Blockade of the KATP channel Kir6.2 by memantine represents a novel mechanism relevant to Alzheimer's disease therapy.

Here, we report a novel target of the drug memantine, ATP-sensitive K+ (KATP) channels, potentially relevant to memory improvement. We confirmed that memantine antagonizes memory impairment in Alzheimer's model APP23 mice. Memantine increased CaMKII activity in the APP23 mouse hippocampus, and memantine-induced enhancement of hippocampal long-term potentiation (LTP) and CaMKII activity was totally abolished by treatment with pinacidil, a specific opener of KATP channels. Memantine also inhibited Kir6.1 and Kir6.2 KATP channels and elevated intracellular Ca2+ concentrations in neuro2A cells overexpressing Kir6.1 or Kir6.2. Kir6.2 was preferentially expressed at postsynaptic regions of hippocampal neurons, whereas Kir6.1 was predominant in dendrites and cell bodies of pyramidal neurons. Finally, we confirmed that Kir6.2 mutant mice exhibit severe memory deficits and impaired hippocampal LTP, impairments that cannot be rescued by memantine administration. Altogether, our studies show that memantine modulates Kir6.2 activity, and that the Kir6.2 channel is a novel target for therapeutics to improve memory impairment in Alzheimer disease patients.

Nefiracetam and galantamine modulation of excitatory and inhibitory synaptic transmission via stimulation of neuronal nicotinic acetylcholine receptors in rat cortical neurons.

The cholinergic and glutamatergic systems are known to be downregulated in the brain of Alzheimer's disease patients. Galantamine and nefiracetam have been shown to potentiate the phasic activity of nicotinic acetylcholine receptors (nAChRs) in the brain. Stimulation of nAChRs is also known to cause release of various neurotransmitters including glutamate and gamma-aminobutyric acid (GABA). We have previously reported that nefiracetam and galantamine potentiate the activity of nAChRs. Therefore, nefiracetam and galantamine are hypothesized to cause stimulations of the glutamate and GABA systems via stimulation of nAChRs. The present study was set out to test this hypothesis by measuring the effects of these drugs on spontaneous miniature excitatory postsynaptic currents (mEPSCs) and spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded by the whole-cell patch clamp technique from rat cortical neurons in primary cultures. Acetylcholine (ACh) at 30 nM generated a steady inward current and increased the frequency of mEPSCs and mIPSCs. Nefiracetam at 10 nM plus 30 nM ACh increased the frequency of mEPSCs and mIPSCs beyond the levels increased by ACh alone. The potentiating action of nefiracetam was abolished by dihydro-beta-erythroidine. None of these treatments affected the amplitude of mEPSCs or mIPSCs. Galantamine at 1 muM plus ACh did not significantly potentiate the frequency. Nefiracetam at 10 nM had no effect on neurons that did not respond to 30 nM ACh. It was concluded that the nefiracetam released glutamate via stimulation of the alpha4beta2 nAChRs.

Synaptic and behavioral interactions of oseltamivir (Tamiflu) with neurostimulants.

Oseltamivir (Tamiflu), a neuraminidase inhibitor, is widely used for treatment of influenza. Because abnormal behaviors have been observed in some Japanese teenagers following oseltamivir use, its safety has been questioned. Oseltamivir is known to alter neuronal function and behavior in animals, particularly when administered in combination with ethanol. Based on this, it has been hypothesized that interactions of oseltamivir with other drugs may result in altered CNS excitability in this study. It has been found that injection of ephedrine and caffeine overcame inactivity induced by oseltamivir and ethanol but did not alter changes in novelty seeking behavior in a Y-maze test. In ex-vivo hippocampal slices, oseltamivir carboxylate (OTC), an active form of oseltamivir, alters excitability in the absence of ethanol. In slices pretreated with OTC, long-term depression (LTD), a form of synaptic plasticity that is correlated with Y-maze performance was not altered if caffeine or ephedrine was administered individually. However, LTD could not be induced in slices pretreated with OTC if caffeine and ephedrine were administered simultaneously. These observations suggest that combination of oseltamivir with other neurostimulants may alter synaptic plasticity and this may contribute to behavioral changes associated with the drug.

