Polychlorocycloalkane insecticide-induced convulsions in mice in relation to disruption of the GABA-regulated chloride ionophore

Investigating open access new approach methods (NAM) to assess biological points of departure: A case study with 4 neurotoxic pesticides

Open access new approach methods (NAM) in the US EPA ToxCast program and NTP Integrated Chemical Environment (ICE) were used to investigate activities of four neurotoxic pesticides: endosulfan, fipronil, propyzamide and carbaryl. Concordance of in vivo regulatory points of departure (POD) adjusted for interspecies extrapolation (AdjPOD) to modelled human Administered Equivalent Dose (AEDHuman) was assessed using 3-compartment or Adult/Fetal PBTK in vitro to in vivo extrapolation. Model inputs were from Tier 1 (High throughput transcriptomics: HTTr, high throughput phenotypic profiling: HTPP) and Tier 2 (single target: ToxCast) assays. HTTr identified gene expression signatures associated with potential neurotoxicity for endosulfan, propyzamide and carbaryl in non-neuronal MCF-7 and HepaRG cells. The HTPP assay in U-2 OS cells detected potent effects on DNA endpoints for endosulfan and carbaryl, and mitochondria with fipronil (propyzamide was inactive). The most potent ToxCast assays were concordant with specific components of each chemical mode of action (MOA). Predictive adult IVIVE models produced fold differences (FD) < 10 between the AEDHuman and the measured in vivo AdjPOD. The 3-compartment model was concordant (i.e., smallest FD) for endosulfan, fipronil and carbaryl, and PBTK was concordant for propyzamide. The most potent AEDHuman predictions for each chemical showed HTTr, HTPP and ToxCast were mainly concordant with in vivo AdjPODs but assays were less concordant with MOAs. This was likely due to the cell types used for testing and/or lack of metabolic capabilities and pathways available in vivo. The Fetal PBTK model had larger FDs than adult models and was less predictive overall.

Pesticides and tremor: An overview of association, mechanisms and confounders

Pesticides are a heterogeneous class of chemicals mainly used for the protection of crops from pests. Because of their very widespread use, acute or/and chronic exposure to these chemicals can lead to a plethora of sequelae inflicting diseases, many of which involve the nervous system. Tremor has been associated with pesticide exposure in human and animal studies. This review is aimed at assessing the studies currently available on the association between the various types of pesticides/insecticides and tremor, while also accounting for potential confounding factors. To our knowledge, this is the first coherent review on the subject. After appraising the available evidence, we call for more intensive research on this topic, as well as intonate the need of implementing future preventive measures to protect the exposed populations and to reduce potential disabilities and social drawbacks.

Novel neurosteroid pregnanolone pyroglutamate suppresses neurotoxicity syndrome induced by tetramethylenedisulfotetramine but is ineffective in a rodent model of infantile spasms

BACKGROUND

Neurosteroids are investigated as effective antidotes for the poisoning induced by tetramethylenedisulfotetramine (TMDT) as well as treatments for epileptic spasms during infancy. Both these conditions are quite resistant to pharmacotherapy; thus, a search for new treatments is warranted.

METHODS

In this study, we determined the efficacy of two novel neurosteroids, pregnanolone glutamate (PAG) and pregnanolone pyroglutamate (PPG), and tested these drugs in doses of 1-10 mg/kg (ip) against the TMDT syndrome and in our rodent model of infantile spasms.

RESULTS

Only PPG in doses 5 and 10 mg/kg suppressed the severity of the TMDT syndrome and TMDT-induced lethality, while the 1 mg/kg dose was without an effect. Interestingly, the 1 mg/kg dose of PPG in combination with 1 mg/kg of diazepam was also effective against TMDT poisoning. Neither PAG nor PPG were effective against experimental spasms in the N-methyl-D-aspartate (NMDA)-triggered model of infantile spasms.

CONCLUSIONS

While evidence suggests that PAG can act through multiple actions which include allosteric inhibition of NMDA-induced and glycine receptor-evoked currents as well as augmentation of ɣ-aminobutyric acid subtype A (GABAA) receptor-induced currents, the agent appears to neither have the appropriate mechanistic signature for activity in the infantile spasm model, nor the adequate potency, relative to PPG, for ameliorating the TMDT syndrome. The full mechanisms of action of PPG, which may become a potent TMDT antidote either alone or in combination with diazepam are yet unknown and thus require further investigation.

Modelling Anxiety Disorders Following Chemical Exposures

The effects of kindling and inverse benzodiazepine receptor agonist -carbolines on animal models of anxiety are briefly reviewed in relation to affective disorder associated with chemical exposure. Recent experimental results are described. In the present study, cats were given the inverse benzodiazepine receptor agonist, FG-7142, a powerful anxiogenic compound in humans and animals. Neural transmission in pathways involved in defensive behavior in the cat was monitored using evoked potential techniques. Change in these pathways was related to behavioral changes induced by the drug. It was found that a single dose offG-7142 lastingly increased defensive response to rodents for at least 40 days after drug administration. Behavioral change was specific to defensive response, since approach-attack behavior remained unchanged, replicating previous studies. The benzodiazepine receptor antagonist, Flumazenil, reversed the increase in defensiveness in a drug-dependent manner, replicating previous findings. Increased defensiveness was paralleled by a delayed onset potentiation of neural transmission between the amygdala and the medial hypothalamus of the left hemisphere. Potentiation in the left hemisphere was transient, decaying between 6 and 12 days after the drug. There was a longer lasting potentiation (LTP) of activity evoked in the left and right amygdalo-periacqueductal gray pathways and in the right amygdalo-medial hypothalamic pathway. Potentiation in these pathways appeared at the time of behavioral change. Potentiation of the right amygdalo-periacqueductal gray and right amygdalo-medial hypothalamic pathways persisted until the end of the experiment. In contrast, potentiation of the left amygdalo-periacqueductal gray pathway faded by 40 days after the drug. Flumazenil decreased potentiation only in the right amygdalo-periacqueductal gray pathway. These data strongly suggest that lasting affective change is mediated by lasting changes in particular efferents of the amygdala of the right hemisphere. Behavioral and physiological effects offG-7142 were blocked by the N-methyl-D-Aspartate (NMDA) receptor blocker, AP7. The data suggest that failure of neural inhibition induced by FG-7142 engages NMDA receptor processes to produce lasting potentiation of transmission in neural circuits that mediate defensive response with behavioral consequences. Since FG-7142 interferes with GABA mediated neural inhibition and is proconvulsant, its action might mimic the action of other environmental chemicals with similar properties, such as chlorinated hydrocarbon insecticides. The relationship of the present data to the literature on the neural and behavioral effects of insecticide exposure is discussed. The significance of these findings for multiple chemical sensitivity disorder is also briefly discussed.

Susceptibility of larval zebrafish to the seizurogenic activity of GABA type A receptor antagonists

Previous studies demonstrated that pentylenetetrazole (PTZ), a GABA type A receptor (GABA_AR) antagonist, elicits seizure-like phenotypes in larval zebrafish (Danio rerio). Here, we determined whether the GABA_AR antagonists, tetramethylenedisulfotetramine (TETS) and picrotoxin (PTX), both listed as credible chemical threat agents, similarly trigger seizures in zebrafish larvae. Larvae of three, routinely used laboratory zebrafish lines, Tropical 5D, NHGRI and Tupfel long fin, were exposed to varying concentrations of PTZ (used as a positive control), PTX or TETS for 20 min at 5 days post fertilization (dpf). Acute exposure to PTZ, PTX or TETS triggered seizure behavior in the absence of morbidity or mortality. While the concentration-effect relationship for seizure behavior was similar across zebrafish lines for each GABA_AR antagonist, significantly less TETS was required to trigger seizures relative to PTX or PTZ. Recordings of extracellular field potentials in the optic tectum of 5 dpf Tropical 5D zebrafish confirmed that all three GABA_AR antagonists elicited extracellular spiking patterns consistent with seizure activity, although the pattern varied between chemicals. Post-exposure treatment with the GABA_AR positive allosteric modulators (PAMs), diazepam, midazolam or allopregnanolone, attenuated seizure behavior and activity but did not completely normalize electrical field recordings in the optic tectum. These data are consistent with observations of seizure responses in mammalian models exposed to these same GABA_AR antagonists and PAMs, further validating larval zebrafish as a higher throughput-screening platform for antiseizure therapeutics, and demonstrating its appropriateness for identifying improved countermeasures for TETS and other convulsant chemical threat agents that trigger seizures via GABA_AR antagonism.

