and MAPK activation.

Activation of mitogen-activated protein kinase by muscarinic receptors in astroglial cells: role in DNA synthesis and effect of ethanol

Mitogen-activated protein kinase (MAPK) can be phosphorylated by mitogens binding to G-protein-coupled receptors and is considered a major pathway involved in cell proliferation. In this study, we report on the activation of MAPK by muscarinic acetylcholine receptors in astroglial cells, namely the 1321N1 human astrocytoma cell line, primary rat cortical astrocytes, and fetal human astrocytes. Carbachol caused a rapid and transient phorphorylation of MAPK (ERK1/2) in all cell types, with an increase in MAPK activity, without changing the levels of MAPK proteins. Human astrocytoma cells were used to characterize the effect of carbachol on MAPK. Experiments with M2- and M3-receptor subtype-selective antagonists, and with pertussis toxin, indicated that the M3 subtype is responsible for activating MAPK in glial cells. Pretreatment of cells with the protein kinase C (PKC) inhibitor bisindolylmaleimide I, or downregulation of PKC by 24-h treatment with the phorbol ester TPA inhibited carbachol-induced MAPK activation. Additional experiments with PKC alpha- or PKC epsilon-specific compounds indicated that the epsilon isozyme of PKC is primarily involved in MAPK activation by carbachol. Chelation of calcium also inhibited MAPK activation by carbachol. Two MEK (MAPK kinase) inhibitors inhibited carbachol-induced DNA synthesis but only at concentrations that exceeded those sufficient to block carbachol-induced MAPK activation. Ethanol (< or =200 mM) had no effect on MAPK when present alone and did not affect carbachol-induced MAPK activation under various experimental conditions, although it inhibits carbachol-induced DNA synthesis at low concentrations (10-100 mM). These results suggest that activation of MAPK by carbachol may be necessary but not sufficient for its mitogenic effect in astroglial cells, and that does not represent a target for ethanol-induced inhibition of DNA synthesis elicited by muscarinic receptors.

Inorganic lead stimulates DNA synthesis in human astrocytoma cells: role of protein kinase Calpha

As lead has been shown to activate protein kinase C (PKC), and gliomas are reported to be highly dependent on PKC for their proliferation, this study was undertaken to investigate whether lead may act as a mitogen in human astrocytoma cells, and to determine the role of PKC in this effect. Lead acetate (from 100 nM to 100 microM) induced a concentration- and time-dependent increase in DNA synthesis, as measured by incorporation of [methyl-3H]thymidine into cell DNA, without causing any cytotoxicity. Flow cytometric analysis showed that lead was able to stimulate the cell cycle transition from the G0/G1 phase to the S/G2 phase, resulting in increased percentage of cells in the latter phase. Western blot analyses showed that lead induced translocation of PKCalpha, but not of PKCepsilon or PKCzeta, from the cytosolic to the particulate fraction, with a concomitant increase in PKC enzyme activity. Prolonged exposure to lead caused down-regulation of PKCalpha, but not of PKCepsilon. The effect of lead on DNA synthesis was mediated through PKC as evidenced by the finding that two PKC inhibitors, GF 109203X and staurosporine, as well as down-regulation of PKC through prolonged treatment with 12-O-tetradecanoylphorbol 13-acetate, blocked lead-induced DNA synthesis. Further experiments using a pseudosubstrate peptide targeting classical PKCs and selective down-regulation of specific PKC isoforms indicated that the effect of lead on DNA synthesis was mediated by PKCalpha. Altogether, these results suggest that lead stimulates DNA synthesis in human astrocytoma cells by a mechanism that involves activation of PKCalpha.

Activation of phosphatidylinositol 3 kinase by muscarinic receptors in astrocytoma cells

Stimulation of Gq-coupled acetylcholine muscarinic receptors leads to proliferation of astroglial cells, but the signal transduction pathway(s) that mediate this mitogenic response have not been fully elucidated. In this study, we report on the ability of carbachol to stimulate the phosphorylation of Akt/PKB, an important target of phosphatidylinositol 3 kinase (PI3 kinase) in 1321N1 human astrocytoma cells. Carbachol induced a dose-dependent phosphorylation of Ser473 on Akt, peaking after 15 min. This effect was mediated by activation of the M3 subtype of muscarinic receptors and was inhibited by two PI3 kinase inhibitors. Inhibitors of protein kinase C, mitogen-activated protein kinase and p70S6 kinase, had no effect on carbachol-induced Akt phosphorylation. Carbachol-induced DNA synthesis was strongly inhibited by two PI3 kinase inhibitors, wortmannin and LY294002, suggesting that PI3 kinase activation plays an important role in carbachol-induced proliferation 1321N1 astrocytoma cells.

