Biomedical subjects

K S Hsu

Publications and source records attributed to K S Hsu.

At least 37 records · Page 2Linked to original sources

PubMed1997

Nitric oxide signalling is required for the generation of anoxia-induced long-term potentiation in the hippocampus.

The involvement of nitric oxide in anoxia-induced long-term potentiation (anoxic LTP) of synaptic transmission was investigated in CA1 neurons of rat hippocampal slices using intracellular recording techniques in vitro. In response to superfusion of an anoxic artificial cerebral spinal fluid saturated with 95% N2-5% CO2, the excitatory postsynaptic potential (EPSP) generated in hippocampal CA1 neurons by stimulation of the Schaffer collateral/commissural afferent pathway was completely abolished within 10 min of anoxia. On return to reoxygenated medium, the EPSP returned to the control value within 10 min and was subsequently and progressively potentiated to reach a plateau 15-20 min after return to oxygen. This anoxia-induced persistent increase in synaptic transmission lasted for more than 1 h. Application of the nitric oxide synthase inhibitors 7-nitroindazole (7-NI) or L-N(G)-nitroarginine (NOARG) produced no effects on the baseline EPSP amplitude, but effectively attenuated the anoxic LTP. The inhibitory effects of both 7-NI and NOARG on the anoxic LTP were blocked by L-arginine, a substrate for nitric oxide synthase. These results suggest that nitric oxide is required for the generation of anoxia-induced LTP of glutamatergic synaptic transmission in the CA1 region of the rat hippocampus.

Animals↗

PubMed1996

Inhibition of calcium channels in rat hippocampal CA1 neurons by conantokin-T.

Effects of conantokin-T, a 21 amino acid peptide toxin isolated from the fish-hunting cone snail Conus tulipa, on the high-voltage-activated Ca2+ channel currents were studied in acutely dissociated rat hippocampal CA1 neurons using whole-cell voltage clamp-recording technique with 5 mM Ba2+ as the charge carrier. Conantokin-T inhibited the whole-cell Ba2+ current (IBa) in a concentration-dependent manner. The nimodipine (20 microM) and omega-agatoxin-IVA (0.2 microM) block of IBa were abolished in the presence of conantokin-T (3 microM); however, conantokin-T (3 microM) did not affect the block of IBa induced by 3 microM omega-conotoxin-GVIA. These results indicate that conantokin-T is a potent but wide-spectrum Ca2+ channel antagonist.

Animals↗

PubMed1996

Inhibition of N-type calcium currents by lamotrigine in rat amygdalar neurones.

Lamotrigine (LAG) is a new anticonvulsant drug for the treatment of partial and secondarily generalized seizures. The present study was aimed at elucidating the possible involvement of Ca2+ channels in the action of LAG using whole-cell patch clamp recordings in acutely dissociated amygdalar neurones. Whole-cell Ca2+ currents (ICa) were elicited by 200 ms step commands from -70 mV to -10 mV. Application of LAG reduced the ICa by an average of 40.3 +/- 3.2%. The inhibition of ICa by LAG was markedly reduced or eliminated in the presence of the N-type Ca2+ channel blocker omega-conotoxin-GVIA (1 microM). These results suggest that LAG may exert its anticonvulsant effect through inhibition of presynaptic N-type Ca2+ channels, thereby reducing glutamate release.

Amygdala↗

PubMed1996

Mutual inhibitory effects between dopamine and carbachol on the excitatory synaptic transmission in the rat neostriatum.

