Maitotoxin induces a calcium-dependent membrane depolarization in GH4C1 pituitary cells via activation of type L voltage-dependent calcium channels

Phytoplankton Toxins and Their Potential Therapeutic Applications: A Journey toward the Quest for Potent Pharmaceuticals

Phytoplankton are prominent organisms that contain numerous bioactive substances and secondary metabolites, including toxins, which can be valuable to pharmaceutical, nutraceutical, and biotechnological industries. Studies on toxins produced by phytoplankton such as cyanobacteria, diatoms, and dinoflagellates have become more prevalent in recent years and have sparked much interest in this field of research. Because of their richness and complexity, they have great potential as medicinal remedies and biological exploratory probes. Unfortunately, such toxins are still at the preclinical and clinical stages of development. Phytoplankton toxins are harmful to other organisms and are hazardous to animals and human health. However, they may be effective as therapeutic pharmacological agents for numerous disorders, including dyslipidemia, obesity, cancer, diabetes, and hypertension. In this review, we have focused on the properties of different toxins produced by phytoplankton, as well as their beneficial effects and potential biomedical applications. The anticancer properties exhibited by phytoplankton toxins are mainly attributed to their apoptotic effects. As a result, phytoplankton toxins are a promising strategy for avoiding postponement or cancer treatment. Moreover, they also displayed promising applications in other ailments and diseases such as Alzheimer’s disease, diabetes, AIDS, fungal, bacterial, schizophrenia, inflammation, allergy, osteoporosis, asthma, and pain. Preclinical and clinical applications of phytoplankton toxins, as well as future directions of their enhanced nano-formulations for improved clinical efficacy, have also been reviewed.

Biotechnological and Pharmacological Applications of Biotoxins and Other Bioactive Molecules from Dinoflagellates

The long-lasting interest in bioactive molecules (namely toxins) produced by (microalga) dinoflagellates has risen in recent years. Exhibiting wide diversity and complexity, said compounds are well-recognized for their biological features, with great potential for use as pharmaceutical therapies and biological research probes. Unfortunately, provision of those compounds is still far from sufficient, especially in view of an increasing demand for preclinical testing. Despite the difficulties to establish dinoflagellate cultures and obtain reasonable productivities of such compounds, intensive research has permitted a number of advances in the field. This paper accordingly reviews the characteristics of some of the most important biotoxins (and other bioactive substances) produced by dinoflagellates. It also presents and discusses (to some length) the main advances pertaining to dinoflagellate production, from bench to large scale-with an emphasis on material published since the latest review available on the subject. Such advances encompass improvements in nutrient formulation and light supply as major operational conditions; they have permitted adaptation of classical designs, and aided the development of novel configurations for dinoflagellate growth-even though shearing-related issues remain a major challenge.

Identification by automated screening of a small molecule that selectively eliminates neural stem cells derived from hESCs but not dopamine neurons

Background

We have previously described fundamental differences in the biology of stem cells as compared to other dividing cell populations. We reasoned therefore that a differential screen using US Food and Drug Administration (FDA)-approved compounds may identify either selective survival factors or specific toxins and may be useful for the therapeutically-driven manufacturing of cells in vitro and possibly in vivo.

Methodology/Principal Findings

In this study we report on optimized methods for feeder-free culture of hESCs and hESC-derived neural stem cells (NSCs) to facilitate automated screening. We show that we are able to measure ATP as an indicator of metabolic activity in an automated screening assay. With this optimized platform we screened a collection of FDA-approved drugs to identify compounds that have differential toxicity to hESCs and their neural derivatives. Nine compounds were identified to be specifically toxic for NSCs to a greater extent than for hESCs. Six of these initial hits were retested and verified by large-scale cell culture to determine dose-responsive NSC toxicity. One of the compounds retested, amiodarone HCL, was further tested for possible effects on postmitotic neurons, a likely target for transplant therapy. Amiodarone HCL was found to be selectively toxic to NSCs but not to differentiated neurons or glial cells. Treated and untreated NSCs and neurons were then interrogated with global gene expression analysis to explore the mechanisms of action of amiodarone HCl. The gene expression analysis suggests that activation of cell-type specific cationic channels may underlie the toxicity of the drug.