The Synapse as a Central Target for Neurodevelopmental Susceptibility to Pesticides

The developmental period of the nervous system is carefully orchestrated and highly vulnerable to alterations. One crucial factor of a properly-functioning nervous system is the synapse, as synaptic signaling is critical for the formation and maturation of neural circuits. Studies show that genetic and environmental impacts can affect diverse components of synaptic function. Importantly, synaptic dysfunction is known to be associated with neurologic and psychiatric disorders, as well as more subtle cognitive, psychomotor, and sensory defects. Given the importance of the synapse in numerous domains, we wanted to delineate the effects of pesticide exposure on synaptic function. In this review, we summarize current epidemiologic and molecular studies that demonstrate organochlorine, organophosphate, and pyrethroid pesticide exposures target the developing synapse. We postulate that the synapse plays a central role in synaptic vulnerability to pesticide exposure during neurodevelopment, and the synapse is a worthy candidate for investigating more subtle effects of chronic pesticide exposure in future studies.

A Comparison of ToxCast Test Results with In Vivo and Other In Vitro Endpoints for Neuro, Endocrine, and Developmental Toxicities: A Case Study Using Endosulfan and Methidathion

INTRODUCTION

The U.S. Environmental Protection Agency's (EPA's) Toxicity Forecaster (ToxCast) is a potential tool for chemical prioritization, hazard identification, and risk assessment. We conducted a case study to compare ToxCast data with endpoints from other in vitro and in vivo studies for two data-rich pesticides: endosulfan and methidathion.

METHODS

ToxCast assays for endocrine disruption, development (zebrafish), and neurotoxicity were qualitatively compared to traditional neurotoxicity, developmental and reproductive toxicity findings. We also used in vitro-in vivo extrapolation to convert half-maximal activity concentrations in active ToxCast assays to rat oral equivalent doses, and quantitatively compared these to the lowest observable effect level (LOEL) from in vivo studies.

RESULTS

Endosulfan was inactive for GABAA R, unlike in vivo; but active with dopamine transporter assays and was neurotoxic in zebrafish as expected. Methidathion was not active for these endpoints in vivo or in vitro. Acetylcholinesterase inhibition was ToxCast-inactive, although both pesticides are inhibitors in vivo. ToxCast results were generally inactive for endosulfan estrogen receptor agonism and androgen receptor antagonism unlike in vivo. Calculated oral equivalent doses for estrogen receptor and androgen receptor pathways and for zebrafish assays for both compounds were generally consistent with in vivo LOELs. Endosulfan showed neurotoxicity and both pesticides showed developmental effects in the zebrafish assays, although methidathion is not developmentally toxic in vivo.

CONCLUSIONS

ToxCast's predictions showed concordance on some endpoints and nonconcordance, consisting mainly of false inactives, in several critical endpoints, likely due to a lack of metabolic activation and limitations in assay design. Zebrafish assays were good predictors of developmental toxicity and neurotoxicity for endosulfan.

The neurotoxicity of organochlorine and pyrethroid pesticides

Organochlorine and pyrethroid compounds represent an old and a new class, respectively, of insecticides. Organochlorines such as DDT, dieldrin, or chlordecone, have been banned, primarily because of environmental issues. DDT is still used in certain countries to fight malaria-bearing mosquitoes, while lindane still finds some limited used against head lice. In contrast, pyrethroids find widespread use because of their efficacy, low environmental persistence, and relatively low mammalian toxicity. Like all insecticides, organochlorines and pyrethroids target the nervous system of insects and of nontarget species. All pyrethroids and DDT interact with the sodium channel; by keeping it open longer, they increase the likelihood of action potentials developing, thus creating a condition of hyperexcitability, whose main clinical sign is tremors. Most other organochlorines (except chlordecone), as well as certain (type II) pyrethroids, block the chloride channels of the GABA-A receptor, and cause seizures. Evidence of an association between exposure to organochlorine and pyrethroid insecticides and neurodegenerative diseases (e.g., Parkinson's disease) is weak, at best.

Tetramethylenedisulfotetramine alters Ca²⁺ dynamics in cultured hippocampal neurons: mitigation by NMDA receptor blockade and GABA(A) receptor-positive modulation

Tetramethylenedisulfotetramine (TETS) is a potent convulsant that is considered a chemical threat agent. We characterized TETS as an activator of spontaneous Ca²⁺ oscillations and electrical burst discharges in mouse hippocampal neuronal cultures at 13-17 days in vitro using FLIPR Fluo-4 fluorescence measurements and extracellular microelectrode array recording. Acute exposure to TETS (≥ 2 µM) reversibly altered the pattern of spontaneous neuronal discharges, producing clustered burst firing and an overall increase in discharge frequency. TETS also dramatically affected Ca²⁺ dynamics causing an immediate but transient elevation of neuronal intracellular Ca²⁺ followed by decreased frequency of Ca²⁺ oscillations but greater peak amplitude. The effect on Ca²⁺ dynamics was similar to that elicited by picrotoxin and bicuculline, supporting the view that TETS acts by inhibiting type A gamma-aminobutyric acid (GABA(A)) receptor function. The effect of TETS on Ca²⁺ dynamics requires activation of N-methyl-D-aspartic acid (NMDA) receptors, because the changes induced by TETS were prevented by MK-801 block of NMDA receptors, but not nifedipine block of L-type Ca²⁺ channels. Pretreatment with the GABA(A) receptor-positive modulators diazepam and allopregnanolone partially mitigated TETS-induced changes in Ca²⁺ dynamics. Moreover, low, minimally effective concentrations of diazepam (0.1 µM) and allopregnanolone (0.1 µM), when administered together, were highly effective in suppressing TETS-induced alterations in Ca²⁺ dynamics, suggesting that the combination of positive modulators of synaptic and extrasynaptic GABA(A) receptors may have therapeutic potential. These rapid throughput in vitro assays may assist in the identification of single agents or combinations that have utility in the treatment of TETS intoxication.

Characterization of seizures induced by acute and repeated exposure to tetramethylenedisulfotetramine

Tetramethylenedisulfotetramine (tetramine; TETS) is a potent convulsant poison that is considered to be a chemical threat agent. To provide a basis for the investigation of antidotes for TETS-induced seizures, we characterized the convulsant activity of TETS in mice and rats when administered by the intraperitoneal, intravenous, oral, and intraventricular routes as a single acute dose and with repeated sublethal doses. In mice, parenteral and oral TETS caused immobility, myoclonic body jerks, clonic seizures of the forelimbs and/or hindlimbs, tonic seizures, and death. The CD₅₀ values for clonic and tonic seizures after oral administration were 0.11 and 0.22 mg/kg, respectively. Intraventricular administration of TETS (5-100 μg) in rats also caused clonic-tonic seizures and death. In mice, repeated sublethal doses of TETS at intervals of 2, 24, and 48 h failed to result in the development of persistent enhanced seizure responsivity ("kindling") as was observed with repeated pentylenetetrazol treatment. In mice, sublethal doses of TETS that produced clonic seizures did not cause observable structural brain damage as assessed with routine histology and Fluoro-Jade B staining 7 days after treatment. However, 1 to 3 days after a single convulsant dose of TETS the expression of glial fibrillary acidic protein, an astrocyte marker, and ionized calcium binding adaptor molecule 1, a microglia marker, were markedly increased in cortex and hippocampus. Although TETS doses that are compatible with survival are not associated with overt evidence of cellular injury or neurodegeneration, there is transient reactive astrocytosis and microglial activation, indicating that brain inflammatory responses are provoked.

