Selective effects of dieldrin on the GABAA receptor-channel subunits expressed in human embryonic kidney cells

Alternative-splicing in the exon-10 region of GABA(A) receptor beta(2) subunit gene: relationships between novel isoforms and psychotic disorders

Background

Non-coding single nucleotide polymorphisms (SNPs) in GABRB2, the gene for β₂-subunit of gamma-aminobutyric acid type A (GABA_A) receptor, have been associated with schizophrenia (SCZ) and quantitatively correlated to mRNA expression and alternative splicing.

Methods and Findings

Expression of the Exon 10 region of GABRB2 from minigene constructs revealed this region to be an “alternative splicing hotspot” that readily gave rise to differently spliced isoforms depending on intron sequences. This led to a search in human brain cDNA libraries, and the discovery of two novel isoforms, β_2S1 and β_2S2, bearing variations in the neighborhood of Exon-10. Quantitative real-time PCR analysis of postmortem brain samples showed increased β_2S1 expression and decreased β_2S2 expression in both SCZ and bipolar disorder (BPD) compared to controls. Disease-control differences were significantly correlated with SNP rs187269 in BPD males for both β_2S1 and β_2S2 expressions, and significantly correlated with SNPs rs2546620 and rs187269 in SCZ males for β_2S2 expression. Moreover, site-directed mutagenesis indicated that Thr³⁶⁵, a potential phosphorylation site in Exon-10, played a key role in determining the time profile of the ATP-dependent electrophysiological current run-down.

Conclusion

This study therefore provided experimental evidence for the importance of non-coding sequences in the Exon-10 region in GABRB2 with respect to β₂-subunit splicing diversity and the etiologies of SCZ and BPD.

Mutational analysis of molecular requirements for the actions of general anaesthetics at the gamma-aminobutyric acidA receptor subtype, alpha1beta2gamma2

BACKGROUND

Amino acids in the beta subunit contribute to the action of general anaesthetics on GABA(A) receptors. We have now characterized the phenotypic effect of two beta subunit mutations in the most abundant GABA(A) receptor subtype, alpha1beta2gamma2.

RESULTS

The beta2(N265M) mutation in M2 decreased the modulatory actions of propofol, etomidate and enflurane, but not of alphaxalone, while the direct actions of propofol, etomidate and alphaxalone were impaired. The beta2(M286W) mutation in M3 decreased the modulatory actions of propofol, etomidate and enflurane, but not of alphaxalone, whereas the direct action of propofol and etomidate, but not of alphaxalone, was impaired.

CONCLUSIONS

We found that the actions of general anaesthetics at alpha1beta2(N265M)gamma2 and alpha1beta2(M286W)gamma2 GABA(A) receptors are similar to those previously observed at alpha2beta3(N265M)gamma2 and alpha2beta3(M286W)gamma2 GABA(A) recpetors, respectively, with the notable exceptions that the direct action of propofol was decreased in alpha1beta2(M286W)gamma2 receptors but indistinguishable form wild type in alpha2beta3(M286W)gamma2 receptors and that the direct action of alphaxalone was decreased in alpha1beta2(N265M)gamma2 but not alpha2beta3(N265M)gamma2 receptors and indistinguishable form wild type in alpha1beta2(M286W)gamma2 receptors but increased in alpha2beta3(M286W)gamma2 receptors. Thus, selected phenotypic consequences of these two mutations are GABA(A) receptor subtype-specific.