Conclusions/Significance

In conclusion, we have developed a screening strategy that allows us to rapidly identify clinically approved drugs for use in a Chemistry, Manufacture and Control protocol that can be safely used to deplete unwanted contaminating precursor cells from a differentiated cell product. Our results also suggest that such a strategy is rich in the potential of identifying lineage specific reagents and provides additional evidence for the utility of stem cells in screening and discovery paradigms.

Actin cytoskeleton of rabbit intestinal cells is a target for potent marine phycotoxins

Biotoxins produced by harmful marine microalgae (phycotoxins) can be accumulated into seafood, representing a great risk for public health. Some of these phycotoxins are responsible for a variety of gastrointestinal disturbances; however, the relationship between their mechanism of action and toxicity in intestinal cells is still unknown. The actin cytoskeleton is an important and highly complicated structure in intestinal cells, and on that basis our aim has been to investigate the effect of representative phycotoxins on the enterocyte cytoskeleton. We have quantified for the first time the loss of enterocyte microfilament network induced by each toxin and recorded fluorescence images using a laser-scanning cytometer and confocal microscopy. Our data show that pectenotoxin-6, maitotoxin, palytoxin and ostreocin-D cause a significant reduction in the actin cytoskeleton. In addition, we found that the potency of maitotoxin, palytoxin and ostreocin-D to damage filamentous actin is related to Ca2+ influx in enterocytes. Those results identify the cytoskeleton as an early target for the toxic effect of those toxins.

A toxic extract of the marine phytoflagellate Prymnesium parvum induces calcium-dependent release of glutamate from rat brain synaptosomes

Blooms of the marine phytoflagellate Prymnesium parvum produced mass mortality of fish in Norway and many other parts of the world. The effects of a purified algae extract of P. parvum on transmitter release from rat brain synaptosomes were studied to characterize its toxic action. Synaptosomes are detached nerve terminals and represent a simple system that has retained the machinery for uptake, synthesis, storage, and release of neurotransmitters. A crude methanol extract of P. parvum was purified by reverse-phase column for fast protein liquid chromatography (FPLC). The purified extract stimulated Ca2+-dependent spontaneous release of glutamate in a concentration-dependent manner. The release was increased by addition of extracellular Ca2+. The release of glutamate was suppressed by the Ca2+-channel blockers flunarizine (10 microM), diltiazem (10 microM), and verapamil (10 microM). The stimulation of release of glutamate from rat brain synaptosomes induced by the toxin may be due to an ionophorelike property of the algae extract such as previously reported for the potent algal toxin maitotoxin. At high concentrations the toxin primarily acts as a powerful lytic agent.

Pharmacological modulators of voltage-gated calcium channels and their therapeutical application

Calcium channels (CCs) play an important role in the transduction of action potential to the cytosol. An influx of Ca(2+) is essential for muscle contraction, neurotransmitter, and hormonal release. Level of cytosolic Ca(2+) controls activities of many enzymes and regulatory proteins. Voltage-gated calcium channels (VGCCs) serve as sensors for membrane depolarization. Blood pressure reduction is due to relaxation of actomyosine filaments in vascular smooth muscles. Calcium channel blockers (CCBs) are traditionally used for treatment of cardiovascular diseases. Neurotransmitter release from presynaptic neurons is triggered by Ca(2+) influx. Blockers of neuronal CCs may be applied for pain treatment. Overload of neurons by Ca(2+) is toxic. CCBs may be applied for prevention of some neurodegenerative disorders.

Reporter gene assays for algal-derived toxins

We have modified the cell-based directed cytotoxicity assay for sodium channel and calcium channel active phycotoxins using a c-fos-luciferase reporter gene construct. In this report we describe the conceptual basis to the development of reporter gene assays for algal-derived toxins and summarize both published and unpublished data using this method. N2A mouse neuroblastoma cells, which express voltage-dependent sodium channels, were stably transfected with the reporter gene c-fos-luc, which contains the firefly luciferase gene under the transcriptional regulation of the human c-fos response element. The characteristics of the N2A reporter gene assay were determined by dose response with brevetoxin and ciguatoxin. Brevetoxin-1 and ciguatoxin-1 induced c-fos-luc with an EC50 of 4.6 and 3.0 ng ml(-1), respectively. Saxitoxin caused a concentration-dependent inhibition of brevetoxin-1 induction of c-fos-luc with an EC50 of 3.5 ng ml(-1). GH4C1 rat pituitary cells, which lack voltage-dependent sodium channels but express voltage-dependent calcium channels, were also stably transfected with the c-fos-luc. GH4C1 cells expressing c-fos-luciferase were responsive to maitotoxin (1 ng ml(-1)) and a putative toxin produced by Pfiesteria piscicida. Although reporter gene assays are not designed to replace existing detection methods used to measure toxin activity in seafood, they do provide a valuable means to screen algal cultures for toxin activity, to conduct assay-guided fractionation and to characterize pharmacologic properties of algal toxins.

