Search PubMed⌕ Search

Biomedical subjects

T Narahashi

Publications and source records attributed to T Narahashi.

At least 37 records · Page 2Linked to original sources
PubMed1999

Neuronal nicotinic acetylcholine receptors: a new target site of ethanol.

Whereas a variety of neuroreceptors and ion channels have been demonstrated to be affected by ethanol including GABAA receptors, NMDA receptors, non-NMDA glutamate receptors, 5-HT3 receptors and voltage-gated calcium channels, neuronal nicotinic acetylcholine receptors (nnAChRs) have recently emerged as a new target site of ethanol. The nnAChRs are different from the muscle type nicotinic AChRs with respect to their molecular architecture and pharmacology. This article briefly reviews the structure, distribution and function of nnAChRs for which a considerable amount of information has been rapidly accumulated during the past 5-10 years. The potent and unique action of ethanol on nnAChRs has been unveiled only during the past few years. Most recent developments along this line of ethanol action are discussed in this paper.

Animals↗
PubMed1999

Anisatin modulation of the gamma-aminobutyric acid receptor-channel in rat dorsal root ganglion neurons.

1. Anisatin, a toxic, insecticidally active component of Sikimi plant, is known to act on the GABA system. In order to elucidate the mechanism of anisatin interaction with the GABA system, whole-cell and single-channel patch clamp experiments were performed with rat dorsal root ganglion neurons in primary culture. 2. Repeated co-applications of GABA and anisatin suppressed GABA-induced whole-cell currents with an EC50 of 1.10 microM. No recovery of currents was observed after washout with anisatin-free solution. 3. However, pre-application of anisatin through the bath had no effect on GABA-induced currents. The decay phase of currents was accelerated by anisatin. These results indicate that anisatin suppression of GABA-induced currents requires opening of the channels and is use-dependent. 4. Anisatin suppression of GABA-induced currents was not voltage dependent. 5. Picrotoxinin attenuated anisatin suppression of GABA-induced currents. [3H]-EBOB binding to rat brain membranes was competitively inhibited by anisatin. These data indicated that anisatin bound to the picrotoxinin site. 6. At the single-channel level, anisatin did not alter the open time but prolonged the closed time. The burst duration was reduced and channel openings per burst were decreased indicating that anisatin decreased the probability of openings.

Affinity Labels↗
PubMed1999

Ion channel modulation as the basis for neuroprotective action of MS-153.

MS-153, (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline, is a new neuroprotective drug. Recent data in the literature suggest that it inhibits glutamate accumulation occurring during ischemia and the translocation of protein kinase C gamma (PKC gamma). The present study was undertaken to prove the hypothesis that MS-153 blocks neuroreceptors and ion channels involved in glutamate accumulation. Neurons isolated from rat dorsal root ganglia and frontal cortex were used for recording channel currents by the whole-cell patch clamp technique. The effects of bath-applied MS-153 were examined on tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels and high voltage-gated calcium channels of dorsal root ganglion neurons, and channels activated by glutamate, N-methyl-D-aspartate (NMDA), kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxarole propionic acid (AMPA), gamma-aminobutyric acid (GABA) and acetylcholine (ACh) in cortical neurons. MS-153 at a concentration of 300 microM had no effect on either tetrodotoxin-sensitive or tetrodotoxin-resistant sodium channels. High voltage-gated calcium channels were either suppressed or not affected by 1-300 microM MS-153. The variable blocking effect of MS-153 was due to the variable activity of intracellular components in individual neurons, especially that of PKC, whose translocation is known to be inhibited by MS-153. When 100 nM phorbol 12-myristate-13-acetate (PMA) was applied to neurons, MS-153 suppressed the calcium channel current more frequently. Calphostin C (0.5 microM), a specific PKC inhibitor, applied intracellularly via recording patch pipette, completely abolished MS-153 suppression of the calcium channel current. Currents induced by glutamate, NMDA, kainate, AMPA, GABA or ACh were not affected by MS-153 at 300 microM. It was concluded that MS-153 inhibited high voltage-gated calcium channels through interactions with PKC, thereby preventing massive release of glutamate from nerve terminals in ischemic conditions.

Animals↗
PubMed1999

Ethanol modulation of nicotinic acetylcholine receptor currents in cultured cortical neurons.