Differential actions of insecticides on target sites: basis for selective toxicity.

Whereas the selective toxicity of insecticides between insects and mammals has a long history of studies, it is now becoming abundantly clear that, in many cases, the differential action of insecticides on insects and mammalian target receptor sites is an important factor. In this paper, we first introduce the mechanism of action and the selective toxicity of pyrethroids as a prototype of study. Then, a more detailed account is given for fipronil, based primarily on our recent studies. Pyrethroids keep the sodium channels open for a prolonged period of time, causing elevation of the depolarizing after-potential. Once the after-potential reaches the threshold for excitation, repetitive after-discharges are produced, resulting in hyperexcitation of intoxicated animals. Only about 1% of sodium channels needs to be modified to produce hyperexcitation, indicating a high degree of toxicity amplification from sodium channels to animals. Pyrethroids were >1000-fold more potent on cockroach sodium channels than rat sodium channels, and this forms the most significant factor to explain the selective toxicity of pyrethroids in insects over mammals. Fipronil, a phenylpyrazole, is known to act on the gamma-aminobutyric acid receptor to block the chloride channel. It is effective against certain species of insects that have become resistant to most insecticides, including those acting on the gamma-aminobutyric acid receptor, and is much more toxic to insects than to mammals. Recently, fipronil has been found to block glutamate-activated chloride channels in cockroach neurons in a potent manner. Since mammals are devoid of this type of chloride channel, fipronil block of the glutamate-activated chloride channel is deemed responsible, at least partially, for the higher selective toxicity to insects over mammals and for the lack of cross-resistance.

Cytoplasmic localization of p63 is associated with poor patient survival in lung adenocarcinoma.

AIMS: To determine the significance of p63 protein expression in the development and progression of lung adenocarcinoma. METHODS AND RESULTS: The expression of p63 was immunohistochemically investigated in 92 cases of lung adenocarcinoma with a maximum diameter of 30 mm or less. p63 expression was observed not only in the nuclei (46/92 cases, 50%), but also in the cytoplasm of neoplastic cells (47/92, 51%). Nuclear localization of p63 was correlated with nuclear accumulation of p53 (P=0.0120), whereas the presence of nuclear p63 had no apparent effect on patient survival. Cytoplasmic localization of p63 was found to be correlated with shorter survival periods by univariate and multivariate analyses (P=0.0486 and P=0.0488, respectively) and the relation was independent of clinicopathological factors. Cytoplasmic localization of p63 was further confirmed by immunoblots of the cytoplasmic fraction of HLC-1, a lung adenocarcinoma cell line which predominately expressed DeltaNp63alpha transcript relative to TAp63 transcript by quantitative reverse transcriptase-polymerase chain reaction. CONCLUSIONS: Cytoplasmic expression of p63 is an adverse prognostic factor in patients with adenocarcinoma of the lung.

Kinetics of modulation of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels by tetramethrin and deltamethrin.

Pyrethroid insecticides may be classified into two groups: type I pyrethroids lack a cyano group in the alpha-position, whereas type II pyrethroids have a cyano group. Both types prolong the sodium channel current thereby causing hyperexcitability, yet details of modulation of current kinetics remain largely to be seen. The mechanism of pyrethroid modulation of sodium currents was studied by the whole-cell patch-clamp technique with rat dorsal root ganglion neurons. Both deltamethrin (type II) and tetramethrin (type I) acted on both tetrodotoxin-sensitive and tetrodotoxin-resistant channels in a qualitatively similar manner and some quantitative differences were derived from different kinetics. During repetitive stimulation in the presence of deltamethrin, leak current increased due to accumulation of prolonged tail currents, explaining the apparent use-dependent modification. For tetramethrin-modified channels, such accumulation was much less because of faster kinetics. Slowing of the kinetics of sodium channel activation by deltamethrin was revealed even after the fast inactivation had been removed by papain. The kinetics of deltamethrin-modified sodium channels was fitted better by the equation that contained two activation components than that with one component. Deltamethrin caused a large shift of the conductance-voltage curve in the direction of hyperpolarization. Cell-attached patch-clamp experiments revealed that deltamethrin had much smaller mobility in the cell membrane than tetramethrin. It was concluded that the apparent use dependence of deltamethrin modification of sodium channels was due primarily to the accumulation of prolonged tail currents during repetitive stimulation and that the sodium channel activation mechanism is the major target of pyrethroids.