Maitotoxin-induced calcium entry in human lymphocytes: modulation by yessotoxin, Ca(2+) channel blockers and kinases

We have studied the effect of the ciguatera-related toxin maitotoxin (MTX) on the cytosolic free calcium concentration ([Ca(2+)]i) of human peripheral blood lymphocytes loaded with the fluorescent probe Fura2 and the regulation of MTX action by different drugs known to interfere in cellular Ca(2+) signalling mechanisms and by the marine phycotoxin yessotoxin (YTX). MTX produced a concentration-dependent elevation of [Ca(2+)]i in a Ca(2+)-containing medium. This effect was stimulated by pretreatment with YTX 1 microM and NiCl(2) 15 microM. The voltage-independent Ca(2+) channel antagonist 1-[beta-[3-(4-methoxyphenyl)propoxyl]-4-methoxyphenyl]-1H-imidazole hydrochloride (SKF96365) blocked the MTX-induced [Ca(2+)]i elevation, while the L-type channel blocker nifedipine had no effect. Pretreatment with NiCl(2) or nifedipine did not modify YTX-induced potentiation of MTX effect, and SKF96365-induced inhibition was reduced in the presence of YTX, which suggest different pathways to act on [Ca(2+)]i. Preincubation with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide.2HCl (H-89) or genistein (10 microM) also had no effect on the MTX-induced [Ca(2+)]i increment. In contrast, the PKC inhibitor bisindolilmaleimide I (GF109203X 1 microM) potentiated the MTX effect, whereas phosphatidylinositol (PI) 3-kinase inhibition with wortmannin (10 nM) reduced the MTX-elicited Ca(2+) entry. In summary, MTX produced Ca(2+) influx into human lymphocytes through a SKF96365-sensitive, nifedipine-insensitive pathway. The MTX-induced [Ca(2+)]i elevation was stimulated by the marine toxin YTX through a mechanism insensitive to SKF96365, nifedipine or NiCl(2). It was also stimulated by the divalent cation Ni(2+) and PKC inhibition and was partially inhibited by PI 3-kinase inhibition.

Effects of simple aromatic compounds and flavonoids on Ca2+ fluxes in rat pituitary GH(4)C(1) cells

The biological activity of phenolic compounds from plants is well documented in vitro, but little is known about the possible effect of simple aromatic compounds and flavonoids on voltage-operated Ca2+ channels (VOCCs). In pituitary cells, several intracellular pathways may regulate the activity of VOCCs. In this study, we investigated the effect of nine phenylpropanes and metanes, and 20 flavonoids on high K(+)-induced 45Ca2+ entry in clonal rat pituitary GH(4)C(1) cells. At the highest dose tested (20 microg/ml), flavone (a flavone) inhibited 45Ca2+ entry by 63.5%, naringenin (a flavanone) by 56.3% and genistein (an isoflavone) by 54.6%. The phenylmetane derivative octyl gallate was the most potent compound tested, with an IC(50) value of 15.0 microg/ml. The IC(50) value for the reference compound verapamil hydrochloride was 3.0 microg/ml. In sharp contrast to the above, the flavonols quercetin and morin potentiated 45Ca2+ entry. At 20 microg/ml, quercetin increased 45Ca2+ entry by 54.1% and morin by 48.0%. Quercetin increased the cellular cAMP content in a concentration-dependent manner. H 89, an inhibitor of protein kinase A, inhibited the effect of quercetin on 45Ca2+ entry. The results thus suggest that the effect of quercetin is the result of a protein kinase A-mediated activation of VOCCs. Quercetin induced a rapid and marked increase in both the transient (143.1+/-4.2%) and delayed (198.8+/-10.0%) Ca2+ currents, measured by the whole cell patch clamp technique. The onset of the inhibitory effect of octyl gallate was slow, but resulted in an almost complete inhibition of both Ca2+ currents.