Pharmacological and functional characterization of muscarinic receptor subtypes in developing oligodendrocytes

This study focused on the molecular and pharmacological characterization of muscarinic acetylcholine receptors expressed by progenitors and differentiated oligodendrocytes. We also analyzed the role of muscarinic receptors in regulating downstream signal transduction pathways and the functional significance of receptor expression in oligodendrocytes. RT-PCR analysis revealed the expression of transcripts for M3, and to a lesser extent M4, followed by M1, M2 and M5 receptor subtypes in both progenitors and differentiated oligodendrocytes. Competition binding experiments using [(3)H]N-methylscopolamine and several antagonists, as well as inhibition of carbachol-mediated phosphoinositide hydrolysis, showed that M3 is the main subtype expressed in these cells. In progenitors the activation of p42/44-mitogen-activated protein kinase (MAPK) and cAMP-response element binding protein (CREB) as well as c-fos mRNA expression were blocked by the M3 relatively selective antagonist, 4-DAMP, and its irreversible analogue, 4-DAMP-mustard. Carbachol increased proliferation of progenitors, an effect prevented by atropine and 4-DAMP, as well as by the MAPK kinase inhibitor PD98059. These results indicate that carbachol modulates oligodendrocyte progenitor proliferation through M3 receptors, involving activation of a MAPK signaling pathway. Receptor density and phosphoinositide hydrolysis are down-regulated during oligodendrocyte differentiation. Functional consequences of these events are a reduction in carbachol-stimulated p42/44(MAPK) and CREB phosphorylation, as well as induction of c-fos.

Adenosine nucleotides acting at the human P2Y1 receptor stimulate mitogen-activated protein kinases and induce apoptosis

For the widely distributed P2Y receptors for nucleotides, the transductional and functional responses downstream of their coupling to G proteins are poorly characterized. Here we describe apoptotic induction and the associated differential stimulation of mitogen-activated protein (MAP) kinase family members by the human P2Y(1) receptor. The potent P2Y(1) receptor agonist, 2-methylthio-ADP (2-MeSADP), stimulated the extracellular-signal regulated kinases (ERK1/2) (EC(50) approximately 5 nm) as well as several, but not all isoforms detected, of the stress-activated protein kinase (SAPK) family. Phospho-isoforms of p38 were unaffected. The induced kinase activity was blocked by the P2Y(1) receptor-selective antagonist, adenosine-2'-phosphate-5'-phosphate, but unaffected by pertussis toxin. In addition, the endogenous ligand ADP, and significantly also 2-MeSATP, induced concentration-dependent phosphorylation changes in the same MAP kinase family members. The sustained activation of ERK1/2 was associated with Elk-1 phosphorylation that was abolished by the MEK1 inhibitor, PD 98059. However, the concomitant transient activation of the SAPKs was not sufficient to induce c-Jun or ATF-2 phosphorylation. The transient phase of the ERK activity was partially inhibited either by the phosphatidylinositol 3-kinase inhibitor, LY 294002, or the PKC inhibitor, Gö 6976. In addition, the Src inhibitor, PP1, or expression of dominant negative Ras also attenuated the transient phase of ERK phosphorylation. In contrast, inhibition of Ras or Src had no effect on the sustained ERK activity, which was critically dependent on phosphatidylinositol 3-kinase. The transient SAPK activity was suppressed by expression of a dominant negative form of MKK4. Furthermore, this kinase-deficient mutant inhibited 2-MeSADP-induced caspase-3 stimulation and the associated decrease in cell number. In conclusion, adenosine di- and triphosphate stimulation of the human P2Y(1) receptor can transiently activate the Ras-ERK cascade via the cooperative effects of phosphatidylinositol 3-kinase, Src and PKC. The sustained ERK stimulation, via a Ras-insensitive pathway, culminates in Elk-1 activation without inducing a proliferation effect. The transient SAPK activity did not evoke transcription factor phosphorylation but was required for the P2Y(1) receptor-mediated apoptotic function.

Analytical cytology: applications to neurotoxicology

This unit describes methods for analyzing the effects of neurotoxicants on cell cycle regulation by dual parameter flow cytometry and on cell signaling by quantifying intracellular calcium concentrations within individual cells by scanning confocal laser microscopy or using the fluorescent calcium probe fluo-3.

Activation of phosphatidylinositol-3 kinase (PI-3K) and extracellular regulated kinases (Erk1/2) is involved in muscarinic receptor-mediated DNA synthesis in neural progenitor cells

Muscarinic acetylcholine receptor (mAChR), a member of the G-protein-coupled receptors (GPCRs) gene superfamily, has been shown to mediate the effects of acetylcholine on differentiation and proliferation in the CNS. However, the mechanism or mechanisms whereby mAChRs regulate cell proliferation remain poorly understood. Here we show that in vitro bFGF-expanded neural progenitor cells dissociated from rat cortical neuroepithelium express muscarinic acetylcholine receptor subtype mRNAs. We demonstrate that stimulation of these mAChRs with carbachol, a muscarinic agonist, activated extracellular-regulated kinases (Erk1/2) and phosphatidylinositol-3 kinase (PI-3K). This, in turn, stimulated DNA synthesis in neural progenitor cells. MEK inhibitor PD98059 and PI-3K inhibitors wortmannin and LY294002 inhibited a carbachol-induced increase in DNA synthesis. These findings indicate that the activation of both PI-3 kinase and MEK signaling pathways via muscarinic receptors is involved in stimulating DNA synthesis in the neural progenitor cells during early neurogenesis.