Ethanol, at physiologically relevant concentrations, significantly enhanced high-affinity neuronal nicotinic acetylcholine receptor (NnAChR) currents insensitive to alpha-bungarotoxin (alpha-BuTX-ICs) in cultured rat cortical neurons in a fast and reversible manner, as determined by standard whole-cell patch-clamp recording techniques. The enhancement was (mean +/- S.D.) 7.7 +/- 5% to 192 +/- 52% upon coapplication of 3 to 300 mM ethanol with 1 to 3 microM ACh. No plateau for this ethanol-induced enhancement of alpha-BuTX-ICs was reached. The maximal alpha-BuTX-IC evoked by very high concentrations of ACh also was increased upon coapplication of ethanol. In contrast, ethanol weakly inhibited low-affinity NnAChR currents sensitive to alpha-BuTX (alpha-BuTX-SCs) (5 +/- 4% to 29 +/- 6% inhibition by 10 to 300 mM ethanol at 300 to 1000 microM ACh). This neuronal preparation also enabled comparison of ethanol action on NnAChRs with its action on N-methyl-D-aspartate receptor currents and gamma-aminobutyric acid receptor currents within the same neurons. Ethanol (100 mM) was more potent at enhancing NnAChR alpha-BuTX-ICs (61 +/- 9% enhancement) than it was at enhancing gamma-aminobutyric acid receptor current (3 +/- 3% enhancement-not statistically significant) or at inhibiting N-methyl-D-aspartate receptor currents (approximately 35 +/- 7% inhibition). Thus, NnAChRs, particularly those insensitive to alpha-BuTX, may be sensitive conduits through which ethanol can mediate some of its actions in the brain.

Acetylcholine↗
PubMed1999

Agonist and potentiation actions of n-octanol on gamma-aminobutyric acid type A receptors.

The n-octanol effects on the gamma-aminobutyric acid type A (GABAA) receptor were studied in human embryonic kidney 293 cells transfected with alpha1, beta2, and gamma2S subunit cDNAs. GABA-evoked currents had an EC50 of 13.3 +/- 1.7 microM and a Hill coefficient (nH) of 1.4 +/- 0.1. n-Octanol was also capable of evoking a small current with an EC50 of 1000 microM and an nH of 2. In addition, n-octanol modulated GABA-induced currents in a concentration-dependent manner. Coapplications of n-octanol increased peak currents evoked by 3 microM GABA with an EC50 of 190 microM and an nH of 1.8. The extent of potentiation decreased with increasing GABA concentrations and no potentiation was observed when n-octanol was coapplied with 1000 microM GABA. One-minute preapplication of 1000 microM n-octanol slightly potentiated 3 microM GABA-induced current, whereas it suppressed 300 microM GABA-induced current to 16% of the control, suggesting that 84% of the receptors underwent desensitization. Two models were used to explain n-octanol agonistic and potentiating actions on the alpha1beta2gamma2S GABAA receptor: n-octanol binds to multiple sites to exert multiple actions, or n-octanol acts as a partial agonist to manifest these actions. The partial agonist model is unique because it is a simpler model to explain n-octanol actions on the GABAA receptor.

1-Octanol↗
PubMed1999

Ethanol-nicotine interactions at alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors in rat cortical neurons.

Numerous studies have indicated a correlation between ethanol intake and cigarette smoking in heavy drinkers. We have studied the underlying pharmacological basis of this relationship using cultured rat cortical neurons. These neurons express nicotinic receptors having characteristics similar to those described for the alpha4beta2 subunit combination. In the presence of alpha-bungarotoxin both acetylcholine (ACh) and nicotine evoked currents with respective EC50 values of 4.3 and 3.4 microM. The maximal nicotine-activated response, however, was only 56% that of the maximal ACh current. It was previously shown that 10 to 100 mM of ethanol potentiated ACh-mediated currents in these neurons. We demonstrate that 100 mM ethanol similarly potentiates currents evoked by 300 nM (40%) and 1 microM nicotine 61%). This suggests that an ethanol-induced potentiation of nicotinic currents may enhance the acute positive reinforcement associated with nicotine and could increase tobacco use during heavy ethanol intake. However, further experimentation indicated that the continuous perfusion of 30, 100, or 300 nM nicotine desensitizes ACh-evoked currents by 38, 54, and 62%, respectively, with little direct receptor-channel activation. The residual ACh currents of nicotine-desensitized receptor channels were potentiated by 100 mM ethanol to nearly the extent as were the undesensitized control responses. We propose that the opposing effect of ethanol on nicotine-induced desensitization could also explain the increased tobacco use observed with excessive drinking. Thus, ethanol has a dual effect regarding nicotine. It enhances acute nicotine-mediated receptor activation, although opposing the net effect of nicotine-induced receptor channel desensitization.