The interactions between dopamine and carbachol on the excitatory synaptic transmission were studied in rat neostriatal slices using an intracellular recording method. Excitatory postsynaptic potentials (EPSPs) were evoked by cortical stimulation. Application of dopamine (DA; 0.1 microM) or carbachol (0.1 microM) produced a dramatic and reversible inhibition of the EPSP amplitude. The inhibitory effect induced by carbachol was markedly attenuated in the presence of either DA (0.1 microM) or the selective D2 dopaminergic receptor agonist (+/-)-2-(N-phenylethyl-N-propyl) amino-5-hydroxytertralin (PPHT; 0.1 microM), but not by the D1 dopaminergic receptor agonist (+/-)-7, 8-dihydroxy-3-allyl-1-phenyl-2, 3, 4, 5-tetrahydro-1H-3-benzazepine (SKF-38393; 0.1 microM) or the D3 dopaminergic receptor agonist R(-)-(4aS, 10bS)-3, 4, 4a, 10b-tetrahydro-4-propyl-2H, 5H-[1] benzogyrano-[4,3-b]-1, 4-oxazin-9-ol (PD-128,907; 0.1 microM). Conversely, muscarinic receptor activation with carbachol (0.1 microM) also completely abolished the DA-induced depression of the EPSP amplitude. In addition, the inhibitory effect of DA on the carbachol-induced depression of the EPSP amplitude was antagonized by sulpiride (1 microM), a selective D2 dopaminergic receptor antagonist. However, D1 dopaminergic receptor antagonist (+/-)-7-bromo-8-hydroxy-3-methyl-1-phenyl-2, 3, 4, 5-tetrahydro-3-benzazepine (SKF-83566; 1 microM) did not affect DA's inhibition. Rp-adenosine-3',5'-cyclic monophosphothioate (Rp-cAMPS; 25 microM), a potent inhibitor of cAMP-dependent protein kinase A (PKA), alone decreased the amplitude of EPSP below baseline values and mimicked the inhibitory effect of DA on the carbachol-induced depression of the EPSP amplitude. Based on these findings, we conclude that the inhibitory effects of D2 dopaminergic receptor and muscarinic receptor activation on the excitatory synaptic transmission in the neostriatum are non-additive and therefore are antagonistic interactions. furthermore, the effect of muscarinic receptor stimulation will depend on the extent of D2 dopaminergic receptor activation and the modulation of the cellular PKA-dependent messenger system seems to contribute to their interactions.

Animals↗

PubMed1996

Blockade of isoproterenol-induced synaptic potentiation by tetra-9-aminoacridine in the rat amygdala.

The effects of tetrahydro-9-aminoacridine (THA) on beta-adrenoceptor activation-induced synaptic potentiation were studied in brain slices of the rat amygdala using intracellular recording techniques. To exclude the involvement of N-methyl-D-aspartate (NMDA) receptors, all the experiments were performed in the presence of NMDA receptor antagonist, D-APV (50 microM). Bath application of isoproterenol (Iso; 15 microM) results in a long-lasting enhancement of the amplitude of excitatory postsynaptic potentials (EPSPs) to 200 +/- 6% of baseline. Forskolin, which directly activates adenyl cyclase, produces a similar effect suggesting that Iso may act through a cyclic AMP-dependent mechanism. Pretreatment of the slices with THA (300 microM) completely abolishes the Iso- and forskolin-induced synaptic potentiation. We hypothesize that the locus of THA/beta-adrenoceptor interaction is presynaptic; the underlying mechanism is likely due to THA's depression of transmitter release via a presynaptic blockade of voltage-dependent Ca2+ channels.

Adrenergic beta-Agonists↗

PubMed1996

Isoproterenol potentiates synaptic transmission primarily by enhancing presynaptic calcium influx via P- and/or Q-type calcium channels in the rat amygdala.

The effects of selective beta-adrenergic receptor agonist isoproterenol (Iso) on neuronal excitability and synaptic transmission were investigated in brain slices of rat amygdala. Iso (15 microM) produced a long-lasting enhancement of the EPSP that was not blocked by pretreatment with 20 microM D-2-amino-5-phosphonovalerate (D-APV) alone or D-APV in combination with kynuretic acid (1 mM). The sensitivity of postsynaptic neurons to the glutamate receptor agonist AMPA was unchanged by Iso pretreatment. Superfusion of Iso reversibly blocked the after-hyperpolarization (AHP) that followed a depolarizing current pulse and caused more action potential firing. Intracellular application of a selective inhibitor of the catalytic subunit of cAMP-dependent protein kinase A blocked the effect of Iso on the AHP, whereas Iso-induced potentiation was entirely normal in the same neuron. In addition, Iso decreased the magnitude of paired-pulse facilitation, which is consistent with a presynaptic mode of action. Substituting the Mg2+ for Ca2+ in the medium completely abolished the Iso-induced enhancement of the EPSP. The effect of Iso also was blocked by low concentrations of omega-agatoxin-IVA, but not by nifedipine or omega-conotoxin-GVIA. These results suggest that Iso enhances synaptic transmission in the amygdala via a presynaptic site of action: the mechanism underlying the potentiating effect likely is attributable to an increased Ca2+ influx through P- and/or Q-type Ca2+ channels.