The effect of Portuguese Man-of-war (Physalia physalis) venom on calcium, sodium and potassium fluxes of cultured embryonic chick heart cells

Portuguese Man-of-war venom markedly increases calcium (45Ca2+) influx into primary, cultured, embryonic chick heart cells. This action is dose-dependent, but is unaffected by organic calcium blockers (diltiazem, verapamil, nifedipine, nimodipine and mibefradil). On the other hand, certain trivalent (La3+, Gd3+) and divalent (Zn2+, Ni2+, Cu2+, Mn2+) metals inhibit venom-induced calcium influx. Sodium (22Na+) influx into chick heart cells is also significantly increased by Man-of-war venom. Flecainide does not block venom-induced sodium influx. The efflux of the potassium analogue, 86Rb+, from heart cells is also significantly increased by the venom. The venom, however, has little or no effect on rubidium (86Rb+) or 2-deoxy-D-[2-3H] glucose influx.

Maitotoxin-induced free Ca changes in single rat hepatocytes

We report the results using bioluminescent and fluorescent indicators to investigate maitotoxin-induced free Ca changes in single rat hepatocytes. Maitotoxin generated a steadily rising free Ca increase after a long lag period. The free Ca increase was dependent on extracellular calcium and could be antagonised by chelation of extracellular calcium or the inclusion of nickel in the superfusate. Manganese-induced quench of cytoplasmic Fura2 dextran revealed an accelerated rate of calcium entry during the final period of the lag phase, immediately prior to the free Ca increase. Imaging experiments demonstrated a markedly different part of free Ca mobilisation compared with glycogenolytic stimuli. Moreover, the use of a combination of hormonal stimuli and maitotoxin revealed that some cells could exhibit free Ca oscillations despite steadily rising intracellular free Ca level. The significance of these observations in terms of the mechanism of action of maitotoxin and the mechanism of free Ca transient generation is discussed.

Membrane depolarization in LA-N-1 cells. The effect of maitotoxin is Ca(2+)- and Na(+)-dependent

We investigated the influence of ion compositions on the membrane potential in LA-N-1 human neuroblastoma cells using bisoxonol as a potential-sensitive fluorescent dye. The ability of K+, ouabain, veratridine, and maitotoxin to induce membrane depolarization was evaluated. Increasing concentrations of K+ ions from 10 to 50 mM caused a dose-dependent increase of bisoxonol fluorescence, which was completely independent on Na+ and Ca2+. Ouabain (5 mM), an inhibitor of the Na+, K(+)-ATPase, failed to induce membrane depolarization. Veratridine (40 and 100 microM), a Na+ channel activator, only in the presence of 10 micrograms of Leiurus scorpion venom reduced the membrane potential. Maitotoxin (MTX) from 3 to 10 ng/mL depolarized LA-N-1 cells in a dose-dependent manner, and produced a rapid and sustained increase of intracellular free calcium monitored by means of fluorescent probe fura-2. The MTX-induced depolarization and the increase in cytosolic free calcium concentration were dependent on extracellular Ca2+ ions. On the other hand, Na+ ions also seem to be, although only partially, implicated in the MTX effects, since both the blockade of tetrodotoxin (TTX)-sensitive voltage-operated Na+ channels and the removal of Na+ ions were able to reduce the depolarization. In conclusion, our data indicate that the depolarizing action of MTX on LA-N-1 cells is Ca(2+)- and Na(+)-dependent, although the latter only partially, and that this effect is dependent on Ca2+ influx into the cells likely through a voltage-insensitive calcium-entry system.