Intracellular signal transduction pathways as targets for neurotoxicants

The multiple cascades of signal transduction pathways that lead from receptors on the cell membrane to the nucleus, thus translating extracellular signals into changes in gene expression, may represent important targets for neurotoxic compounds. Among the biochemical steps and pathways that have been investigated are the metabolism of cyclic nucleotides, the formation of nitric oxide, the metabolism of membrane phospholipids, the activation of a multitude of protein kinases and the induction of transcription factors. This brief review will focus on the interactions of three known neurotoxicants, lead, ethanol and polychlorinated biphenyls, with signal transduction pathways, particularly the family of protein kinase C isozymes, and discusses how such effects may be involved in their neurotoxicity.

Modulation of DNA synthesis by muscarinic cholinergic receptors

Acetylcholine muscarinic receptors are a family of five G-protein-coupled receptors widely distributed in the central nervous system and in peripheral organs. Activation of certain subtypes of muscarinic receptors (M1, M3, M5) has been found to modulate DNA synthesis in a number of cell types. In several cell types acetylcholine, by activating endogenous or transfected muscarinic receptors, can indeed elicit cell proliferation. In other cell types, however, or under different experimental conditions, activation of muscarinic receptors has no effect, or inhibits DNA synthesis. A large number of intracellular pathways are being investigated to define the mechanisms involved in these effects of muscarinic receptors; these include among others, phospholipase D, protein kinases C and mitogen-activated-protein kinases. The ability of acetylcholine to modulate DNA synthesis through muscarinic receptors may be relevant in the context of brain development and neoplastic growth.

Inhibition of astroglial cell proliferation by alcohols: interference with the protein kinase C-phospholipase D signaling pathway

Ethanol inhibits astroglial cell proliferation, an effect that may contribute to the development of alcoholic embryopathy in humans. In the present study, we investigated inhibitory effects of ethanol and butanol isomers (1-, 2- and t-butanol) on astroglial cell proliferation induced by the strongly mitogenic phorbol ester, 4beta-phorbol-12alpha,13beta-dibutyrate (PDB). 4beta-Phorbol-12alpha,13beta-dibutyrate (PDB) induced a 10-fold increase of [3H] thymidine incorporation in cortical astrocytes prepared from newborn rats (EC50: 70 nM) which was blocked by Ro 31-8220, a cell-permeable protein kinase C (PKC) inhibitor. Ethanol blocked PDB-induced astroglial proliferation in a concentration-dependent manner; significant effects were already seen at 0.1% (v/v). Concomitantly, ethanol caused the formation of phosphatidylethanol (PEth) by phospholipase D (PLD) and reduced PLD-mediated formation of phosphatidic acid (PA). The butanols also inhibited the mitogenic action of phorbol ester; the inhibitory potency of the butanols was 1-butanol > 2-butanol > t-butanol. The same range of potencies was observed for the inhibitory activity of the butanols towards protein kinase C activity measured in vitro. At 0.3% concentration, 1-butanol potently suppressed the PDB-induced formation of phosphatidic acid while 2- and t-butanol were less active. Taken together, our results suggest that ethanol and 1-butanol exert a specific inhibitory effect on PKC-dependent astroglial cell proliferation by synergistically inhibiting PKC activity and the PLD signaling pathway.

Possible role of protein kinase C zeta in muscarinic receptor-induced proliferation of astrocytoma cells

Recent studies have shown that protein kinase C zeta (PKC zeta) is part of a pathway that plays a key role in a wide range of physiological processes including mitogenesis, cell survival, and transcriptional regulation. Most studies on PKC zeta have been done by stimulating cells with tyrosine kinase receptor agonists, or by transfecting the cells with either constitutively active PKC zeta or negative mutants of PKC zeta. Less is known about the ability of endogenous G-protein-coupled receptors to generate a mitogenic signal through activation of endogenous PKC zeta. In the present paper, we showed that in 123-1N1 human astrocytoma cells, which express the G-protein-coupled M2, M3, and M5 muscarinic receptors, PKC zeta is activated by carbachol in a concentration-dependent manner, resulting in the translocation of PKC zeta from the cytoplasm to granules in the perinuclear region. The effect of carbachol was long-lasting (up to 24 hr) and appeared to be mediated by activation of M3 muscarinic receptors. A selective PKC zeta inhibitor peptide (peptide Z) inhibited PKC zeta translocation as well as carbachol-induced DNA synthesis. Inhibition of both phosphatidylinositol 3-kinase and phospholipase D decreased carbachol-induced [(3)H]thymidine incorporation and blocked carbachol-induced PKC zeta translocation, suggesting an involvement of both pathways in these effects.

Presence of aquaporin-4 and muscarinic receptors in astrocytes and ependymal cells in rat brain: a clue to a common function?