Acetylcholine↗
PubMed1999

Effects of the oxadiazine insecticide indoxacarb, DPX-MP062, on neuronal nicotinic acetylcholine receptors in mammalian neurons.

The effects of the novel oxadiazine insecticide DPX-MP062 and its metabolite (DCJW) on neuronal nicotinic acetylcholine receptors (AChRs) were investigated using the whole-cell patch clamp technique in rat embryonic cerebral cortical neurons in primary culture. DPX-MP062, applied at concentrations of 1 and 10 microM to the bath, reduced the peak amplitude of ACh-induced, rapidly decaying currents to 46.8+/-8.8% (n=9) and 15.7+/-5.0% (n=4) of the control, respectively. The effect was irreversible after washing with drug-free solution. DCJW at either 1 microM or 10 microM had similar actions but the potency was much less than that of DPX-MP062. The slowly desensitizing currents induced by low concentrations of ACh (0.1-10 microM) were augmented and those induced by high concentrations of ACh (100-1000 microM) were inhibited by 10 microM DPX-MP062 with great acceleration of the current decay in a time-dependent manner. These effects were use independent and reversible after washing with drug-free solution. In contrast, DCJW at 10 microM did not show significant effects on peak amplitude and decay phase of the slowly desensitizing ACh-induced current in cortical neurons. These results indicate that the oxadiazine insecticide DPX-MP062 has potent modulating actions on neuronal nicotinic AChRs. The neuronal nicotinic AChR could be one of the primary target sites of the insecticide in mammals.

Acetylcholine↗
PubMed1998

Ion channel modulation as the basis for general anesthesia.

(1) Modulation of the function of the GABA(A) and neuronal nicotinic acetylcholine receptor channels caused by general anesthetics and modulation of the GABA(A) receptor-channel by halothane, enflurane, isoflurane, and n-octanol was channel state-dependent. (3) Halothane modulation of the GABA(A) receptor was independent of subunits, but n-octanol modulation was subunit-dependent. (4) Ethanol at 30-100 microM was very potent in accelerating the desensitization of currents induced by acetylcholine. (5) The ethanol modulation was subunit- and state-dependent, occurring in the alpha3beta4 combination but only weakly in the alpha3beta2 combination. (6) In contrast, halothane at 430 microM (approximately 1 MAC) potently suppressed ACh-induced currents in the alpha3beta2 subunit combination.

Anesthesia, General↗
PubMed1998

Dieldrin and picrotoxinin modulation of GABA(A) receptor single channels.

The insecticide dieldrin is known to suppress the GABA(A) receptor-channel complex in a manner similar to that of picrotoxin. To elucidate the more detailed mechanisms of dieldrin and picrotoxin interactions with the GABA system, single-channel patch clamp experiments were performed using rat dorsal root ganglion neurons in primary culture. Dieldrin did not alter the open time distribution and mean current amplitude or the distribution of burst duration and the mean burst duration. However, the mean closed time was prolonged indicating that dieldrin decreased the channel open probability. Previous studies have demonstrated that dieldrin and picrotoxin share the common binding site on the GABA receptor. Thus, the effects of picrotoxinin on the GABA(A) receptor single channels were also examined. Dieldrin and picrotoxinin had similar effects at the single-channel level. These changes of single-channel parameters explain the suppressive effects of these chemicals on GABA-induced whole-cell currents.

Animals↗
PubMed1998

Alcohol modulation of single GABA(A) receptor-channel kinetics.