Effects of caffeine on the influx of extracellular calcium in GH4C1 pituitary cells

Caffeine increases intracellular Ca2+ concentrations ([Ca2+]i) in a variety of cell types by triggering the mobilization of Ca2+ from intracellular Ca2+ stores. Caffeine also can change [Ca2+]i by affecting Ca2+ influx through voltage-operated Ca2+ channels (VOCCs). In the present study, we investigated the effects of caffeine on Ca2+ entry in GH4C1 pituitary cells. Pretreatment of the cells with caffeine attenuated the high K+-evoked influx of 45Ca2+ in a dose-dependent manner. This inhibition was not secondary to the caffeine-evoked elevation of [Ca2+]i because caffeine was able to inhibit VOCCs also in the presence of the intracellular Ca2+ chelator BAPTA. However, the inhibitory effect of caffeine on 45Ca2+ entry appeared to be dependent on the degree of depolarization of the plasma membrane. Only in cells depolarized with relatively high concentrations of K+ (20, 35, and 50 mM) was the caffeine-induced inhibition observed. A similar inhibitory effect of caffeine on the high K+-evoked calcium and barium entry was observed in experiments using Fura 2. Neither IBMX, forskolin nor dibutyryl cAMP reduced the enhanced [Ca2+]i induced by 50 mM K+, suggesting that the effect of caffeine was not due to increased intracellular cAMP. Furthermore, high doses of caffeine inhibited the plateau level of the TRH-induced increase in [Ca2+]i, which is caused partly by influx of Ca2+ through VOCCs. The inhibitory effect of caffeine was, in part, due to an hyperpolarization of the plasma membrane observed at high doses of caffeine. On the other hand, low doses of caffeine enhanced depolarization-evoked Ba2+ entry as well as the TRH-evoked plateau level of [Ca2+]i. We conclude that caffeine has a dual effect on Ca2+ entry through activated VOCCs in GH4C1 cells: at low concentrations caffeine enhances Ca2+ entry, whereas high concentrations of caffeine block Ca2+ entry.

Redox modulation of calcium entry and release of intracellular calcium by thimerosal in GH4C1 pituitary cells

In the present work we have investigated the actions of the oxidizing sulfhydryl reagent thimerosal on different mechanisms which regulate intracellular free Ca2+ concentration ([Ca2+]i) in GH4C1 pituitary cells. In intact Fura-2 loaded cells, low concentrations of thimerosal potentiated the spike phase of the TRH-induced (thyrotropin-releasing hormone) rise in [Ca2+]i, whereas high thimerosal concentrations inhibited it. The effect of thimerosal on the plateau phase was always inhibitory. The effect of thimerosal on the IP3-induced calcium release (IICR) was studied in permeabilized cells using the Ca2+ indicator Fluo-3. A low concentration of thimerosal (10 microM) stimulated IICR: the Ca2+ release induced by 300 nM inositol-1,4,5-trisphosphate (IP3) was enhanced in cells treated with thimerosal for 1 or 6 min (67 +/- 11 nM and 34 +/- 5 nM, respectively) as compared to control cells (17 +/- 2 nM). On the other hand, a high concentration of thimerosal (100 microM) inhibited IICR: when IP3 (10 microM) was added after a 5 min preincubation with thimerosal, the IP3-induced rise in [Ca2+]i (46 +/- 14 nM) was 57% smaller as compared with that seen in control cells (106 +/- 10 nM). The effect of thimerosal on the voltage-operated Ca2+ channels (VOCCs) was studied by depolarizing intact Fura-2 loaded cells by addition of 20 mM K+ to the cuvette. The depolarization-evoked increase in [Ca2+]i was inhibited in a dose-dependent manner by thimerosal. Direct evidence for an inhibitory effect of thimerosal on VOCCs was obtained by using the whole-cell configuration of the patch-clamp technique: thimerosal (100 microM) potently inhibited the Ba2+ currents through VOCCs. In addition, our results indicated that thimerosal inhibited the caffeine-induced increase in [Ca2+]i, and activated a capacitative Ca2+ entry pathway. The actions of thimerosal were apparently due to its oxidizing activity because the effects were mostly reversed by the thiol-reducing agent dithiothreitol (DTT). We conclude that, in GH4C1 pituitary cells, the mobilization of intracellular calcium and the different Ca2+ entry pathways are sensitive to redox modulation.

Molecular pharmacology of voltage-dependent calcium channels

Voltage-dependent Ca2+ channels serve as the only link to transduce membrane depolarization into cellular Ca(2+)-dependent reactions. A wide variety of chemical substances that have the ability to modulate Ca2+ channels have been demonstrated both for their clinic utility and for importance in elucidating the molecular basis of various biological responses. Recently, introduction of molecular biology to pharmacology has brought a great deal of information about the molecular basis of drug action in Ca2+ channels. In this review, we attempt to overview recent progress in understanding the interactions between Ca2+ channels and their blockers, namely Ca2+ antagonists, from a molecular and structural point of view.