DDT and Other Chlorinated Insecticides

The use of organochlorine insecticides such as DDT, lindane and cyclodieneshas declined markedly worldwide over the last decades. Most are now banned or not used. At an acute toxicity level they have been relatively safe in use for humans. However, the greatest concerns are their persistence in people, wildlife and the environment due to their slow metabolism. Although their carcinogenicity for humans has not been supported by strong epidemiological evidence, their potential to be modulators of endocrine and immune function at levels remaining in the environment or associated with residual spraying of DDT continue to be of concern. At present, DDT is still allowed by the United Nations for combating malaria, with continual monitoring and assessment where possible. The toxicological consequences of exposure of animals and people to DDT is discussed as well as some analogues and other insecticides such as lindane, dieldrin and chlordecone that, although little used, continue to persist in surroundings and people. Because of circumstances of world health brought about by climate change or human activities that have yet to develop, there may come a time when the importance of some may re-emerge.

The environmental pollutant endosulfan disrupts cerebral cortical function at low doses

Endosulfan can induce convulsions that could lead to brain damage. The variability and lack of specificity of neurological signs and symptoms in the pre-convulsive stages makes early diagnosis difficult. We sought to determine if electrophysiological exploration of the cerebral cortex could yield objective signs of endosulfan intoxication at levels that do not elicit convulsions. Endosulfan was administered intravenously to Sprague-Dawley adult rats under urethane anesthesia at doses from 0.5 to 4mg/kg. EEG power and the evoked potentials (EP) to forepaw electrical stimulation were studied over the contralateral (S1CL) and homolateral (S1HL) cortical somatosensory areas and the contralateral visual area (V1CL). At each area, five EP waves were measured. Arterial blood pressure, heart rate and body temperature were also recorded. Endosulfan induced a dose-related increase in EPs at all sites. At S1CL, EP peak amplitude was greater than baseline at 1, 2 and 4mg/kg for the first negative, second positive and third negative waves, and at 2 and 4mg/kg for the first and third positive waves. Similar but less marked trends were observed at S1HL and V1CL. A shift of EEG power to higher frequencies (alpha and beta EEG bands) was only present at 4mg/kg. In conclusion, endosulfan induced a large increase of cortical evoked potentials amplitudes at doses that did not elicit convulsions. These responses could be used as a non-invasive diagnostic tool to detect low-level endosulfan intoxication in humans and to help establish the NOAEL and LOAEL levels of this pollutant.

Human health risk assessment of endosulfan. I: Toxicology and hazard identification

Endosulfan is persistent in the environment and toxic to wildlife. Legal mandates necessitate that a risk assessments be performed for endosulfan by the California Department of Pesticide Regulation (CDPR) and the United States Environmental Protection Agency (USEPA). This hazard identification (hazard ID) compared critical no-observed effect levels (NOEL) for acute, subchronic and chronic exposure intervals between the agencies. NOELs were discussed in light of their application to numerous exposure scenarios (occupational, general population and dietary). Only the acute oral NOELs differed between CDPR (0.7 mg/kg/day) and USEPA (1.5 mg/kg/day). Pregnant rabbits were considered by CDPR to be more responsive to low gavage doses of endosulfan than non-pregnant female or male rats in the acute study selected by USEPA. NOELs for other exposure routes and durations were similar between agencies. CDPR and USEPA concurred that a Food Quality Protection Act (FQPA, 1996) Safety Factor is not needed after evaluating all studies including a Developmental Neurotoxicity study. The SF was reduced to 1x. NOELs generated from this hazard ID will be used to calculate the Margins of Exposure for all scenarios and subsequently the risk characterization for endosulfan.

Immunologic and neurodevelopmental susceptibilities of autism

Symposium 5 focused on research approaches that are aimed at understanding common patterns of immunological and neurological dysfunction contributing to neurodevelopmental disorders such as autism and ADHD. The session focused on genetic, epigenetic, and environmental factors that might act in concert to influence autism risk, severity and co-morbidities, and immunological and neurobiological targets as etiologic contributors. The immune system of children at risk of autism may be therefore especially susceptible to psychological stressors, exposure to chemical triggers, and infectious agents. Identifying early biomarkers of risk provides tangible approaches toward designing studies in animals and humans that yield a better understanding of environmental risk factors, and can help identify rational intervention strategies to mitigate these risks.

Exposure to mixtures of endosulfan and zineb induces apoptotic and necrotic cell death in SH-SY5Y neuroblastoma cells,in vitro

A number of epidemiological studies have demonstrated a strong association between the incidence of Parkinson's disease and pesticide exposure. Earlier it was demonstrated that exposure to the pesticides endosulfan and zineb, alone and in combination, caused neurodegeneration in vivo. It was hypothesized that these pesticides cause neurotoxicity, in part, by enhancing apoptotic cell death. SH-SY5Y human neuroblastoma cells, which retain a catecholaminergic phenotype, were exposed to endosulfan, zineb or a combination of these chemicals, in vitro. For mixture studies, concentrations of pesticides (100 microM each) were chosen based on LC(25) (lethal concentration) that would result in minimum cell death. Exposure to a mixture of pesticides exhibited significantly (P < or = 0.05) higher toxicity than each one alone. Both pesticides were found to cause apoptotic cell death that was concentration (50-400 microM) dependent. A flow cytometric (7-aminoactinomycin D) assay was used to distinguish live, early apoptotic and late apoptotic/necrotic populations. Exposure to mixtures of the pesticides enhanced both early apoptosis and late apoptosis/necrosis compared with either chemical alone. Visual evaluation using a DNA ladder assay and a fluorescence Annexin V/PI assay confirmed the contribution of both apoptotic and necrotic processes. These findings suggest that the cytotoxicity of endosulfan and zineb, both individually and in mixtures, is associated with the occurrence of early and late apoptotic/necrotic processes in SH-SY5Y human neuroblastoma cells and support the contention that pesticide-induced neuronal cell death leading to neurodegenerative disease may, at least in part, be associated with early and late apoptosis of dopaminergic neurons.

Selective effects of insecticides on nigrostriatal dopaminergic nerve pathways

A degeneration of the nigrostriatal pathway is a primary component of Parkinson's disease (PD), and we have investigated the actions of insecticides on this pathway. For in vivo exposures, C57BL/6 mice were treated three times over a 2-week period with heptachlor, the pyrethroids deltamethrin and permethrin, or chlorpyrifos. One day after the last treatment, we observed that heptachlor and the pyrethroids increased maximal [3H]dopamine uptake in striatal synaptosomes from treated mice, with dose-dependent changes in Vmax displaying a bell-shaped curve. Western blot analysis confirmed increased levels of dopamine transporter (DAT) protein in the striatum of mice treated with heptachlor and permethrin. In contrast, we observed a small, but statistically significant decrease in dopamine uptake by 100 mg/kg chlorpyrifos. For heptachlor, doses that upregulated DAT expression had little or no effect on serotonin transport. Permethrin did cause an upregulation of serotonin transport, but required a 30-fold greater dose than that effective on dopamine uptake. Other evidence of specificity was found in transmitter release assays, where heptachlor and deltamethrin released dopamine from striatal terminals with greater potency than other transmitter types. These findings confirm that insecticides possess specificity for effects on striatal dopaminergic neurotransmission.