Using combined double immunofluorescence and laser confocal microscopy, we studied the common cellular localization of cholinergic muscarinic receptors (mAChRs) and aquaporin-4 water channels (AQP4) in the cortex, the corpus callosum and in ependymal cells of the rat brain. In the cortex, AQP4 staining was restricted to the perivascular end-feet of astrocytes. It was more widely distributed on the astrocytes of the corpus callosum. On astrocytes, mAChRs were often present in regions immunoreactive to AQP4. Ependymal cells bordering the third ventricle were also stained by both antibodies. The double staining of mAChRs with AQP4 on two different cell-types might indicate that further interactions exist which may be important in the regulation of water and electrolyte movements in the brain.

Effect of ethanol on muscarinic receptor-induced calcium responses in astroglia

The effects of ethanol on muscarinic receptor-mediated calcium responses were investigated in individual primary rat astrocytes and human 132 1N1 astrocytoma cells using indo-1/AM and image cytometry. After a 30-min incubation, carbachol-induced calcium responses were inhibited only at 100 or 250 mM ethanol. The effects of ethanol were more pronounced and occurred at lower concentrations with longer exposures, with significant inhibition seen at 10 mM following a 24-hr incubation. Thapsigargin- and glutamate-induced responses were unaffected by ethanol, indicating some selectivity in this inhibition. Upon removal of ethanol, inhibition of calcium responses persisted for up to 6-12 hr, with carbachol responses returning to control levels by 24 hr after washout. Ethanol exposure did not affect muscarinic-receptor binding in astrocytoma cells, but inhibited carbachol-induced IP(3) formation. Inhibition of (3)H-thymidine incorporation by ethanol also persisted upon removal of the alcohol, with a time-dependency similar to that of the calcium responses. These results indicate that ethanol inhibits muscarinic receptor-induced calcium responses in astroglia in a concentration- and duration-dependent manner. They also show that co-incubation with ethanol is not necessary for this effect, suggesting that long-term exposure to ethanol may modify, in a reversible manner, the coupling of muscarinic receptors with its effector. This effect of ethanol may play a role in ethanol's inhibition of carbachol-induced thymidine incorporation.

Acetylcholine stimulates cortical precursor cell proliferation in vitro via muscarinic receptor activation and MAP kinase phosphorylation

Increasing evidence has shown that some neurotransmitters act as growth-regulatory signals during brain development. Here we report a role for the classical neurotransmitter acetylcholine (ACh) to stimulate proliferation of neural stem cells and stem cell-derived progenitor cells during neural cell lineage progression in vitro. Neuroepithelial cells in the ventricular zone of the embryonic rat cortex were found to express the m2 subtype of the muscarinic receptor. Neural precursor cells dissociated from the embryonic rat cortical neuroepithelium were expanded in culture with basic fibroblast growth factor (bFGF). reverse transcriptase-polymerase chain reaction (RT-PCR) revealed the presence of m2, m3 and m4 muscarinic receptor subtype transcripts, while immunocytochemistry demonstrated m2 protein. ACh and carbachol induced an increase in cytosolic Ca2+ and membrane currents in proliferating (BrdU+) cells, both of which were abolished by atropine. Exposure of bFGF-deprived precursor cells to muscarinic agonists not only increased both cell number and DNA synthesis, but also enhanced differentiation of neurons. These effects were blocked by atropine, indicating the involvement of muscarinic ACh receptors. The growth-stimulating effects were also antagonized by a panel of inhibitors of second messengers, including 1,2-bis-(O-aminophenoxy)-ethane-N,N,N', N'-tetraacetic acid (BAPTA-AM) to chelate cytosolic Ca2+, EGTA to complex extracellular Ca2+, pertussis toxin, which uncouples certain G-proteins, the protein kinase C inhibitor H7 and the mitogen-activated protein kinase (MAPK) inhibitor PD98059. Muscarinic agonists activated MAPK, which was significantly inhibited by atropine and the same panel of inhibitors. Thus, muscarinic receptors expressed by neural precursors transduce a growth-regulatory signal during neurogenesis via pathways involving pertussis toxin-sensitive G-proteins, Ca2+ signalling, protein kinase C activation, MAPK phosphorylation and DNA synthesis.

Exposure to 60-Hz magnetic fields and proliferation of human astrocytoma cells in vitro

Epidemiological studies have suggested that exposure to electric and magnetic fields (EMF) may be associated with an increased incidence of brain tumors, most notably astrocytomas. However, potential cellular or molecular mechanisms involved in these effects of EMF are not known. In this study we investigated whether exposure to 60-Hz sinusoidal magnetic fields (0.3-1.2 G for 3-72 h) would cause proliferation of human astrocytoma cells. Sixty-Hertz magnetic fields (MF) caused a time- and dose-dependent increase in proliferation of astrocytoma cells, measured by (3)H-thymidine incorporation and by flow cytometry, and strongly potentiated the effect of two agonists (the muscarinic agonist carbachol and the phorbol ester PMA). However, MF had no effect on DNA synthesis of rat cortical astrocytes, i.e., of similar, nontransformed cells. To determine the amount of heating induced by MF, temperatures were also recorded in the medium. Both 1.2 G MF and a sham exposure caused a 0.7 degrees C temperature increase in the medium; however, (3)H-thymidine incorporation induced by sham exposure was significantly less than that caused by MF. GF 109203X, a rather specific protein kinase C (PKC) inhibitor, and down-regulation of PKC inhibited the effect of MF on basal and on agonist-stimulated (3)H-thymidine incorporation. These data indicate that MF can increase the proliferation of human astrocytoma cells and strongly potentiate the effects of two agonists. These findings may provide a biological basis for the observed epidemiological associations between MF exposure and brain tumors.