Maitotoxin-elevated cytosolic free calcium in GH4C1 rat pituitary cells nimodipine-sensitive and -insensitive mechanisms

Maitotoxin includes an extracellular Ca2+-dependent membrane depolarization predominantly via activation of L-type voltage-dependent Ca2+ channels (L-VDCC) in GH4C1 rat pituitary cells. In contract to studies employing intracellular dyes, electrophysiological studies have indicated that maitotoxin activates voltage-independent conductances. In the present study, we used fura-2 calcium digital analysis to investigate the actions of very low concentrations of maitotoxin on cytosolic free calcium ([Ca2+]i) in GH4C1 cells in an effort to distinguish different calcium entry mechanisms. Maitotoxin at concentrations as low as 0.01 ng/mL elevated [Ca2+]i 35 +/- 3% and induced membrane depolarization. The concentration dependency for maitotoxin-elevated [Ca2+]i was biphasic with the first phase maximal at 0.05 to 0.5 ng/mL and the minimum EC50 of the second phase about 2.0 ng/mL. Nimodipine (100 nM), a dihydropyridine antagonist of L-VDCC, prevented the [Ca+2]i increase and depolarization induced by up to 0.1 ng/mL maitotoxin, but not at higher concentration (0.5 ng/mL) of maitotoxin. This indicates that lower concentrations (0.1 ng/mL) of maitotoxin require L-VDCC, whereas higher concentrations (>-0.5 ng/mL) of maitotoxin may require additional ionic mechanisms. Maitotoxin (0.5 ng/mL) induced 45Ca2+ uptake and depolarization in Ltk-cells which lack VDCC. Reducing extracellular Cl- from 123 to 5.8 microM increased the magnitude of membrane depolarization by maitotoxin (0.5 ng/mL), which suggests that a Cl- conductance participated in depolarization induced by higher maitotoxin concentrations. Taken together, our results indicate that maitotoxin activates at least two ionic mechanisms. At lower concentrations of maitotoxin, the primary ionic mechanism requires the activation of L-VDCC; however, at higher maitotoxin concentrations, additional ionic mechanisms are involve in the entry of extracellular Ca2+. This latter mechanism may represent the voltage-independent pathway evident under voltage clamp conditions.

Maitotoxin, a calcium channel activator, inhibits cell cycle progression through the G1/S and G2/M transitions and prevents CDC2 kinase activation in GH4C1 cells

Calcium regulates progression through several checkpoints in the cell cycle, including the G1/S-phase transition, G2/M-phase transition, and exit from mitosis. In the GH4C1 rat pituitary cell line, calcium mobilizing polypeptides and calcium channel activation inhibit cell proliferation. This report examines the effects of maitotoxin (MTX), an activator of type L voltage-dependent calcium channels (L-VDCC), on calcium influx and cell cycle progression in GH4C1 cells. MTX causes both a block from G1 to S-phase and a concentration-dependent accumulation of cells in G2+M. MTX does not increase the mitotic index; thus, sustained calcium channel activation by MTX results in an accumulation of cells in G2. In order to temporally localize the MTX-induced G2 block relative to cell cycle regulatory events at the G2/M transition, we assessed the relative activity of two cell cycle regulatory protein kinases, CDC2 and CDK2, in MTX-treated cells. CDC2-specific histone kinase activity in MTX-treated cells is lower than either in cells blocked in mitosis with the microtubule destabilizing agent demecolcine or in randomly cycling cells. In contrast, the activity of CDK2 is highest in MTX-treated cells, consistent with a G2 block prior to CDC2 activation. Together, these results implicate with a G2 block prior to CDC2 activation. Together, these results implicate calcium as an intracellular signal required for progression through G2 phase of the cell cycle prior to CDC2 kinase activation.