Selective effects of cyclodiene insecticides on dopamine release in mammalian synaptosomes

Cyclodiene insecticides release labeled neurotransmitter in striatal and cortical synaptosome preparations under nondepolarizing conditions, typically showing half-maximal potencies for release in the low micromolar range. This level of potency is similar to those reported for inhibition of 36Cl- influx at the gamma-aminobutyric acid (GABA)(A) receptor, their consensus target site. A wide variety of other GABA(A) antagonists, including picrotoxinin and bicuculline, did not cause significant dopamine release, which obviated direct involvement of the GABA(A) receptor as a possible site of action. Release assays with different transmitters indicated that striatal dopaminergic terminals are severalfold more sensitive to release than other neurotransmitter types. The selective sensitivity of nigrostriatal dopaminergic nerve terminals to insecticidal organochlorines provides biochemical evidence supporting an epidemiological linkage between exposure to environmental toxicants and Parkinsonism.

Effect of three chlorinated pesticides on hsromega stress gene in transgenic Drosophila melanogaster

Expression of hsromega stress gene in the third-instar larvae of 951-lacZ2 (hsromega-lacZ having-844pb sequence) and 498-lacZ1 (hrsomega-lacZ having -498bp sequence) strains of Drosophila melanogaster at LC(50) and lower dietary concentrations of hexachlorocyclohexane (HCH) pentachlorophenol (PCP), and endosulfan was examined in relation to larval mortality by beta galactosidase activity, vital dye staining, and salivary gland polytene chromosome puffing. Our results showed that both HCH and PCP at lower concentrations evoked strong hsromega stress gene expression in the larval tissues while endosulfan did not. On the other hand, puffing data revealed that endosulfan at lower doses, induced well-developed puff at the resident site (93D) of the hsromega gene but the transgenic sites (30B in 951-lacZ2 and 44B in 498-lacZ1 strain) did not show any well-developed puff. Regression in hsromega stress gene expression in 951-lacZ2 strain at LC(50) concentrations of HCH and PCP after 48 h was concurrent with extensive tissue damage as evident by trypan blue staining. Similarly, strong hsromega expression was accompanied by insignificant trypan blue staining in the larval tissues of this strain after shorter duration of exposure (2-12 h) to these toxicants. Although endosulfan under similar experimental condition did not induce hsromega, strong trypan blue staining indicated extensive tissue damage after 48 h of exposure. The present study suggests that all the three toxicants pose cytotoxic potential to Drosophila. While protective role of this stress gene was evident at the initial stages of exposure, extensive tissue damage in the later stages of intoxication accompanied by autorepression of hsromega led to larval mortality. The study further suggests that -844bp upstream sequence of the gene is adequate for hsromega inducibility against HCH and PCP but not for endosulfan for which responsive elements may be searched further upstream.

Role of human GABA(A) receptor beta3 subunit in insecticide toxicity

The gamma-aminobutyric acid type A (GABA(A)) receptor is the target for the major insecticides alpha-endosulfan, lindane, and fipronil and for many analogs. Their action as chloride channel blockers is directly measured by binding studies with [(3)H]ethynylbicycloorthobenzoate ([(3)H]EBOB). This study tests the hypothesis that GABA(A) receptor subunit composition determines the sensitivity and selectivity of insecticide toxicity. Human receptor subtypes were expressed individually (alpha1, alpha6, beta1, beta3, and gamma2) and in combination in insect Sf9 cells. Binding parameters were similar for [(3)H]EBOB in the beta3 homooligomer, alpha1beta3gamma2 heterooligomer, and native brain membranes, but toxicological profiles were very different. Surprisingly, alpha-endosulfan, lindane, and fipronil were all remarkably potent on the recombinant beta3 homooligomeric receptor (IC50 values of 0.5-2.4 nM), whereas they were similar in potency on the alpha1beta3gamma2 subtype (IC50 values of 16-33 nM) and highly selective on the native receptor (IC50 values of 7.3, 306, and 2470 nM, respectively). The selectivity order for 29 insecticides and convulsants as IC50 ratios for native/beta3 or alpha1beta3gamma2/beta3 was as follows: fipronil > lindane > 19 other insecticides including alpha-endosulfan and picrotoxinin > 4 trioxabicyclooctanes and dithianes (almost nonselective) > tetramethylenedisulfotetramine, 4-chlorophenylsilatrane, or alpha-thujone. Specificity between mammals and insects at the target site (fipronil > lindane > alpha-endosulfan) paralleled that for toxicity. Potency at the native receptor is more predictive for inhibition of GABA-stimulated chloride uptake than that at the beta3 or alpha1beta3gamma2 receptors. Therefore, the beta3 subunit contains the insecticide target and other subunits differentially modulate the binding to confer compound-dependent specificity and selective toxicity.

Regional modification of [(3)H]Ethynylbicycloorthobenzoate binding in mouse brain GABA(A) receptor by endosulfan, fipronil, and avermectin B(1a)

[(3)H]Ethynylbicycloorthobenzoate ([(3)H]EBOB), a high affinity radioligand for the noncompetitive blocker site of the GABA(A) receptor, is used here for quantitative autoradiography to determine regional binding in mouse brain and the effects on this binding of administering toxic doses of endosulfan, fipronil, and avermectin B(1a) (AVM). Animals were euthanized 4-8 min after 1 LD50 or 2 LD50 doses of the two channel blockers and 32 min after 1 LD50 or 4 LD50 doses of the channel activator AVM. Specific binding of [(3)H]EBOB was determined for 20-microm brain sections as the difference in labeling on incubation with 2 nM [(3)H]EBOB either alone (total binding) or with 5 microM alpha-endosulfan (nonspecific binding). The highest specific labeling was observed for layers I and IV of the cerebral cortex, the globus pallidus, and the medial septal nucleus/nucleus of the vertical limb of the diagonal band. Dose-dependent inhibition by endosulfan was highest in the nucleus accumbens and least in the cerebellum and periaqueductal gray matter. Fipronil had much less effect on binding even at severely toxic doses. AVM increased [(3)H]EBOB binding in most regions and was the only one of the three agents inhibiting in vitro [(3)H]strychnine binding to the glycine receptor. In summary, the noncompetitive blocker site was strongly inhibited with dose dependence and regional selectively by alpha-endosulfan but was generally poorly inhibited or activated by fipronil and was activated by avermectin.

Protection by conventional and new antiepileptic drugs against lindane-induced seizures and lethal effects in mice

Toxic effects caused by the organochlorine xenobiotic lindane may result from too excessive antiscabicidal treatment and in cases of accidental or intentional poisoning. Predominant symptoms of lindane intoxication concern the central nervous system, e.g. different manifestation of hyperexcitability and epileptiform activity. The inhibition of GABA-ergic neurotransmission seems to be responsible for the convulsant activity of lindane. This study was intended to compare the protective activity of conventional and new antiepileptic drugs against convulsions and lethal effects elicited by lindane administration in mice. Diazepam, clonazepam and phenobarbital protected against full seizure pattern and lethal effects evoked by lindane. Carbamazepine, phenytoin, gabapentin, felbamate and lamotrigine inhibited only lindane-induced tonic convulsions and mortality. It may be concluded that apart from benzodiazepines, phenobarbital and, to a lesser extent, carbamazepine, phenytoin, gabapentin, felbamate and lamotrigine could be used in lindane poisoning. Vigabatrin proved completely ineffective against seizures or lethal effects elicited by lindane.