Dual action of n-alcohols on neuronal nicotinic acetylcholine receptors.

Alcohol is known to modulate the activity of a variety of neuroreceptors and ion channels. Recently, neuronal nicotinic acetylcholine receptors (nnAChRs) have become a specific focus of study because not only are they potently modulated by alcohol but also they regulate the release of various transmitters, including gamma-aminobutyric acid (GABA) and dopamine, which play an important role in the behavioral effects of ethanol. Whereas the potency of normal alcohols (n-alcohols) to potentiate GABA(A) receptors and to inhibit N-methyl-D-aspartate receptors increases with carbon chain length, we have found that n-alcohols, depending on the carbon chain length, exert a dual action, potentiation and inhibition, on nnAChRs in primary cultured rat cortical neurons. The mechanism of dual action of n-alcohols on nnAChRs was further analyzed using human embryonic kidney cells expressing the alpha 4 beta 2 subunits. Shorter chain alcohols from methanol to n-propanol potentiated acetylcholine (ACh)-induced currents, whereas longer chain alcohols from n-pentanol to n-dodecanol inhibited the currents. n-Butanol either potentiated or inhibited the currents depending on the concentrations of ACh and butanol. The parameters for both potentiation (log EC(200)) and inhibition (log IC(50)) were linearly related to carbon number, albeit with different slopes. The slope for potentiation was -0.299, indicating a change in free energy change (Delta Delta G) of 405 cal/mol/methylene group, whereas the slope for inhibition was -0.584, indicating a Delta Delta G of 792 cal/mol. These results suggest that potentiating and inhibitory actions are exerted through two different binding sites. Ethanol decreased the potency of n-octanol to inhibit ACh currents, possibly resulting from an allosteric mechanism.

Interaction of tetramethrin and deltamethrin at the single sodium channel in rat hippocampal neurons.

Type I and type II pyrethroids are known to modulate the sodium channel to cause persistent openings during depolarization and upon repolarization. Although there are some similarities between the two types of pyrethroids in their actions on sodium channels, the pattern of modification of sodium currents is different between the two types of pyrethroids. In the present study, interactions of the type I pyrethroid tetramethrin and the type II pyrethroid deltamethrin at rat hippocampal neuron sodium channels were investigated using the inside-out single-channel patch clamp technique. Deltamethrin-modified sodium channels opened much longer than tetramethrin-modified sodium channels. When 10 microM tetramethrin was applied to membrane patches that had been exposed to 10 microM deltamethrin, deltamethrin-modified prolonged single sodium currents disappeared and were replaced by shorter openings which were characteristic of tetramethrin-modified channel openings. These single-channel data are compatible with previous whole-cell competition study between type I and type II pyrethroids. These results are interpreted as being due to the displacement of the type II pyrethroid molecule by the type I pyrethroid molecule from the same binding site or to the allosteric interaction of the two pyrethroid molecules at separate sodium channel sites.

Post-stroke dementia. Nootropic drug modulation of neuronal nicotinic acetylcholine receptors.

