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  • 1
    Electronic Resource
    Electronic Resource
    Follicular Tissues Reduce Drug Effects on Ion Channels in Oocytes of Xenopus laevis (1997)
    Oxford, UK : Blackwell Publishing Ltd
    European journal of neuroscience 9 (1997), S. 0 
    Details
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The influence of follicular tissues on drug effects on ion channels in Xenopus oocytes was tested by investigating the pharmacological properties of a cloned potassium channel in oocytes with and without follicular tissues. The data show that the efficacy of blocking agents (ranging from metal ions to peptides) is drastically reduced by the follicular tissues (reductions by as much as 90% and increases of the IC50 values up to 30–fold). Furthermore, the time course of the blocking effect was slowed down by the tissues (increases of the t50 values up to 40–fold). The described impairment could be mitigated, but not abolished by partial removal of the follicular tissues (so-called defolliculation, leaving only the vitelline envelope and part of the follicle cells on the oocyte surface). The results indicate that the follicular tissues can induce significant errors in pharmacological measurements on membrane proteins in Xenopus oocytes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1460-9568.1997.tb01636.x
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  • 2
    Electronic Resource
    Electronic Resource
    Diversity of Potassium Channels Contributing to Differences in Brain Area-specific Seizure Susceptibility: Sensitivity of Different Potassium Channels to the Epileptogenic Agent Pentylenetetrazol (1997)
    Oxford, UK : Blackwell Publishing Ltd
    European journal of neuroscience 9 (1997), S. 0 
    Details
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The effect of the epileptogenic agent pentylenetetrazol on eight cloned voltage-operated mammalian potassium channels (expressed in oocytes of Xenopus laevis) was investigated in order to contribute to an explanation for the brain area-specific differences in seizure susceptibility. Pentylenetetrazol increased the potassium currents at more negative and decreased them at more positive potentials for the channels of the Kvl gene family, whereas for the other channels the currents were decreased over the whole potential range. The sensitivities of the different potassium channels to the epileptogenic agent were different. At a potential of 0 mV, for example, there were strong reductions for the Kvl.1, Kvl.4 and Kv2.1 currents, whereas the decrease was smaller for the Kvl.3 and Kvl.6 currents and was negligible for the Kvl.2, Kvl.5 and Kv3.4 currents. Correlating these data with the distribution patterns of the potassium channels in the hippocampus, the neocortex and the cerebellum (representing examples of brain areas of distinct seizure susceptibility) revealed that in brain areas with higher seizure susceptibility the overall sensitivity of the potassium channels to the epileptogenic agent is augmented. As a whole, the findings give the first evidence that the differences in distributions and properties of potassium channels contribute to differences in the seizure susceptibility of brain areas.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1460-9568.1997.tb01408.x
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  • 3
    Electronic Resource
    Electronic Resource
    Evidence for a melatonin receptor within pancreatic islets of neonate rats: functional, autoradiographic, and molecular investigations (2000)
    Copenhagen : Munksgaard
    Journal of pineal research 28 (2000), S. 0 
    Details
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: In a recent perifusion investigation, we showed that the pineal secretory product melatonin reduces insulin secretion from isolated pancreatic islets of neonate rats stimulated with potassium chloride (KCl), glucose, and forskolin. This effect of melatonin was reproduced with doses ranging from 200 pmol/L to 5 μmol/L. Because it is generally accepted that melatonin exerts some of its biological effects through specific, high-affinity pertussis-toxin-sensitive G-protein-coupled receptors, we blocked the putative melatonin receptor of pancreatic islets using both the non-hydrolyzable guanosine triphosphate analog guanosine 5′-O-(3-thiotriphosphate) (GTPΓS, 30 μmol/L) and the melatonin antagonist luzindole (10 μmol/L). Both GTPΓS and luzindole caused a near normalization of the melatonin-induced inhibition of the forskolin-stimulated insulin secretion. To localize putative melatonin receptors within the pancreatic islets autoradiographic studies were additionally carried out. These investigations showed specific binding of 2-[125I]iodomelatonin, which were in exact correspondence with the localization of the islets. In addition, gray-level analysis showed that unlabeled melatonin was able to reduce the binding of 2-[125I]iodomelatonin in a dose-dependent manner. Concentrations of unlabeled melatonin of 10−9 mol/L produced a 50% reduction in specific binding, whereas concentrations of 10−6 mol/L displaced the binding completely. Likewise, the results of molecular investigations showed that the rat pancreas contains a melatonin receptor, since reverse transcription polymerase chain reaction (RT-PCR) experiments, using specific primers for the rat melatonin receptor Mel1a, showed that mRNA for this melatonin receptor type is expressed in pancreatic tissue of newborn rats. In summary, it may be said that our functional, autoradiographic, and molecular results indicate that the Mel1a receptor is located on the pancreatic islets, possibly in the beta cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1034/j.1600-079X.2001.280305.x