Excitatory amino acid antagonists alleviate convulsive and toxic properties of lindane in mice

Pesticides acting at GABAA receptors may induce convulsions in man and animals, but the mechanisms responsible for their convulsant activity are not fully explained. The following excitatory amino acid antagonists were studied for their protective action in mice intoxicated with chlorinated hydrocarbon insecticide lindane (gamma-hexachlorocyclohexane): the competitive NMDA antagonist: 3-(2-carboxypiperazine-4-yl)propenyl-1- phosphonic acid (D-CPPene, 20 mg/kg), the non-competitive NMDA antagonist: dizocilpine (MK-801, 0.4 mg/kg), the glycine site antagonist of NMDA receptor: 2-phenyl-1,3-propane-diol dicarbamate (felbamate, 400 mg/kg) and the competitive AMPA antagonist: 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX, 100 mg/kg). Systemic administration of an antagonist prior to lindane resulted in a strong anticonvulsant effect. D-CPPene, MK-801 and NBQX produced a marked increase of CD50 values of lindane for clonic convulsions. All the antagonists protected animals against tonic convulsions. Toxicity of lindane was potently reduced, as assessed 2, 24 and 120 hr after administration of the pesticide. Our results demonstrate that excitatory amino acid antagonists reduce convulsant properties and toxicity of lindane, suggesting that excitatory amino acid neurotransmission may be involved in its central action.

Role of GABA receptor complex in low dose lindane (HCH) induced neurotoxicity: neurobehavioural, neurochemical and electrophysiological studies

Lindane is widely used as an insecticide and scabicide in mammals. High doses in chronic exposures caused hyperexcitability and convulsions and impaired motor activity involving GABA-ergic mechanism. To investigate the role of GABA/Benzodiazepine mechanism in the neurotoxicity of low doses of lindane, rats were administered 2, 3, or 5 mg/kg orally for 90 days and behavioural, electrophysiological, and neurochemical studies were conducted. The animals exposed to lindane exhibited increased geotaxis and decreased spontaneous drug-induced locomotor activity (which further potentiated by phenobarbitone and increased after leptazol). The EEG of the treated rats showed high voltage slow-wave activity (HVSA) patterns with occasional spindles (9-10 HZ-amplitude of 100 uv). A significant increase (p < 0.01) in GABA levels in cerebellum and significant increase in benzodiazepine receptors in cerebellar membrane measured by (3H)flunitrazepam binding were observed in the animals exposed to 3 and 5 mg lindane. The study suggests that low dose chronic exposure of lindane causes neurobehavioral, neurochemical, and electrophysiological effects involving GABA-ergic mechanism(s).

Allosteric interactions between gamma-aminobutyric acid, benzodiazepine and picrotoxinin binding sites in primary cultures of cerebellar granule cells. Differential effects induced by gamma- and delta-hexachlorocyclohexane

Allosterism between gamma-aminobutyric acid (GABA), benzodiazepine and picrotoxinin recognition sites on the GABAA receptor was studied in primary cultures of cerebellar granule cells. The increase in [3H]flunitrazepam binding induced by GABA was inhibited by bicuculline and picrotoxinin and the decrease in [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding mediated by GABA was reverted by bicuculline. The effects of hexachlorocyclohexanes (the convulsant gamma- and the depressant delta-isomers, both acting at the picrotoxinin recognition site) on GABA and benzodiazepine sites were studied. delta-Hexachlorocyclohexane, but not the gamma-isomer (lindane), increased [3H]flunitrazepam binding in a concentration-dependent manner (EC50: 8.3 microM). This increase in [3H]flunitrazepam binding was reduced by bicuculline and picrotoxinin. The gamma-isomer reduced the increase in [3H]flunitrazepam binding induced by GABA or delta-hexachlorocyclohexane. Neither delta- nor gamma-hexachlorocyclohexane inhibited [3H]GABA binding. Moreover, the inhibition of [35S]TBPS binding induced by delta-hexachlorocyclohexane was not reverted by bicuculline. The results obtained in this study in vitro agree with the pharmacological properties and the effects of gamma- and delta-hexachlorocyclohexane in vivo. It is concluded that delta-hexachlorocyclohexane acts as a positive allosteric modulator and gamma-hexachlorocyclohexane acts as a non-competitive antagonist of the GABAA receptor.

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.

Repeated exposure to lindane leads to behavioral sensitization and facilitates electrical kindling

Repeated intermittent exposure to some chemicals produces behavioral sensitization and seizure induction through a kindling mechanism. Although many pesticides are convulsant at high dosages, the persistent neurological effects of chronic low level exposure are unclear. The impact of intermittent exposure to lindane on behavioral seizure development and subsequent electrical kindling was assessed in the present study. Rats were administered lindane (0 or 10 mg/kg, po) for 30 days, or 3 times/week for 10 weeks. Enhanced behavioral responsiveness to lindane (myoclonic jerks, clonic seizures) emerged over the course of dosing and persisted 2 to 4 weeks after the last dose. The incidence of generalized convulsions was increased from 0% to 15% between the first and final day of dosing. In addition, electrographic recordings from the amygdala revealed brief rhythmic bursts and isolated interictal spike and wave discharge in the absence of overt behavioral seizures. Electrical kindling of the amygdala, beginning 4 to 6 weeks after the final dose, was facilitated. In contrast, prior administration of a single convulsive dose of lindane (20 mg/kg) was without effect on kindling development. These data indicate that repeated exposure to subconvulsant doses of lindane produces a persistent alteration in the central nervous system as evidenced by an enhanced susceptibility to kindled seizures. The pattern of behavioral development whereby the sensitivity is built up gradually over time is suggestive of a chemical kindling mechanism. Savings in the number of stimulation sessions required to induce electrical kindling following a history of lindane treatment provides further evidence that prior lindane exposure may lead to a state of partial kindling. Thus, intermittent subconvulsive lindane treatment induces alterations in limbic excitability that persist for at least 1 month.

Seizure thresholds in kindled animals are reduced by the pesticides lindane and endosulfan

Lindane and endosulfan are chlorinated hydrocarbon pesticides that bind to the gamma-aminobutyric-acidA (GABAA) receptor ionophore complex. We have previously demonstrated development of a persistent predisposition to the seizure-inducing properties of these pesticides following repeated administration of initially subconvulsive doses. A subsequent facilitation of electrical kindling development indicated that a chemical kindling mechanism may contribute to this enhanced sensitivity. We tested the bidirectionality of kindling transfer induced by chemical and electrical means by comparing the convulsive thresholds of lindane and endosulfan in electrically kindled and nonkindled animals. Amygdala kindled, implanted/nonstimulated, and nonimplanted controls were administered 0, 5, 10, or 20 mg/kg of lindane or endosulfan in corn oil by gavage, and each animal's behavior was scored for the presence of myoclonic jerks (MCJ) and clonic seizures. Dose-related increases in the number of animals displaying convulsions in response to both of these pesticides were observed. The dose of lindane (4.5 mg/kg) and endosulfan (4.3 mg/kg) producing MCJs in 50% of the animals (ED50) was reduced by more than 60% in kindled animals relative to controls. A second experiment evaluated the generalized seizure threshold in previously kindled animals treated with low doses of endosulfan (0, 2.5, and 5 mg/kg, po by gavage). Endosulfan reduced the intensity of electrical stimulation required to evoke seizures in amygdala kindled animals. These data indicate that these chlorinated hydrocarbon insecticides reduce seizure thresholds in amygdala-kindled animals with acute administration, and together with previous findings (see ref. 20) demonstrate bidirectional transfer between chemical and electrical kindling.(ABSTRACT TRUNCATED AT 250 WORDS)