Nefiracetam is a new pyrrolidone nootropic drug that is being developed for clinical use in the treatment of post-stroke vascular-type and Alzheimer's-type dementia. Among a few neuroreceptors that have been identified as potential targets of nootropics, neuronal nicotinic acetylcholine receptors (nnAChRs) are deemed the most important since they are related to learning, memory, and Alzheimer's disease dementia. We have recently found potent stimulating action of nefiracetam on nnAChRs. Rat cortical neurons in long-term primary culture expressed nnAChRs. Whole-cell patch clamp experiments revealed two types of currents induced by ACh, alpha-bungarotoxin (alpha-BuTX)-sensitive, rapidly desensitizing, alpha 7-type currents and alpha-BuTX-insensitive, slowly desensitizing, alpha 4 beta 2-type currents. Although alpha 7-type currents were only weakly inhibited by nefiracetam, alpha 4 beta 2-type currents were potently and efficaciously potentiated by nefiracetam. Nefiracetam at 0.1 nM reversibly potentiated ACh-induced currents to 200-300% of control. Very high concentrations (about 10 microM) also potentiated these currents, but to a lesser extent, indicative of the bell-shaped dose-response relationship known to occur for nefiracetam, even in animal behavior experiments. Three specific inhibitors of each of PKA and PKC did not prevent nefiracetam from potentiating ACh-induced currents, indicating that these protein kinases are not involved in nefiracetam action. Pretreatment with pertussis toxin did not alter nefiracetam potentiation, indicating Gi/Go proteins are not involved. Pretreatment with cholera toxin did abolish nefiracetam potentiation. Thus, nefiracetam potentiation is mediated via Gs proteins. In conclusion, nefiracetam stimulates alpha 4 beta 2-type nnAChRs via Gs proteins at nanomolar concentrations. The potentiation of alpha 4 beta 2-type nnAChRs is thought to be at least partially responsible for cognitive enhancing action.

Mechanisms of alcohol-nicotine interactions: alcoholics versus smokers.

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Toshio Narahashi and Bo Söderpalm. The presentations were (1) Nicotinic mechanisms and ethanol reinforcement: Behavioral and neurochemical studies, by Bo Söderpalm, M. Ericson, P. Olausson, and J. A. Engel; (2) Chronic nicotine and ethanol: Differential regulation in gene expression of nicotinic acetylcholine receptor subunits, by X. Zhang and A. Nordberg; (3) Nicotine-ethanol interactions at neuronal nicotinic acetylcholine receptors, by Toshio Narahashi, William Marszalec, and Gary L. Aistrup; (4) Relapse prevention in alcoholics by cigarette smoking? Treatment outcome in an observational study with acamprosate, by L.G. Schmidt, U. Kalouti, M. Smolka, and M. Soyka; and (5) Effect of nicotine on voluntary ethanol intake and development of alcohol dependence in male rats, by L. Hedlund and G. Wahlström.

Neuroreceptors and ion channels as targets of alcohol.

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Toshio Narahashi and Kinya Kuriyama. The presentations were (1) Modulation of neuroreceptors and ion channels by alcohol, by T. Narahashi; (2) Inhibition by ethanol of NMDA and AMPA receptor-channels, by P. Illes, K. Wirkner, W. Fischer, K. Mühlberg, P. Scheibler, and C. Allgaier; (3) Effects of ethanol on metabotropic glutamate receptors, by K. Minami; (4) Acute alcohol actions on the 5-HT3 ligand-gated ion channel, by D. Lovinger; (5) Inhibition of NMDA receptors by MK801 attenuates ethanol-induced taurine release from the hippocampus, by F. Lallemand, R.J. Ward, and P. DeWitte; and (6) Effect of ethanol on voltage-operated Ca2+ channels in hepatic stellate cells, by T. Itatsu, Y. Takei, H. Oide, M. Hirose, X. E. Wang, S. Watanabe, M. Tateyama, R. Ochi, and N. Sato.

Nootropic drug modulation of neuronal nicotinic acetylcholine receptors in rat cortical neurons.