Action Potentials↗

PubMed1996

Presynaptic inhibition of excitatory neurotransmission by lamotrigine in the rat amygdalar neurons.

Lamotrigine (LAG) is a new antiepileptic drug which is licensed as adjunctive therapy for partial and secondary generalized seizures. In the present study, the mechanisms responsible for its antiepileptic effect were studied in rat amygdaloid slices using intracellular recording and whole-cell patch clamp techniques. Bath application of LAG (50 microM) reversibly suppressed the excitatory postsynaptic potentials (EPSPs) and currents (EPSCs) evoked by stimulating ventral endopyriform nucleus. Synaptic response mediated by the N-methyl-D-aspartate (NMDA) receptor (EPSPNMDA) was isolated pharmacologically by application of a solution containing non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX,10 microM) and gamma-aminobutyric acidA receptor antagonist bicuculline (20 microM). LAG produced a parallel inhibition of EPSPNMDA. Postsynaptic depolarization induced by alpha-amino-5-methyl-4-isoxazole propionate (AMPA) was not altered by LAG. In addition, LAG increased the ratio of the second pulse response to the first pulse response (P2/P1), which is consistent with a presynaptic mode of action. The L-type Ca+2 channel blocker nifedipine (20 microM) had no effect on LAG-induced presynaptic inhibition. However, the depressant effect of LAG was markedly reduced in slices pretreated with N-type Ca+2 channel blocker omega-conotoxin-GVIA (omega-CgTX-GVIA, 1 microM) or a broad spectrum Ca+2 channel blocker omega-conotoxin-MVIIC (omega-CgTX-MVIIC, 1 microM). It is concluded that a reduction in omega-CgTX-GVIA-sensitive Ca+2 currents largely contributes to LAG-induced presynaptic inhibition.

Amygdala↗

PubMed1996

Carbachol induces inward current in neostriatal neurons through M1-like muscarinic receptors.

The effects of carbachol on rat neostriatal neurons were examined in the slice and the freshly dissociated neuron preparations using intracellular and whole-cell voltage-clamp recording methods. Superfusion of carbachol (30 microM) produced a depolarization concomitant with an increase in the rate of spontaneous action potentials. This depolarization was associated with an increase in the input resistance. The carbachol-induced membrane depolarization was blocked by pirenzepine (1 microM), a selective M1 muscarinic receptor antagonist. In other experiments, we observed that carbachol induced a transient inward current on the freshly dissociated neostriatal neuron at a holding potential of -60 mV in a concentration-dependent manner underlying the whole-cell voltage-clamp mode. The inward current caused by carbachol was not reduced by tetrodotoxin (1 microM), calcium-free recording solution or Cd2+ (100 microM). However, it was blocked by Ba2+ (100 microM). In addition, the carbachol-induced inward current reversed polarity at about the potassium equilibrium potential. The whole-cell membrane inward current in response to voltage-clamp step from -90 to -140 mV was reduced by 30 microM carbachol. With stronger hyperpolarization beyond the potassium equilibrium potential, carbachol produced a progressively greater reduction in membrane current. This inhibitory effect was also abolished by Ba2+ (100 microM). A concentration of 30 microM carbachol-induced inward current could be reversibly antagonized by the M1 muscarinic receptor antagonist pirenzepine (0.1-1 microM), with an estimated IC50 of 0.3 microM. However, other muscarinic receptor subtype (M2 or M3) antagonists could also block the carbachol-induced inward current. The rank order of antagonist potency was: pirenzepine (M1 antagonist) > 4-diphenylacetoxy-N,N-methyl-piperidine methiodide (M3/M1 antagonist) > gallamine (M2 antagonist). Based on these pharmacological data, we concluded that carbachol can act at M1-like muscarinic receptors to reduce the membrane K+ conductances and excite the neostriatal neurons.