Lindane cytotoxicity in cultured neocortical neurons is ameliorated by GABA and flunitrazepam

The effect of lindane (gamma-hexachlorocyclohexane) on [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding and GABA-stimulated 36Cl- influx was investigated in cultured cerebral cortical neurons. In addition, the cytotoxic action of lindane as well as a protection by GABA and flunitrazepam were studied together with the ability of lindane to increase the intracellular concentration of free Ca2+. Lindane was found to be toxic to the neurons, an effect that could be completely prevented by the simultaneous presence of GABA (0.1 microM) and flunitrazepam (100 microM) and reduced by GABA alone. An interaction with the GABA receptor-gated chloride channel was demonstrated by an inhibitory action of lindane on [35S]TBPS binding (IC50 188 +/- 51 nM) and on GABA-stimulated 36Cl- influx in the neurons. Lindane only marginally increased the intracellular Ca2+ concentration in the neurons. It is concluded that the cytotoxic action of lindane is mediated through interaction with GABA receptors in a manner essentially independent of changes in intracellular Ca2+ homeostasis.

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

We have recently demonstrated that the cyclodiene insecticide dieldrin modulates the kinetics of the GABAA receptor-chloride channel complex of rat dorsal root ganglion neurons in a complex manner, causing both stimulatory and inhibitory effects. We now report that the differential effects of dieldrin on the GABA-induced chloride current of human embryonic kidney cells expressing three different combinations of alpha, beta and gamma subunits. The EC50 values for GABA induction of current were estimated to be 9.8 microM for the alpha 1 beta 2 gamma 2s combination, 2.0 microM for the alpha 1 beta 2 combination and 3.0 microM for the alpha 6 beta 2 gamma 2s combination. When co-applied with GABA, dieldrin exerted a dual effect, enhancement and suppression, on the GABA-induced chloride currents in the alpha 1 beta 2 gamma 2s and alpha 6 beta 2 gamma 2s combinations. However, only suppression was observed in the alpha 1 beta 2 combination, indicating that the gamma subunit is necessary for dieldrin's enhancing effect. Dieldrin was more efficacious in enhancing the current in the alpha 6 beta 2 gamma 2s combination than in the alpha 1 beta 2 gamma 2s combination, indicating some specific role of alpha subunits in the dieldrin enhancement of current. Dieldrin suppressed the GABA-induced current in a non-competitive manner, with an EC50 value of 2.1 microM for alpha 1 beta 2 gamma 2s, 2.8 microM for alpha 1 beta 2 and 1.0 microM for alpha 6 beta 2 gamma 2s combination. These results indicated that dieldrin suppression did not require specific subunit combinations among the three tested.

Toxicology, mode of action and target site-mediated resistance to insecticides acting on chloride channels

1. The discovery that the avermectins and cyclodienes affected the chloride channels of excitable membranes generated intense research interest among academic and industrial scientists. 2. The results of biochemical and neurophysiological studies indicate that the gamma-aminobutyric acid (GABA)-gated chloride channel is an important, if not the primary site of action for these compounds. 3. The action of insecticides on the functional properties of the GABA receptor differs by structural class. The cyclodienes block the chloride ion channel and the avermectins activate it. 4. Blockage of the GABA-gated chloride channel by cyclodienes reduces neuronal inhibition, which leads to hyper-excitation of the central nervous system, convulsions, and death. For avermectins, activation of the channel suppresses neuronal activity, resulting in ataxia, paralysis and death. Although actions on the GABA-gated chloride channel can explain many of the effects of these compounds, there is evidence supporting the participation of other ligand- and voltage-gated chloride channels in the overall intoxication process. This consideration is especially true for the avermectins. 5. Several structural series of experimental insecticides have been synthesized which possess a blocking action on the GABA-gated chloride channel similar to that of the cyclodienes. 6. Resistance to cyclodienes usually occurs through an altered target site, and extends to all experimental compounds that block chloride channels. However, the resistance does not afford protection against the avermectins. 7. The continued search for new insecticides directed against chloride channels may lead to compounds with less environmental impact and greater selectivity than that of the cyclodienes. Given the pre-selection for resistance by the cyclodienes, new compounds with a similar mode of action must be used judiciously in order to suppress or delay the re-emergence of widespread resistance.

Inhibition of t-[35S]butylbicyclophosphorothionate binding by convulsant agents in primary cultures of cerebellar neurons

The characteristics of the picrotoxinin binding site present on the gamma-aminobutyric acidA (GABAA) receptor were studied in neurons using primary cultures of cerebellar granule cells. The binding properties of these sites in intact cultured cells were compared with those measured in cultured cell membrane preparations. t-[35S]Butylbicyclophosphorothionate (TBPS) binding was performed in cultured rat cerebellar neurons grown for 13 days. Binding parameters (Kd and Bmax) were similar to those reported in the literature determined using brain membranes. However, equilibrium was reached faster when using intact cultured neurons. Convulsant compounds like picrotoxinin (PTX) and pentylenetetrazol (PTZ) competitively inhibited the binding of TBPS in this in vitro system. Convulsant organochlorine pesticides (gamma-hexachlorocyclohexane gamma-HCH or lindane and the cyclodienes aldrin, endrin, dieldrin and alpha-endosulfan) competitively inhibited [35S]TBPS binding in cerebellar neuronal cultures. Inhibitory affinity constant (Ki) values were in the nanomolar range, alpha-endosulfan and endrin being the most potent inhibitors corresponding to their high toxicity in mammals. Stereospecificity was also shown for HCH isomers, the non-convulsant isomers (alpha- and delta-HCH) being 15-30 times less potent in inhibiting [35S]TBPS binding than the convulsant gamma-HCH, while the beta-isomer was inactive.

Lindane administration to the rat induces modifications in the regional cerebral binding of [3H]Muscimol, [3H]-flunitrazepam, and t-[35S]butylbicyclophosphorothionate: an autoradiographic study

The effect of lindane administration on the specific binding of ligands to different sites on the GABAA receptor-ionophore complex was studied in the rat brain by receptor mapping autoradiography. [3H]Muscimol (Mus), [3H]flunitrazepam (Flu), and t-[35S]butylbicyclophosphorothionate (TBPS) were used as specific ligands of GABA, benzodiazepine, and picrotoxinin binding sites, respectively. Rats received a single oral dose of 30 mg/kg lindane and they were classified into two groups according to the absence or presence of convulsions. Vehicle-treated groups acted as controls. The effect of the xenobiotic on ligand binding was measured in different brain areas and nuclei 12 min or 5 h after its administration. Lindane induced a generalized decrease in [35S]TBPS binding, which was present shortly after dosing. In addition, [3H]Flu binding was increased in lindane-treated animals, this modification also appearing shortly after administration but diminishing during the studied time. Finally, lindane induced a decrease in [3H]Mus binding, which became more evident over time. These modifications were observed both in the presence and in the absence of convulsions. However, an increase in [3H]-Mus binding was detected shortly after lindane-induced convulsions. The observed decrease in [35S]TBPS binding is in agreement with the postulated action of lindane at the picrotoxinin binding site of the GABAA receptor chloride channel. The effects observed on the binding of [3H]Flu and [3H]Mus may be secondary to the action of lindane as an allosteric antagonist of the GABAA receptor.