Alcohol modulation of single-channel kinetics of GABA(A) receptor currents was studied with rat dorsal root ganglion neurons using the excised outside-out patch clamp technique. GABA (1 microM) alone or GABA (1 microM) plus ethanol (30-300 mM) or n-Octanol (30-300 microM) were applied by pressure ejection to evoke single-channel currents. The main single-channel conductance was not changed by either ethanol or n-Octanol at 25 pS. Both alcohols exerted the same effects on the single-channel kinetics, although n-Octanol was more potent than ethanol. The frequency of openings, the mean open time, the percentage of open time, the frequency of bursts, and the mean burst duration were all increased, but the mean closed time was decreased. These changes in channel kinetics account for the increase in whole-cell current amplitude caused by ethanol and n-Octanol.

1-Octanol↗
PubMed1998

Symposium overview: chemical modulation of neuroreceptors and channels via intracellular components.

Whereas the roles of G proteins and protein kinases in various neuroreceptors and ion channels have been studied extensively, their roles in the actions of drugs and toxicants on these receptors and channels remain to be elucidated. Almost all drugs and toxicants exert multiple actions on multiple target sites, and there is no reason to assume that a chemical modulates a receptor/channel via a single mechanism. In fact, experimental evidence is slowly but steadily being accumulated to indicate that certain drugs and toxicants modulate neuroreceptor/channel functions through interactions with intracellular components such as G proteins and protein kinases. Multiple actions of a toxicant on various receptors/channels may be explained on the basis of its interaction with the G protein/kinase system that is a common denominator of the target sites. This is a virgin field that promises a quantum leap in the coming years. Each presentation and discussion will focus on expected future developments and potential significance in the field of neurotoxicology.

Animals↗
PubMed1998

Potent modulation of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels by the type II pyrethroid deltamethrin.

In both tetrodotoxin-resistant (TTX-R) and tetrodotoxin-sensitive (TTX-S) sodium channels, deltamethrin greatly prolonged the current during step depolarizing pulse and caused a large and prolonged slow tail current. The activation was shifted by 20 mV in the hyperpolarizing direction. These changes in channel kinetics account for the prolongation of action potential, membrane depolarization and spontaneous discharges in the deltamethrin-treated neurons. The slow tail current of TTX-S sodium channels rose and decayed slowly, showing a hook. By contrast, the slow tail current of TTX-R channels occurred quickly upon step repolarization. The slow tail current in deltamethrin-treated cells developed slowly during a depolarizing pulse, with a time constant in the order of several milliseconds. The percentages of sodium channels modified by deltamethrin were measured as a function of the deltamethrin concentration. The EC50 values were 0.53 microM and 0.37 microM for TTX-S and TTX-R sodium channels, respectively. However, when compared at the level of 5% modification, the potency of deltamethrin for TTX-R channels was 40 to 50 times higher than that for TTX-S channels. Deltamethrin-induced large and prolonged tail current was hardly reversed after prolonged washout with drug-free solution. However, after application of tetramethrin, it was converted into a much shorter tail current. Washout with solution devoid of tetramethrin and deltamethrin resulted in rapid reappearance of the deltamethrin-type tail current. These results suggest that the deltamethrin and tetramethrin share a binding site on the sodium channel and that the slow onset and offset of deltamethrin action are controlled by the rates at which deltamethrin moves and unbinds from the membrane lipid phase rather than by the rate of deltamethrin binding to the sodium channel site.

Action Potentials↗
PubMed1998

Modulation of the neuronal nicotinic acetylcholine receptor-channel by the nitromethylene heterocycle imidacloprid.

Nitromethylene heterocycle insecticides are known to act on the nicotinic acetylcholine receptor-channel. The effects of the nitromethylene heterocycle, imidacloprid, on the nicotinic acetylcholine receptor-channel of clonal rat phaeochromocytoma (PC12) cells were studied using whole-cell and single-channel patch clamp methods. Imidacloprid suppressed carbachol-induced whole-cell currents in a dose-dependent manner, and this compound itself generated small currents. Multiple conductance states of single-channel currents were also evoked by imidacloprid at the nicotinic acetylcholine receptor-channels. The most frequently generated single-channel currents showed two conductance states, 25.4 and 9.8 pS, which were identical to the conductance states of acetylcholine-generated currents. The mean open time and burst duration of the main conductance currents induced by imidacloprid were shorter than those induced by acetylcholine. Co-application of imidacloprid and acetylcholine caused some interactions at the two conductance states. Mean open time and mean burst duration of the main conductance state currents evoked by acetylcholine were decreased by the co-application of imidacloprid as compared with those induced by acetylcholine alone. In conclusion, imidacloprid has both multiple agonist and antagonist effects on the neuronal nicotinic acetylcholine receptor-channels.