Nefiracetam (DM-9384) is a new pyrrolidone nootropic drug being developed for the treatment of Alzheimer's type and poststroke vascular-type dementia. Because the cholinergic system plays an important role in cognitive functions and Alzheimer's disease dementia, the present study was conducted to elucidate the mechanism of action of nefiracetam and aniracetam on neuronal nicotinic acetylcholine receptors (nnAChRs). Currents were recorded from rat cortical neurons in long-term primary culture using the whole-cell, patch-clamp technique. Two types of currents were evoked by acetylcholine (ACh): alpha-bungarotoxin-sensitive, alpha 7-type currents and alpha-bungarotoxin-insensitive, alpha 4 beta 2-type currents. Although nefiracetam and aniracetam inhibited alpha 7-type currents only weakly, these nootropic agents potentiated alpha 4 beta 2-type currents in a very potent and efficacious manner. Nefiracetam at 1 nM and aniracetam at 0.1 nM reversibly potentiated alpha 4 beta 2-type currents to 200 to 300% of control. Nefiracetam at very high concentrations (approximately 10 microM) also potentiated alpha 4 beta 2-type currents but to a lesser extent, indicative of a bell-shaped dose-response relationship. Nefiracetam markedly increased the saturating responses induced by high concentrations of ACh. However, human alpha 4 beta 2 subunits expressed in human embryonic kidney cells were inhibited rather than potentiated by nefiracetam. The specific protein kinase A inhibitors (H-89, KT5720, and peptide 5-24) and protein kinase C inhibitors (chelerythrine, calphostin C, and peptide 19--63) did not prevent nefiracetam from potentiating alpha 4 beta 2-type currents, indicating that these protein kinases are not involved in nefiracetam action. The nefiracetam potentiating action was not affected by 24-h pretreatment of neurons with pertussis toxin, but was abolished by cholera toxin. Therefore, G(s) proteins, but not G(i)/G(o) proteins, are involved in nefiracetam potentiation. These results indicate that nnAChRs are an important site of action of nefiracetam and G(s) proteins may be its crucial target.

Modulation of neuronal nicotinic acetylcholine receptors by halothane in rat cortical neurons.

Inhalational general anesthetics have recently been shown to inhibit neuronal nicotinic acetylcholine (ACh) receptors (nnAChRs) expressed in Xenopus laevis oocytes and in molluscan neurons. However, drug actions on these systems are not necessarily the same as those seen on native mammalian neurons. Thus, we analyzed the detailed mechanisms of action of halothane on nnAChRs using rat cortical neurons in long-term primary culture. Currents induced by applications of ACh via a U-tube system were recorded by the whole-cell, patch-clamp technique. ACh evoked two types of currents, alpha-bungarotoxin-sensitive, fast desensitizing (alpha 7-type) currents and alpha-bungarotoxin-insensitive, slowly desensitizing (alpha 4 beta 2-type) currents. Halothane suppressed alpha 4 beta 2-type currents more than alpha 7-type currents with IC(50) values of 105 and 552 microM, respectively. Halothane shifted the ACh dose-response curve for the alpha 4 beta 2-type currents in the direction of lower ACh concentrations and slowed its apparent rate of desensitization. The rate of recovery after washout from halothane block was much faster than the rate of recovery from ACh desensitization. Thus, the halothane block was not caused by receptor desensitization. Chlorisondamine, an irreversible open channel blocker for nnAChRs, caused a time-dependent block that was attenuated by halothane. These results could be accounted for by kinetic simulation based on a model in which halothane causes flickering block of open channels, as seen in muscle nAChRs. Halothane block of nnAChRs is deemed to play an important role in anesthesia via a direct action on the receptor and an indirect action to suppress transmitter release.

Fipronil modulation of gamma-aminobutyric acid(A) receptors in rat dorsal root ganglion neurons.