Differential actions of fipronil and dieldrin insecticides on GABA-gated chloride channels in cockroach neurons

Fipronil and dieldrin are known to inhibit GABA receptors in both mammals and insects. However, the mechanism of selective toxicity of these insecticides between mammals and insects remains to be seen. One possible mechanism is that insect GABA receptors are more sensitive than mammalian GABAA receptors to fipronil and dieldrin. We examined differential actions of fipronil and dieldrin on GABA-gated chloride channels in insects and compared them with the data on mammalian GABAA receptors. Neurons were acutely dissociated from the American cockroach thoracic ganglia, and currents evoked by GABA were recorded by the whole-cell patch-clamp technique. GABA-evoked currents were carried by chloride ions, blocked by picrotoxinin, but not by bicuculline. Fipronil inhibited GABA currents with an IC50 value of 28 nM, whereas dieldrin exhibited a dual action potentiation with an EC50 value of 4 nM followed by inhibition with an IC50 value of 16 nM. Fipronil and dieldrin acted on the resting receptor at comparable rates, whereas fipronil blocked the activated receptor 10 times faster than dieldrin. Fipronil inhibition was partially reversible, whereas dieldrin inhibition was irreversible. Fipronil was 59 times more potent on cockroach GABA receptors than on rat GABAA receptors. However, the potentiating and inhibitory potencies of dieldrin in cockroach GABA receptors were comparable with those in rat GABAA receptors. It was concluded that the higher toxicity of fipronil in insects than in mammals is due partially to the higher sensitivity of GABA receptors. The mechanism of dieldrin's selective toxicity must lie in factors other than the sensitivity of GABA receptors.

Pharmacological characterization of the homomeric and heteromeric UNC-49 GABA receptors in C. elegans

(1) UNC-49B and UNC-49C are gamma-aminobutyric acid (GABA) receptor subunits encoded by the Caenorhabditis elegans unc-49 gene. UNC-49B forms a homomeric GABA receptor, or can co-assemble with UNC-49C to form a heteromeric receptor. The pharmacological properties of UNC-49B homomers and UNC-49B/C heteromers were investigated in Xenopus oocytes. (2) The UNC-49 subunits are most closely related to the bicuculline- and benzodiazepine-insensitive RDL GABA receptors of insects. Consistent with this classification, bicuculline (10 micro M) did not inhibit, nor did diazepam (10 micro M) enhance UNC-49B homomeric or UNC-49B/C heteromeric receptors. (3) The UNC-49C subunit strongly affects the pharmacology of UNC-49B/C heteromeric receptors. UNC-49B homomers were much more picrotoxin sensitive than UNC-49B/C heteromers (IC(50)=0.9+/-0.2 micro M and 166+/-42 micro M, respectively). Pentobarbitone enhancement was greater for UNC-49B homomers compared to UNC-49B/C heteromers. Propofol (50 micro M) slightly enhanced UNC-49B homomers but slightly inhibited UNC-49B/C heteromers. Penicillin G (10 mM) inhibited UNC-49B homomers less strongly than UNC-49B/C heteromers (30% compared to 53% inhibition, respectively). (4) Several aspects of UNC-49 pharmacology were unusual. Picrotoxin sensitivity strongly correlates with dieldrin sensitivity, yet UNC-49B homomers were highly dieldrin resistant. The enhancing neurosteroid pregnanolone (5beta-pregnan-3alpha-ol-20-one; 10 micro M) strongly inhibited both UNC-49 receptors. Alphaxalone (10 micro M), another enhancing neurosteroid, did not affect UNC-49B homomers, but slightly inhibited UNC-49B/C heteromers. (5) UNC-49 subunits and mammalian GABA(A) receptor alpha, beta, and gamma subunit classes all share roughly the same degree of sequence similarity. Thus, although they are most similar to other invertebrate GABA receptors, the UNC-49 receptors share significant structural and pharmacological overlap with mammalian GABA(A) receptors.