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  • 4
    Electronic Resource
    Electronic Resource
    Gene expression and functional characterization of melatonin receptors in the spinal cord of the rat: implications for pain modulation (2003)
    Oxford, UK : Munksgaard International Publishers
    Journal of pineal research 35 (2003), S. 0 
    Details
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Recently, a species-dependent distribution of melatonin binding sites have been found in lamina I–V and lamina X of the spinal cord. In order to learn more about the function of spinal melatonin receptors, we investigated (i) the gene expression for melatonin receptor subtypes in lumbar and thoracal spinal cord tissue by means of the reverse-transcriptase polymerase chain reaction (RT-PCR) technique, and (ii) the electrophysiological and pharmacological properties of melatonin receptors heterologously expressed in Xenopus oocytes after injection of spinal cord mRNA by means of the voltage clamp technique. Because ample evidence indicates an antinociceptive effect of melatonin, (iii) the role of spinal melatonin receptors for maintaining mechanical and thermal hyperalgesia was studied in a rat model for postoperative pain. The RT-PCR data revealed that transcripts for MT1 and MT2 melatonin receptors are present in the dorsal and ventral horn of lumbar and thoracal spinal cord tissue. Injection of mRNA from lumbar spinal cord tissue into Xenopus oocytes led to the functional reconstitution of melatonin receptors which activate calcium-dependent chloride inward currents. Melatonin responses were abolished by simultaneous administration of the antagonists, 2-phenylmelatonin and luzindole and were unaffected by the MT2 antagonist 4-phenyl-2-propionamidotetralin. Intrathecal administration of different melatonin doses (10–100 nmol) did not inhibit mechanical or thermal hyperalgesia. However, intrathecal application of a low dose of morphine together with melatonin caused a brief antinociceptive effect suggesting an enhanced morphine analgesia by melatonin. In conclusion, the present study demonstrated for the first time the presence of transcripts of MT1 and MT2 receptors located in the dorsal and ventral horn of the spinal cord. Furthermore, spinal melatonin enhanced the antinociceptive effect of morphine indicating that melatonin acts as a neuromodulator in the spinal cord.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1034/j.1600-079X.2003.00047.x
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  • 5
    Electronic Resource
    Electronic Resource
    Receptor (MT1) mediated influence of melatonin on cAMP concentration and insulin secretion of rat insulinoma cells INS-1 (2002)
    Oxford, UK : Munksgaard International Publishers
    Journal of pineal research 33 (2002), S. 0 
    Details
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Recent functional, autoradiographic, and molecular investigations have shown that the pineal secretory product melatonin reduces the forskolin-stimulated insulin secretion from isolated pancreatic islets of neonate rats. Autoradiographic and binding studies as well as reverse transcriptase-polymerase chain reaction (RT-PCR) experiments proved that these effects are mediated through specific, high-affinity pertussis-toxin-sensitive Gi-protein-coupled MT1 receptors and subsequent inhibition of the adenylyl cyclase/cyclic adenosine monophosphate (cAMP) system. This hypothesis was proved by blocking the intracellular signal transduction pathway using the non-hydrolyzable guanosine triphosphate analog guanosine 5′-O-(3-thiotriphosphate) (GTPγS) or the competitive melatonin receptor antagonist luzindole. Both GTPγS and luzindole diminished the melatonin effect. We have published these prior results elsewhere. So far, however, no information is available on both whether the MT1 receptors are located on the β-cells and whether the consecutive functional reactions are based on a direct influence of melatonin on the insulin producing β-cells. In order to examine this question, we used a glucose responsive insulin producing insulinoma cell line INS-1 isolated from rats. Comparable with the results of islets the competitive receptor antagonist luzindole diminished the insulin-decreasing