GABA receptor molecules of insects

Receptors for 4-aminobutyric acid (GABA) have been identified in both central and peripheral nervous systems of several invertebrate phyla. To date, much of the information derived from physiological and biochemical studies on insect GABA receptors relates to GABA-gated chloride channels that show some similarities with vertebrate GABAA receptors. Like their vertebrate central nervous system (CNS) counterparts, agonist activation of such insect GABA receptors leads to a rapid, picrotoxin-sensitive increase in chloride ion conductance across the cell membrane. In insects, responses to GABA can be modulated by certain benzodiazepines and barbiturates. However, recent studies have detected a number of striking pharmacological differences between GABA-gated chloride channels of insects and vertebrates. Receptor binding, electrophysiological and 36Cl- flux assays have indicated that many insect receptors of this type are insensitive to the vertebrate GABAA antagonists bicuculline and pitrazepin. Benzodiazepine binding sites coupled to insect GABA receptors display a pharmacological profile distinct from that of corresponding sites in vertebrate CNS. Receptor binding studies have also demonstrated differences between convulsant binding sites of insect and vertebrate receptors. Insect GABA receptor molecules are important target sites for several chemically-distinct classes of insecticidally-active molecules. By characterizing these pharmacological properties in detail, it may prove possible to exploit differences between vertebrate and insect GABA receptors in the rational design of novel, more selective pest control agents. The recent application of the powerful techniques of molecular biology has revealed a diversity of vertebrate GABAA receptor subunits and their respective isoforms that can assemble in vivo to form a multiplicity of receptor subtypes. Molecular cloning of insect GABA receptor subunits will not only enhance our understanding of invertebrate neurotransmitter receptor diversity but will also permit the precise identification of the sites of action of pest control agents.

Repeated lindane exposure in the rat results in changes in spontaneous motor activity at 2 weeks post-exposure

Male Wistar rats were given 25 doses of lindane, 10 mg/kg per day, 1 ml/kg p.o. in olive oil, or control vehicle. Two weeks after the last dose, the animals were assessed for modifications in spontaneous motor activity, plus-maze behavior, shuttle-box active avoidance acquisition, brain regional concentrations of biogenic amines and metabolites, and regional [35S]TBPS binding. Rats treated with lindane showed an increase in spontaneous motor activity. Although no additional behavioral or neurochemical modifications were found, the changes in activity observed at 2 weeks post-exposure further demonstrate the need to assess for long lasting neurobehavioral sequelae of repeated lindane exposure.

Intrinsic lethality of chloride-channel-directed insecticides and convulsants in mammals

The intrinsic lethality of a series of chloride-channel-directed convulsants and insecticides was determined by intracerebral injection into mice. The toxicities of the cyclodiene insecticides and picrotoxinin were potentiated by intracerebral injection, when compared to their reported intraperitoneal LD50s. In contrast, the toxicities of lindane, abamectin, and the bicyclic convulsants TBPS and a TBOB analog were approximately the same by either route of administration. These results suggest that the brain is the primary target site for the cyclodienes and picrotoxinin, while the peripheral nervous system may be relatively more important in the toxic action of lindane, abamectin, and bicyclic convulsants such as TBPS. With the exception of picrotoxinin these compounds showed, overall, a good correlation between acute intracerebral toxicity and potency for inhibiting GABA-dependent chloride uptake. The evidence that multiple chloride-channel subtypes serve as targets for these compounds and the potential impact this had on the results of these studies are discussed.

Proconvulsant activity of endosulfan in amygdala kindling

The proconvulsant properties of the chlorinated hydrocarbon insecticide, endosulfan, were investigated using electrical kindling of the amygdala. Male rats were implanted with electrodes in the amygdala and stimulated once daily with a standard kindling stimulus 60-90 min following endosulfan (0, 2.5, 5.0 mg/kg, PO). No alterations were observed in either the threshold to induce an afterdischarge (AD) or the duration of clonus upon seizure generalization. Endosulfan significantly reduced the number of stimulations required to produce Stage 5 generalized seizures. Seizures prior to stimulation were evident in a subset of animals from both dosage groups and were never observed in controls. The presence of kindled seizures was maintained in the absence of further dosing, as amygdala stimulation 2-4 weeks after the last endosulfan treatment resulted in generalized seizures in all animals. These results suggest that faster kindling rates induced by endosulfan are not readily attributable to transient toxicant-related increases in excitability of the nervous system. It was concluded that endosulfan has proconvulsant properties that may be related to an action on GABA within the central nervous system.

A characterization of chemical kindling with the pesticide endosulfan

In a companion article (see this issue), the proconvulsant properties of the pesticide endosulfan in electrical kindling of the amygdala are described. In the present report, an evaluation of the chemical kindling properties of endosulfan is presented. Repeated administration (3 times per week for a total of 21 doses) of endosulfan (5 and 10 mg/kg in corn oil, PO) was found to induce behavioral seizures in rats. Behavioral seizure development was most apparent in the high dose group (10 mg/kg). Heightened seizure responsiveness to a challenge dose was maintained following a two-week, drug-free period, arguing against cumulative toxicity as a mechanism for seizure induction. Electrical kindling induced by once daily stimulation of the amygdala began approximately 4 weeks after the final dose of endosulfan. In the absence of further dosing, a significant facilitation in the rate of kindling development was evident as a function of prior treatment with endosulfan. An enhancement in the rate of kindling was also evident in the low dose group (5 mg/kg) in the absence of clonic seizure development during dosing. A history of endosulfan treatment failed to affect threshold for inducing an afterdischarge (AD) and had equivocal effects on the development of AD with repeated stimulation. Pretreatment with a single high dose of endosulfan 2 weeks prior to electrical kindling was without effect on the development of the kindled response. Endosulfan has been reported to decrease binding of gamma-amino-butyric acid (GABA). Chemical kindling with endosulfan may result from the interaction of this pesticide with GABA-mediated neurotransmission in the central nervous system (CNS).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of different hexachlorocyclohexanes and cyclodienes on glucose uptake and inositol phospholipid synthesis in rat brain cortex

The inositol lipids from rat brain miniprisms were deacylated and separated by anion-exchange chromatography in order to determine whether or not gamma-hexachlorocyclohexane (gamma-HCH, lindane) and related compounds affect the different phosphatidylinositols. The incorporation of myo-[2-3H]inositol into phosphatidylinositol, phosphatidylinositol monophosphate and phosphatidylinositol bisphosphate were inhibited by lindane and its delta-HCH isomer. The inhibitory effects on phosphatidylinositol synthesis are not prominent in alpha-HCH and they are not significant with the beta-HCH and cyclodienes. The results presented here indicate that the inhibitory effect of lindane and delta-HCH on the phosphatidylinositol metabolism was no exclusively due to an interference with glucose transport. Lindane-treated miniprisms showed decreased myo-[2-3H]inositol uptake and, proportionately, an even greater inhibition of inositol phospholipid synthesis. Cellular uptake can, therefore, not account for all of the lindane inhibition.

Toxicokinetics of Ro 5-4864, lindane and picrotoxin compared

The effects produced by IP administration of these three agents in the rat were compared because of in vitro evidence that each modulates the picrotoxinin site of the GABAA receptor. For each, hypothermia had the lowest threshold and convulsions the next, with hypophagia produced only by the highest dose of either Ro 5-4864 or lindane. Convulsant effects had a shorter latency and a shorter duration than did hypothermia. Hypophagia, when present, lasted the longest. Myoclonus was the seizure type with the lowest threshold for all three agents. At the highest dose, lindane produced a high incidence of maximal clonic (hopping) seizures, whereas Ro 5-4864 and picrotoxin produced a high incidence of maximal tonic seizures instead. On a mole/kg basis, picrotoxin was 40 times more effective than the other two agents and produced seizures which started later, peaked later, and persisted longest. Ro 5-4864 and lindane were effective at equimolar concentrations and, in combination, produced effects which suggested either dose-addition or synergism. The data are consistent with the hypothesis that the toxic effects of both Ro 5-4864 and lindane may be attributable, at least in part, to an action at a subpopulation of GABAA receptors.