Comparison of base-excision repair capacity in proliferating and differentiated PC 12 cells following acute challenge with dieldrin

Dieldrin, an organochlorine pesticide and known neurotoxicant, is ubiquitously distributed in the environment. Dieldrin depletes brain monoamines in some animal species and is toxic for dopaminergic neurons in vitro. Dieldrin interferes with mitochondrial electron transport and increases generation of superoxide anion. Reactive oxygen species have been shown to produce oxidative lesions to DNA bases, i.e., 8-hydroxy-2'-deoxyguanosine (8-oxodGuo). Accumulation of 8-oxodGuo has been shown to be promutagenic in proliferating cells, and can lead to degeneration in fully differentiated cells. The objective of this study was to determine the effects of dieldrin exposure on the activity of the enzyme responsible for removing 8-oxodGuo, OGG1, from undifferentiated (untreated with NGF) and differentiated (NGF-treated) PC 12 cells. Proliferating PC 12 cells exhibited a mild upregulation of glycosylase activity, reaching a maximum by 1 h and returning to baseline by 6 h. Differentiated (+) NGF cells showed a time-dependent decline in activity reaching a nadir at 3 h with a return towards baseline by 6 h. Levels of the damaged base, 8-oxodGuo, in the differentiated PC12 cells appeared to be regulated by the activity of OGG1. In contrast, levels of the damaged base in actively proliferating cells were independent of the OGG1 activity. This difference between actively dividing and differentiated cells in the regulation of base-excision repair and DNA damage accumulation explains, in part, the vulnerability of postmitotic neurons to oxidative stresses and neurotoxins.

The central nervous system convulsant pentylenetetrazole stimulates gamma-aminobutyric acid (GABA)-activated current in picrotoxin-resistant GABA(A) receptors in HEK293 cells

We tested the ability of the central nervous system convulsant pentylenetetrazole (PTZ) to inhibit gamma-aminobutyric acid (GABA)-gated current in receptors expressing a mutation that rendered them resistant to picrotoxin. Consistent with previous reports, receptors expressing beta2(T246F), along with alpha3 and gamma2 subunits, resulted in a greatly diminished sensitivity to picrotoxin. Sensitivity to PTZ was completely abolished in the mutant receptor, confirming the hypothesis that PTZ acts at the picrotoxin site. Quite unexpected, however, was our finding that PTZ elicited marked stimulation (up to 400% of control) in the mutated receptors. This stimulatory effect was not mediated via an interaction with the benzodiazepine site, as preincubation with the benzodiazepine antagonist flumazenil did not block the PTZ-induced stimulation. Our results reveal the existence of a novel stimulatory domain of PTZ in GABA(A) receptors.

Influence of subunit configuration on the interaction of picrotoxin-site ligands with recombinant GABA(A) receptors

We have assessed the interaction of picrotoxin and a putative picrotoxin-site ligand [4-dimethyl-3-t-butylcarboxyl-4,5-dihydro (1, 5-a) quinoxaline] (U-93631) with varying configurations of recombinant GABA(A) receptors, using the whole-cell patch clamp technique. In alpha2beta2gamma2 GABA(A) receptors, coapplication of picrotoxin with GABA had minimal effects on initial GABA-activated Cl(-) current amplitude, and subsequently enhanced decay of GABA-activated Cl(-) currents. The half-maximal inhibitory concentration (IC(50)) for picrotoxin in alpha2beta2gamma2 receptors was 10.3+/-1.6 microM. The alpha subunit isoform did not affect picrotoxin-induced inhibition, as IC(50) values for alpha3beta2gamma2 (5.1+/-0.7 microM) and alpha6beta2gamma2 receptors (7.2+/-0.4 microM) were comparable to those obtained in alpha2beta2gamma2 receptors. Interestingly, in receptors lacking an alpha subunit (beta2gamma2 configuration), picrotoxin had a markedly lower IC(50) (0.5+/-0.05 microM) compared to alpha-containing receptors. The inhibitory profile was generally similar for the presumed picrotoxin-site ligand U-93631, i.e., IC(50) values were comparable in all alphabetagamma-containing receptors, but the IC(50) in beta2gamma2 receptors was greater than 10-fold lower. In addition, a modest but significant initial stimulation of GABA-activated current by U-93631 was observed in alpha2beta2gamma2 and beta2gamma2 receptors. A mutation in the second transmembrane domain, shown previously to abolish picrotoxin sensitivity, also greatly attenuated sensitivity to U-93631. Moreover, incubation of receptors with excess U-93631 hindered picrotoxin's ability to gain access to its binding site; both results indicate that U-93631 interacts at the picrotoxin site of the receptor. Our results indicate the presence of an alpha subunit hinders the ability of picrotoxin to block the GABA(A) receptor, and thus provides additional insight into the site of action of picrotoxin. In addition, we have shown that domains important for the actions of picrotoxin also affect U-93631. Thus, this compound should prove to be a useful ligand for analysis of the convulsant site of this receptor.