effect of melatonin. In addition, our RT-PCR experiments, using specific primers for the rat melatonin receptor MT1 showed that this melatonin receptor mRNA is also expressed in the INS-1 cells. Furthermore we radioimmunologically analyzed the forskolin-stimulated cAMP concentration in the superfusate. Similar to insulin secretion, the cAMP concentration was significantly reduced by melatonin. Following the hypothesis that cAMP is actively secreted from INS-1 cells by an energy-dependent mechanism based on either a OAT1/ROAT1 like anion exchanger or MDR-like transport systems, we used probenecid (p-[dipropylsulfamoyl] benzoic acid), a known inhibitor of cAMP extrusion. Probenecid blocks the export of cAMP by acting on transport mechanisms which are as yet not completely understood. Consistently, insulin secretion was increased and cAMP concentration diminished. The application of the phosphodiesterase inhibitor IBMX (3-isobutyl-1-methylxanthine) caused a marked rise of insulin secretion as well as cAMP concentration in the perifusate. From these data we conclude that the MT1 receptor is located on the INS-1 cell and therefore in general on pancreatic β-cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1034/j.1600-079X.2002.02919.x
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  • 6
    Electronic Resource
    Electronic Resource
    Expression and functional characterization of the mt1 melatonin receptor from rat brain in Xenopus oocytes: evidence for coupling to the phosphoinositol pathway (2001)
    Copenhagen : Munksgaard International Publishers
    Journal of pineal research 30 (2001), S. 0 
    Details
    ISSN: 1600-079X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Melatonin-sensitive receptors were expressed in Xenopus laevis oocytes following an injection of mRNA from rat brain. The administration of 0.1–100 μmol/L melatonin to voltage-clamped oocytes activates calcium-dependent chloride currents via a pertussis toxin-sensitive G protein and the phosphoinositol pathway. To determine which melatonin receptor type (mt1, MT2, MT3) is functionally expressed in the Xenopus oocytes, we used (i) agonists and antagonists of different receptor types to characterize the pharmacological profile of the expressed receptors and (ii) a strategy of inhibiting melatonin receptor function by antisense oligonucleotides. During pharmacological screening administration of the agonists 2-iodomelatonin and 2-iodo-N-butanoyl-5-methoxytryptamine (IbMT) to the oocytes resulted in oscillatory membrane currents, whereas the administration of the MT3 agonist 5-methoxycarbonylamino-N-acetyltryptamine (GR135,531) exerted no detectable membrane currents. The melatonin response was abolished by a preceding administration of the antagonists 2-phenylmelatonin and luzindole but was unaffected by the MT3 antagonist prazosin and the MT2 antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT). In the antisense experiments, in the control group the melatonin response occurred in 45 of 54 mRNA-injected oocytes (83%). Co-injection of the antisense oligonucleotide, corresponding to the mt1 receptor mRNA, caused a marked and significant reduction in the expression level (13%; P〈0.001). In conclusion, the results demonstrate that injection of mRNA from rat brain in Xenopus oocytes induced the expression of the mt1 receptor which is coupled to the phosphoinositol pathway.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1034/j.1600-079X.2001.300302.x
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  • 7
    Electronic Resource
    Electronic Resource
    Lead-induced blockade of kainate-sensitive receptor channels (1995)
    Springer
    Naunyn-Schmiedeberg's archives of pharmacology 353 (1995), S. 42-45 
    Details
    ISSN: 1432-1912
    Keywords: Lead ; Glutamate receptor ; AMPA Kainate ; Toxicity ; Xenopus oocytes ; Membrane currents
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The effects of bivalent lead on ion channels activated by kainate and α-amino-3-hydroxy-5-methyl-4-isoxazolpropionate (AMPA) were studied using Xenopus oocytes microinjected with mRNA from rat brain. Lead reduced kainate-induced membrane currents in a reversible and dose-dependent manner, without affecting membrane currents induced by AMPA. Lead decreased the kainate currents with a concentration of 0.1 μmol/l to 0.93 ± 0.01 and with a concentration of 100 μmol/l to 0.41 ± 0.04 of the control values. The blocking effect of lead on kainate responses was voltage dependent. The inhibition was strongest at - 90 mV to - 70 mV and became weaker at more positive membrane potentials. The effect of lead on the kainate-induced membrane currents remained unchanged when the concentration of kainate was increased. Hence lead probably represents a noncompetitive channel-blocking agent for non-N-methyl-d-aspartate (NMDA) receptor channels activated by kainate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00168914
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  • 8
    Electronic Resource