Functional acetylcholine muscarinic receptor subtypes in human brain microcirculation: identification and cellular localization

Acetylcholine is an important regulator of local cerebral blood flow. There is, however, limited information available on the possible sites of action of this neurotransmitter on brain intraparenchymal microvessels. In this study, a combination of molecular and functional approaches was used to identify which of the five muscarinic acetylcholine receptors (mAChR) are present in human brain microvessels and their intimately associated astroglial cells. Microvessel and capillary fractions isolated from human cerebral cortex were found by reverse transcriptase-polymerase chain reaction to express m2, m3, and, occasionally, m1 and m5 receptor subtypes. To localize these receptors to a specific cellular compartment of the vessel wall, cultures of human brain microvascular endothelial and smooth muscle cells were used, together with cultured human brain astrocytes. Endothelial cells invariably expressed m2 and m5 receptors, and occasionally the m1 receptor; smooth muscle cells exhibited messages for all except the m4 mAChR subtypes, whereas messages for all five muscarinic receptors were identified in astrocytes. In all three cell types studied, acetylcholine induced a pirenzepine-sensitive increase (62% to 176%, P<0.05 to 0.01) in inositol trisphosphate, suggesting functional coupling of m1, m3, or m5 mAChR to a phospholipase C signaling cascade. Similarly, coupling of m2 or m4 mAChR to adenylate cyclase inhibition in endothelial cells and astrocytes, but not in smooth muscle cells, was demonstrated by the ability of carbachol to significantly reduce (44% to 50%, P<0.05 to 0.01) the forskolin-stimulated increase in cAMP levels. This effect was reversed by the mAChR antagonist AFDX 384. The results indicate that microvessels are able to respond to neurally released acetylcholine and that mAChR, distributed in different vascular and astroglial compartments, could regulate cortical perfusion and, possibly, blood-brain barrier permeability, functions that could become jeopardized in neurodegenerative disorders such as Alzheimer's disease.

Adrenergic modulation of astroglial phospholipase D activity and cell proliferation

As phospholipase D (PLD) activation has been associated with mitogenic signalling in several cell types, we tested an association between adrenergic activation of PLD and cellular proliferation in primary cultures of rat cortical astrocytes. In 2-week old cultures, PLD activation by noradrenaline (EC50: 0.49 microM) was inhibited by prazosin, a specific antagonist at alpha1-adrenergic receptors (IC50: 0.23 microM). Adrenergic PLD activation was not affected by genistein, an inhibitor of tyrosine kinases, or by Ro 31-8220, an inhibitor of protein kinase C (PKC), but was dose-dependently depressed in the presence of brefeldin A (1-100 microg/ml), an inhibitor of ARF activation. In experiments measuring cell proliferation, noradrenaline potently (EC50: 20 nM) reduced [3H]thymidine incorporation to 20-30% of basal values. This action was mimicked by the beta-specific agonist isoprenaline and was inhibited by the beta-antagonist propranolol in a concentration-dependent manner. The alpha1-adrenergic agonists, phenylephrine and methoxamine, also reduced DNA synthesis. The adrenergic inhibition of astroglial DNA synthesis was not reduced, but further potentiated in the presence of brefeldin A, ethanol, and 1- and 2-butanol; 1-butanol, a substrate of PLD, was equally effective as 2-butanol, a non-substrate. We conclude that adrenergic PLD activation in astrocytes is not involved in mitogenic signalling. The involvement of ARF in the activation of PLD via alpha1-adrenoceptors indicates a role in protein trafficking.

Muscarinic cholinergic receptor signal transduction as a potential target for the developmental neurotoxicity of ethanol

Central nervous system dysfunctions (most notably mental retardation and microcephaly) are among the most significant effects of in utero exposure to ethanol. Ethanol has been shown to cause alterations of both neuronal and glial cells, including cell loss, and changes in their migration and maturation. Here, we propose that one of the potential targets for the developmental neurotoxicity of ethanol may be represented by the signal transduction systems activated by cholinergic muscarinic receptors. Ethanol has been shown to inhibit second messenger systems activated by various G-protein-coupled receptors, including certain subtypes of muscarinic receptors. Although the roles of muscarinic receptors in brain development have not been fully elucidated, two potentially relevant effects have been discovered in the past few years. By activating muscarinic receptors coupled to phospholipid metabolism, acetylcholine can induce proliferation of glial cells, and act as a trophic factor in developing neurons by preventing apoptotic cell death. Ethanol has been shown to inhibit both actions of acetylcholine in vitro. These effects of ethanol may lead to a decreased number of glial cells and to a loss of neurons, which have been observed following in vivo alcohol exposure. In turn, these may be the basis of microencephaly and cognitive disturbances in children diagnosed with Fetal Alcohol Syndrome.