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.

GABAA receptors mediate trophic effects of GABA on embryonic brainstem monoamine neurons in vitro

The inhibitory neurotransmitter GABA may act as a trophic signal for developing monoamine neurons in embryonic rat brain, because GABA neurons and their receptors appear in brainstem during generation of monoamine neurons. To test this hypothesis, we used dissociated cell cultures from embryonic day 14 rat brainstem, which contains developing serotonin (5-HT), noradrenaline (tyrosine hydroxylase; TH), and GABA neurons. Immunocytochemistry and reverse transcription-PCR (RT-PCR) revealed the presence of multiple alpha, beta, gamma, and delta subunits in these cultures. Competitive RT-PCR demonstrated high levels of beta3 subunit transcripts. Expression of functional GABAA receptors was demonstrated using 36Cl- flux assays. To investigate GABAergic regulation of neuronal survival and growth, cultures were treated for 1-3 d in vitro with 10 microM GABA and/or GABAA antagonist (bicuculline or the pesticide dieldrin). The effects of treatments were quantified by analysis of immunoreactive 5-HT, TH, and GABA neurons. GABAA receptor ligands differentially regulated neuronal survival and growth depending on neurotransmitter phenotype. GABA exerted positive effects on monoamine neurons, which were countered by bicuculline (and dieldrin, 5-HT neurons only). By itself, bicuculline produced inhibitory effects on both 5-HT and TH neurons, whereas dieldrin potently inhibited 5-HT neurons only. GABA neurons responded positively to both antagonists, but more strongly to bicuculline. Taken together, these results demonstrate that the activation/inhibition of GABAA receptors produces opposite effects on the development of embryonic monoamine and GABA neurons. This suggests that these neurotransmitter phenotypes may express GABAA receptors that differ in fundamental ways, and these differences determine the developmental responses of these cells to GABAergic stimuli.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

Pharmacology of recombinant gamma-aminobutyric acidA receptors rendered diazepam-insensitive by point-mutated alpha-subunits

Amino acids in the alpha- and gamma-subunits contribute to the benzodiazepine binding site of GABA(A)-receptors. We show that the mutation of a conserved histidine residue in the N-terminal extracellular segment (alpha1H101R, alpha2H101R, alpha3H126R, and alpha5H105R) results not only in diazepam-insensitivity of the respective alphaxbeta2,3gamma2-receptors but also in an increased potentiation of the GABA-induced currents by the partial agonist bretazenil. Furthermore, Ro 15-4513, an inverse agonist at wild-type receptors, acts as an agonist at all mutant receptors. This conserved molecular switch can be exploited to identify the pharmacological significance of specific GABA(A)-receptor subtypes in vivo.

Regulation of GABA(A) receptor subunit mRNA expression by the pesticide dieldrin in embryonic brainstem cultures: a quantitative, competitive reverse transcription-polymerase chain reaction study