The gamma-aminobutyric acid (GABA) receptor is an important site of action of a variety of chemicals, including barbiturates, benzodiazepines, picrotoxin, bicuculline, general anesthetics, alcohols, and certain insecticides. Fipronil is the first phenylpyrazole insecticide introduced for pest control. It is effective against some insects that have become resistant to the existing insecticides. To elucidate the mechanism of fipronil interaction with the mammalian GABA system, whole-cell patch-clamp experiments were performed using rat dorsal root ganglion neurons in primary culture. Fipronil suppressed the GABA-induced whole-cell currents reversibly in both closed and activated states. The IC(50) values and Hill coefficients for fipronil block of the GABA(A) receptor were estimated to be 1.66 +/- 0.18 microM and 1.23 +/- 0.14 for the closed receptor, respectively, and 1.61 +/- 0.14 microM and 0.96 +/- 0.06 for the activated receptor, respectively. The association rate and dissociation rate constants of fipronil effect were estimated to be 673 +/- 220 M(-1) s(-1) and 0.018 +/- 0.0035 s(-1) for the closed GABA(A) receptor, respectively, and 6600 +/- 380 M(-1) s(-1) and 0.11 +/- 0.0054 s(-1) for the activated GABA(A) receptor, respectively. Thus, both the association and dissociation rate constants of fipronil for the activated GABA(A) receptor are approximately 10 times as large as those for the closed receptor. Experiments with coapplication of fipronil and picrotoxinin indicated that they did not compete for the same binding site to block the receptor. It is concluded that although fipronil binds to the GABA(A) receptor without activation, channel opening facilitates fipronil binding to and unbinding from the receptor.

Modulation of tetrodotoxin-resistant sodium channels by dihydropyrazole insecticide RH-3421 in rat dorsal root ganglion neurons.

The effects of the dihydropyrazole insecticide RH-3421 on the retrodotoxin-resistant (TTX-R) voltage-gated sodium channels in rat dorsal root ganglion (DRG) neurons were studied using the whole-cell patch clamp technique. RH-3421 at 10 nM to 1 microM completely blocked action potentials. The sodium currents were irreversibly suppressed by 1 microM RH-3421 in a time- and a dose-dependent manner and the IC50 value of RH-3421 was estimated to be 0.7 microM after 10 min of application. RH-3421 blocked the sodium currents to the same extent over the entire range of test potentials. The sodium conductance-voltage curve was not shifted along the voltage axis by 1 microM RH-3421 application In contrast, both fast and slow steady-state sodium channel inactivation curves were shifted in the hyperpolarizing direction in the presence of 1 microM RH-3421. It was concluded that RH-3421 bound to the resting and inactivated sodium channels to cause block with a higher affinity for the latter state.

Symposium overview: mechanism of action of nicotine on neuronal acetylcholine receptors, from molecule to behavior.

Nicotine has long been known to interact with nicotinic acetylcholine (ACh) receptors since Langley used it extensively to chart sympathetic ganglia a century ago. It has also been used as an effective insecticide. However, it was not until the 1990s that the significance of nicotine was increasingly recognized from the toxicological, pharmacological, and environmental points of view. This is partly because studies of neuronal nicotinic ACh receptors are rapidly emerging from orphan status, fueled by several lines of research. Since Alzheimer's disease is known to be associated with down-regulation of cholinergic activity in the brain, a variety of nicotine derivatives are being tested and developed for treatment of the disease. Public awareness of the adverse effects of nicotine has reached the highest level recently. Since insect resistance to insecticides is one of the most serious issues in the pest-control arena, it is an urgent requirement to develop new insecticides that act on target sites not shared by the existing insecticides. The neuronal nicotinic ACh receptor is one of them, and new nicotinoids are being developed. Thus, the time is ripe to discuss the mechanism of action of nicotine from a variety of angles, including the molecular, physiological, and behavioral points of view. This Symposium covered a wide area of nicotine studies: genetic, genomic, and functional aspects of nicotinic ACh receptors were studied, as related to anthelmintics and insecticides; interactions between ethanol and nicotine out the ACh receptor were analyzed, in an attempt to explain the well-known heavy drinker-heavy smoker correlation; the mechanisms that underlie the desensitization of ACh receptors were studied as related to nicotine action; selective pharmacological profiles of nicotine, and descriptions of some derivatives were described; and chronic nicotine infusion effects on memory were examined using animal models.

Temperature dependence of pyrethroid modification of single sodium channels in rat hippocampal neurons.