Perforated-patch recording with gramicidin avoids artifactual changes in intracellular chloride concentration

The antibiotic gramicidin, when incorporated into lipid membranes, forms pores that are exclusively permeable to monovalent cations and small unchanged molecules. We report the use of gramicidin for perforated patch-clamp recordings in the whole-cell mode. Recordings were performed in cultured rat spinal cord dorsal horn neurons. Cells had stable resting potentials and series resistances for times routinely exceeding 60 min. To test if intracellular chloride concentration ([Cl]i) remains stable with this technique, we measured responses to agonists of glycine and GABAA receptors, both of which gate chloride conductances. The driving force for these responses remained stable at values that differed significantly from values that would be expected if [Cl-]i were biased towards pipette [Cl-]. We conclude that gramicidin perforated-patch recording, in addition to other properties of the perforated-patch recording technique, has the advantage of not altering [Cl-]i. It is, therefore, an electrophysiological method particularly suitable for studies of anionic channels when [Cl-]i is a variable of interest, as well as for studies of homeostatic [Cl-]i regulation.

Effects of remoxipride and raclopride on prolactin release from clonal pituitary tumour cells

The dopamine D2 receptor antagonist remoxipride (30 microM) stimulated prolactin release from the prolactin-producing rat pituitary tumour cell strains GH3 and GH4C1 as well as from transfected GH4C1 cells expressing the short isoform of the rat D2 receptor (GH4ZR7). The effect of remoxipride on prolactin release is probably not due to an interaction with D2 receptors since GH4C1 cells, in contrast to GH3 and GH4ZR7 cells, are completely devoid of D2 receptors; in contrast, we have previously shown that the D2 antagonist haloperidol causes prolactin release from D2 receptor-expressing cells, only. Exposure of GH3 cells to the inhibitor of intracellular Ca2+ mobilization, 8-(diethylamino)-octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) prevented the prolactin-releasing effect of remoxipride whereas pretreatment with the membrane Ca2+ channel antagonist nimodipine did not influence the response. The D2 receptor antagonist raclopride counteracted the reduction of VIP-stimulated prolactin release induced by the D2 agonist quinpirole but caused no prolactin release per se.

Maitotoxin increases voltage independent chloride and sodium currents in GH4C1 rat pituitary cells

Maitotoxin (MTX) is a 3,424 dalton polyether marine toxin that causes influx of calcium through type L voltage-dependent calcium channels (L-VDCC) in GH4C1 rat pituitary cells, presumably as the result of membrane depolarization. In this study we have investigated the ionic conductances responsible for MTX-induced depolarization under voltage clamp conditions using the perforated and ruptured patch methods. MTX-induced steady-state voltage independent currents of nearly 400 pS/pF within seconds of addition to the bath. Ion substitution experiments demonstrated these currents are consistent with the conductance of sodium and chloride, but not calcium, ions. MTX induction of the voltage-independent chloride conductance in GH4C1 cells occurred concurrently without modification of L-VDCC currents. Pretreatment with nimodipine eliminated voltage activation of L-VDCC, and reduced by two thirds the voltage independent current. Analysis as a function of time of MTX exposure revealed that the first 60 sec of MTX-induced currents were not affected by nimodipine pretreatment, but subsequent additional currents were prevented. This indicates that the initial currents induced by MTX occur independently of L-VDCC mediated calcium entry, but full activation of these currents by MTX likely requires the involvement L-VDCC. Taken together this work identifies a voltage-independent sodium/chloride conductance as an initial action of MTX, one that may promote the sequence of ionic events leading to activation of L-VDCC and massive calcium entry.

Development of rapid and sensitive high throughput pharmacologic assays for marine phycotoxins

The lack of rapid, high throughput assays is a major obstacle to many aspects of research on marine phycotoxins. Here we describe the application of microplate scintillation technology to develop high throughput assays for several classes of marine phycotoxin based on their differential pharmacologic actions. High throughput "drug discovery" format microplate receptor binding assays developed for brevetoxins/ciguatoxins and for domoic acid are described. Analysis for brevetoxins/ciguatoxins is carried out by binding competition with [3H] PbTx-3 for site 5 on the voltage dependent sodium channel in rat brain synaptosomes. Analysis of domoic acid is based on binding competition with [3H] kainic acid for the kainate/quisqualate glutamate receptor using frog brain synaptosomes. In addition, a high throughput microplate 45Ca flux assay for determination of maitotoxins is described. These microplate assays can be completed within 3 hours, have sensitivities of less than 1 ng, and can analyze dozens of samples simultaneously. The assays have been demonstrated to be useful for assessing algal toxicity and for assay-guided purification of toxins, and are applicable to the detection of biotoxins in seafood.