Cyclodiene organochlorine pesticides, such as dieldrin, inhibit gamma-aminobutyric acid (GABA)ergic neurotransmission by blocking the Cl- channel of GABA(A) receptors. This action may make the developing nervous system especially vulnerable to these neurotoxins, which could interfere with the trophic actions of GABA on developing neurons and alter expression of GABA(A) receptors. We have used an in vitro model to determine whether exposure to dieldrin alters developmental expression of GABA(A) receptor subunit mRNA transcripts. Dissociated cell cultures were prepared from embryonic day 14 (E14) brainstem and cultured in serum-containing medium for 1 day in vitro (DIV), then treated for 2 DIV with 10 microM dieldrin in serum-free medium. This dose was based on preliminary experiments and previous studies (Nagata et al.: Brain Res 645:19-26, 1994; Pomes et al.: J Pharmacol Exp Ther 271:1616-1623, 1994). Absolute amounts of alpha1, beta3, gamma1, gamma2S and gamma2L mRNA transcripts were quantified in these cultures by quantitative, competitive reverse transcription-polymerase chain reaction (RT-PCR) using subunit-selective internal standards. The most abundant GABA(A) subunit transcript was beta3, which was much more highly expressed than gamma2S, gamma1, gamma2L, or alpha1 subunit mRNAs. Dieldrin differentially regulated expression of these transcripts. Levels of beta3 subunit transcripts were significantly increased (by 300%) by dieldrin, whereas expression of gamma2S and gamma2L transcripts were decreased (by 50% and 40%, respectively). However, dieldrin did not alter the ratio of gamma2S to gamma2L transcripts, indicating that it did not affect alternative splicing of gamma2 transcripts. Dieldrin appeared to increase expression of alpha1 subunit transcripts, but this effect was not statistically significant. Dieldrin did not significantly alter expression of gamma1 subunit transcripts. These results support the hypothesis that in utero exposure to cyclodiene pesticides could pose a risk to the developing brain by virtue of their ability to alter gene expression of GABA(A) receptor subunits, which could produce GABA(A) receptors with altered functional properties.

Differential effects of hexachlorocyclohexane isomers on the GABA receptor subunits expressed in human embryonic kidney cell line

We have recently demonstrated by patch clamp experiments that the four isomers of hexachlorocyclohexane (HCH), alpha-, beta-, gamma- and delta-HCH insecticides, modulated the kinetics of the GABAA receptor-chloride channel complex of rat dorsal root ganglion neurons. The present paper reports the differential effects of HCH isomers of the GABA-induced chloride currents in three combinations of alpha, beta and gamma subunits of GABAA receptor expressed in human embryonic kidney cells. When co-applied with GABA, gamma-HCH strongly suppressed the peak amplitude of GABA-induced current, and delta-HCH strongly enhanced it in the alpha 1 beta 2 gamma 2s, alpha 1 beta 2, alpha 6 beta 2 gamma 2s combinations in a dose-dependent manner. There was little or no difference in the dose dependence of the effects between gamma- and delta-HCH in any of the three subunit combinations. However, alpha- and beta-HCH showed differential effects on GABA-induced chloride currents in the three subunit combinations tested. alpha-HCH showed enhancing effects on the peak current in alpha 1 beta 2 gamma 2s, small enhancing effects on alpha 1 beta 2, and biphasic effects on alpha 6 beta 2 gamma 2s subunit combinations. beta-HCH had little or no effect on the peak current in alpha 1 beta 2 gamma 2s and alpha 1 beta 2 combinations, but suppressed currents in the alpha 6 beta2 gamma 2s subunit combination in a dose-dependent manner. The differential actions of HCH isomers may produce variable effects on different regions of the nervous systems and in different species of animals.