Differential presynaptic effects of hexachlorocyclohexane isomers on noradrenaline release in cerebral cortex

To investigate presynaptic effects of hexachlorocyclohexane (HCH) isomers, the release of noradrenaline (NA) in brain tissue was analyzed using rat cerebral cortical slices preloaded with [3H]-NA. gamma-HCH (lindane) 50 microM significantly enhanced the [3H]-NA release evoked by 15-25 mM K+. alpha- and beta-HCH (50 microM) did not produce any significant effect on K(+)-evoked [3H]-NA release. delta-HCH (50 microM) induced a significant decrease of the 25 mM K(+)-evoked release of [3H]-NA. The effect of the gamma- and delta-HCH isomers on the presynaptic action of the alpha 2-agonist clonidine and the alpha 2-antagonist yohimbine was also studied. The presynaptic inhibitory effect of clonidine and the stimulatory effect of yohimbine on [3H]-NA release was attenuated by lindane and delta-HCH, respectively. These results are consistent with a presynaptic action of the HCH isomers on noradrenergic release processes.

Prolonged exposure to GABA activates GABA-gated chloride channels in the presence of channel-blocking convulsants

1. In assays of 36Cl- uptake into mouse brain vesicles, 100 microM GABA markedly increased both the initial rate of 36Cl- uptake and the total amount of chloride taken up over a 120-sec incubation period. Specific GABA-dependent 36Cl- uptake (the difference between total and background uptake) was essentially complete within 15 sec of incubation. 2. Incubation with GABA following preincubation with 10 microM endrin, a polychlorocycloalkane insecticide and established blocker of GABA-gated chloride channels, showed a stimulation of uptake over background levels that was much slower in onset than that observed with GABA alone but nevertheless achieved virtually the same level of stimulation above background levels after 90 sec of incubation with GABA. 3. In electrophysiological assays of GABA receptors expressed in Xenopus oocytes following injection with rat brain mRNA, endrin (20 microM) effectively blocked the transient currents elicited by brief exposure of oocytes to GABA (200 microM). However, prolonged exposure to GABA in the absence of perfusion produced a large, slowly-developing inward current. 4. The actions of several known GABA antagonists were also compared as inhibitors of GABA-dependent 36Cl- uptake into mouse brain vesicles at short (4 sec) and long (120 sec) incubation times using concentrations of inhibitors known to produce approximately 70-90% inhibition of GABA-dependent chloride uptake in 4-sec incubations. Picrotoxinin and TBPS, like endrin, were completely ineffective as inhibitors in 120-sec incubations. In contrast, bicuculline was almost as effective at 120 sec as at 4 sec, and avermectin Bla produced approximately 50% inhibition of the GABA response after 120 sec.(ABSTRACT TRUNCATED AT 250 WORDS)

Lindane inhibition of [35S]TBPS binding to the GABAA receptor in rat brain

The inhibition of [35S]t-butylbicyclophosphorothionate [( 35S]TBPS) binding to the GABAA receptor by the insecticide gamma-hexachlorocyclohexane, lindane, was studied in several brain regions and using different membrane preparation methods, both in vitro and after dosing the animals with the chemical. In the latter studies, the amount of lindane remaining in the membrane suspensions used for binding assays was determined. In vitro data showed values of IC50 from 150 to 1675 nM, varying in function of the membrane preparation method used. This may account for the discrepancies in IC50 values found in the literature. IC50 values within the range of 150-250 nM were determined using extensively washed membranes from several brain regions, so no evidence arose for brain regional differences in the affinity of lindane for the TBPS binding site. After different schedules of acute treatment with lindane, we found a manifest relationship between the extent of the observable inhibition of [35S]TBPS binding and the lindane amount remaining in the membrane suspensions used for binding assays. This relationship was in good agreement with the in vitro data, so no support for an in vivo acute regulation of the binding site was obtained.

Mechanisms of toxic action and structure-activity relationships for organochlorine and synthetic pyrethroid insecticides

The mechanisms and sites of action of organochlorine (DDT-types and chlorinated alicyclics) and synthetic pyrethroid insecticides are presented with discussion of symptoms, physiological effects, and selectivity. The structural requirements for toxicity are assessed, and structure-activity relationships are considered for each subclass. Lipophilicity is important for all the groups because it facilitates delivery of these neurotoxicants to the site of action in the nerve. Steric factors including molecular volume, shape, and isomeric configuration greatly influence toxicity. Electronic parameters also have been demonstrated to affect biological activity in some of the groups of insecticides, e.g., Hammett's sigma and Taft's sigma * as indicators of electronegativity. New synthetic pyrethroids continue to be developed, with varied structures and different physicochemical and biological properties.

Survey of the QSAR and in vitro approaches for developing non-animal methods to supersede the in vivo LD50 test

Quantitative structure-activity relationship (QSAR) studies and in vitro studies in which correlations with LD50 have been sought are reviewed. QSAR methods have shown some success in relating LD50 to certain physicochemical properties of the compound, particularly lipophilicity, but have been less successful in correlating LD50 with electronic properties of molecules (related to reactivity) or structural variables. It is concluded that insufficient evidence is available to determine whether QSAR methods can be of general use in predicting the acute toxicity (LD50) of chemicals, and that until further work is undertaken to develop QSARs for a much wider range of homologous series of compounds, this situation is unlikely to be resolved. New chemical descriptors that are more directly relevant to the mechanism of toxic action of the chemical should be identified. Cytotoxicity in vitro is poorly correlated with LD50, but good correlations have been obtained between toxicity in vivo and in vitro, using systems in which the toxic endpoint reflects the probable mechanism(s) of acute toxicity of the test chemical (e.g. the assessment of neurotoxins using neural cell systems). Therefore, it seems that the successful application of in vitro methods requires a better understanding of the mechanisms of acute toxicity in vivo and the development of mammalian cell culture systems that can model more closely the metabolic fate of the chemicals in vivo.

Microcomputer adaptation of the wheel-shaped activity monitor: effects of lindane

The development of microcomputers with increased power and memory capacity may allow for the spreading of the techniques of microanalysis of animal behavior in neurotoxicology. The present work describes the adaptation of the wheel-shaped activity monitor to a personal computer system (IBM-PC/XT/AT-compatible). The system has been used to study the effects elicited by a single 30 mg/kg dose of the organochlorine insecticide gamma-hexachlorocyclohexane (lindane) on the spontaneous behavior of rats. Lindane induced complex changes in behavior, the most prominent being a disruption of the temporal pattern of activity and changes in the local activity/locomotor activity ratios and in place preferences in the monitor. Effects on body weight and number of fecal boluses were also observed.

Effects of long-term aldrin administration on seizure susceptibility of rats

The effects of long-term administration of sub-convulsive doses of aldrin, an organochlorinated pesticide, on seizure susceptibility of rats were studied. Results show that treatment with the pesticide made the rats more susceptible to sound-induced seizures; an increment in post-seizure depression time after maximal electroshock administration was also observed in aldrin-treated animals. Aldrin administration displaced the control dose-response curves constructed for amphetamine and pentylenetetrazol to the left and right, respectively; no differences were detected between control and aldrin-treated rats for strychnine, picrotoxin or 3-mercaptopropionic acid. The results, discussed in the light of the effects of aldrin on biogenic amines or on the electrophysiology of neurones suggested an involvement of both noradrenergic and alteration excitability of the CNS with the observed increments on seizure susceptibility.