Effects of ethanol on calcium homeostasis in the nervous system: implications for astrocytes

Ethanol is a major health concern, with neurotoxicity occurring after both in utero exposure and adult alcohol abuse. Despite a large amount of research, the mechanism(s) underlying the neurotoxicity of ethanol remain unknown. One of the cellular aspects that has been investigated in relationship to the neuroteratogenicity and neurotoxicity of ethanol is the maintenance of calcium homeostasis. Studies in neuronal cells and other cells have shown that ethanol can alter intracellular calcium levels and affect voltage and receptor-operated calcium channels, as well as G protein-mediated calcium responses. Despite increasing evidence of the important roles of glial cells in the nervous systems, few studies exist on the potential effects of ethanol on calcium homeostasis in these cells. This brief review discusses a number of reported effects of alcohol on calcium responses that may be relevant to astrocytes' functions.

The effect of pregnancy and contractile activity on bladder muscarinic receptor subtypes

In the rabbit bladder, pregnancy and prolonged bladder contractions decrease both muscarinic receptor density and contractile response, whereas newborns show enhanced muscarinic contractile response. Although the M(2) receptor predominates in rabbit bladder, we and others have shown that the affinity of a series of subtype selective muscarinic antagonists for inhibition of muscarinic agonist-induced contractions is most consistent with the pharmacologically defined M(3) receptor directly mediating smooth muscle contraction. Bladders from fetal rabbits, gravid rabbits, and male rabbits exposed to 4 hr of induced spontaneous contractions were used to determine whether changes in receptor density and contractility are due to a selective decrease in either the M(2) or M(3) muscarinic receptor subtype. To determine organ specificity, the heart and uterus were also studied. Gravid rabbits of 3 weeks' gestation and their fetal rabbits were studied. In male rabbits, bladder contractions were induced for 4 hr by ligating the catheterized penis at its base. Muscarinic receptor density and subtype distribution were determined by radioligand binding and immunoprecipitation. Receptor density was 24% lower in gravid bladder body, unchanged in gravid bladder base, 54% lower in gravid uterus, 115% higher in fetal bladders, and 34% lower after induced bladder contractions. Immunoprecipitation showed greater M(2) receptors than M(3) in all tissues studied, whereas M(l) and M(4) receptors were undetectable. There was no difference from control in the ratio of M(2) to M(3) receptor in any tissues except that a greater proportion of M(3) receptors was found in male vs. female bladders. Changes in contractile response to cholinergic stimulation in the gravid, fetal, and experimental detrusor instability model are associated with changes in total receptor density and not solely with changes in the M(3) receptor subtype that mediates bladder smooth muscle contraction. Neurourol. Urodynam. 18:511-520, 1999.

Developmental and aging aspects of the cholinergic innervation of the olfactory bulb

The olfactory bulb is a limbic paleocortex which receives monosynaptic sensory afferents from the olfactory mucosa, and a strong direct cholinergic input from the basal forebrain. This review focuses on the rat olfactory bulb as a suitable model to study cholinergic involvements in cortical processing, during development, adulthood and aging. Anatomical and biochemical data show that cholinergic influences upon the bulbar neuronal network are exerted through several types of target cells and receptors (muscarinic and nicotinic). Functional data indicate that cholinergic afferents to the olfactory bulb are involved in local events related to olfactory learning. Neurodegenerative disorders such as Alzheimer's disease involve early olfactory deficits and typical histopathological lesions in the olfactory bulb. In summary, with its exclusively extrinsic cholinergic innervation and direct sensory input, the rat olfactory bulb offers the opportunity to study the cellular and molecular mechanisms of cholinergic influences on cortical processing, in both normal and pathological conditions.

Cell proliferation in human prostatic smooth muscle cells involves the modulation of protein kinase C isozymes

We have examined the role of protein kinase C in the regulation of foetal-calf serum-stimulated cell proliferation in human prostatic smooth muscle cells. The data showed that the proliferative effect to foetal-calf serum (10%, v/v) was partially inhibited by 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo (2,3-a) pyrrolo (3,4-c)-carbazole (Go-6976), a selective Ca2+-dependent protein kinase C inhibitor, suggesting that Ca2+-dependent protein kinase C isozymes might play roles in this proliferative regulation. Additionally, foetal-calf serum caused a significant translocation of protein kinase C-betaII and -epsilon from a cytosolic to a membrane distribution. These findings combined with the aforementioned functional experiments suggest that foetal-calf serum-stimulated cell proliferation might involve the activation of protein kinase C-betaII in human prostatic smooth muscle cells; however, the role of protein kinase C-epsilon in mediating cellular functions other than cell proliferation remains further investigation in these cells.