Pyrethroid modulation of sodium channels is unique in the sense that it is highly dependent on temperature, the potency being augmented by lowering the temperature. To elucidate the mechanisms underlying the negative temperature dependence of pyrethroid action, single sodium channel currents were recorded from cultured rat hippocampal neurons using the inside-out configuration of patch-clamp technique, and the effects of the pyrethroid tetramethrin were compared at 22 and 12 degrees C. Tetramethrin-modified sodium channels opened with short closures and/or transitions to subconductance levels at 22 and 12 degrees C. The time constants of the burst length histograms for tetramethrin-modified channels upon depolarization to -60 mV were 7. 69 and 14.46 msec at 22 and 12 degrees C, respectively (Q(10) = 0. 53). Tetramethrin at 10 microm modified 17 and 23% of channels at 22 and 12 degrees C, respectively, indicating that the sensitivity of the sodium channel of rat hippocampal neurons to tetramethrin was almost the same as that of tetrodotoxin-sensitive sodium channels of rat dorsal root ganglion neurons and rat cerebellar Purkinje neurons. The time constants for burst length in tetramethrin-modified sodium channels upon repolarization to -100 mV from -30 mV were 8.26 and 68. 80 msec at 22 and 12 degrees C (Q(10) = 0.12), respectively. The prolongation of tetramethrin-modified whole-cell sodium tail currents upon repolarization at lower temperature was ascribed to a prolongation of opening of each channel. Simple state models were introduced to interpret behaviors of tetramethrin-modified sodium channels. The Q(10) values for transition rate constants upon repolarization were extremely large, indicating that temperature had a profound effect on tetramethrin-modified sodium channels.

Basis of variable sensitivities of GABA(A) receptors to ethanol.

BACKGROUND: The GABA(A) system is believed to be one of the crucial target sites for ethanol. However, in the literature, data using various preparations yielded controversial conclusions regarding the ethanol potency to modulate the activity of GABA(A) receptors. We have previously shown that the potency of n-alcohols to potentiate GABA-induced currents is correlated with their carbon chain length. This correlation was further compared among four cell types in an attempt to explain the variable potencies of ethanol to potentiate GABA responses. METHODS: Whole-cell patch clamp experiments were performed to determine and compare the potencies of n-alcohols in potentiating GABA-induced currents in rat dorsal root ganglion (DRG) neurons, human embryonic kidney cells expressing the rat alpha1beta2gamma2S or alpha1beta2gamma2L subunits, and rat cortical neurons. RESULTS: The GABA(A) receptors of the four cell types tested were all sensitive to n-alcohols, albeit with different potencies and efficacies. The effective concentration to increase GABA-induced currents to 125% of control (EC125) was correlated with the carbon chain length of n-alcohols, but slopes for this relationship are different among DRG neurons, the alpha1beta2gamma2S, and alpha1beta2gamma2L subunits. Thus, the potencies of lower alcohols such as ethanol differed among these cell types although higher alcohols such as n-octanol were almost equally potent. In cortical neurons, however, the relationship was shifted in the direction of longer carbon chains, indicating that their sensitivity was lower than those of the other three cell types. The ethanol EC125 values as obtained by experiments or those by extrapolation (in parenthesis) from the EC125-carbon chain length relationship were: 169 (103) mM for DRG neurons, 501 (333) mM for the alpha1beta2gamma2L subunits, 781 (674) mM for the alpha1beta2gamma2S subunits, and (1897) mM for cortical neurons. CONCLUSIONS: It was concluded that the GABA(A) receptors of these four cell types were basically sensitive to n-alcohols including ethanol but the sensitivity curve was shifted to the lower side in the order of decreasing sensitivity of DRG neurons > alpha1beta2gamma2L > alpha1/beta2gamma2S > cortical neurons.

Neuroreceptors and ion channels as the basis for drug action: past, present, and future.