Acetylcholine↗
PubMed1998

Ethanol modulation of excitatory and inhibitory synaptic interactions in cultured cortical neurons.

The effects of ethanol on spontaneous excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) were studied in a culture of embryonic rat cortical neurons. In these experiments, EPSCs and IPSCs were recorded concurrently as inward and outward currents, respectively. These spontaneous currents were dominated by a slow (<1 Hz) repetitive pattern of prolonged N-methyl D-aspartate (NMDA)-EPSCs and co-occurring IPSCs when Mg2+ was left out of the perfusate. A 3- to 5-min bath perfusion of 100 mM ethanol reduced the average integrated EPSC by 65%, while simultaneously potentiating IPSCs by about 3-fold. EPSC frequency was also reduced by about one-third. NMDA-mediated EPSCs were inhibited more than non-NMDA currents. A perfusion of 30 mM ethanol was less effective and probably represents a threshold concentration for these effects. The ethanol inhibition of currents evoked by directly applied glutamate or NMDA to these cells was much less than that observed for spontaneous EPSCs. Currents evoked by exogenous gamma-aminobutyric acid (GABA) application were never potentiated by ethanol. When spontaneous NMDA-EPSCs were blocked with an NMDA antagonist, ethanol no longer potentiated the IPSCs. However, benzodiazepine treatment increased these IPSCs 2-fold. In other experiments, spontaneous IPSCs were blocked by a GABA(A) antagonist. Here, the EPSCs occurred as groups of repetitive bursts. Ethanol decreased the total number of EPSCs per burst but did not decrease their overall amplitude, as in the control recordings. Thus, the way in which ethanol affects concurrently recorded spontaneous EPSCs and IPSCs appears different from the way in which it affects isolated GABA- and NMDA-evoked currents. In addition, the antagonist studies show that concurrently activated NMDA and GABA channels each tend to limit the responses of the other. Thus, the overall effect of ethanol on spontaneous activity may result, in part, by a modification of this synaptic interaction.

Animals↗
PubMed1997

Direct actions of anticholinesterases on the neuronal nicotinic acetylcholine receptor channels.

Recent studies have suggested that anticholinesterases including organophosphates and carbamates act directly on the nicotinic acetylcholine receptor (AChR) channel. We performed whole-cell and single-channel patch-clamp experiments to elucidate the mechanism of action of anticholinesterases on the nicotinic AChR in rat clonal phaeochromocytoma (PC12) cells. Neostigmine and carbaryl showed a biphasic effect; enhancement and suppression of carbachol-induced whole-cell currents. The currents induced by 100 microM carbachol was enhanced by the first co-application with 10 or 100 microM neostigmine, and the current was eventually suppressed below the control level during repeated co-applications. The decay phase of current was accelerated by neostigmine. Carbaryl at 0.1 microM greatly potentiated the carbachol-induced current, and at higher concentrations (0.3-3 microM), current was suppressed. In single-channel experiments, these compounds increased the short closures or gaps during channel opening without changing the single-channel conductance. Mean open time and burst duration were decreased in the presence of neostigmine and carbaryl. These results indicate that neostigmine and carbaryl directly block the nicotinic AChR channel.

Acetylcholine↗
PubMed1997

G-proteins are involved in riluzole inhibition of high voltage-activated calcium channels in rat dorsal root ganglion neurons.

Effects of riluzole on high voltage-activated (HVA) calcium channels of rat dorsal root ganglion neurons were studied using the whole-cell patch-clamp technique. Riluzole at 30 microM inhibited the HVA currents. The onset and offset of riluzole inhibitory effect were slow usually taking more than 3 min. Riluzole inhibition of the HVA currents was abolished and partially reduced by addition of 500 microM GDP-beta-S and 1 mM N-ethylmaleimide, respectively, to the pipette solution. Pre-treatment with pertussis toxin or application of depolarizing pre-pulses did not affect riluzole's inhibitory effect on the HVA currents. Riluzole inhibition of the HVA currents was also blocked by internal application of 50 microg/ml protein kinase A inhibitory peptide. It was concluded that pertussis toxin-insensitive G-proteins and protein kinase A may be involved in riluzole inhibition of the HVA currents.

Animals↗