Animals↗

PubMed1996

Thromboxane A2 agonist modulation of excitatory synaptic transmission in the rat hippocampal slice.

1. The effects of the selective thromboxane A2 (TXA2) receptor agonist I-BOP on neuronal excitability and synaptic transmission were studied in the CAl neurones of rat hippocampal slices by an intracellular recording technique. 2. Superfusion of I-BOP (0.5 microM) resulted in a biphasic change of the excitatory postsynaptic potential (e.p.s.p.), which was blocked by pretreatment with SQ 29548, a specific antagonist of TXA2 receptors. The inhibitory phase of I-BOP on the e.p.s.p. was accompanied by a decrease in neuronal membrane input resistance. 3. The sensitivity of postsynaptic neurones to glutamate receptor agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartate (NMDA), was unchanged by I-BOP (0.5 microM) pretreatment. 4. Bath application of Ba2+ (0.5 mM) prevented both the I-BOP-induced reduction of the neuronal membrane input resistance and the blockade of e.p.s.p. induced by I-BOP. 5. Intracellular dialysis of the hippocampal CA1 neurones with GDP (10 mM) significantly attenuated the I-BOP inhibition of e.p.s.p. and membrane input resistance. Incubation of the slices with either pertussis toxin (PTX, 5 micrograms ml-1 for 12 h) or cholera toxin (CTX, 5 micrograms ml-1 for 12 h) did not affect the biphasic action of I-BOP on the e.p.s.p. or the reduction of membrane input resistance induced by I-BOP. 6. Pretreatment of the slices with the protein kinase C (PKC) inhibitor, NPC-15437 (20 microM), abolished the biphasic modulation by I-BOP (0.5 microM) of the e.p.s.p. Intracellular application of a specific PKC inhibitor, PKCI 19-36 (20 microM), completely inhibited the I-BOP reduction of e.p.s.p. The specific cyclic AMP-dependent protein kinase (PKA) inhibitor, Rp-cyclic adenosine 3',5'-monophosphate (Rp-cyclic AMPS, 25 microM), had no effect on the I-BOP action. 7. In this study we have demonstrated, for the first time, the existence of functional TXA2 receptors in the hippocampus which mediate the effects of a TXA2 agonist on neuronal excitability and synaptic transmission. Activation of the presynaptic TXA2 receptors may stimulate the release of glutamate. Conversely, activation of postsynaptic TXA2 receptors leads to inhibition of synaptic transmission resulting from a decrease in the membrane input resistance of the neurones. The pre- and postsynaptic actions of the TXA2 agonist are both mediated by PTX- and CTX-insensitive G-protein-coupled activation of PKC pathways.

Animals↗

PubMed1996

TXA2 agonists inhibit high-voltage-activated calcium channels in rat hippocampal CA1 neurons.

Whole cell voltage clamp recordings were used to investigate the effects of thromboxane A2 (TXA2) agonists on the voltage-dependent Ca2+ currents in rat hippocampal CA1 neurons. TXA2 agonists [1S-[1 alpha, 2 beta(5Z), 3 alpha(1E, 3S*)4 alpha ]]-7-[3-[3-hydroxy-4-(4'-iodophenoxy)-1-butenyl]-7-oxabicyclo [2,2,1]heptan-2-yl]-5-heptenoic acid (I-BOP) and U-46619, reversibly suppressed the whole cell Ca2+ currents in a concentration-dependent manner. The effect was blocked by specific TXA2 receptor antagonist, SQ-29548. I-BOP as well as U-46619 inhibited both omega-conotoxin GVIA (CgTx)-sensitive and nimodipine sensitive Ca2+ currents but had no effect on CgTx/nimodipine insensitive Ca2+ currents. The I-BOP and U-46619 inhibition of Ca2+ currents was blocked by internal dialysis of hippocampal neurons with specific protein kinase C (PKC) inhibitors, NPC-15437 and PKC inhibitor-(19-36). Pretreatment of hippocampal neurons with either 5 micrograms/ml pertussis toxin (PTX) or 5 micrograms/ml cholera toxin (CTX) did not significantly affect the suppression of the Ca2+ currents by I-BOP and U-46619. Dialyzing with 1 mM guanosine 5'-O-(3-thiotriphosphate) or 1 mM GDP significantly attenuated the I-BOP or U-46619 action. These results demonstrate that TXA2 agonists inhibit both CgTx- and nimodipine-sensitive Ca2+ currents but not CgTx/nimodipine-insensitive currents in rat hippocampal CA1 neurons via a PTX- and CTX-insensitive G protein-coupled activation of the PKC pathway.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗

PubMed1996

Characterization of dopamine receptors mediating inhibition of excitatory synaptic transmission in the rat hippocampal slice.

1. The effect of dopamine (DA) on the excitatory synaptic transmission was studied in the CA1 neurons of rat hippocampal slices using intracellular recording technique. 2. Depolarizing excitatory postsynaptic potentials (EPSPs) were evoked by stimulation of the Schaffer collateral-commissural pathway. Superfusion of DA (0.03-1 microM) reversibly decreased the EPSP in a concentration-dependent manner and with an estimated IC50 of 0.3 microM. The sensitivity of postsynaptic neurons to the glutamate-receptor agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid or N-methyl-D-aspartate was unchanged by DA (0.3 microM) pretreatment. In addition, DA (0.3 microM) increased the magnitude of paired-pulse facilitation, a phenomenon attributed to an increase in the amount of transmitter released in response to the second stimulus. 3. The reduction of DA (0.3 microM) on the EPSP was antagonized by sulpiride (1-10 nM), a selective D2-receptor antagonist. However, D1-receptor antagonist, SKF-83566 (1-10 microM), did not significantly affect the reduction of DA (0.3 microM) on the EPSP. 4. (+/-)-2-(N-Phenylethyl-N-propyl)amino-5-hydroxytetralin (1 microM), an agonist of D2 receptor, mimicked the inhibitory effect of DA on the EPSP. However, neither the D1-receptor agonist SKF-38393 (1 microM) nor the D3-receptor agonist (PD-128,907 (1 microM) affected the EPSP. 5. Incubation of hippocampal slices with pertussis toxin (PTX, 5 micrograms/ml) for 12 h prevented the reduction of EPSP induced by DA (0.3 microM). 6. Rp-adenosine-3',5'-cyclic monophosphothioate (25 microM), a potent inhibitor of protein kinase A (PKA), alone decreased the amplitude of EPSP below baseline values and prevented the subsequent reduction by DA (0.3 microM). 7. These results indicate that DA at a low concentration (< or = 0.3 microM) reduces the excitatory response of hippocampal CA1 neurons after synaptic stimulation via the activation of presynaptic D2 receptors. The presynaptic action of DA is mediated by a PTX-sensitive Gi-proteins-coupled to PKA pathway.

Animals↗

PubMed1996

L-deprenyl (selegiline) decreases excitatory synaptic transmission in the rat hippocampus via a dopaminergic mechanism.

The effect of L-deprenyl (selegiline) on the excitatory synaptic transmission was characterized in the CA1 neurons of rat hippocampal slices by using a intracellular recording technique. Superfusion of L-deprenyl (0.1-10 microM) reversibly decreased the EPSP, which was evoked by orthodromic stimulation of the Schaffer collateral-commissural afferent pathway in a concentration-dependent manner. The sensitivity of postsynaptic neurons to the glutamate receptor agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid or N-methyl-D-aspartate, was not affected by L-deprenyl (1 microM) pretreatment. In addition, L-deprenyl (1 microM) clearly increased the magnitude of paired-pulse facilitation regardless of the interstimulus intervals of 20 to 300 msec used. The ability of L-deprenyl to decrease the EPSP amplitude was not observed in the dopamine-depleted rats. Pargyline and 4-phenylpyridine, the monoamine oxidase type B inhibitors, mimicked the depressant effect of L-deprenyl on the EPSP. Moreover, the reduction of L-deprenyl (1 microM) on the EPSP amplitude was specifically antagonized by sulpiride (0.01-0.1 microM), a selective dopamine D2 receptor antagonist. However, the dopamine D1 receptor antagonist, SKF-83566 (1-10 microM), did not significantly affect L-deprenyl's action. These results indicate that the monoamine oxidase type B inhibitory ability leading to an increase of the dopaminergic tonus in the hippocampus is involved in the L-deprenyl-induced depression of excitatory synaptic transmission in the CA1 region of the rat hippocampus. Moreover, application of L-deprenyl (1 and 10 microM) also reversibly suppressed the epileptiform activity evoked by picrotoxin.