This article summarizes the development of cellular neuropharmacology and neurotoxicology, based primarily on my own research. The progress of this field depends at least in part on the theoretical and technological developments of excitable cell physiology, biophysics, and biochemistry. First, a brief historical development is described. Second, my earlier studies of the mechanism of action of insecticides on the nervous system are introduced. The most significant is the early discovery of the increase in depolarizing after-potential caused by DDT and pyrethroids. This laid the foundation of subsequent analyses of sodium channel modulation as the major mechanism of action of DDT/pyrethroids. Third, my initial contributions to cellular neuropharmacology are described. The discovery of the potent and selective block of sodium channels by tetrodotoxin aroused interest not only in using this toxin and other chemicals as useful laboratory tools but also in studying receptors/channels as important targets of various drugs. Using internally perfused squid giant axons, pioneering studies of local anesthetic action led to the conclusion that these anesthetics block the sodium channel from inside the nerve membrane in the cationic form. Fourth, a few examples of my more recent studies using voltage-clamp and patch-clamp techniques are described. Pyrethroid modulation of sodium channels was analyzed in great detail, including single-channel kinetics, toxicity amplification from channels to animal behaviors, temperature dependence, selective toxicity, and vitamin E antagonism. The neuroprotective drug riluzole blocked sodium channels and high-voltage-activated calcium channels, thereby preventing excess stimulation of N-methyl-D-aspartate receptors and massive influx of calcium, thereby retarding spread of infarction in the brain. Neuronal nicotinic acetylcholine receptors have received much attention recently, and I launched an extensive study of the mechanism whereby alcohols and general anesthetics modulate their activity. Ethanol potently stimulates the alpha-bungarotoxin-insensitive, alpha4beta2-type acetylcholine receptors, thereby causing release of various transmitters; this leads to a cascade of multisynaptic events and behavioral changes. Inhalational general anesthetics augment the activity of gamma-aminobutyric acid(A) receptors and inhibit the activity of alpha4beta2-type acetylcholine receptors, causing a variety of clinical syndromes. Fifth, one of the possible future directions of cellular neuropharmacology and neurotoxicology is discussed. Emphasis is placed on the three-dimensional structure-activity relationship, in particular how changes in the molecular structure of drugs and receptors/channels result in kinetic changes in the function of receptors/channels.

Alpha-thujone (the active component of absinthe): gamma-aminobutyric acid type A receptor modulation and metabolic detoxification.

Alpha-thujone is the toxic agent in absinthe, a liqueur popular in the 19th and early 20th centuries that has adverse health effects. It is also the active ingredient of wormwood oil and some other herbal medicines and is reported to have antinociceptive, insecticidal, and anthelmintic activity. This study elucidates the mechanism of alpha-thujone neurotoxicity and identifies its major metabolites and their role in the poisoning process. Four observations establish that alpha-thujone is a modulator of the gamma-aminobutyric acid (GABA) type A receptor. First, the poisoning signs (and their alleviation by diazepam and phenobarbital) in mice are similar to those of the classical antagonist picrotoxinin. Second, a strain of Drosophila specifically resistant to chloride channel blockers is also tolerant to alpha-thujone. Third, alpha-thujone is a competitive inhibitor of [(3)H]ethynylbicycloorthobenzoate binding to mouse brain membranes. Most definitively, GABA-induced peak currents in rat dorsal root ganglion neurons are suppressed by alpha-thujone with complete reversal after washout. alpha-Thujone is quickly metabolized in vitro by mouse liver microsomes with NADPH (cytochrome P450) forming 7-hydroxy-alpha-thujone as the major product plus five minor ones (4-hydroxy-alpha-thujone, 4-hydroxy-beta-thujone, two other hydroxythujones, and 7,8-dehydro-alpha-thujone), several of which also are detected in the brain of mice treated i.p. with alpha-thujone. The major 7-hydroxy metabolite attains much higher brain levels than alpha-thujone but is less toxic to mice and Drosophila and less potent in the binding assay. The other metabolites assayed are also detoxification products. Thus, alpha-thujone in absinthe and herbal medicines is a rapid-acting and readily detoxified modulator of the GABA-gated chloride channel.