The role of protein kinase C alpha and epsilon isozymes in DNA synthesis induced by muscarinic receptors in a glial cell line

Acetylcholine has been shown to induce proliferation of human astrocytoma cells by activating muscarinic receptors, particularly the m3 subtype. In the present study the role of protein kinase C in DNA synthesis induced by carbachol has been investigated. Carbachol-induced [methyl-3H]thymidine incorporation was inhibited by the protein kinase C inhibitors GF 109203X and staurosporine. However, carbachol-induced DNA synthesis was only partially reduced by protein kinase C down-regulation by phorbol 12-myristate 13-acetate (PMA), and maximal concentrations of carbachol and PMA had an additive effect on [methyl-3H]thymidine incorporation. Exposure for 24 h to maximally effective concentrations of carbachol did not induce down-regulation of protein kinase C alpha, and caused a small but significant down-regulation of protein kinase C epsilon; cells exposed for 24 h to carbachol were still able to respond with protein kinase C translocation to PMA stimulation. Carbachol caused a significant increase of phorbol ester binding, but did not stimulate protein kinase C alpha translocation, while it caused a short-lasting translocation of protein kinase C epsilon; however, protein kinase C epsilon translocation was not correlated with the time-course of carbachol-induced increase in [methyl-3H]thymidine incorporation. On the other hand, the time-course of translocation/down-regulation of protein kinase C alpha and protein kinase C epsilon induced by PMA was in good correlation with the time-course of PMA-induced [methyl-3H]thymidine incorporation. These results suggest that protein kinase C alpha may not be involved in DNA synthesis induced by muscarinic receptors stimulation in 132-1N1 astrocytoma cells, while protein kinase C epsilon appears to play a role in the initial exit from G0/G1 phase, though it cannot be considered the major determinant for sustained proliferation.

Signal transduction in environmental neurotoxicity

Signal transduction is the process by which specific information is transferred from the cell surface to the cytosol and ultimately to the nucleus, leading to changes in gene expression. Since these chains of biochemical and molecular steps control the normal function of each cell, disruption of these processes would have a significant impact on cell physiology. Some of the major signal transduction pathways are briefly reviewed. The interactions of four chemicals (lead, ethanol, polychlorinated biphenyls, and trimethyltin) with different cell signaling systems, particularly the phospholipid hydrolysis/protein kinase C pathway, are discussed. The possible causal relationship of such cellular and molecular interactions with known signs and symptoms of neurotoxicity are highlighted.

Effect of chronic nitric oxide deficiency on angiotensin II-induced hypertrophy of rat basilar artery

BACKGROUND AND PURPOSE

Although in vitro studies suggest that nitric oxide has an inhibitory effect on cellular proliferation and migration, in vivo experiments failed to support this conclusion. The present study was designed to determine the effect of endogenous nitric oxide on angiotensin II-induced hypertrophy of small arteries in vivo.

METHODS

Angiotensin II (200 ng/kg per minute), alone or in combination with N(omega)-nitro-L-arginine methyl ester (L-NAME) (60 mg/kg per day), was administered for 2 weeks in normotensive rats. Basilar arteries were harvested, and their geometry was determined in perfused and pressurized conditions.

RESULTS

Angiotensin II increased media thickness, media-lumen ratio, and cross-sectional area of the arteries, confirming the presence of hypertrophic remodeling. The concomitant administration of L-NAME, an inhibitor of nitric oxide synthesis, prevented vascular hypertrophy. The remodeling of the basilar artery geometry in the combined treatment was of eutrophic nature, similar to that observed with the administration of L-NAME alone.

CONCLUSIONS

Our results suggest that endogenous nitric oxide does not inhibit angiotensin II-induced vascular hypertrophy in vivo. Nitric oxide may even be a necessary factor for hypertrophy to develop.

Regulation of kappa-opioid receptor mRNA level by cyclic AMP and growth factors in cultured rat glial cells

The mRNA of the kappa-opioid receptor (KOR) has been found recently in cultured astrocytes and in microglia. By using RT-PCR and Southern hybridization, we confirmed these observations and, in addition, we observed that KOR mRNA was expressed in oligodendrocytes and in the precursors of astrocytes and oligodendrocytes. KOR mRNA level was the highest in the immature astrocytes and decreased with their maturation. Very few data are available on the regulation of KOR level by extracellular signals. Therefore, we examined the effect of three growth factors known to be present in the adult brain, basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF-BB) and leukemia inhibitory factor (LIF) and of two cyclic AMP (cAMP) generating systems, the cAMP analog, 8-(4-chlorophenylthio)-cAMP and forskolin, on this level. It was found that in astrocytes, KOR mRNA level decreased dramatically under the effect of cAMP and less under the effect of bFGF while it did not change significantly after LIF treatment. In oligodendrocytes, it also decreased with cAMP, but increased under the effect of bFGF and PDGF-BB. In microglia, a decrease was observed with cAMP and lipopolysaccharides (LPS), the most used activators of macrophages. These results shed new evidence on the expression of opioid receptor mRNA in the glial cells of the rat CNS. The regulation of KOR mRNA level under the effect of extracellular signals suggests that opioids take part in dynamic processes in glial cells, possibly related to glial-neuron communication.