Animals↗

PubMed1995

Muscarinic depression of excitatory synaptic transmission mediated by the presynaptic M3 receptors in the rat neostriatum.

The effect of carbachol on the excitatory synaptic transmission was studied in rat neostriatal neurons using intracellular and whole-cell voltage clamp-recording methods. Depolarizing excitatory postsynaptic potentials (EPSPs) were evoked by cortical stimulation. Superfusion of carbachol (0.01-3 microM) reversibly decreases the EPSP amplitude in a concentration-dependent manner and with an estimated IC50 of 0.3 microM. While, neither the N-methyl-D-aspartate (NMDA, 100 microM)- nor (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 100 microM)-induced response was affected by carbachol (0.1 microM). In addition, the inhibitory effect induced by carbachol at a low concentration of 0.1 microM was attenuated by 4-diphenylacetoxy-N,N-methyl-piperidine (4-DAMP), a selective M3 muscarinic receptor antagonist. However, other muscarinic subtype (M1 or M2) antagonists could also block the inhibitory effect by carbachol 0.1 microM. The rank order of antagonist potency was: 4-DAMP (M3 antagonist) > methoctramine (M2 antagonist) > pirenzepine (M1 antagonist). Based on these findings, we conclude that carbachol at a low concentration (< or = 0.1 microM) reduced the excitatory response of neostriatal neurons following cortical stimulation via presynaptic M3 muscarinic receptors located on the terminals of corticostriatal neurons.

Animals↗

PubMed1995

Presynaptic D2 dopaminergic receptors mediate inhibition of excitatory synaptic transmission in rat neostriatum.

The effect of dopamine (DA) on excitatory synaptic transmission was studied in rat neostriatal neurons using intracellular- and whole-cell voltage clamp-recording methods. Depolarizing excitatory postsynaptic potentials (EPSPs) were evoked by cortical stimulation. Superfusion of DA (0.01-10 microM) reversibly decreases EPSP in a concentration-dependent manner and with a estimated IC50 of 0.3 microM. In addition, the inhibitory effect induced by DA at a low concentration (0.1 microM) was antagonized by sulpiride (1-10 nM), a selective D2 dopaminergic receptor antagonist. However, D1 dopaminergic receptor antagonist SKF-83566 (1-5 microM) did not affect the blocking effect by DA 0.1 microM. Based on these findings, we conclude that DA at a low concentration (< or = 0.1 microM) reduced the excitatory response of neostriatal neurons following cortical stimulation via the activation of D2, but not D1 dopaminergic receptors, located on the terminals of corticostriatal neurons.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗

PubMed1995

Voltage- and use-dependent block by 1-methyl-4-phenylpyridinium ion (MPP+) of N-methyl-D-aspartate-activated currents in rat hippocampal neurons.

The effects of 1-methyl-4-phenylpyridinium ion (MPP+) on N-methyl-D-aspartate (NMDA) receptor-channel complex were studied in rat hippocampal neurons using intracellular- and whole-cell voltage clamp-recording techniques. Intracellular recordings were made from CA1 pyramidal cells of rat hippocampal slices in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and picrotoxin (PTX; 50 microM) which block non-NMDA and GABA receptors, respectively. Superfusion of of MPP+ reversibly decreases the pharmacologically isolated NMDA receptor-mediated excitatory postsynaptic potential (EPSP(NMDA)) in a concentration-dependent manner. In other experiments, we observed that MPP+ attenuated NMDA-evoked whole-cell currents in a voltage- and use-dependent manner and was not dependent on the extracellular glycine or spermine concentration on neurons freshly dissociated from rat hippocampi CA1 region. These results suggest that MPP+ applied at micromolar concentrations, is non-competitive NMDA receptor antagonist in rat hippocampal neurons.