Neuronal ion channels as the target sites of insecticides

Certain types of neuronal ions channels have been demonstrated to be the major target sites of insecticides. The insecticide-channel interactions that have been studied most extensively are pyrethroid actions on the voltage-gated sodium channel and cyclodiene/lindane actions on the GABAA receptor chloride channel complex. With the exception of organophosphate and carbamate insecticides which inhibit acetylcholinesterases, most insecticide commercially developed act on the sodium channel and the GABA system. Pyrethroids show the kinetics of both activation and inactivation gates of sodium channels resulting in prolonged openings of individual channels. This causes membrane depolarization, repetitive discharges and synaptic disturbances leading to hyperexcitatory symptoms of poisoning in animals. Only a very small fraction (approximately 1%) of sodium channel population is required to be modified by pyrethroids to produce severe hyperexcitatory symptoms. This toxicity amplification theory applies to pharmacological and toxicological action of other drugs that go through a threshold phenomenon. Selective toxicity of pyrethroids between invertebrates and mammals can be explained based largely on the responses of sodium channels and partly on metabolic degradation. The pyrethroid-sodium channel interaction is also supported by Na+ uptake and batrachotoxin binding experiments. Cyclodienes and lindane exert a dual action on the GABAA system, the initial transient stimulation being followed by a suppression. The stimulation requires the presence of the gamma 2 subunit. The suppression of the GABA system is also documented by Cl- flux and ligand binding experiments. It appears that the sodium channel and the GABA system merit continuing efforts for development of newer and better insecticides. Nitromethylene heterocycles including imidacloprid act on nicotinic acetylcholine receptors. Insect receptors are more sensitive to these compounds than mammalian receptors. Single-channel analyses of the nicotinic acetylcholine receptor of PC12 cells have shown that imidacloprid increases the activity of subconductance state currents and decreases that of main conductance state currents. This may explain the imidacloprid suppression of acetylcholine responses.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

Differential effects of hexachlorocyclohexane isomers on the GABA receptor-chloride channel complex in rat dorsal root ganglion neurons

The GABAA receptor-chloride channel complex has recently been demonstrated by patch clamp experiments to be the target of cyclodiene insecticides. We have now examined the effects of four isomers of hexachlorocyclohexane (HCH), alpha-, beta-, gamma- and delta-HCH, on the GABAA receptor-chloride channel complex of rat dorsal root ganglion neurons using patch clamp techniques. When co-applied with 10 microM GABA, 1 microM gamma-HCH slightly enhanced and then suppressed the GABA-induced chloride current. The desensitization of the current was greatly accelerated by gamma-HCH in a dose-dependent manner. The acceleration of desensitization and the suppression of sustained component of current by gamma-HCH occurred at lower concentration ranges than those for the suppression of peak current. When 10 microM delta-HCH was co-applied with 10 microM GABA, current was greatly enhanced and then suppressed, and the level of enhancement was much higher than that of gamma-HCH. alpha- and beta-HCH had little or no effect on the GABA-induced chloride current. The differential actions of these isomers on GABA-activated chloride currents account for variable symptoms of poisoning in insects and mammals.

Sodium channels and GABAA receptor-channel complex as targets of environmental toxicants

Voltage-activated sodium channels and GABAA receptor-chloride channel complex are among the most important target sites of various environmental neurotoxicants. Pyrethroids keep the sodium channels open for prolonged periods of time leading to hyperexcitation of the entire nervous system. In rat cerebellar Purkinje neurons and dorsal root ganglion neurons, only about 1% of sodium channel population needed to be modified by the pyrethroid tetramethrin to increase the depolarizing after-potential to the level of the threshold membrane potential for generation of repetitive after-discharges. This concept of toxicity amplification is applicable to other chemicals that go through a threshold phenomenon to exert their effects. The potency of pyrethroids on neuronal sodium channels increased with lowering the temperature with a Q10 value of 0.2. The selective pyrethroid toxicity between mammals and insects can be quantitatively explained on the basis of the differences in 5 factors, i.e. the intrinsic sodium channel sensitivity, the sodium channel modification due to temperature difference, the reversibility of sodium channel, the detoxication of pyrethroids, and body size. These 5 factors are multiplied to approximately 2000 which is in the same order of magnitude as that of the difference in LD50. Dieldrin had a dual action on the GABAA receptor-chloride channel complex of rat dorsal root ganglion neurons. The initial transient potentiation of GABA-induced currents after application of dieldrin was followed by a suppression. Dieldrin-induced potentiation of current was observed only when the gamma 2 subunit was present in embryonic kidney cells (HEK-293) transfected with GABA receptor subunits. Dieldrin-induced suppression was observed in the presence and absence of the gamma 2 subunit. The dieldrin suppression of GABA-induced currents is deemed directly responsible for hyperactive symptoms of poisoning in animals.