Heterogeneity of astrocyte resting membrane potentials and intercellular coupling revealed by whole-cell and gramicidin-perforated patch recordings from cultured neocortical and hippocampal slice astrocytes

Astrocytes are thought to regulate the extracellular potassium concentration by mechanisms involving both voltage-dependent and transport-mediated ion fluxes combined with intercellular communication via gap junctions. Mechanisms regulating resting membrane potential (RMP) play a fundamental role in determining glial contribution to buffering of extracellular potassium and uptake of potentially toxic neurotransmitters. We have investigated the passive electrophysiological properties of cultured neocortical astrocytes and astrocytes recorded in hippocampal slices from 18-25 d postnatal rats. These experiments revealed a wide range of astrocyte RMPs that were independent of developmental factors, length of culturing, cellular morphology, the electrophysiological techniques used (whole-cell vs perforated recording), cell-specific expression of Na+/2HCO3- co-transporters, or voltage-dependent Na+ channels. Exposure of cultured astrocytes to differentiation-inducing factors (such as cAMP) or inhibition of proliferation (by serum deprivation) did not significantly influence RMP. Expression of ATP-sensitive potassium channels was absent in these glia; thus, K(ATP)-related mechanisms did not contribute to cell resting potential. In both cultured and slice astrocytes, spontaneous electrophysiological changes were commonly observed. These reversible events, which resulted in differential sensitivity to potassium channel blockers (cesium and barium) and sudden current-voltage profile changes, were attributable to dynamic changes in cell-to-cell coupling, as confirmed by recordings from isolated pairs of cells. We conclude that the heterogeneity of astrocytic RMP and intercellular coupling both in culture and in situ are intrinsic properties of glia that may contribute to transcellular transport of potassium. We propose a model in which spatial buffering may be facilitated by heterogeneous mechanisms controlling glial RMP in combination with dynamic changes in intercellular coupling.

Xestospongins: potent membrane permeable blockers of the inositol 1,4,5-trisphosphate receptor

Xestospongins (Xe's) A, C, D, araguspongine B, and demethylxestospongin B, a group of macrocyclic bis-1-oxaquinolizidines isolated from the Australian sponge, Xestospongia species, are shown to be potent blockers of IP3-mediated Ca2+ release from endoplasmic reticulum vesicles of rabbit cerebellum. XeC blocks IP3-induced Ca2+ release (IC50 = 358 nM) without interacting with the IP3-binding site, suggesting a mechanism that is independent of the IP3 effector site. Analysis of Pheochromocytoma cells and primary astrocytes loaded with Ca2+-sensitive dye reveals that XeC selectively blocks bradykinin- and carbamylcholine-induced Ca2+ efflux from endoplasmic reticulum stores. Xe's represent a new class of potent, membrane permeable IP3 receptor blockers exhibiting a high selectivity over ryanodine receptors. Xe's are a valuable tool for investigating the structure and function of IP3 receptors and Ca2+ signaling in neuronal and nonneuronal cells.

Coupling of the thrombin receptor to G12 may account for selective effects of thrombin on gene expression and DNA synthesis in 1321N1 astrocytoma cells

In 1321N1 astrocytoma cells, thrombin, but not carbachol, induces AP-1-mediated gene expression and DNA synthesis. To understand the divergent effects of these G protein-coupled receptor agonists on cellular responses, we examined Gq-dependent signaling events induced by thrombin receptor and muscarinic acetylcholine receptor stimulation. Thrombin and carbachol induce comparable changes in phosphoinositide and phosphatidylcholine hydrolysis, mobilization of intracellular Ca2+, diglyceride generation, and redistribution of protein kinase C; thus, activation of these Gq-signaling pathways appears to be insufficient for gene expression and mitogenesis. Thrombin increases Ras and mitogen-activated protein kinase activation to a greater extent than carbachol in 1321N1 cells. The effects of thrombin are not mediated through Gi, since ribosylation of Gi/Go proteins by pertussis toxin does not prevent thrombin-induced gene expression or thrombin-stimulated DNA synthesis. We recently reported that the pertussis toxin-insensitive G12 protein is required for thrombin-induced DNA synthesis. We demonstrate here, using transfection of receptors and G proteins in COS-7 cells, that G alpha 12 selectively couples the thrombin receptor to AP-1-mediated gene expression. This does not appear to result from increased mitogen-activated protein kinase activity but may reflect activation of a tyrosine kinase pathway. We suggest that preferential coupling of the thrombin receptor to G12 accounts for the selective ability of thrombin to stimulate Ras, mitogen-activated protein kinase, gene expression, and mitogenesis in 1321N1 cells.