1-Methyl-4-phenylpyridinium↗

PubMed1995

Relaxation of rat thoracic aorta induced by pyridine.

The pharmacological and toxicological activity of pyridine was determined in rat thoracic aorta. Pyridine inhibited norepinephrine (3 microM)-induced phasic and tonic contractions in the thoracic aorta as well as the endothelium-denuded aorta of the rat. The tonic pre-contraction elicited by norepinephrine was also relaxed by the addition of pyridine and this relaxing effect was not affected by indomethacin (20 microM), NG-monomethyl-L-arginine acetate (50 microM) or methylene blue (50 microM). In high-K+ medium (80 mM), pyridine inhibited the Ca2+ concentration-dependent vasocontraction. Moreover, in Ca(2+)-free medium, the norepinephrine (3 microM)-induced phasic contraction was also suppressed by pyridine, while the caffeine (10 mM)-induced contraction remained unaffected. The cAMP and cGMP levels of rat aorta were not changed by pyridine. The 45Ca2+ influx elicited by either norepinephrine or high-K+ was inhibited by pyridine in a concentration-dependent manner. All of these findings indicated that pyridine relaxes rat thoracic aorta by virtue of its Ca2+ channel-blocking properties in vascular smooth muscle.

Animals↗

PubMed1995

Antagonism by paraquat of the relaxing effects of acetylcholine and A23187 in rat thoracic aorta.

Paraquat, a widely used herbicide, has been reported to be capable of producing superoxide. In the present paper, therefore, the possibility of paraquat inhibiting endothelium-dependent relaxation, mediated by the production of nitric oxide, was tested. The relaxing effects of acetylcholine and A23187, but not that of sodium nitroprusside, in the rat thoracic aorta were found to be inhibited by paraquat in a concentration-dependent manner. In contrast, paraquat was totally inactive with regard to the aortic contractions induced by either norepinephrine or prostaglandin F2 alpha. The inhibitory action of paraquat could be antagonized by superoxide dismutase but not by catalase and indomethacin. All of these findings indicate that superoxide anions produced by paraquat in the endothelium contribute to a decrease in the relaxation response to acetylcholine and A23187 by interfering with endothelium-derived nitric oxide.

Acetylcholine↗

PubMed1994

Modulation of the nicotinic acetylcholine receptor channels by spermine in Xenopus muscle cell culture.

The modulation by endogenous polyamine spermine on acetylcholine (ACh)-induced currents was evaluated using a outside-out patch and whole-cell voltage-clamp recordings from cultured Xenopus muscle cells. Spermine potentiated the ACh-induced whole-cell currents at micromolar concentrations but is less effective at millimolar concentrations at a holding potential of -60 mV. It produced a voltage-dependent potentiation of ACh-induced whole-cell currents; the degree of potentiation by 100 microM spermine was 18.3 +/- 1.7% at a holding potential of -60 mV while 108.2 +/- 2.9% increase at +60 mV. Arcaine, a putative competitive antagonist at the polyamine-binding site on the N-methyl-D-aspartate (NMDA) receptor-channel complex, reduced the spermine enhancement of ACh-induced whole-cell currents. Glycine (10 microM) had no effect on the potentiation of ACh-induced whole-cell currents by spermine. In single-channel recordings, spermine (1 microM to 1 mM) increased the opening frequency of both low- and high-conductance ACh channels and at higher concentrations (> or = 100 microM) it produced, in addition, a voltage-dependent decrease in channel amplitude and average open time of both types of ACh channels that limits its enhancing action. It is likely that these two actions of spermine, due to differences in concentration- and voltage-dependence, are mediated by different binding sites on the nicotinic ACh receptor-channel complex of the muscle cells.

Acetylcholine↗