Interaction of picrotoxin with GABAA receptor channel-lining residues probed in cysteine mutants

We used the substituted-cysteine-accessibility method to identify the channel-lining residues in a region (257-261) near the putative cytoplasmic end of the M2 membrane-spanning segment of the rat gamma-aminobutyric acid type A (GABAA) receptor alpha 1 subunit. The residues alpha 1Val257 and alpha 1Thr261 were accessible to charged, sulfhydryl-specific reagents applied extracellularly in both the open and closed states. The accessibility of alpha 1V257C and alpha 1T261C in the closed state implies that the gate must be at least as close to the cytoplasmic end of the channel as alpha 1Val257. Also, the positively charged reagent methanethiosulfonate ethylammonium penetrated from the extracellular end of the channel to alpha 1T261C, with which it reacted, indicating that the anion-selectivity filter is closer to the cytoplasmic end of the channel than this residue is. Co-application of picrotoxin prevented the sulfhydryl reagents from reacting with alpha 1V257C but did not prevent reaction with the more extracellular residue alpha 1T261C. Picrotoxin protection of alpha 1V257C may be due to steric block by picrotoxin bound in the channel at the level of alpha 1Val257; however, if this protection is allosteric, it is not due to the induction of the resting closed state in which alpha 1V257C was accessible to sulfhydryl reagent.

Stable high expression of human gamma-aminobutyric acidA receptors composed of alpha and beta subunits

Multiple classes of pharmacological agents including benzodiazepines, cage convulsants like t-butylbicyclophosphorothionate (TBPS), barbiturates and neuroactive steroids allosterically modulate the gamma-aminobutyric acidA receptor-chloride ionophore complex (GRC). The function of benzodiazepines requires a GRC comprised of alpha, beta and gamma subunits, while TBPS, barbiturates and neuroactive steroids will allosterically modulate GRCs comprised of only alpha and beta subunits. Binary alpha beta complexes are still hypothesized to be expressed in the mammalian brain particularly during development and could contribute to the pharmacological action of neuroactive steroids and barbiturates. In order to examine binary alpha beta complexes we report here the establishment of stable cell lines that express high levels of human GABAA receptors comprised of alpha 1 beta 1, alpha 2 beta 1 and alpha 3 beta 1 subunit combinations. The apparent potencies for allosteric modulation of [35S]TBPS for most naturally occurring neuroactive steroids for the binary subunit combinations was similar to that of the gamma-containing subunit combinations. Also discussed is the usefulness of these cell lines for the biophysical analysis of the GABAA receptor stoichiometry.

2,3-Butanedione monoxime protects mice against the convulsant effect of picrotoxin by facilitating GABA-activated currents

While adult mice receiving picrotoxin (PTX) alone responded with clonic and tonic-clonic seizures, this response was greatly suppressed for mice simultaneously injected with 2,3-butanedione monoxime (BDM). For example, 60% and 10% of the mice convulsed when injected (i.p.) with 3.0 mg/kg PTX alone or PTX plus 205 mg/kg of BDM, respectively. In contrast, a non-oxime analogue of BDM, 2,3-butanedione (BTD), did not have this anticonvulsant effect. In order to explore the basis for the anticonvulsant effect of BDM, we recorded GABA-activated currents (IGABA) of frontal cortical as well as ventromedial hypothalamic neurons before, during and after exposure to this oxime. BDM had a biphasic effect on IGABA. That is, high concentrations (100 microM-40 mM) decreased and lower concentrations (0.01 microM-0.001 microM) potentiated IGABA; these effects of BDM reversed upon washout of the oxime. In contrast, BTD had no effect on IGABA. Finally, when 0.001 microM BDM, 10-30 microM PTX and GABA were co-applied the inhibitory effect of the toxin on IGABA was markedly suppressed. These data suggest that the anticonvulsant effect of oximes involves facilitation of the inhibitory action of GABA.