    Electronic Resource
    Characterization of ion currents elicited by a stream of fluid during spontaneous and ligand-induced chloride current oscillation in Xenopus laevis oocytes (1998)
    Springer
    Pflügers Archiv 436 (1998), S. 49-55 
    Details
    ISSN: 1432-2013
    Keywords: Key words Xenopus laevis ; Oocytes ; Mechanosensitivity ; Inositol trisphosphate ; Calcium ; Chloride current oscillations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  During Ca2+-activated C– current oscillations a mechanical deformation of the Xenopus laevis oocyte by a fluid stream evokes transient inward currents of high amplitude (stream evoked inward current, I i,st). This current can be observed either in native or RNA-injected oocytes expressing ligand-controlled ion channels from rat brain. I i,st reversed at the equilibrium potential of chloride and was blocked by 9-anthracene carboxylic acid (2 mM). Power spectral analysis of the oscillations did not reveal a correlation between the features of the oscillations and the amplitude of I i,st. Antagonists of stretch-activated cation channels [gadolinium (100 µM) and lanthanum (1mM)] did not block I i,st. Calcium channel blockers [cobalt and manganese (10 mM)] did not inhibited I i,st and I i,st could also be elicited in calcium-free medium. Preloading oocytes with pertussis toxin (PTX) for 17 h prevented current oscillations and I i,st caffeine (10 mM), an antagonist of the liberation of calcium from intracellular stores, inhibited I i,st. Our results proride evidence for modulation of the mechanosensitivity of chloride currents by activation of intracellular second messenger cascades.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050603
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  • 9
    Electronic Resource
    Electronic Resource
    Effects of n-hexane and its metabolites on cloned voltage-operated neuronal potassium channels (1997)
    Springer
    Archives of toxicology 71 (1997), S. 238-242 
    Details
    ISSN: 1432-0738
    Keywords: Key words n-Hexane  ;  Hexane metabolites  ;   Potassium current  ;  Oocyte
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract In order to study the mechanisms of acute n-hexane␣intoxication, the effects of n-hexane and its metabolites 2-hexanol, methyl-n-butyl ketone, 2,5-hexanediol and 2,5-hexanedione on the cloned voltage-operated potassium channels Kv1.1, Kv1.4, Kv2.1 and Kv3.4 were investigated with electrophysiological techniques in the expression system of Xenopus oocytes. n-Hexane had no effect at any channel, whereas some of its metabolites led to reductions of the potassium currents. The greatest effects obtained were caused by 2-hexanol at the Kv2.1 channel, resulting in reductions of 13% at 0␣mV with a concentration of 500 mg/l and IC50 of ca. 3500 mg/l. The reduction appeared to be caused by a shift of the current-voltage relation to the right. Methyl-n-butyl ketone showed smaller effects, whereas 2,5-hexanedione and 2,5-hexandiol were nearly ineffective. Concerning the different potassium channels, the sensitivity to the metabolites differed. The metabolites showed greatest sensitivity towards the Kv2.1 channel and lowest sensitivity towards the Kv3.4 channel. Since the n-hexane metabolite concentrations in the brain during acute n-hexane intoxication are unknown, the relevance of the data is still unclear. The size of the effects and the currently available data on tissue concentration, however, make it more likely that the action of n-hexane and its metabolites on voltage-operated potassium channels is not a major mechanism for acute neurotoxicity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002040050382
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  • 10
    Electronic Resource
    Electronic Resource
    Effects of 2-phenoxyethanol on N-methyl-d-aspartate (NMDA) receptor-mediated ion currents (1999)
    Springer
    Archives of toxicology 73 (1999), S. 55-59 
    Details
    ISSN: 1432-0738
    Keywords: Key words N-methyl-d-aspartate ; Glutamate receptor ; Glycol ether ; 2-Phenoxyethanol ; Ethylene glycol monophenyl ether
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The actions were examined of 17 frequently used glycol ether compounds on the glutamate receptor-mediated ion currents. The receptors were expressed in Xenopus oocytes by injection of rat brain mRNA. Most of the 17 glycol ethers exerted no effects on the glutamate subreceptors activated by kainate and N-methyl-d-aspartate (NMDA), whereas 2-phenoxyethanol (ethylene glycol monophenyl ether) caused a considerable reduction of NMDA-induced membrane currents in a reversible and concentration-dependent manner. The threshold concentration of the ethylene glycol monophenyl ether effect was 〈10 μmol/l. The concentration for a 50% inhibition (IC50) was ∼360 μmol/l. The results indicate a neurotoxic potential for 2-phenoxyethanol.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002040050586
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